The ArmstrongHumidification

Handbook

Handbook HB-501

2

Although humidity is invisible to our eyes,we can easily observe its effects. Inhuman terms, we are more comfortableand more efficient with proper humid-ification. In business and industrial environ-ments, the performance of equipmentand materials is enhanced by effectivelyapplying humidity control.

Maintaining indoor air quality throughhumidity management can lower energycosts, increase productivity, save laborand maintenance costs, and ensureproduct quality. In short, humidificationcan provide a better environment andimprove the quality of life and work.

Armstrong has been sharing know-howin humidification application since 1938.Through the design, manufacturing, andapplication of humidification equipment

Humidification Plays an Essential Role

Controlled humidification helps protecthumidity-sensitive materials, personnel,delicate machinery, and equipment.Beyond the important issues of comfortand process control, humidity controlcan help safeguard against explosiveatmospheres. You can’t afford NOTto humidify. And the best way to protectyour investment is through provenhumidification strategies and solutionspioneered by Armstrong.

Armstrong has led the way to countlesssavings in energy, time and money.Armstrong also provides computersoftware, video tapes, and other educa-tional materials to aid in humidificationequipment selection, sizing, installation,and maintenance.

Armstrong offers the newly updatedHumidification Handbook as a problem-solving, educational aid for those involvedwith the design, installation, and mainten-ance of environmental control systemsin all types of buildings. In addition, youmay request a free copy of Armstrong’sSoftware Program 2 (Humidifier Sizingand Selection) for step-by-step sizing ofyour own installation.

Your specific humidification questionscan be answered by your ArmstrongRepresentative. Additional support fromArmstrong International humidificationspecialists is available to assist withdifficult or unusual problems.

© 1995 Armstrong International, Inc.

3

Why Humidification is Important 4-7

How Humidity Affects Materials 8-9

Determining Humidity Requirements of Materials 10

How Psychrometrics Help in Humidification 11-13

How Humidifiers Work 14-17

Considerations in Selecting Steam Humidifiers 18-21

Basic Application Principles 22-25

Sizing Considerations 26-28

Steam Humidifiers in Central Systems 29-34

Installation Tips 35

Application of Unit Humidifiers for Direct Discharge 36-38

Conclusion 39

Table of Contents

4

Humidity and TemperatureHumidity is water vapor or moisture contentalways present in the air. Humidity isdefinable as an absolute measure: theamount of water vapor in a unit of air. Butthis measure of humidity does not indicatehow dry or damp the air is. This can onlybe done by computing the ratio of theactual partial vapor pressure to thesaturated partial vapor pressure atthe same temperature. This is relativehumidity, expressed by the formula:

vpa = actual vapor pressurevps = vapor pressure at saturation

t = dry-bulb temperature

For practical purposes, at temperaturesand pressures normally encountered inbuilding systems, relative humidity isconsidered as the amount of water vaporin the air compared to the amount the aircan hold at a given temperature.

“At a given temperature” is the key tounderstanding relative humidity. Warm airhas the capacity to hold more moisture thancold air. For example, 10,000 cubic feet of70°F air can hold 80,550 grains of moisture.The same 10,000 cubic feet of air at 10°Fcan hold only 7,760 grains of moisture.

Humidification is simply the addition ofwater to air. However, humidity exerts apowerful influence on environmental andphysiological factors. Improper humiditylevels (either too high or too low) cancause discomfort for people, and candamage many kinds of equipment andmaterials. Conversely, the proper type ofhumidification equipment and controlscan help you achieve effective, economi-cal, and trouble-free control of humidity.

As we consider the importance of humidityamong other environmental factors—temperature, cleanliness, air movement,and thermal radiation—it is important toremember that humidity is perhaps theleast evident to human perception. Mostof us will recognize and react more quicklyto temperature changes, odors or heavydust in the air, drafts, or radiant heat.Since relative humidity interrelates withthese variables, it becomes a vitalingredient in total environmental control.

If the 10,000 cubic feet of 10°F air held 5,820grains of moisture, its relative humidity wouldbe 75%. If your heating system raises thetemperature of this air to 70°F with no moistureadded, it will still contain 5,820 grains of

Why Humidification is Important

Relative Humidity(RH):The ratio of the vapor pressure (ormole fraction) of water vapor in the airto the vapor pressure (or mole fraction)of saturated air at the same dry-bulbtemperature and pressure.

Sensible Heat:Heat that when added to or taken awayfrom a substance causes a change intemperature or, in other words, is “sensed”by a thermometer. Measured in Btu.

Latent Heat:Heat that when added to or taken awayfrom a substance causes or accompa-nies a phase change for that substance.This heat does not register on a ther-mometer, hence its name “latent” orhidden. Measured in Btu.

Dew Point:The temperature at which condensationoccurs (100%RH) when air is cooled ata constant pressure without adding ortaking away water vapor.

Evaporative Cooling:A process in which liquid water isevaporated into air. The liquid absorbsthe heat necessary for the evaporationprocess from the air, thus, there is areduction in air temperature and anincrease in the actual water vaporcontent of the air.

Enthalpy:Also called heat content, this is the sumof the internal energy and the productof the volume times the pressure.Measured in Btu/lb.

Hygroscopic Materials:Materials capable of absorbing orgiving up moisture.

Phase:The states of existence for a sub-stance, solid, liquid, or gas (vapor).

moisture. However, at 70°F, 10,000 cubic feetof air can hold 80,550 grains of moisture. Sothe 5,820 grains it actually holds give it arelative humidity of slightly more than 7%.That’s very dry...drier than the Sahara Desert.

70°F80,550 Grains

10°F7,760 Grains

RH =vpavps

Glossary

t

5

Evaporative CoolingWe’ve discussed the effects of changingtemperature on relative humidity. AlteringRH can also cause temperature to change.For every pound of moisture evaporatedby the air, the heat of vaporizationreduces the sensible heat in the air byabout 1,000 Btu. This can be moistureabsorbed from people or from wood,paper, textiles, and other hygroscopicmaterials in the building. Conversely, ifhygroscopic materials absorb moisturefrom humid air, the heat of vaporizationcan be released to the air, raising thesensible heat.

Dew PointCondensation will form on windowswhenever the temperature of the glasssurface is below the dew point of the air.Table 5-2, from data presented in theASHRAE Handbook & Product Directory,indicates combinations of indoor relativehumidity and outside temperature at whichcondensation will form. Induction units,commonly used below windows in modernbuildings to blow heated air across theglass, permit carrying higher relativehumidities without visible condensation.

Air Movement and HumidityAnother variable, air movement in theform of infiltration and exfiltration from thebuilding, influences the relationship betweentemperature and relative humidity. Typically,one to three times every hour (and manymore times with forced air make-up orexhaust) cold outdoor air replaces yourindoor air. Your heating system heats thiscold, moist outdoor air, producing warm,dry indoor air.

Per lbDry Air°F

-10- 5

05

10

1520253031

3233343536

3738394041

4243444546

474849

Table 5-1. Grains of Water per Cubic Foot of Saturated Air and per Pound of Dry Airat Various Temperatures. (Abstracted from ASHRAE Handbook)

Percu ft

0.284660.369170.47500

0.6090.776

0.9841.2421.5581.9462.033

2.1242.2032.2882.3762.469

2.5632.6602.7602.8632.970

3.0813.1963.3153.4363.562

3.6923.8263.964

Per lbDry Air

3.21864.22105.5000

7.129.18

11.7715.0119.0524.0725.21

26.4027.5228.6629.8331.07

32.3333.6234.9736.3637.80

39.3140.8842.4844.1445.87

47.6649.5051.42

°F Percu ft

Per lbDry Air

Percu ft°F Per lb

Dry Air °F Percu ft

4.4074.5614.722

525354

57.5859.7461.99 82

83848586

11.75

12.1112.4912.8713.27

166.4

172.1178.0184.0190.3

5150 4.106 53.38

4.255 55.45

5.2345.4155.602

6.1966.4076.622

7.3087.5717.798

8.5888.8679.153

575859

626364

676869

727374

69.2371.8274.48

83.0286.0389.18

99.19102.8106.4

118.2122.4126.6

77 10.06 140.4

78798081

9.4489.749

7576

131.1135.7

8.0558.319

7071

110.2114.2

6.8457.074

6566

92.4095.76

5.7955.993

6061

77.2180.08

4.8895.060

5556

64.3466.75

87

949596

104

13.67

16.7917.2817.80

22.32

196.7

247.5255.6264.0

341.5105 22.95 352.6

97

98

18.31

18.85

272.7

281.799

100101102

103

19.3919.9520.52

21.11

21.71

290.9300.5310.3

320.4

330.8

106 23.60 364.0

93 16.31 239.5

88 14.08 203.389909192

14.5114.9415.3915.84

210.1217.1224.4231.8

10.3810.7111.0411.39

145.3150.3155.5160.9

440.4454.5469.0483.9

566.5

1250.91473.51743.02072.72480.8

3664.54550.75780.67581.0

112113114115

120

145150155160165

175180185190

27.8128.5729.3430.13

34.38

64.0471.9980.7790.43101.0

125.4139.2154.3170.7

375.8387.9400.3413.3

107 24.26108109110

24.9325.6226.34

426.4111 27.07

662.6774.9907.9

1064.7

125130135140

39.1344.4150.3056.81

2996.0170 112.6

10493.015827.0

195200

188.6207.9

Double Glass(Storm Windows

or Thermal Glass)

38%42%49%56%63%71%

SingleGlass

11%16%21%28%37%48%

OutdoorTemperature

-100

+10+20+30+40

Table 5-2. Relative Humidities at WhichCondensation Will Appear on Windows at70°F When Glass Surface is Unheated

6

Energy ConservationWith Controlled RHIndoor relative humidity as we havecomputed it is called Theoretical IndoorRelative Humidity (TIRH). It virtually neverexists. RH observed on a measuringdevice known as a hygrometer will almostalways exceed the TIRH. Why? Dry air isthirsty air. It seeks to draw moisture fromany source it can. Thus it will soak upmoisture from any hygroscopic materials(such as wood, paper, foodstuffs, leather,etc.) and dry out the nasal passages andskin of human beings in the building.

But is this free “humidification”? No, it isthe most expensive kind there is whentranslated into terms of human comfort,material deterioration, and productiondifficulties. Moreover, it requires the sameamount of energy whether the moistureis absorbed from people and materialsor added to the air by an efficienthumidification system.

The true energy required for a humidificationsystem is calculated from what the actualhumidity level will be in the building, NOTfrom the theoretical level. In virtually allcases, the cost of controlling RH at thedesired level will be nominal in terms ofadditional energy load, and in some casesmay result in reduced energy consumption.

A major convention center in the CentralUnited States reported that it experienceda decrease in overall steam consumptionwhen it added steam humidification. Fromone heating season with no humidificationto the next with humidifiers operating, thesteam consumption for humidificationwas 1,803,000 lbs, while the steam forheating decreased by 2,486,000 lbs in thesame period. The decreased (metered)

consumption occurred despite 7.2% colderweather from the previous year. Therecords from this installation indicate that itis possible to reduce the total amount ofsteam required for environmental control bymaintaining a higher, controlled relativehumidity.

Let’s examine a theoretical system usingenthalpy (heat content) as our base.■ Assume a winter day with outside

temperature of 0°F at 75% RH.■ The enthalpy of the air is .6 Btu/lb dry

air (DA).■ If the air is heated to 72°F without

adding moisture, the enthalpy becomes18 Btu/lb DA.

■ Theoretical relative humidity becomes3.75%, but actual RH will be about 25%.

■ At 72°F and 25% RH the enthalpy is22 Btu/lb DA.

■ The additional moisture is derivedfrom hygroscopic materials andpeople in the area.

But what about the additional energy—the difference between the 18 Btu/lb DAand 22 Btu/lb DA? This 22% increase mustcome from the heating system to compen-sate for the evaporative cooling effect. If ahumidification system is used and moistureadded to achieve a comfortable 35% RH,the enthalpy is 23.6 Btu/lb DA.

This is only a 7% increase over the“inevitable” energy load of 22 Btu/lb DA—substantially less than the theoreticalincrease of 31% from 3.75% RH (18 Btu/lbDA) to 35% RH (23.6 Btu/lb DA) at 72°F.If the temperature was only 68°F at 35%RH (because people can be comfortableat a lower temperature with higher humiditylevels), the enthalpy is 21.8 Btu/lb DA, ora slight decrease in energy.

Problems With Dry AirDry air can cause a variety of costly,troublesome, and sometimes dangerousproblems. If you are not familiar with theeffects of dry air, the cause of theseproblems may not be obvious. You shouldbe concerned if you are processing orhandling hygroscopic materials such aswood, paper, textile fibers, leather, orchemicals. Dry air and/or fluctuatinghumidity can cause serious productionproblems and/or material deterioration.

Static electricity can accumulate in dryatmospheric conditions and interfere withefficient operation of production machineryor electronic office machines. Where static-prone materials such as paper, films,computer disks, and other plastics arehandled, dry air intensely aggravates thestatic problem. In potentially explosiveatmospheres, dry air and its resultantstatic electricity accumulations can beextremely dangerous.

Why Humidification is Important, Continued...

°F °F

INDOOR OUTDOOR

120

100

80

60

40

20

0

20

40

120

100

80

60

40

20

0

20

40

50

40

30

20

10

0

10

20

30

40

°C

7

Humidity and Human ComfortStudies indicate people are generally mostcomfortable when relative humidity ismaintained between 35% and 55%. Whenair is dry, moisture evaporates more readilyfrom the skin, producing a feeling of chillinesseven with temperatures of 75°F or more.Because human perception of RH is oftensensed as temperature differential, it’spossible to achieve comfortable conditionswith proper humidity control at lowertemperatures. The savings in heatingcosts is typically very significant over thecourse of just a single heating season.

The Need for HumidityControl in Today’sElectronic WorkplaceElectronics are revolutionizing the wayyour office and plant floor operates,communicates, collects data, and main-tains equipment. In the office, xerographiccopies, phone systems, word processors,and typewriters, even wall thermostats areelectronically controlled. Personal computersand CRT access points are sproutingeverywhere. What’s more, office decorhas far more work stations incorporatingwall panels and furniture with natural andsynthetic fabric than ever before.

In manufacturing areas, more machinesare electronically controlled. In fact, yousee more control rooms (just to houseelectronic control systems) than in previousyears.

All this means that the nature of today’sbusiness makes proper humidification avirtual necessity.

Why Improper HumidificationThreatens SensitiveElectronic EquipmentCentral to all electronic circuits today isthe IC (integrated circuit) or “chip.” Theheart of the IC is a wafer-thin miniaturecircuit engraved in semiconductormaterial. Electronic components—andchips in particular—can be overstressedby electrical transients (voltage spikes).This may cause cratering and melting ofminute areas of the semiconductor,leading to operational upsets, loss ofmemory, or permanent failure. Thedamage may be immediate or thecomponent may fail sooner than anidentical part not exposed to an electricaltransient.

A major cause of voltage spikes iselectrostatic discharge (ESD). Althoughof extremely short duration, transients canbe lethal to the wafer-thin surfaces ofsemiconductors. ESD may deliver voltageas high as lightning, and it strikes faster.

ESD is a particularly dangerous phenom-enon because you are the source of thesetransients. It is the static electricity whichbuilds up on your body. The jolt you getfrom touching a doorknob or shakingsomeone’s hand is ESD. Table 7-1 belowshows voltages which can be generatedby everyday activities.

Voltage accumulates on surfaces (in thiscase, the human body), and when thesurface approaches another at a lowervoltage a discharge of electrical voltageoccurs. Note the humidity levels at whichthese voltages may be generated. As thelevel of humidity rises, voltages arereduced because a film of moisture forms Figure 7-1.

Effect of humidity on electrostatic voltages

Integrated circuit damaged by ESD.(Photo courtesy of Motorola Semiconductor, Inc.)

on surfaces, conducting the charges to theground. Although the 65%-90% RH citedin Table 7-1 is impractical for office areas,any increase in humidity will yield asignificant reduction in ESD events.

ESD Damage is Not OnlyPossible but ProbableA study of personnel ESD events in apoorly controlled room with a wool carpetwas conducted for 16 months. Thestrength of the ESD event was measuredin current (amps). Results indicate, forexample, that a current discharge of 0.3amps is 100 times more likely to occurat 10%-20% RH than at 45%-50% RH.In other words, the higher the relativehumidity, the lower the occurrence andseverity of ESD.

In addition to the risk of damage toelectronic devices from static electricitycharges, there are grave risks associatedwith sparks from static charges in manyprocess applications. Static electricity isextremely dangerous in the presence ofgases, volatile liquids, or explosive dustssuch as is found in munitions plants, paintspray booths, printing plants, pharmaceuti-cal plants, and other places.

While many static control products(special mats, carpeting, sprays, straps,etc.) are available, bear in mind thathumidification is a passive static-controlmeans. It is working to control static all thetime—not just when someone remembers.

Walking across carpetWalking over vinyl floorWorker at benchVinyl envelopes for work instructionsCommon poly bag picked up from benchWork chair padded with polyurethane foam

Means of StaticGeneratioo

35,00012,0006,0007,000

20,00018,000

1,500250100600

1,2001,500

Electrostatic Voltages

10%-20%Relative Humidity

65%-90%Relative Humidity

Table 7-1. Effect of Humidity on Electrostatic Voltages

8

Paper and Paper ProductsEvery production superintendent in thepaper industry is, by experience, familiarwith the excessive scrap losses andcustomer complaints that can result fromthe following wintertime headaches:

1. Curling of stock.2. Cracking or breaking at creases of

folding boxes, cartons, corrugated andsolid fiber containers.

3. Loss of package and containerstrength.

4. Production delays when sheets fail togo through machines smoothly due tostatic electricity.

5. Gluing failures.

All of the above wintertime problemshave a common cause—dry or curlingpaper caused by low indoor relativehumidities.

Whenever you heat air, without addingmoisture, its RH drops. Table 8-1 showsthat 0°F outside air at 75% RH will have a

How Humidity Affects MaterialsWhy Humidification is Important, Continued...

Figure 9-1. Effects of moisture content infolding paper. Sheet on left has propermoisture. Sheet on right lacks enoughmoisture—is dry and brittle—breaks on fold.

relative humidity of only 4.4% when heatedto 70°F indoors. Even though the theoreti-cal RH should be 4.4% in your plant, theactual observed humidity will be muchhigher because of the moisture given off bythe paper. This type of humidification is veryexpensive in terms of stock and production.

The RH of surrounding air governs themoisture content of paper, as shown inTable 9-1. The fibrils in paper take onmoisture when the paper is drier than thesurrounding air and give up moisture whenthe conditions are reversed.

A paper moisture content range of 5%-7%is essential to maintain satisfactory strengthand workability of paper. This requires anindoor RH of about 40%-50%, dependingupon the composition of the paper.

Moisture contents of different types of paperswill vary slightly from those shown in thetable but will follow an identical pattern.

Changes in moisture content thus causepaper to become thicker or thinner, flatteror curlier, harder or softer, larger orsmaller, limp or brittle.

Effect of Indoor Heating Upon RH andMoisture Content of Kraft Wrapping Paper.

NOTE: This table assumes an outdoor relativehumidity of 75%. When outdoor RH is less,as is common, indoor RH will also be less.

Indoor temperatures higher than 70°F will alsocause lower relative humidities.

Indoor Temperature 70°F

OutdoorTemperature

Degrees

-20-10010203040506070

IndoorRelative

Humidity %

1.52.54.47.211.618.126.838.354.075.0

Approx.Moisture

Content ofPaper

0.50.81.22.23.34.35.36.48.011.6

Table 8-1. How Indoor Heating ReducesIndoor RH and Dries Out Paper

9

PrintingThe dry air problems found in papermanufacturing are equally common tothe printing industry.

Paper curling, generally caused by theexpansion and contraction of an unpro-tected sheet of paper, takes place whentoo dry an atmosphere draws moisturefrom the ex-posedsurfacewhichshrinksand curls.The curlwill be withthe grain ofthe sheet. Thistrouble is most pronounced with verylightweight stocks or with cover stocks,and coated-one-side papers.

Wood Products, Woodworking,and Furniture ManufactureLike all hygroscopic materials, wood takeson or gives off moisture as the RH of thesurrounding air varies. When, at any giventemperature and relative humidity, thewood finally stops absorbing or liberating

moisture, it is said to havereached its equilibriummoisture content (EMC).The moisture in the

wood is then “in balance”with the moisture in the air.

It is generally not practical to holdindoor RH as high during the cold months

as it is during the warm months. However,when the cold season sets in, humidifierspermit a gradual reduction of RH andEMC to a practical minimum working level.Under this controlled condition, warpingand cracking will not occur.

Leather ProcessingRH maintained uniformly in the 40%-60%range (higher in muller rooms) reducescracking, minimizes loss of pliability, helpsmaintain quality and appearance, andreduces the dust problem in the plant.

OfficesRH maintained at 30%-40% stopssplitting, checking, shrinkage, and gluejoint failure in paneling and furnishings,adds life to carpeting and draperies.Electronic office equipment such ascomputers, xerographic copiers, andphone systems require a constant RHof 40%-50% to guard against harmfulelectrical transients (see page 7).

Libraries and MuseumsRelative humidity maintained uniformly at40%-55% in storage rooms, vaults, andgalleries prolongs the life of valuablecollections by stabilizing the pliability ofglue, starch and casein. The embrittle-ment of fibers in paper, canvas, papyrus,leather bindings, etc., is minimized.

Relative Humidity %Material Description

10 20 30 40 50 60 70 80 90

M.F. Newsprint Wood Pulp 24% Ash 2.1 3.2 4.0 4.7 5.3 6.1 7.2 8.7 10.6

HMF Writing Wood Pulp 3% Ash 3.0 4.2 5.2 6.2 7.2 8.3 9.9 11.9 14.2

White Bond Rag 1% Ash 2.4 3.7 4.7 5.5 6.5 7.5 8.8 10.8 13.2Com. Ledger 75% Rag 1% Ash 3.2 4.2 5.0 5.6 6.2 6.9 8.1 10.3 13.9

Kraft Wrapping Coniferous 3.2 4.6 5.7 6.6 7.6 8.9 10.5 12.6 14.9

Table 9-1. Moisture Content of Paper at Various Relative Humidities

10

moisture loss and materials deteriorationand/or production problems that result.

vary greatly from one material to the next.We will discuss typical hygroscopic materialswhich require specific RH levels to avoid

Determining Humidity Requirements of MaterialsNo single level of relative humidity providesadequate moisture content in all hygroscopicmaterials. Moisture content requirements

Table 10-1. Recommended Relative Humidities

Abstracted from the ASHRAE Handbook Systems and Applications

TeaPackaging 65 65

TobaccoCigar & cigarette making 70-75 55-65Softening 90 85-88Stemming & stripping 75-85 70-75Packing & shipping 73-75 65Filler tobacco casing

& conditioning 75 75Filter tobacco storage

& preparation 77 70Wrapper tobacco storage

& conditioning 75 75

PharmaceuticalsPowder storage (prior to mfg)* *Manufactured powder storage

& packing areas 75 35Milling room 75 35Tablet compressing 75 35Tablet coating room 75 35Effervescent tablets & powders75 20Hypodermic tablets 75 30Colloids 75 30-50Cough drops 75 40Glandular products 75 5-10Ampoule manufacturing 75 35-50Gelatin capsules 75 35Capsule storage 75 35Microanalysis 75 50Biological manufacturing 75 35Liver extracts 75 35Serums 75 50Animal rooms 75-80 50Small animal rooms 75-78 50

* Store in sealed plastic containers in sealed drums.

Photographic ProcessingPhoto Studio

Dressing room 72-74 40-50Studio (camera room) 72-74 40-50Film darkroom 70-72 45-55Print darkroom 70-72 45-55Drying room 90-100 35-45Finishing room 72-75 40-55

Storage roomb/w film & paper 72-75 40-60color film & paper 40-50 40-50

Motion picture studio 72 40-55

Static Electricity ControlTextiles, paper, explosive control > 55

Clean Rooms & Spaces 45

Data Processing 72 45-50

Paper ProcessingFinishing area 70-75 40-45Test laboratory 73 50

Switchgear:Fuse & cutout assembly 73 50Capacitor winding 73 50Paper storage 73 50

Conductor wrapping with yarn75 65-70Lightning arrester assembly 68 20-40Thermal circuit breakers

assembly & test 75 30-60High-voltage transformer repair79 55Water wheel generators:

Thrust runner lapping 70 30-50Rectifiers:

Processing selenium &copper oxide plates 73 30-40

FurStorage 40-50 55-65

GumManufacturing 77 33Rolling 68 63Stripping 72 53Breaking 73 47Wrapping 73 58

LeatherDrying 68-125 75Storage, winter room temp. 50-60 40-60

Lenses (Optical)Fusing 75 45Grinding 80 80

MatchesManufacture 72-73 50Drying 70-75 60Storage 60-63 50

MushroomsSpawn added 60-72 nearly sat.Growing period 50-60 80Storage 32-35 80-85

Paint ApplicationOils, paints: Paint spraying 60-90 80

PlasticsManufacturing areas:

Thermosetting moldingcompounds 80 25-30

Cellophane wrapping 75-80 45-65

PlywoodHot pressing (resin) 90 60Cold pressing 90 15-25

Rubber-Dipped GoodsCementing 80 25-30*Dipping surgical articles 75-80 25-30*Storage prior to manufacture60-75 40-50*Laboratory (ASTM Standard) 73.4 50*

* Dew point of air must be below evaporationtemperature of solvent

Residences 70-72 30

Libraries & MuseumsArchival 55-65 35Art storage 60-72 50Stuffed fur animals 40-50 50

Communication CentersTelephone terminals 72-78 40-50Radio & TV studios 74-78 30-40

General Commercial & Public Buildings70-74 20-30

(including cafeterias, restaurants, airport terminals,office buildings, & bowling centers)

Hospitals & Health FacilitiesGeneral clinical areas 72 30-60Surgical area

Operating rooms 68-76 50-60Recovery rooms 75 50-60

ObstetricalFull-term nursery 75 30-60Special care nursery 75-80 30-60

Industrial Hygroscopic MaterialsAbrasive

Manufacture 79 50Ceramics

Refractory 110-150 50-90Molding room 80 60-70Clay storage 60-80 35-65Decalcomania production 75-80 48Decorating room 75-80 48

CerealPackaging 75-80 45-50

DistillingStorage

Grain 6 35-40Liquid yeast 32-33

General manufacturing 60-75 45-60Aging 65-72 50-60

Electrical ProductsElectronics & X-ray:

Coil & transformer winding72 15Semi conductor

assembly 68 40-50Electrical instruments:

Manufacture& laboratory 70 50-55

Thermostat assembly& calibration 75 50-55

Humidistat assembly& calibration 75 50-55

Small mechanisms:Close tolerance assembly72 40-45Meter assembly & test 75 60-63

PROCESS OR PRODUCT Temp.°F %RH PROCESS OR PRODUCT Temp.°F %RHPROCESS OR PRODUCT Temp.°F %RH

11

How Psychrometrics Help in Humidification

Psychrometrics is the measurement ofthermodynamic properties in moist air.As a problem-solving tool psychrometricsexcel in clearly showing how changes inheating, cooling, humidification, anddehumidification can affect the propertiesof moist air. Psychrometric data is neededto solve various problems and processesrelating to air distribution.

Most complex problems relating to heating,cooling and humidification are combina-tions of relatively simple problems. Thepsychrometric chart illustrates these pro-cesses in graphic form, clearly showing howchanges affect the properties of moist air.

One of the reasons psychrometric data isparticularly important today is traceable tothe way most new buildings (and manyolder ones) are heated. The lower ducttemperatures (55°F and below) used innew buildings make accurate humidity

control more difficult to achieve. (This isbecause low duct temperatures have alimited ability to absorb moisture. Addingmoisture via the central air handlingsystem must compensate for reheatingof air before it leaves the duct.)

For such applications, booster humid-ification must sometimes be accomplishedin the duct of the zone after it has reachedits final temperature (reheated).

To maintain typical conditions of 70°F and50% RH, duct humidities will be very high(75% RH and above). To keep the ductfrom becoming saturated, a duct high limithumidistat is used, and becomes in thesecases the main controller of the humidifier.Since this humidistat is in close proximityto the humidifier, and air is constantlymoving, and must be controlled close tosaturation, the humidifier output controlmust be fast, accurate and repeatable.

TOTAL HEAT∆H T

S E N S I B L E H E A T∆H s

ENTHALPY∆h

HUMIDITY RATIO∆W

500

0-1

000

~200

0–

50003000

2000

1500

1.0

0.8

0.6

0.5

0.4

0.3

0.2

0.1

1.0

2.04.08.0

-8.0-4.0-2.0-1.0

-0.5-0.4-0.3-0.2-0.1

605040

3020

10

0120

110100

90

8070

201510

5

05

1015

25

3035

4045

°C

°F

5

29

5

31

5

30

1020

1010990

980

970

960

950

10301040

MILLIBARS

INCHES

6050

40

3020

100

9080

70

5545

3525

15

65

7585

95

HUMIDITY

%RELATIVE HUMIDTY

28

10

BAROMETER

1000

THERMOMETER

SATURAT

ION T

EMPERAT

URE F

12.5

13.0

13.5

14.0

14.5 VOLU

ME

CU

. FT. PE

R LB

. DR

Y A

IR15.0

90%

80%

70

%60

%

50%

40%

30%

20

10 RELATIVE HUMIDITY

120

115

110

105

100

DR

Y B

ULB

TE

MP

ER

ATU

RE

— F

95908580757065605550454035

65

60

55

50

4540

353025

70

75

80 WET BULB TEMPERATURE

85

90

35

40

45

50

55

60

65

70

75

80

85

15

15 20 25

30

35

40

45

50

55

60

6055

10

20

25

30

35

40

45

50

ENTHALPY (h

) BTU P

ER BOUND O

F DRY A

IR

ENTHALPY (h) BTU PER BOUND OF DRY AIR

.028

.026

.024

.022

.020

.018

.016

.014

.012

.010

.008

.006

.004

.002

HU

MID

ITY

RAT

IO (

W)

— P

OU

ND

S M

OIS

TU

RE

PE

R P

OU

ND

DR

Y A

IR

12

The psychrometric chart is a graphicalrepresentation of the thermodynamicproperties which impact moist air.

It consists of eight major components:

Using the Psychrometric Chart

1. Humidity ratio values are plottedvertically along the right-hand margin,beginning with 0 at the bottom andextending to .03 at the top.

2. Enthalpy , or total heat, is plotted withoblique lines, at intervals of 5 Btu/lb of dryair, extending from upper left to lower right.

3. Dry-bulb temperature lines areplotted vertically at 1°F intervals.

4. Wet-bulb temperature lines areindicated obliquely and fall almost parallelto enthalpy lines. They are shown at 1°Fintervals.

5. Relative humidity lines curve acrossthe chart from left to right at intervals of10%. They begin at the bottom at 10%and end at the top with the saturation curve(100%).

7. Two-phase region includes a narrow,cross-hatched area to the left of thesaturation region indicating a mixture ofcondensed water in equilibrium.

6. Volume lines indicating cubic feet perpound of dry air are plotted at intervalsof .5 cubic foot.

8. The protractor at the upper left of thechart contains two scales. One is for theratio of enthalpy difference. The other isfor a ratio of sensible heat to the totalheat. The protractor establishes the angleof a line on the chart along which aprocess will follow.

How Psychrometrics Help in Humidification, Continued...

0.030

0.028

0.026

0.024

0.022

0.020

0.018

0.016

0.014

0.012

0.010

0.008

0.006

0.004

0.002

0 HUM

IDIT

Y RA

TIO,

POU

NDS

MOI

STUR

E PE

R PO

UND

DRY

AIR

10

15

20

25

30

35

40

45

50

55

ENTH

ALPY (H

) BTU

per p

ound

of dr

y air

35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 12025 30 35 40

4550

55

60

65

70

75

80

85

90

90%

80%

70%

60%

50%

40%

30%

20%

10%

100%

14.5Volum

e(cubic

feet per poundof dry

air)15.0

14.0

13.0

13.5

12.5

Saturat

ionTe

mperat

ure°F

TOTAL HEAT∆H T

S E N S I B L E H E A T∆H s

ENTHALPY∆h

HUMIDITY RATIO∆W

500

0-1

000

~200

0–

50003000

2000

1500

1.0

0.8

0.6

0.5

0.4

0.3

0.2

0.1

1.0

2.04.08.0

-8.0-4.0-2.0-1.0

-0.5-0.4-0.3-0.2-0.1

13

SATURAT

ION T

EMPERAT

URE F

12.5

13.0

13.5

14.0

14.5 VOLU

ME

CU

. FT. PE

R LB

. DR

Y A

IR15.0

90%

80%

70

%60

%

50%

40%

30%

20

10 RELATIVE HUMIDITY

120

115

110

105

100

DR

Y B

ULB

TE

MP

ER

ATU

RE

— F

95908580757065605550454035

65

60

55

50

4540

353025

70

75

80 WET BULB TEMPERATURE

85

90

35

40

45

50

55

60

65

70

75

80

85

15

15 20 25

30

35

40

45

50

55

60

6055

10

20

25

30

35

40

45

50

ENTHALPY (h

) BTU P

ER BOUND O

F DRY A

IR

ENTHALPY (h) BTU PER BOUND OF DRY AIR

.028

.026

.024

.022

.020

.018

.016

.014

.012

.010

.008

.006

.004

.002

HU

MID

ITY

RAT

IO (

W)

— P

OU

ND

S M

OIS

TU

RE

PE

R P

OU

ND

DR

Y A

IR

SATURAT

ION T

EMPERAT

URE F12.5

13.0

13.5

14.0

14.5 VOLU

ME

CU

. FT. PE

R LB

. DR

Y A

IR15.0

90%

80%

70

%60

%

50%

40%

30%

20

10 RELATIVE HUMIDITY

120

115

110

105

100

DR

Y B

ULB

TE

MP

ER

ATU

RE

— F

95908580757065605550454035

65

60

55

50

4540

353025

70

75

80 WET BULB TEMPERATURE

85

90

35

40

45

50

55

60

65

70

75

80

85

15

15 20 25

30

35

40

45

50

55

60

6055

10

20

25

30

35

40

45

50

ENTHALPY (h

) BTU P

ER BOUND O

F DRY A

IR

ENTHALPY (h) BTU PER BOUND OF DRY AIR

.028

.026

.024

.022

.020

.018

.016

.014

.012

.010

.008

.006

.004

.002

HU

MID

ITY

RAT

IO (

W)

— P

OU

ND

S M

OIS

TU

RE

PE

R P

OU

ND

DR

Y A

IR

TOTAL HEAT∆H T

S E N S I B L E H E A T∆H s

ENTHALPY∆h

HUMIDITY RATIO∆W

500

0-1

000

~200

0–

50003000

2000

1500

1.0

0.8

0.6

0.5

0.4

0.3

0.2

0.1

1.0

2.04.08.0

-8.0-4.0-2.0-1.0

-0.5-0.4-0.3-0.2-0.1

Example 1Given the conditions of 75°F drybulb and 50% RH, determine the dewpoint, volume and humidity contentin grains per cubic foot of dry air.

Solution:1. Locate the state point, where the

75°F dry-bulb line intersects the50% RH line. Call this state pointnumber 1.

2. Project horizontally to the left tothe saturation curve and read 55°F(dew point).

3. Project horizontally to the right andread .0092 pounds of moisture perpound of dry air.

4. Draw a line through the state pointparallel to 13.5 volume line andestimate a volume of 13.68 cubicfeet per pound of dry air.

5. Solve for grains per cubic foot byconverting:

0.0092 x 7,000 ÷ 13.68 = 4.71 grains/cu ftSee also Table 27-5 for quick values.

Example 2Determine resultant RH when 55°Fair at 80% RH is heated to a tem-perature of 75°F.

Solution:1. Locate the state point where 55°F

dry-bulb line intersects 80% RHline. Call this state point number 2.

2. Project horizontally to the right tointersect the 75°F dry-bulb line at40% RH. Call this state point 3.

3. Observe that if air is delivered toa system at state point 2, that areheat operation can deliver it toan area at state point 3.

4. If state point 1 (example 1) isdesired in the area, then boosterhumidification is needed.

TOTAL HEAT∆H T

S E N S I B L E H E A T∆H s

ENTHALPY∆h

HUMIDITY RATIO∆W

500

0-1

000

~200

0–

50003000

2000

1500

1.0

0.8

0.6

0.5

0.4

0.3

0.2

0.1

1.0

2.04.08.0

-8.0-4.0-2.0-1.0

-0.5-0.4-0.3-0.2-0.1

No. 3No. 2

No. 1

14

Steam HumidificationUnlike other humidification methods,steam humidifiers have a minimal effecton dry-bulb (DB) temperatures. The steamhumidifier discharges ready-made watervapor. This water vapor does not requireany additional heat as it mixes with the airand increases relative humidity. Steam ispure water vapor existing at 212°F (100°C).This high temperature creates a percep-tion that steam, when discharged into theair, will actually increase air temperature.This is a common misconception. In truth,as the humidifier discharges steam intothe air, a steam/air mixture is established.In this mixture steam temperature willrapidly decrease to essentially the airtemperature.

How Humidifiers Work

Direct Steam InjectionHumidifiersThe most common form of steamhumidifier is the direct steam injectiontype. From a maintenance point of view,direct steam humidification systemsrequire very little upkeep. The steamsupply itself acts as a cleaning agent tokeep system components free of mineraldeposits that can clog water spray andevaporative pan systems.

Response to control and pinpoint controlof output are two other advantages of thedirect steam humidification method. Sincesteam is ready-made water vapor, it needsonly to be mixed with air to satisfy thedemands of the system. In addition, directsteam humidifiers can meter output bymeans of a modulating control valve. Asthe system responds to control, it canposition the valve anywhere from closedto fully open. As a result, direct steamhumidifiers can respond more quickly andprecisely to fluctuating demand.

The high temperatures inherent in steamhumidification make it virtually a sterilemedium. Assuming boiler makeup wateris of satisfactory quality and there is nocondensation, dripping or spitting in theducts, no bacteria or odors will bedisseminated with steam humidification.

Corrosion is rarely a concern with aproperly installed steam system. Scaleand sediment—whether formed in the unitor entrained in the supply steam—aredrained from the humidifier through thesteam trap.

Steam-to-Steam HumidifiersSteam-to-steam humidifiers use a heatexchanger and the heat of treated steam tocreate a secondary steam for humidificationfrom untreated water. The secondarysteam is typically at atmospheric pressure,placing increased importance onequipment location.

Maintenance of steam-to-steam humidifiersis dependent on water quality. Impuritiessuch as calcium, magnesium and ironcan deposit as scale, requiring frequentcleaning. Response to control is slowerthan with direct steam because of thetime required to boil the water.

Electronic Steam Humidifiers(Electrode)Electronic steam humidifiers are usedwhen a source of steam is not available.Electricity and water create steam atatmospheric pressure. Electrode-type unitspass electrical current through water toprovide proportional output. Use with puredemineralized, deionized or distilled wateralone will generally not provide sufficientconductivity for electrode units.

Water quality affects the operation andmaintenance of electrode-type humidifiers.Use with hard water requires more frequentcleaning, and pure softened water canshorten electrode life. Microprocessor-based diagnostics assist withtroubleshooting.

The psychrometric chart helps illustratethat steam humidification is a constant DBprocess. Starting from a point on any DBtemperature line, steam humidification willcause movement straight up along theconstant DB line. The example illustratesthat 70°F DB is constant as we increaseRH from 30%-50%. This is true becausesteam contains the necessary heat(enthalpy) to add moisture withoutincreasing or decreasing DBtemperature. Actual resultsutilizing high pressure steamor large RH increases(more than 50%) increaseDB by 1° to 2°F. As a result,no additional heating or airconditioning load occurs.

70° DB

50%

30%

Cross Section of Manifold

Dry Steam

Direct Steam HumidificationFigure 14-1.

Steam-to-Steam HumidificationFigure 14-2.

15

Electrode units are easily adaptable todifferent control signals and offer fullmodulated output. However, the need toboil the water means control will notcompare with direct-injection units.

Electronic Steam Humidifiers(Ionic Bed)Ionic bed electronic humidifiers typicallyuse immersed resistance heating elementsto boil water. Since current does not passthrough water, conductivity is not aconcern. Therefore, ionic bed technologymakes the humidifier versatile enough toaccommodate various water qualities.These units work by using ionic bedcartridges containing a fibrous media toattract solids from water as its temperaturerises, minimizing the buildup of solidsinside the humidifier. Water quality doesnot affect operation, and maintenancetypically consists of simply replacing thecartridges.

Ionic bed humidifiers are adaptable todifferent control signals and offer fullmodulated output. Control is affected bythe need to boil the water, however.

Water SprayThe water spray process can createpotential temperature control problems. Inorder to become water vapor or humidity,water requires approximately 1,000 Btuper pound to vaporize. This heat must bedrawn from the air, where it will hopefullyvaporize. If not enough heat is availablequickly enough, the water remains aliquid. This unvaporized water can result inoverhumidification, and the water can“plate out” on surfaces, creating a sanita-tion hazard.

Water spray contains virtually none of theheat of vaporization required to increasethe RH of the air to desired conditions.For this reason, water spray humidificationis a virtually constant enthalpy process.However, as the psychrometric exampleillustrates, DB temperature changes as weincrease RH from 30%-50%. The result ofthis loss of DB temperature is an in-creased heating load to maintain 70°F.

Water SprayFigure 15-2.

70° DB

50%

30%

ENTHALPY

70° DB

50%

30%

Heat Taken From AirTo Evaporate Moisture

Water SupplyUnder Pressure

Water Mist

Response of water spray humidifiersto control is slow due to the need forevaporation to take place before humidi-fied air can be circulated. On/off controlof output means imprecise response tosystem demand and continual dangerof saturation. Water spray systems candistribute large amounts of bacteria, andunevaporated water discharge cancollect in ducts, around drains and drippans, and on eliminator plates, encour-aging the growth of algae and bacteria.Corrosion is another ongoing problemwith water spray humidification. Scaleand sediment can collect on nozzles,ductwork, eliminator plates, etc., leadingto corrosion and high maintenance costs.

Evaporative PanThe evaporative pan method uses steam,hot water or electricity to provide energyfor heating coils which in turn heat waterand create water vapor. This method ismost effective when installed in smallercapacity environments either in the airhandling system or individually withinthe area(s) to be humidified.

Evaporative pan humidification canincrease dry-bulb temperature as mea-sured on the psychrometric chart. Thisunwanted temperature change may occuras air is forced across the warmed waterin the pan. The increase in DB can causedamaging results in process applicationsand increase the need for humidity control.The psychrometric chart helps illustratethat evaporative pan humidification is nota constant DB process. This exampleshows DB temperature increasing as wemove from 30%-50% RH. To maintain aconstant DB of 70°F some cooling load(air conditioning) is required.

Figure 15-1.Electronic Steam Humidification

16

How Humidifiers Work, Continued...

Cost ComparisonsTo fairly evaluate the costs of selecting ahumidification system, you should includeinstallation, operating, and maintenancecosts as well as initial costs. Total humidi-fication costs are typically far less thanheating or cooling system costs.

Initial costs, of course, vary with the sizeof the units. Priced on a capacity basis,larger capacity units are the mosteconomical, regardless of the type ofhumidifier, i.e.: one humidifier capable ofdelivering 1,000 pounds of humidificationper hour costs less than two 500 lbs/hrunits of the same type.

Direct steam humidifiers will provide thehighest capacity per first cost dollar; waterspray and evaporative pan are the leasteconomical, assuming capacity needs of75 lbs/hr or more.

Installation costs for the various typescannot be accurately formulated becausethe proximity of water, steam and electricityto humidifiers varies greatly amonginstallations. Operating costs are low fordirect steam and slightly higher for steam-to-steam. Water spray and evaporative panoperating costs are also low. Energy costsare higher for electronic humidifiers.

Direct steam humidifiers have the lowestmaintenance costs. Ionic bed electronichumidifiers are designed specifically tominimize maintenance while adapting tovarious water qualities. Maintenance costsfor other types can vary widely, dependingon water quality and applications.

These are the principal considerations inselecting a humidification system. Table16-1 summarizes the capabilities of eachhumidifier type.

Heated Water

Water Pan

Water VaporFloatValve Steam, Hot Water

Or Electricity In

Auto Valve

Evaporative PanFigure 16-1.

Maintenance of evaporative pan humid-ification systems demands regularcleaning of the heating coils and pan,which are subject to “liming up.”

The use of chemical additives addedeither automatically or manually to thewater in the pan can reduce this problemby as much as half.

Response to control with the evaporativepan method is slow due to the timerequired for evaporation to take placebefore humidified air can be circulated.Output is determined by water tempera-ture and surface area.

Evaporative pan humidifiers can sustainbacteria colonies in the reservoir anddistribute them throughout the humidifiedspace. High water temperatures, watertreatment, and regular cleaning andflushing of the humidifier help to minimizethe problem, however.

Table 16-1. Comparison of Humidification Methods

Maintenance frequency

Subject to severe corrosion and

bacteria problems

Pan subject to corrosion; bacteria

can be present

Effect on temperature

Unit capacity per unit size

Vapor quality

Response to control

Control of output

Sanitation/corrosion

Installation

Operating

Maintenance

Evaporative Pan Water SpraySmall

temperature riseSubstantial

temperature drop

Small Small

Poor

Slow Slow

Weekly to monthly

HighLow

Direct Steam

Small to very large

Excellent

Immediate

Good to excellent

Sterile medium; corrosion free

Annual

Low

Low

Maintenance difficulty

Costs: Price (per unit of capacity)

Low

High

Steam-to-Steam

Virtually no change

Small

Good

Slow

Below average

Bacteria can be present

Monthly

High

Low

High

Medium

Electronic Steam

Small to medium

Good

Fair

Average

Programmed to not promote bacteria

Monthly to quarterly

High

Medium

Varies with availability of steam, water, electricity, etc.

Medium

Low

Ionic Bed Electronic Steam

Small to medium

Good

Fair

Average

Programmed to not promote bacteria

Quarterly to semi-annually

Low to medium

Medium

Medium

Good

Average

High

Low

High

Average

Weekly to bimonthly

High

Medium to high

Low

Very high

17

Recommended ApplicationsSteam: Recommended for virtually allcommercial, institutional and industrialapplications. Where steam is not avail-able, small capacity needs up to 50-75lbs/hr can be met best using ionic bed typeself-contained steam generating units.Above this capacity range, central systemsteam humidifiers are most effectiveand economical. Steam should bespecified with caution where humidificationis used in small, confined areas to addlarge amounts of moisture to hygroscopicmaterials. We recommend that youconsult your Armstrong Representativeregarding applications where theseconditions exist.

Evaporative Pan: Recommended onlyas an alternative to self-contained steamgenerating unit humidifiers for small loadcommercial or institutional applications.Generally not recommended where loadrequirements exceed 50-75 lbs/hr.

Water Spray: Recommended for industrialapplications where evaporative cooling isrequired; typical application is summer-time humidification of textile mills in thesouthern U.S.

The evidence supports the conclusion thatsteam is the best natural medium forhumidification. It provides ready-madevapor produced in the most efficientevaporator possible, the boiler. There isno mineral dust deposited, and becausethere is no liquid moisture present, steamcreates no sanitation problems, will notsupport the growth of algae or bacteria,has no odor and creates no corrosion orresidual mineral scale. Unless there areparticular requirements to an applicationthat can only be met with evaporativepan or water spray methods, steamhumidification will meet system needsmost effectively and economically.

Strainer

Safety Relief Valve

“Packaged” Steam Generator

AutomaticBlowdownSystem

Trap

To Drain

To AdditionalHumidifiers

SteamHumidifier

To Drain

Auto Fill Valve

Figure 17-1. Typical Piping for Boiler-Humidifier InstallationDesign Guidelines—Boiler-Humidifier Combinations

1. Boiler gross output capacityshould be at least 1.5 times thetotal humidification load.

2. Water softeners should be usedon boiler feedwater.

3. Condensate return system is notnecessary (unless required bycircumstances).

4. Boiler pressure should be at15 psig or less.

5. An automatic blowdown systemis desirable.

6. All steam supply piping shouldbe insulated.

7. No limit to size or number ofhumidifiers from one boiler.

With these advantages in mind, engineersspecify steam boilers and generatorssolely for humidification when the buildingto be humidified does not have a steamsupply. The minimum humidification loadwhere this becomes economically feasiblefalls in the range of 50-75 lbs/hr. Steamgenerator capacity is generally specified50% greater than maximum humidificationload, depending on the amount of pipingand number of humidifiers and distributionmanifolds that must be heated. Typicalpiping for boiler-humidifier installations isshown in Fig. 17-1.

Level Control &Low Water Cutoff

18

Considerations in Selecting Steam Humidifiers

Electronic Steam HumidifiersWhen steam is not available, self-contained electronic humidifiers can meetlow-capacity requirements. The primaryconsideration in selecting this typeof humidifier is its ability to work withwide ranges in water quality. Ionic bedelectronic humidifiers are frequentlyselected for this capability.

Direct InjectionSteam HumidifiersAn evaluation of three performancecharacteristics is essential to understandthe advantages steam holds over otherhumidification media:

■ Conditioning■ Control■ Distribution

The humidifier must condition the steamso that it’s completely dry and free ofsignificant particulate matter. Responseto control signals must be immediate,and modulation of output must be precise.Distribution of steam into the air mustbe as uniform as possible. Inadequateperformance in any of these areasmeans the humidifier will not meet thebasic humidification requirements.

Direct injection steam humidifiers areavailable in three basic types: speciallydesigned steam grids, steam cups andthe steam separator.

Specially designed steam grid systemsincorporate advanced engineering inaddressing unique applications wherevapor trail is of prime concern.

Steam cup humidifiers receive steamfrom the side of the cup which theoreticallypermits the condensate to fall by gravityto the steam trap. However, in practicea great deal of the liquid moisture in thesteam goes into the air flow, and thesteam itself is poorly distributed.

The steam separator is a moresophisticated device which, whenproperly designed, meets essentialperformance criteria.

Figure 18-3. Steam Separator Type Humidifier

�� ���

��� �

������

������Integral

Control Valve

Drying Chamber

Separating Chamber

InvertedBucketSteamTrap

Condensate Deflected Downward

Steam JacketedDistribution Manifold

Pneumaticor ElectricOperator

��� STEAM SUPPLY AT SUPPLY

PRESSURE

STEAM AT ATMOSPHERICPRESSURE

CONDENSATE

KEY

Drain

Drain

Strainer

Trap

NOTE: Condensate cannot be lifted or dischargedinto pressurized return.

From Boiler

Figure 18-1. Steam Grid Humidifier

Figure 18-2. Cup Type Steam Humidifier

Drip Leg

1"/12" Slope

1" Air Gap

Header

Air Flow

Control Valve

19

Figure 19-1. Parabolic Plug Metering Valve

Steam ConditioningAs steam moves through supply lines,scale and sediment may be entrained inthe flow—a Y-type strainer is required toremove larger solid particles. Similarly, thecondensation that occurs in the supplylines permits water droplets or even slugsof condensate to be carried into thehumidifier.

Several steps within the humidifierare required to positively prevent thedischarge of liquid moisture and finerparticulate matter along with thehumidifying steam.

The separating chamber in the humidifierbody should provide the volume requiredfor optimum velocity reduction andmaximum separation of steam fromcondensate. Properly separated, the

Chart 19-1. Desirable modified linearcharacteristic curve for valves used undermodulating control. The modification of truelinear characteristics provides more precisecontrol when capacity requirements are verylow and the valve is just cracked off the seat.

7/32"

9/32"

5/16"

11/32"

3/8"

13/32"

15/32"

1/2"

9/16"

5/8"

3/4"

7/8"

EquivalentDiameter

Rangeability

Ratio ofFlow

Max: Mii

Minimum Flowas % of

Maximuu

Valve Size

1"

1

1

1

63:1

69:1

61:1

53:1

44:1

33:1

123:1

105:1

97:1

85:1

75:1

64:1

70:1

59:1

49:1

40:1

31:1

24:1

18:1

59:1

37:1

28:1

21:1

15:1

10:1

1.6

1.4

1.6

1.9

2.3

3.0

0.8

0.9

1.0

1.2

1.3

1.6

1.4

1.7

2.0

2.5

3.2

4.2

5.6

1.7

2.7

3.5

4.8

6.9

10.0

Table 19-1.Steam Humidifier Valve Rangeabilities

1/2"1/4"1/8"

7/16"

1/4"

3/16"5/32"1/8"7/64"3/32"5/64"1/16"

Humidifier control must provide immediateresponse and precise modulation in orderto accurately maintain the required relativehumidity. Faulty control can make itdifficult to provide the desired humiditylevel, and can lead to overloading theducts with moisture and the creation ofwet spots.

Two design factors affect the accuracy ofhumidifier control that can be achieved—the metering valve and the actuator thatpositions the valve.

Precise flow control can be achievedwith a valve designed expressly for thepurpose of adding steam to air. Parabolicplug type valves have been established asbest for this service. They permit a longerstroke than comparable industrial valves,and the plug normally extends into theorifice even with the valve in “full open”position. This facilitates full and accuratemodulation of flow over the completestroke of the valve.

The Control ValveThe parabolic plug design also providesexceptionally high rangeability. Range-ability is the ratio between the maximumcontrollable flow and the minimumcontrollable flow of steam through thevalve. The higher the rangeability of avalve, the more accurately it can controlsteam flow. Rangeabilities of the parabolicplug valves used in Armstrong Series9000 Humidifiers shown in Table 19-1 aretypical of the ratios that can be achievedwith this type of valve.

condensate carries a substantial portion ofthe significant micronic particulates withit to be discharged through the drain trap.

Steam from the separating chamber canstill carry liquid mist which must beremoved. Humidifiers equipped with aninner drying chamber that is jacketed bythe steam in the separating chamber caneffectively re-evaporate any remainingwater droplets before steam is discharged.Similarly, the control valve should beintegral with the humidifier. Both thehumidifier and the distribution pipe shouldbe jacketed by steam at supply pressureand temperature to prevent condensationas steam is discharged.

Only proper design of the humidifier forconditioning of steam can assure theessential levels of sanitation and a cleanatmosphere. These guidelines contributeto better comfort conditions and ensurethat the humidifier meets the vitalphysical requirements of the system.

Control of OutputAs discussed, the duct high-limit humidistatcan become the controlling humidistat dueto psychrometric conditions in the duct. Intypical duct air flow rates, the high limithumidistat senses humidifier output inone second or less. In most applica-tions, humidifiers consistently operateat a fraction of maximum output.

100

90

80

70

60

50

40

30

20

10

00 10 20 30 40 50 60 70 80 90 100

PERCENT OF FULL STROKE

PE

RC

EN

T O

F F

UL

L C

AP

AC

ITY

3/4"Stroke

3/8"

3/8" 3/8" 3/8"

9/32" 7/32"

20

The actuator is another important compo-nent in humidity control. Several types areavailable to provide compatibility withvarious system types. The actuator mustbe able to position the valve in very nearlyidentical relationship to the seat on bothopening and closing strokes. This isessential to provide consistent, accuratemetering of steam discharged by thehumidifier.

By their design, electric motor modulatingactuators provide true linear positioningcharacteristics on both opening andclosing cycles. Pneumatic actuators mayor may not be able to provide the precisepositioning and holding characteristicsessential to accurate control. Rollingdiaphragm type pneumatic actuators arerecommended, providing they meet thefollowing criteria:

Distribution of SteamThe third essential factor in proper humidifierdesign is distribution. Steam must bedischarged as uniformly as possible intothe air to permit the fastest possibleabsorption without creating damp spotsor saturated zones.

In normal ducts, a single distributionmanifold installed across the long dimen-sion will provide good distribution of steam.In large ducts or plenum chambers, it maybe necessary to broaden the pattern ofvapor discharge to achieve the requireddistribution, thus requiring multiple mani-folds from single or multiple humidifiers.

Humidification for industrial areas withoutcentral air handling systems is customarilyachieved with unit humidifiers dischargingsteam directly into the atmosphere. Propermixing of steam and air can be accom-plished in two ways. A dispersing fan maybe mounted on the humidifier or a unitheater can be positioned to absorb anddistribute the water vapor.

Considerations in Selecting Steam Humidifiers,Continued...

Figure 20-1.

Distribution Manifold

Duct Cross Section

SingleDistributionManifold in aNormal Duct

DistributionManifold

Duct Cross Section

Multiple DistributionManifolds in a LargeDuct or Housing

11

10

9

8

7

6

5

4

Opening StrokeClosing Stroke

0 10 20 30 40 50 60 70 80 90 100

CO

NT

RO

L A

IR P

RE

SS

UR

E T

O A

CT

UA

TO

R-P

SIG

PERCENT OF FULL STROKE

Chart 20-1. Desirable OperatingCharacteristic for Pneumatic ActuatorsPosition of valve is very nearly identical onboth opening and closing strokes at any givenair pressure to the actuator.

1. Large diaphragm area—22 sq in ormore—to provide ample lifting force.This permits the use of a spring heavyenough to stabilize both the hysteresiseffect and the flow velocity effect on thepositioning of the valve stem versus airpressure to the actuator.

2. Diaphragm material highly resistantto wear or weakening from continuouscycling.

3. Actuator stroke long enough (inconjunction with valve plug and seatdesign) to provide high rangeabilityratios.

All modulating actuators, whether electricor pneumatic, should incorporate a springreturn. This is necessary to ensure closingthe valve if there is an interruption of poweror control air to the unit.

For industrial in-plant operation and forvery limited duct applications, a solenoidactuator may be used to provide simpleon-off operation. This type of actuatorshould not be specified for duct applica-tions without a detailed analysis of thesystem.

Strainer

Inlet

Drain

Temp.Switch

Trap

SolenoidValve

Steam Out

C

E

Unit Humidifier for Direct Dischargeinto Area Humidified

Figure 20-2.

21

SilencingMaterial

D. The stainless steel metering valve shallbe a parabolic plug, with a full 3/4" stroke.This valve shall provide the high range-abilities required to achieve full andaccurate modulation of steam flow overthe entire stroke of the valve.

E. The internal drying chamber shallreceive total steam flow at essentiallyatmospheric pressure and be jacketedby steam at supply pressure.

F. The silencing chamber shall be steamjacketed and utilize a stainless steelsilencing medium.

G. The distribution manifold shall provideuniform distribution over its entirelength and be jacketed by steam atsupply pressure to assure that vapordischarged is free of water droplets.A full length stainless steel internalsilencing screen shall be provided.

H. Humidifier shall be equipped with aninterlocked temperature switch toprevent the humidifier from operatingbefore start-up condensate is drained.

Suggested Specification:Steam humidifiers for pneumatic orelectric modulating control: humidifier shallbe the steam separator type providing fullseparation ahead of an integral steamjacketed control valve. Steam shall bedischarged through an internal steamjacketed drying chamber, a silencingchamber and a steam jacketed distributionmanifold.

A. Humidifier shall receive steam atsupply pressure and discharge atatmospheric pressure. It shall befurnished with inlet strainer andexternal inverted bucket steam trap.

B. Separating chamber shall be of avolume and design that will disengageand remove all water droplets and allparticulate matter larger than 3microns when humidifier is operatingat maximum capacity.

C. The stainless steel metering valveshall be integral within the body of thehumidifier, and shall be jacketed bysteam at supply pressure and tempera-ture to prevent condensation.

Operating NoiseIn addition to these crucial performancecharacteristics, operating noise is aconsideration in selecting steam humidifi-ers for areas where quiet operation isessential or desirable, i.e., hospitals, officebuildings, schools, etc.

Figure 21-1. Figure 21-2.The noise of escaping steam is generated atthe control valve. Muffling materials around thevalve are necessary to minimize this noise.

Steam moving at high velocity through thedistribution manifold can create loud, high-pitched whistling as it passes the dischargeholes. An internal silencing screen or similarmaterial is required to interrupt the air flow andprevent whistling.

Stainless SteelMesh

22

Several basic principles must be consid-ered in the application of steam humidifi-cation equipment to avoid potentialoperating problems.

Vapor dissipation in air ducts is one ofthese considerations. In the steamhumidification process, pure water vaporat 212°F is mixed with air at a lowertemperature. The mixing of hot steam withcooler air results in heat transfer. Any timeheat is transferred from steam, conden-sation takes place. This condensation isreferred to as visible vapor. When steamis discharged from a manifold in an airduct, it quickly changes from an invisiblegas into visible water particles, and thendissipates to become invisible again.

Basic Application Principles

Visible vapor indicates an area of super-saturation, where the invisible steam gasis condensing into water particles. Whencondensation occurs, the steam gasreleases its latent heat of vaporization(about 1,000 Btu/lb of vapor) to duct air.Then, as the vapor completely mixes withthe duct air, the latent heat previouslygiven off is reabsorbed, converting thevisible vapor back into invisible gas withessentially no change in DB temperature.(See Fig. 22-1).

Clearly, the vapor dissipation in air ductsis very important to proper location oftemperature or humidity controllers. Anycontroller located in or near the visiblevapor pattern will produce inaccurateresults because of pockets of saturatedair. Under typical duct conditions, allcontrollers should be located at least 10 to12 feet downstream of a manifold.However, the following system character-istics will affect the visible vapor pattern,and therefore should be considered incontroller location:

1. Aspect Ratio of Duct. The ratio ofduct height to width is a factor thatinfluences the visible vapor pattern.Fig. 22-2 shows two ducts with equalcross section areas, but with differentaspect ratios. Air velocities, tempera-tures, RH and vapor output from themanifolds are all identical. However,in the taller duct the manifold is shorterand its vapor output comes in contactwith a much smaller percentage of ductair, causing a longer visible vapor pattern.

Figure 22-1. Figure 22-2.Typical dry-bulb (sensible) temperaturevariations within a duct near the humidifiermanifold. As the latent heat of vaporizationis released, the temperature increases (inor near the visible vapor the temperaturemay rise as much as 20° to 30°F).However, as the visible vapor mixes andre-evaporates in the air flow, the heat ofvaporization is reabsorbed and the duct airtemperature returns to its former level.

Visible Vapor

60°F 60°F

ControllerHumidifier Heat Released

Heat Reabsorbed

10 to 12 ft

Visible Vapor

Vapor Contacts 25% of Duct Air

Visible Vapor

Vapor Contacts 75% of Duct Air

Air Flow

Duct Cross Sections

Duct Cross Sections

23

2. Duct Air Temperature. The tempera-ture of the air flow in the duct alsoaffects the length of the visible vaporpattern. Warmer air produces shortervapor pattern, as shown in Fig. 23-1.All other conditions are the same. In75°F duct air, the average vapor outputfrom a manifold produces a visiblevapor pattern shorter than 12 inchesin length.

3. Duct Air Velocity. As the duct airvelocity increases, the length of the visiblevapor pattern increases. Fig. 23-2shows two sections of air ducts with airvelocities of 500 fpm and 2,000 fpm

respectively. Other conditions are thesame: temperature, duct air humidity,duct dimensions and the amount ofsteam released from the identicalmanifolds. The length of the visiblevapor pattern is approximately propor-tional to the velocity of the air in the duct.

4. Number of Manifolds in Duct.In a large duct section requiring thedischarge capacity of two humidifiers,better vapor distribution is achieved byusing two manifolds full across the ductand vertically spaced to divide the ductsection into thirds. The same effect isachieved by using multiple distributionmanifolds from a single humidifierthat has adequate capacity to meet

the requirements. When a quantity ofvapor is distributed among multiplemanifolds, the amount released througheach manifold is smaller, and more of theduct air comes into contact with thevapor. This effect is shown in Fig. 23-3.

5. Duct Air RH. Relative humidityin the duct also affects the visiblevapor. The higher the relative humiditydownstream of the humidifier discharge,the longer the visible vapor trail. Thecloser duct conditions are to saturation,the longer the vapor trails are likely tobe. Fortunately, duct air RH may becontrolled with a duct high-limithumidistat, as shown in Fig. 25-2.

Figure 23-1. Figure. 23-2. Figure 23-3.

Visible Vapor

Duct Air 75 °F

Air Flow

Visible Vapor

Air Flow 500 FPM

Air Flow

Visible Vapor

Duct Air 55 °F

Visible Vapor

Air Flow 2,000 FPM

24

Basic Application Principles,Continued...

Since the use of multiple manifoldsreduces the length of visible vapor, theiruse should be considered whenever anyof the following conditions exist at thehumidifier location:

A. Duct air temperature is below 65°For relative humidity is above 80%.

B. Duct air velocity exceeds 800 fpm.

C. “Final” or “high efficiency” filters arelocated within 10 feet downstreamfrom humidifier.

D. Height of duct section exceeds 36".

E. Visible vapor impinges upon coils,fans, dampers, filters (not final),turning vanes, etc. located down-stream from humidifier.

Table 24-1 and Fig. 24-1 show atypical number of manifolds and typicalspacing between them when duct heightexceeds 36".

Consult your Armstrong Representativefor specific recommendations regard-ing your needs.

The piping arrangement for humidifierswith multiple manifolds varies with thelocation of the manifolds.

When all manifolds are located abovethe humidifier inlet, manifold piping shouldbe as shown in Fig. 24-2.

When one or more manifolds are locatedbelow the humidifier inlet, the manifoldsshould be trapped separately, as shownin Fig. 24-3.

Smaller manifolds, when possible touse, reduce the cost of multiple manifoldinstallations. Care must be taken that thehumidifier capacity does not exceed thecombined capacity of the multiple mani-folds. Piping arrangement is shown inFig. 25-3.

6. Humidifier Manifold too Closeto High Efficiency Filter. Many airhandling systems require the use ofhigh efficiency filters (also called“absolute” or “final” filters). These filtersremove up to 99.97% of all particles0.3 micron in diameter, and up to 100%of larger particles. The significance ofthese filtering qualities is shown in thefollowing table, where particle sizes ofcommon substances are compared.

Strainer

Inverted BucketSteam Trap

ManifoldPipe Adapters

Duct Cross Section

Strainer

Inverted BucketSteam Trap

“X” Equals DuctHeight Divided By

Number of Manifolds

Duct Cross Section

Strainer

InvertedBucket

Steam Trap

F&T TrapDuct Cross Section

Pipe Insulation is Recommended

Manifold Pipe Adapters Standard Pipe With Threads—One End OnlyPipe Insulation is Recommended

Standard Pipe With Threads–One End Only Standard Pipe With Threads–One End Only

X/2

X

X

X/2

Figure 24-1. Figure 24-2. Figure 24-3.

No. of manifolds tobe installed from oneor more humidifiers

Duct height athumidifierlocation

2

3

4

5

37" to 58"

59" to 80"81" to 100"

101" & Over

Table 24-1. Typical Number of Manifoldsfor Various Duct Heights

Consult your Armstrong Representative forspecific recommendations.

Manifold Pipe Adapters

Particles visible to human eye

Human hair

Dust

Pollen

Fog (visible steam vapor)

Mist (water spray)

Industrial fumes

Bacteria

Gas molecules (steam gas)

Particle Sizein MicronsMaterial

10 or more

100

1 to 100

20 to 50

2 to 40

40 to 500

0.1 to 1

0.3 to 10

0.0006

Table 24-2.Typical Particle Sizes of Common Substances

25

StandardPipe WithThreads—One End

OnlyInverted Bucket

Steam Trap

F&T TrapDuct Cross Section

Manifold Pipe AdaptersStrainer

Figure 25-3.

Since water particles present in visiblevapor range from 2 to 40 microns, theseparticles are trapped by high efficiencyfilters. Some types of filters absorbmoisture and expand, reducing air flowthrough the filter material. As a result,the static pressure in the duct rises fromnormal (about 1" water gauge) to as highas 40" wg. When the filter absorbs moisture,it also releases the latent heat of con-densed steam into the duct air.

Header

Strainer Control Valve

CondensateReturn

F&TTrap

Drip Leg Trap

InactiveTubes

ActiveTubes

F&TTrap

MountingFrame

NOTE: Condensate cannot be liftedor discharged into pressurized return.

Figure 25-4. Steam Grid System

Specially Designed Steam Grid SystemsFor applications with particularly limiteddownstream absorption distances, customengineered systems may be considered.The system includes a separator/headerand multiple dispersion tube assemblypackaged with a control valve, strainer,steam supply drip trap and one or twoheader drain traps. Each system iscustomized to provide uniform distributionand shortened wettable vapor trail(See Fig. 25-4.)

When a humidifier manifold is located tooclose to an absolute filter, the filter collectswater vapor, preventing the moisture fromreaching the space to be humidified.Placing the humidifier manifold fartherupstream allows the water vapor tochange into steam gas, which will passunhindered through an absolute filter.

Under most circumstances, the watervapor will dissipate properly if the humidi-fier manifold is located at least 10 feetahead of the final filter. However, if theduct air temperature is low, air velocity ishigh or the duct is tall, multiple manifoldsmay be installed to speed the mixing ofsteam with the duct air. For additionalprotection, install a duct high-limit controllerjust ahead of the final filter to limit themaximum humidity to approximately 90%.(See Fig. 25-2.)

Figure 25-1.

Air Flow

+ ++++++ ++ ++

+ +++ ++

+++

+

++++

+++++

+++

+

+

++++ ++

+++++++

++++++

+++ ++++++++++++

++++

+++ +

++

+++++

++

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

++

+++

+ + + +++ ++

+++

+

+ ++++++ ++ ++

+ +++ ++

+++

+

++++

+++++

++++++ ++

+++++++

++++++

+++ ++++++++++++

++++

+++ +

++

+++++

+

++

+++

+ + + +++ ++

++

+

++

+

+ ++

+ +

++

+

+

++

+

++ +

+ +

+

+ +

+

+++

+

+

+

+

+

+

+

+

+

Active

Condensate

Inactive Tubes(Baffle Tubes)

Steam Nozzle

+

+

+

+

+

+

+

+

+

+

++

+

+

+

+

+

+

+

+

+

++ +

+

+

+

+

+

+

++

+

+

+

+

+

+

+

+

+

++

+

+

+

+

+

+

+

+

+

Not Here

At Least10 ft

HumidifierHigh-Limit Controller

Mounts Here

Preferred Location

Alternate Location

Figure 25-2.

How Steam Grid Systems ShortenImpingement Distances

Dry steam enters each of the dispersiontubes and flows through stainless steelsteam nozzles which extend from thecenter of each tube, before dischargingthrough orifices into the airstream.

Air flow first encounters baffle tubes(See Fig. 25-1) which influence itsflow pattern and increase its velocity.Air traveling around each set of baffletubes encounters opposing flow ofhigh velocity steam exiting the orifices.The result is more uniform distributionand faster absorption of moisture intothe air, resulting in shorter impingementdistances than experienced withtraditional manifolds or dispersion tubes.

26

However, short-cut methods for makingthese calculations or for checking psychro-metric calculations are described below.

Sizing for Primary HumidificationIn sizing duct humidifiers for air handlingsystems, you should know:CFM of air.Design outdoor air temperature andrelative humidity.Required indoor temperature and relativehumidity.Humidifier steam supply pressure.

The formula for load calculation is:

Humidification Load in lbs/hr

Where:CFM = air flow of unhumidified air at

moisture condition R1R2 = moisture content of required

indoor condition air in gr/ft3

R1 = moisture content of air to behumidified (from outdoor condi-tion) in gr/ft3

7,000 = gr/lb conversion60 = min/hr conversion

EXAMPLE, assume:6,800 CFM of outdoor air.Design outdoor air temperature 0°F.Steam pressure 10 psig.Required 40% RH at 70°F.Air controls used.

From Table 27-1, for 70°F final temperature,read 2.456 under 40% and opposite 0°F.This is pounds of vapor per hour for 100 CFM.Then 68 x 2.456 = 167.01 or, call it 167 lbs perhour required for design conditions.

A single humidifier can provide this capacityalthough sequence control for two humidifiersmight be needed to avoid duct condensationon very light loads. Length of distributionmanifold is governed by width of duct wherethe humidifier is to be located.

Sizing Considerations

Psychrometric Considerationsin Ducted SystemsIn practice you may find that areas needhumidification but cannot be satisfactorilyhumidified through the central air handlingsystem. These are often areas havinghigh sensible heat loads that must bebalanced with low duct air temperatures tomaintain design temperature conditions inthe area. Typical examples are dataprocessing rooms or hospital operatingrooms where duct air temperatures maybe held as low as 50°F to maintain adesign condition of 75°F in the room.These low duct air temperatures preventadding enough moisture to the air to meetdesign RH requirements in the room—say, 55% RH.

Using these conditions as an example,duct air at 50°F and 90% RH holds slightlyless than 3.7 grains of moisture per cubicfoot. At 75°F the same 3.7 grains ofmoisture yield a relative humidity of 39%.To achieve design conditions of 55% RHat 75°F, the air must contain 5.2 grains ofmoisture per cubic foot—1.5 grains morethan it psychrometrically can hold at ductair temperature.

For such applications, booster humid-ification must be accomplished in the airof the area after it has reached its finaltemperature. Evaporative pan humidifiersmay be used for this purpose, although wewould recommend using combined steamhumidifier-fan units which can be installedeither within the humidified space orremote ducted to the space. For hospitalapplications, steam humidifier-fan unitsshould include an integral high efficiency(95%) filter to satisfy code requirements.

Determining HumidificationLoads for Air Handling SystemsMost engineers prefer to determinehumidification requirements psychro-metrically on the basis of design condi-tions and humidification requirements.

Sizing for Booster HumidifierAssume that a primary humidifier providesair that will have 40% RH at 70°F, but youwant to maintain 60% RH in a laboratorysupplied with 900 CFM of the air at 40%at 70°F. Refer to Table 27-3 and read 1.38under 60% and opposite 70°F—40%.9 x 1.38 = 12.42 lbs. The humidifier mustbe able to provide this capacity at steamsupply pressure.

Special ConditionsWhen relative humidities must be figuredfor temperature conditions other thanthose given in Tables 27-1 through 27-3,Table 27-5 will prove helpful.

New Condition—55% RH at 77°F.Makeup Air—35% RH at 70°F.From Table E-23:Grains per cu ftNew Condition 5.54Less Grains per cu ft,Makeup Air 2.82Grains to be added 2.72Assume 800 CFM

800 x 2.72 x 60 7,000

NOTE: .857 lb of steam per hour will add1 grain to 100 CFM. Use of this factor simplifiesthe above equation to: 8 x 2.72 x .857 = 18.65.

Where Table 27-5 is used for outdoor airmakeup, assume 75% RH for the outdoorair at 0° to -20°F.

Room to Duct ComparisonsWhen high humidity is needed in a room(70°F-60% RH) and the duct temperatureis lower than the room temperature (50°F),the duct high-limit humidistat often actsas the controlling stat. Duct high-limithumidistats should be set between 70%and 90% RH. We do not recommendsetting the high-limit stat any higher than90% RH. Table 27-4 shows the maximumroom humidity that can be achieved for thegiven duct conditions.

= 18.65 lbs/hr

CFM (R2 - R1) 607,000

=

27

Table 27-3. Booster Humidification Pounds of vapor per hour per100 CFM to secure desired relative humidity with no change in air temperature

Relative Humidity DesiredInitial Condition

RH 40% 45% 50% 55% 60% 65% 70%Temp.

70

70

72

72

75

75

35%

40%

35%

40%

35%

40%

.345

––

.368

––

.405

––

.690

.345

.728

.368

.810

.405

1.03

.69

1.10

.73

1.22

.81

1.38

1.03

1.46

1.10

1.62

1.22

1.72

1.38

1.83

1.46

2.03

1.62

2.07

1.72

2.20

1.83

2.43

2.03

2.42

2.07

2.57

2.20

2.84

2.43

Relative Humidity DesiredOutdoor

emp.

*30

20

10

0

-10

-20

35%

1.165

1.618

1.918

2.111

2.233

2.309

40%

1.510

1.963

2.263

2.456

2.578

2.654

45%

1.855

2.308

2.608

2.801

2.923

2.999

50%

2.200

2.653

2.953

3.146

3.268

3.344

55%

2.545

2.998

3.298

3.591

3.613

3.689

60%

2.891

3.344

3.644

3.837

3.959

4.035

65%

3.236

3.689

3.989

4.182

4.304

4.380

70%

3.581

4.034

4.334

4.527

4.649

4.725

Table 27-1. 70°F Primary Humidification Pounds of vapor required perhour per 100 CFM to secure desired RH at 70°F (outside air 75% saturated)

*30

20

10

0

-10

-20

1.584

2.034

2.334

2.529

2.652

2.727

1.989

2.439

2.739

2.934

3.057

3.132

2.394

2.844

3.144

3.339

3.462

3.537

2.799

3.249

3.549

3.744

3.867

3.942

3.204

3.654

3.954

4.149

4.272

4.347

3.609

4.059

4.359

4.554

4.677

4.752

4.014

4.464

4.764

4.959

5.082

5.157

4.419

4.869

5.169

5.364

5.487

5.562

Table 27-2. 75°F Primary Humidification Pounds of vapor required perhour per 100 CFM to secure desired RH at 75°F (outside air 75% saturated)

Relative Humidity DesiredOutdoor

emp. 35% 40% 45% 50% 55% 60% 65% 70%

Grains per cu ft atRelative Humidity Specified

75%65%60%55%50%45%40%35%

Grainscu ft

SaturatedAir

Temp.

Table 27-5. Grains of Water Vapor per cu ft of Air at VariousTemperatures and Relative Humidities

8.288.037.797.557.317.096.866.656.446.246.045.134.353.673.082.151.46.93.58.36.21.12

7.186.966.756.556.346.145.955.765.585.415.244.453.773.182.671.861.26.81.50.31.19.11

6.626.436.236.045.855.675.495.325.154.994.834.113.482.932.461.721.17.75.47.29.17.10

6.075.895.715.545.365.205.034.884.724.584.433.763.192.692.261.571.07.68.43.26.16.09

5.525.365.195.034.874.724.584.434.294.164.033.422.902.442.051.43.97.62.39.24.14.08

4.974.824.674.534.394.254.123.993.863.743.623.082.612.201.851.29.78.56.35.21.13.07

4.424.284.154.033.903.783.663.553.443.333.222.742.321.961.641.15.68.50.31.19.11.07

3.863.753.633.523.413.313.203.103.012.912.822.402.031.711.441.00.58.43.27.17.10.06

11.0410.7110.3810.069.7499.4489.1538.8678.5688.3198.0556.8455.7954.8894.1062.8631.9461.242.776.475.285.166

807978777675747372717065605550403020100

-10-20

Steam required to add 1 gr per cu ft to 100 CFM:100 x 60 = 6,000 cu ft per hour or 6,000 grains per hour.

You respond to the questions—often witha single keystroke—and the SoftwareProgram 2 can:

■ Calculate humidification load.■ Determine correct humidifier

model number.■ Determine correct orifice size.■ Indicate psychrometric properties

of air.■ Calculate equivalent room humidity

from known duct conditions.■ Print the complete humidification

application specification.

= .857 lb/hr

For a free copy of Armstrong SoftwareProgram 2, contact Armstrong or yourArmstrong Representative.

Table 27-4. Maximum Room RH for Given Duct Conditions

Room RH @ Temperature °FDuctTemperature

°F 68° 70° 72° 75°

DuctRelativeHumidity

(RH)90%85%80%90%85%80%90%85%80%

47%44%42%57%53%50%68%64%60%

44%41%39%53%50%47%63%60%56%

41%39%36%49%46%44%59%56%52%

37%35%33%44%42%39%53%50%47%

50

55

60

Computer Software CanSimplify Humidifier SelectionArmstrong offers a free software programwhich can eliminate the need for time-consuming pencil-and-paper calculations.The Armstrong Software Program 2 runson IBM PC or compatible MS-DOScomputers. Once the user-friendly soft-ware is loaded into your computer, theprogram displays on your monitor a seriesof easy-to-understand questions aboutyour humidification application.

* When outdoor design temperatures exceed 30°F, use Table 27-5 enteringthe table with both outdoor design temperature and outdoor design RH.

6,0007,000

28

Table 28-4. With 75°F Return Air

30% 35% 40% 45% 50% 55%

Max. Humidification Load (given in lbs of vapor/hr/1,000 CFM of total air) Occurs at Outside Air Temp. Shown for Given Inside RH

Max.Load

OutsideAir ˚F

Max.Load

OutsideAir ˚F

Max.Load

OutsideAir ˚F

Max.Load

OutsideAir ˚F

Max.Load

OutsideAir ˚F

Max.Load

OutsideAir ˚F

Mixed AirTemp. ˚F

50556065

Inside RH

35353535

9.77.85.83.9

43434343

12.610.17.65.1

50505050

16.4*13.19.86.5

50555959

20.5*17.0*13.08.6

50556065

24.7*21.2*17.112.3

50556065

28.8*25.4*21.3*16.5*

Table 28-3. With 75°F Return Air

% Outside Air Required at Temperature Shown

-10˚ 0˚ 5˚ 10˚ 15˚ 20˚ 25˚ 30˚ 35˚ 40˚ 45˚ 50˚ 55˚ 60˚ 65˚

DesiredMixed AirTemp. °F

50556065 100

––––––100

67

––––100

7550

––100806040

83675033

71574329

62503725

56443322

50403020

45362718

42332516

38312315

36282114

33262013

30231812

Table 28-2. With 70°F Return Air

30% 35% 40% 45% 50% 55%

Max. Humidification Load (given in lbs of vapor/hr/1,000 CFM of total air) Occurs at Outside Air Temp. Shown for Given Inside RH

21.5*18.1*14.09.2

Max.Load

50556065

OutsideAir ˚F

18.0*14.610.55.7

Max.Load

50556065

OutsideAir ˚F

14.511.17.43.7

Max.Load

50545454

OutsideAir ˚F

11.28.45.62.8

Max.Load

46464646

OutsideAir ˚F

8.66.54.32.2

Max.Load

3939390

OutsideAir ˚F

Max.Load

6.75.03.31.9

OutsideAir ˚F

3030300

Mixed AirTemp. ˚F

50556065

Inside RH

% Outside Air Required at Temperature Shown

65˚

––100

––––

-10˚2519126

0˚2921147

5˚3123157

10˚3325178

15˚3627189

20˚40302010

25˚45332211

30˚50362513

35˚57432914

40˚67503316

45˚80604020

50˚100755025

55˚––

1006733

60˚

––10050

––

DesiredMixed AirTemp. °F

50556065

Table 28-1. With 70°F Return Air

Economizer CyclesFan coil air systems which mix return airand outside air in varying amounts toobtain a given final mixed air temperaturerequire special consideration in determin-ing maximum humidification loads.

Systems of this type usually use a fixedminimum amount of outside air (approxi-mately 10%-30%) when outside air temp-erature is at a maximum design (-10°F).As the outside air temperature increases,more outside air is mixed with return airto achieve a final mixed air temperature(55°F). Since humidification load is afunction of the amount of outside airintroduced (plus its moisture content) themaximum humidification requirement willoccur at some outside air temperatureother than maximum design.

ConditionsTables 28-1 and 28-3 below give thepercent of outside air required to main-tain desired mixed air temperature whenout-side air temperature is as shown.Table 28-1 is used when return air(room air) temperature is at 70°F. Table28-3 is for 75°F return air systems.

Tables 28-2 and 28-4 can be used todetermine maximum humidification loadat the given conditions of mixed airtemperature and required RH, assuming50% RH OSA and 10% minimum OSA.

NOTE: Consideration must be given to over-saturating conditions in lower temperaturesystems.

EXAMPLEGiven conditions that 70°F return airtemperature is mixed with outside air toproduce 55°F constant mixed air temperaturein duct. The design of the space being con-ditioned is 70°F at 40% RH. Total volumeof air through the fan system is 4,000 CFM.Determine maximum humidification load.

From Table 28-2 with 55°F mixed airtemperature and 40% RH space design, themaximum humidification load is 8.4 poundsper 1,000 CFM of total air volume. Thismaximum load occurs when the outside airtemperature is at 46°F. Multiplying 8.4 x 4results in total pounds per hour required inthe 4,000 CFM system. Therefore maximumhumidification load becomes 33.6 pounds ofvapor per hour.

* Humidification loads will exceed 90% RH in duct at temperature indicated. Booster humidification is recommended.

29

Steam Humidifiers in Central Systems

were located between the coil and thefan, it might interfere with the temperaturesensing bulb. The indicated use of a high-limit duct humidity controller shown isoptional. It is advisable if the capacity ofthe humidifier at design loads couldpossibly overload air when outside airmoisture content is higher than the design.The high-limit controller should be 10 to12 feet downstream from the humidifier.Place the high-limit controller where it willsee the same temperature as the humidi-fier. A cooler temperature at the humidifierwould allow saturation if the high-limitcontroller were in warmer air.

This is shown as a pneumatic controlsystem. The fan switch activates thecontrol system and the electric pneumaticrelay bleeds air from the humidifieractuator diaphragm when the fan is off.The following examples also showpneumatic control—if the systems wereelectric, control locations would remainthe same.

Steam pressure to the humidifiers must bekept relatively constant to assure sufficientcapacity. Double-check to be sure you’renot trying to put more moisture into theair stream than it can hold at its existingtemperature. Use of psychrometics canbe a helpful aid in determining moisturepotential in your application.

Proper location of humidifiers in the systemis most important, although sometimes thedesign of the system makes this difficult toachieve. The following examples of typicalsystems demonstrate proper humidifierlocation.

System 1This is a simple ventilating system. Weassume final duct air temperature to beslightly above desired room temperature.The desirable location of the steamjacketed distribution manifold of theprimary humidifier is downstream from thesupply fan. This humidifier would be sizedfor maximum design load. If the humidifier

Proper location, installation and control ofhumidifiers is essential to achieve totallysatisfactory, trouble-free performance.The primary objective is to provide therequired relative humidity without dripping,spitting or condensation. Liquid moisture,even in the form of damp spots, cannot betolerated in the system. Aside from thehazards to the structure caused by waterin the ducts, there is an even more criticalhealth hazard if breeding grounds areprovided for bacteria.

In addition to the need for proper humidi-fier design and performance, several otherfactors deserve close attention. Thehumidifier must be the proper capacity forthe system; properly located in relation toother components of the system; properlyinstalled and piped in a manner that willnot nullify all the other precautions taken.In sizing humidifiers you should be surethat they deliver the amount of steam perhour called for in the design calculations.

SupplyFanNC

NO

OSA

E-P Relay Bleeds Air FromHumidifier Diaphragm When

Fan is Off

Fan Switch EnergizesControl System

Humidifier With SteamJacketed Distribution Manifold

Sized For Max. Design Load

Space Humidity ControlModulates Humidifier Valve

To Maintain Space RH

System 1Figure 29-1.

Ventilation system with primary humidification.

Features of this system andthe following systems include:A. Accurate control is possible because of

immediate response of steam humidifier.B. Control can be modulating electric or

pneumatic (shown).C. No need for drain pans or eliminator

plates; makes location of humidifiermore flexible.

D. Addition of moisture is accomplishedwith no appreciable change in duct dry-bulb temperature.

E. The humidifier’s integral steam jacketedcontrol valve with parabolic plug is accuratelysized to meet capacity requirements.

Glossary of SymbolsEA Exhaust airE-P relay Electric-Pneumatic relayH Humidity controllerM Damper motorMA Mixed airNC Normally closedNO Normally openOSA Outside airRA Return airT Temperature controller

H

H

RA

M

TT

90% RH High-Limit DuctHumidity Controller

10'-12'Min.

30

System 3System 2

50°-60°

Motor

ExhaustFan

Duct Controller

EA Grille

E-P Relay

Humidifier

SupplyFan

ReheatPreheat

NC100%OSA

EA

Controller(Space or EA Duct)

SupplyFan

Preheat Reheat

Motor

100%OSA

NC 50°-60°

V-2

V-1

E-P Relay

Humidifiers—(V-1 has 1/3 DesignCapacity, 4-7 psi Spring Range;V-2 Has 2/3 Capacity,8-13 psi Spring Range

Figure 30-1.100% OSA heat-vent system with primary humidification.

Figure 30-2.100% OSA heat-vent system with sequence control

on primary humidification.

NC Normally closedNO Normally openOSA Outside airRA Return airT Temperature controller

EA Exhaust airE-P relay Electric-Pneumatic relayH Humidity controllerM Damper motorMA Mixed air

Glossary of Symbols

closer moisture input control, particularlywhen operating conditions vary consider-ably from design, thus preventing thepossibility of overrun and duct saturation.With milder outdoor air conditions, V-1 cansatisfy space conditions by introducingonly a portion of the total design capacity.

As the outdoor air becomes colder anddrier, humidifier V-1 will not satisfydemand so the V-2 unit starts to open inresponse to the additional demand. Thisgives much closer control in all kinds ofoutside air conditions, as well as prevent-ing a super-saturated condition in the ductat minimum design. Again the high-limitcontroller is optional but desirable.

Again, the high-limit controller is optionalbut generally recommended.

System 3This system is similar to the previousone. It also shows 100% outside air andpreheat and reheat coils. But here twohumidifiers are used and are controlled insequence from a single space or exhaustair duct humidity controller. The twohumidifiers are indicated as V-1 and V-2.

V-1 will deliver one-third of the totalcapacity with a 4 to 7 psig spring range.V-2 is sized for two-thirds of the capacity,with a spring range of 8 to 13 psig. Thissequencing control arrangement allows

System 2This is a typical 100% outside air systemwith preheat and reheat coils. The preheatcoil heats outside air to a duct tempera-ture controlled at 50° to 60°F. The reheatcoil adds more sensible heat dependingon the space heat requirement. Here thedesirable location for the primary humidi-fier is downstream from the reheat coil tointroduce moisture into the highest levelof dry-bulb air temperature.

Note the humidity controller location inthe exhaust air duct. When a good pilotlocation for a humidity controller is notavailable in the space humidified, oneplaced in the exhaust air duct as closeto the outlet grille as possible serves thepurpose very well.

Steam Humidifiers in Central Systems, Continued...

H

TT

M

T T

M

H

H

H

90% RH High-Limit DuctHumidity Controller

90% RH High-Limit DuctHumidity Controller

10'-12'Min.

10'-12'Min.

31

System 5

System 4Here is another 100% outside air system.In this case, the air leaving the preheatcoil is held at a constant dry-bulb tempera-ture in the 55° to 60°F range. This systemindicates the use of two humidifiers—oneas a primary humidifier and the second asa booster or secondary humidifier.

This system allows a primary humidifier tobe controlled directly from a duct humiditycontroller at a level high enough tomaintain a space condition of about35% RH at a space temperature of 75°F.The booster unit, located downstreamfrom a reheat coil and fan, can then besized and controlled to produce thenecessary moisture to raise the space RHfrom 35% to some higher condition, say55%, where and when desired. This allowsindividual humidity control for each zoneat a higher level than otherwise possible.

This is an important combination becausethe use of the primary unit allows thecapacity of the booster unit to be smallenough so that super saturation andvisible moisture will not occur, even whenthe units are located as close as three feetfrom the discharge grille. Tests indicatethat with capacities in the 8 to 10 lbs/hrrange, three feet minimum distance willnot produce visible vapor.

In this typical air handling system, it wouldnot be psychrometrically possible to intro-duce enough humidity into the air temper-ature downstream from the preheat coil togive the maximum required condition inexcess of 35% RH in the space. SeeExample 2, page 13. The use of both primaryand booster humidifiers is the only methodfor controlling the relative humidity in spaceat any level above approximately 35%.

System 5Here is a single zone packaged heatingand ventilating unit with internal face andby-pass dampers. The humidifier shouldbe positioned downstream from the mixingdampers so that moisture is introducedinto the final leaving air temperatures ofthe heating ventilating unit. This locationpermits a high level of space relativehumidity to be maintained without ductsaturation. It is a preferable location to justahead of the coils because of higher airtemperature and better mixing conditions.Again a high-limit controller is recom-mended to prevent possible duct satura-tion, installed 10 to 12 feet downstreamfrom the humidifier.

System 4

Grille

Preheat

55°-60°

BoosterHumidifier

Space Controller inZones With BoosterHumidifier Maintaining55% RH

To OtherZones

Motor100%OSA

E-P Relay

3'Min

SupplyFan

Primary HumidifierMaintains Duct RH at70% to Give 35% RHin Zones Not UsingBooster Humidifier

E-P Relay

10'-12'Min.

Humidifier

Low-LimitT’Stat

Motor DampersCoils

DuctController

MA 55°

90% RH High-limit Controller

Figure 31-1.100% OSA heat-vent system with primary and booster humidification.

Figure 31-2.Single zone heat-vent unit with internal face and by-pass

dampers—primary humidification.

H T

M

T T

H

Space Temp-HumidityControllers

T H

T

M

H

C

C

C

H

10'-12'Min.

32

System 6This is a multi-zone heating/ventilating unitwith face and by-pass dampers on eachzone. The example shows a method andlocation for primary humidification, but itshould be restricted to design conditionsof “comfort humidification” of, say, 35%.These systems are normally packageunits and it is standard practice to incor-porate the humidifier ahead of the coils asshown. This location of the humidifier willprovide equal moisture distribution in hotor cold decks before heat zone takeoff,but it does limit the amount of moisturethat can be added to the 55° air. Designconditions above 35% RH risk impinge-ment of visible vapor on the coils. SeeExample 2, page 13.

With these units, it is sometimes possibleto use two humidifiers at this location with

supply fan—to ensure good air mixing andto allow the duct controller ample time tosense the condition short of saturation.The use of multiple manifolds will helpprovide good air mixing.

Note that the primary humidifier in thiscase should not be controlled from aspace controller or an exhaust air ductcontroller, but rather from the supply ductcontroller as indicated. Since each zonehas its own temperature-controlled mixingbox, a location of the primary humidifiercontroller in the space or exhaust ductcould not provide accurate control.Further, the distance between thehumidifier and the controller could causedelayed response or override.

baffles between zone takeoff and sizedfor different conditions of relative humidityin their respective sections. Boosterhumidifiers can be used in individualzones for a higher relative humidity whererequired.

System 7Here is a high-velocity dual duct systemwith primary and booster humidificationshown. Like System 6, above, the primaryhumidifier is capable of providing “comforthumidification” only—30% to 35% RH.Because of space limitations, the primaryhumidifier, sized to maintain a ductcondition of, say, 90% RH in the mixedair temperature, can be located as shownahead of the fan. The humidifier should belocated as far as possible upstream—nocloser than three feet from the face of the

Steam Humidifiers in Central Systems, Continued...

MixingBox

Grille

BoosterHumidifier 3'From Grille

Cold Deck

SupplyFan

MA50°-55°

DuctController

for PrimaryHumidifier

Primary HumidifierSized to Maintain

Approx. 90% RH inMixed Air

MA55°

Two or MoreZones to Rightof Line A-A

Controller(Space or RA Duct)

E-P Relay

Humidifier

MotorHot Deck To

OtherZones

Space HumidityController in Each

Zone

System 6Figure 32-1.

Multi-zone heat-vent unit with internal face and by-passdampers for each zone—primary humidification.

System 7

Figure 32-2.High-velocity dual duct system with primary and booster humidification.

NC Normally closedNO Normally openOSA Outside airRA Return airT Temperature controller

EA Exhaust airE-P relay Electric-Pneumatic relayH Humidity controllerM Damper motorMA Mixed air

Glossary of Symbols

C

C

C

H

H

T

M T

T

M

H

T H

A

A

33

Recommended humidifier locations fora vertical discharge draw-thru type airconditioner (Fig. 33-2) are identical to thehorizontal unit. If the alternate locationmust be used, a high-limit controller set at80% is desirable. The humidifier manifoldshould discharge upward, as with thehorizontal discharge unit.

at the fan discharge. In some instancesthis may not be possible. Note that withthe alternate location, the humidifiermanifold is installed to discharge upwardinto the area of greatest air turbulence.This permits the air to achieve optimummixing before reaching the fan blades.A high-limit controller, set at 80%, shouldbe located as shown when the humidifieris installed at the alternate location.

High-LimitController80% RH

RecommendedHumidifierLocation

AlternateLocation

OSA

RA

AlternateLocation Recommended

Humidifier Location

With humidifier installed at recommended location, high-limit duct controller should beset at 90% RH maximum—alternate location at 80% RH maximum.

Figure 33-2. Vertical Discharge

Figure 33-1. Horizontal Discharge

OSA

RA

AlternateLocation Recommended

Humidifier Location

High-LimitController80% RH

Packaged Air ConditionerInstallationsHumidifiers frequently must be installed inpackaged central station air conditioners.This can present some unusual locationrequirements due to the close quarterswithin the packaged units.

In the horizontal discharge draw-thru typepackaged unit shown in Fig. 33-1, therecommended location of the humidifier is

34

In a low pressure blow-thru type, multi-zone,packaged air conditioner (Fig. 34-1), therecommendations are much the same.However, to avoid overloading the colddeck and to avoid impingement ofdischarge, the manifold is installed todischarge upward instead of directly intothe fan discharge.

As with the draw-thru units, a high-limitcontroller set at 90% should be installed.In a high pressure blow-thru type pack-aged unit (Fig. 34-2), again the recom-mended location is as close to the fan aspossible, with the manifold dischargingdirectly into the fan discharge. A high-limitcontroller set at 90% is desirable.

In either high or low pressure systems,where the humidifier is installed at thealternate location, set the high-limithumidity controller at 80% RH.

Steam Humidifiers in Central Systems, Continued...

Alternate LocationRecommended

Humidifier Location

Figure 34-2. High Pressure System

Alternate LocationRecommended

Humidifier Location

Figure 34-1. Low Pressure System

RecommendedHumidifier LocationAlternate Location

OSACold

Hot

RA

Alternate Location RecommendedHumidifier Location

OSACold

Hot

RA

35

8" or Less

NOTE: All dimensions shown in the above figures are based on duct temperature of 65°F or higher and duct velocities of 800 ft/min, or lower.If duct air is cooler or velocities are higher, these dimensions should be greater or multiple manifolds considered.

Don’t Do

10" Min.

CoilsDon’t

less than 3'

Do

Don’t DoDo

LessThan 3'

Do

Fan

Don’t

LessThan10'

Duct Temp.Control

Don’tDo

Do

Don’tDead Air

LessThan 3'

Do Don’t

Disc

harg

e Gr

ille

Don’t

Do

Don’t

DoDon’t

Don’t

Do Elevation

Section A A

A

A

Dead Air

Fan

When it is necessary to place the humidi-fier discharge into a packaged multi-zoneair handling system, install the distributionmanifold into the center of the active airflow and as close to the fan dischargeas possible.

Do not install a distribution manifold closerthan 10 feet upstream from a temperaturecontroller or you may get false signals.

The distribution manifold should neverbe placed within three feet of an air fanintake. The best location is at the fandischarge.

Whenever possible, install the distributionmanifold into the center of the duct.

Installation Do’s and Don’tsIn discussing the systems, we mentioneda few location “do’s and don’ts.” Let’sreview these precautions that may helpto keep you out of trouble. For example,whenever possible, install the distributionmanifold downstream from coils. If youhave more than three feet of distanceavailable between the manifold and thecoil on the upstream side, the manifoldcan be installed at this location (greaterthan three feet for higher velocity systems).

When it is necessary to place the humidi-fier in the coil section ahead of the fan,locate the manifold in the most active airflow and as far upstream from the fan inletas possible.

Don’t risk restriction of the air flow in ducts8" or less in depth. Use an expandedsection as shown.

Always install distribution manifolds asfar upstream from discharge air grillesas possible—never less than three feetupstream.

Always size and install the distributionmanifold to span the widest dimensionof the duct section.

Always select the stream distributionmanifold length that will span themaximum width of the duct.

The manifold should never be installedvertically downward from the humidifier.This presents a condensate drainageproblem in the jacket of the manifold.Vertical upward installation is permissible.

Filte

r

Coil

36

Typical Problem:Design outdoor temperature 0°FIndoor temperature 70°FRH required 40%Air changes per hour 2Steam pressure available 5 psi

Room size 400' x 160' with 10' ceilingNatural ventilationHeated by: Unit heaters-fan on-offcontrol

Step I: Steam required for humidification.Our room contains (400' x 160' x 10') or640,000 cu ft.

From the 70°F Table 36-1, read acrossfrom 0°F outside temperature to the 40%RH column where you find the figure .409lbs of steam/hour per 1,000 cu ft of spacefor each air change. Then, 640 times .409times 2 equals 524 lbs of steam/hourinstalled humidification capacity required.

Step II: Electric or air-controlled units.The large floor area calls for multiplehumidifiers. No explosion hazard hasbeen specified so use of air-controlledunits is not required. Electric units arerecommended.

Step III: Number of humidifiers for job.Divide steam required by capacity ofhumidifiers at steam pressure available.

Application of Unit Humidifiers for Direct Discharge

Pounds of steam per hour, per air change for each 1,000 cu ft of space to secure desired indoorrelative humidity at 70°F with various outdoor temperatures (outside air 75% saturated).

70°F—Relative Humidity Desired Indoors—70°FOutdoorTemp. 75%

.654

.730

.780

.812

.832

.844

70%

.597

.672

.722

.754

.775

.787

65%

.539

.615

.665

.697

.717

.730

60%

.482

.557

.607

.639

.660

.672

55%

.424

.499

.549

.582

.602

.615

50%

.367

.441

.492

.524

.545

.557

45%

.309

.385

.434

.467

.487

.500

40%

.251

.327

.377

.409

.430

.442

35%

.194

.269

.319

.352

.372

.385

30%

.136

.212

.262

.294

.314

.327

25%

.079

.154

.204

.237

.257

.270

Table 36-1. 70°F Humidification

*3020100

-10-20

Step IV: What size humidifier to use. Forthis example, a large number of smallercapacity units is recommended. Largercapacity units could cause condensationon the low ceiling. Also, because of thelarge floor area, the humidistats for fewerunits would be widely spaced which couldresult in less accurate control thandesirable.

Step V: What type humidifier to use.In this example, integral fan units arepreferable to steam jet units installed inconjunction with unit heaters. Since theunit heater fans are on or off to controltemperature, it follows that the humidistat may call for steam when the nearest unitheater is not running. With the low ceiling,the discharge from a steam jet humidifiermight rise to the ceiling and producecondensation. Therefore, the integral fantype should be used.

Step VI: Location of humidifiers. Severalpatterns are possible, and actual locationcan usually conform with the existingsteam supply and return lines to make aneconomical installation with a minimumof new piping.

A survey of your requirements should betaken to determine the amount of steamneeded for humidification, the number,size and type of units required, and thelocation of both humidifier and humiditycontrollers.

Sizing and Location withNatural VentilationThese are the average industrial humid-ification applications with:

Room temperatures—65° to 80°F.Relative humidities—35% to 80%.Natural ventilation—i.e., infiltration aroundwindows and doors.

Selection Data RequiredMinimum Outdoor Temperature: Formost jobs, figure 10°F above the lowestrecorded temperature for your locality.The lowest temperatures are seldomencountered for more than a few hours.■ Indoor Temperature■ RH Desired■ Pressure of Steam Available for

Humidification■ Number of Cubic Feet in Room■ Air Changes Per Hour: air changes

taking place under average conditionsexclusive of air provided for ventilationor regain of hygroscopic materials.

Rooms, 1 side exposed 1Rooms, 2 sides exposed 11/2

Rooms, 3 or 4 sides exposed 2Rooms with no windows or outside doors 1/2 - 3/4

37

rge

Pounds of steam required per hour per air change for each 1,000 cu ft of space to secure desiredindoor relative humidity at 75°F with various outdoor temperatures (outside air 75% saturated).

NOTE: When outdoor design temperatures exceed 30°F, use Table 5-1.

75°F—Relative Humidity Desired Indoors—75°FOutdoorTemp.

*3020100

-10-20

Table 37-1. 75°F Humidification

.129

.204

.254

.286

.307

.319

25%

.196

.271

.321

.354

.374

.387

30%

.264

.339

.389

.421

.442

.454

35%

.331

.406

.456

.489

.509

.522

40%

.399

.474

.524

.556

.577

.589

45%

.466

.541

.591

.624

.644

.657

50%

.533

.609

.659

.691

.711

.724

55%

.601

.676

.726

.759

.779

.792

60%

.668

.744

.794

.826

.846

.859

65%

.736

.811

.861

.894

.914

.927

70%

.803

.879

.929

.961

.981

.994

75%

Figure 37-2. Outlines a typical requirement.Schematic layout of humidifiers in wood-working plant where exhaust fans are used.Arrows indicate air flow induced by fans.Humidifiers are sized for load conditionsimposed by fan. Humidifiers are located to giveuniform distribution of humidity when fans areoff or when fans are running.

Figure 37-1. Where practical, locatehumid-ifiers to minimize piping. Locationsshown in black where steam supply linesare along outer walls; in color wheresupply is in center of room.

In our problem of a 400' x 160' x 10' room,there would likely be steam lines alongboth sides of the room, and humidifierscan be located as shown in black in Fig.37-1. If the supply lines run down thecenter of the room the colored line patternwould be practical. Runouts to integral fanunits in a 160' wide room would be about20' long. If the room were only 60 or 80feet wide, runouts need be no longer thanrequired for actual hookup.

Step VII: Location of humidistat. Thisshould be from 20 to 30 feet away fromthe humidifier and slightly to one side ofthe air stream from the unit. The humidi-stat should “see” its humidifier and be in“active” air. Do not hide it behind a postor in the channel of an H-beam. It mustget a good sample of the air to controlthe humidity.

Sizing and Location withForced VentilationTypical Jobs: Mill and sanding roomsin furniture factories. Here, the problemof selecting and installing humidifiers ismuch the same as previously describedexcept for:

1.Determining the number of air changes.

2.Location of humidifiers and humidistats.

Air Changes: These can be determinedfrom the exhaust fans’ capacities. Thecubic feet per hour capacity of the fans,divided by the cubic feet of space to behumidified, will give the number of airchanges.

Where the capacity of fan or fans is notknown, air changes can be measured withvelometer readings at all open doors,elevator shafts, etc. leading to the roomand with fans operating at full capacity.Your Armstrong Representative candetermine air changes for you.

Humidifier Location: Bear in mindthat humidifiers will have to control thehumidity 24 hours a day, seven days aweek during the heating season. Exhaustfans may operate only 40 hours or 80hours per week. Thus the humidifiers andhumidistats must be located for gooddistribution of humidity during fan-offperiods as well as when the fans areoperating.

A

B

D

E F

C

38

Sizing for High or LowTemperature HumidificationWhere air temperatures are well above75°F or below 70°F, it is impractical touse Tables 36-1 or 37-1. Humidificationrequirements must be figured from Table5-1, page 5, showing grains of water percu ft of saturated air at various tempera-tures. Typical problem: How much steamper hour is required to humidify 60,000 cu ftof space with four air changes per hourto 40% RH when the air temperature is90°F? Assume that any makeup air willcome from outdoors at 0°F, 75% saturated.

Special Purpose IndustrialApplicationsIn some industrial operations, a stratum ofhigh relative humidity is required in closeproximity to a fast moving sheet or film ofpaper, thin gauge plastic, fabric, cello-phane, etc. The objective may be toprevent accumulation of static electricitycharges, or to prevent loss of moisturefrom the material. If the sheet or film ishot, as it very well might be, it tends togive up its moisture very quickly. By usingsteam shower humidifiers expresslyadapted for this application to create alaminar zone of high humidity adjacentto the sheet, moisture loss is preventedand moisture content of the material isproperly maintained.

For this application, the humidifier must beinterlocked with the drive of the machine,and it is essential that the steam bedischarged in a dry state, with no waterdroplets or liquid spray.

Explosion HazardHumidificationSizing air-operated humidifiers for areaswhen explosion hazard exists is doneexactly as for other requirements exceptthat they should be sized for the mostsevere conditions of makeup air, RHrequired and minimum steam pressure.

Humidifiers should be located to get thebest possible dispersal and distributionof vapor in the area.

90°F saturated air = 14.9 gr/cu ft saturated = 5.976 gr/cu ft at 40% RH

Outdoor air 0°F saturated = .475 gr/cu ft

75% saturated = .356 gr/cu ft

5.976 minus .356 = 5.620 grains to be added per cu ft

5.620 X 1,000 7,000

NOTE: 7,000 gr = 1 lb

= .803 lb per M cu ft per air change

With four air changes in a 60,000 cu ftroom, then .803 x 60 x 4, or 193 lbs steamwould be required per hour. Humidifiercapacity required for temperatures below70°F is determined in exactly the samemanner.

NOTE: For high temperature air inparticular, air volume changes dramaticallywith RH. Armstrong Software Program 2will provide greater accuracy in humidifiersizing for these applications.

Application of Unit Humidifiers for Direct Discharge, Continued...

39

Installation Bulletins:Bulletin 544Series 9000 and 1000 Humidifiersfor Air Handling Systems

Bulletin 549Series 9000 Humidifiersfor Direct Area Humidification

Bulletin IB-87Series CS/CSESteam-to-Steam Humidifiers

Bulletin 527Series EHU-700Electronic Steam Humidifiers

Bulletin 526Series EHU-600 Solutionsfor Sensitive Environments

Bulletin 537Series HC-4000HumidiClean™ with patentedionic bed technology

Bulletin 560HumidiPack®

Steam Humidifier System

The Armstrong organization includesspecialists–from researchers to applica-tion engineers–in every aspect ofhumidification. Armstrong Internationalhas been successfully solving humidi-fication problems for over 60 years.When you contact your local ArmstrongRepresentative, he will make available

Your humidification needs can bemet if you follow Armstrong’s fivebasic guidelines:

1. Evaluate the requirementsof the system.

2. Select the most suitable medium.3. Size the humidifiers properly.4. Locate and install the humidifiers

correctly.5. Employ suitable humidity controllers

also properly located.

Conclusion

References

ASHRAE Handbook, 1988—Equipment Volume.

ASHRAE Handbook of Fundamentals, 1989.

ASHRAE Handbook and Product Directory,1987—Systems and Applications Volume.

IBM Installation Planning Manual, April, 1973.

Obert, Edward F. Thermodynamics, 1948.

Static Electricity, National Fire ProtectionAssociation. 1941. U.S. National Bureauof Standards.

Video Tapes:It’s the HumidityThis video tape covers the essentials ofhumidity and outlines the primary reasonsfor humidity control.

Improving Humidificationwith the Series 9000This tape discusses the design of theArmstrong Series 9000 Conditioned-Steamhumidifiers and explains why it’s the key toefficient and reliable steam humidification.

Series EHU-700This tape demonstrates the installation,start-up and maintenance of theArmstrong Series EHU-700Electronic Steam Humidifier.

Ionic Bed TechnologyIonic bed technology is the problem-solvingkey that makes Armstrong’s HumidiClean™

line of humidifiers unique. This tapeexplains how these patented ionicbeds work.

Armstrong Humidification Literature, Software and VideosProduct Literature:Catalog 504Series 9000 and 1000 Conditioned-SteamHumidifiers for air handling systems anddirect area humidification

Bulletins 570 & 571Series CS-10Steam-to-Steam Humidifiers

Bulletin 514Series EHU-700Electronic Steam Humidifiers

Bulletin 516Series EHU-600 Solutions forSensitive Environments

Bulletin 581Series HC-4000 HumidiClean™

with patented ionic bed technology

Bulletin 565HumidiPack®

Steam Humidifier System

Computer Software:Software Program 2Humidifier Sizing and Selection

to you the vast knowledge and experienceof the entire organization. We urge you toinvolve your Armstrong Representative inthe initial planning of your system for agreater payoff in efficiency and economy.

Armstrong International, Inc.

816 Maple Street, P.O. Box 408, Three Rivers, Michigan 49093 - USA Phone: (616) 273-1415 Fax: (616) 278-6555

Parc Industriel Des Hauts-Sarts, B-4040 Herstal/Liege, Belgium Phone: (04) 2409090 Fax: (04) 2481361

Steam Traps \ Humidifiers \ Steam Coils \ Valves \ Water Heaters \ Air Vents \ Pumping TrapsPrinted in U.S.A.Handbook HB-501 30M 11/97 www.armstrong-intl.com

Limited Warranty and Remedy

Computerized SizingGiven the basic parameters of an installation,Armstrong humidification specialists can usecomputers to quickly size equipment for efficienthumidification, especially when complex econo-mizer systems are involved. In addition, you mayrequest a free copy of Armstrong’s SoftwareProgram 2 (Humidifier Sizing and Selection)for step-by-step sizing of your own installation.

Seminar FacilitiesArmstrong conducts comprehensive steam energyconservation seminars at locations in the UnitedStates. Contact your Armstrong Representativefor more information or to schedule a seminar.

Representative/Factory AssistanceArmstrong Representatives have years of practicalexperience in solving humidification problems.What’s more, they’re experts at assessing yourhumidification needs. Backing the Representatives,of course, are Armstrong humidification specialistswho are available to assist with difficult or unusualproblems.

Installation/Operation ManualsArmstrong provides detailed instruction materialsto assist customers in the proper installation andoperation of Armstrong steam equipment.

Application Data SheetsThorough but concise application data sheetsare provided by Armstrong for many of its steamproducts.

Armstrong International, Inc. warrants to the originaluser that those products supplied by it and used in theservice and in the manner for which they are intendedshall be free from defects in materials and workman-ship for a period of one (1) year after installation, butnot longer than fifteen (15) months from date of ship-ment. Except as may be expressly provided for in awritten agreement between Armstrong International,Inc. and the user, which is signed by both parties,Armstrong International, Inc. DOES NOT MAKE ANYOTHER REPRESENTATIONS OR WARRANTIES,EXPRESS OR IMPLIED, INCLUDING, BUT NOTLIMITED TO, ANY IMPLIED WARRANTY OF MER-CHANTABILITY OR ANY IMPLIED WARRANTY OFFITNESS FOR A PARTICULAR PURPOSE.

The sole and exclusive remedy with respect to the abovelimited warranty or with respect to any other claimrelating to the products or to defects or any condition oruse of the products supplied by Armstrong International,Inc. however caused, and whether such claim is basedupon warranty, contract, negligence, strict liability or anyother theory, is limited to Armstrong International, Inc.’srepair or replacement of the part or product, excludingany labor or any other cost to remove or install said partor product or, at Armstrong International, Inc.’s option, torepayment of the purchase price. Notice of any such claimmust be given in writing to Armstrong International, Inc.within fifteen months after the first installation or use ofthe products. In no event shall Armstrong International,Inc. be liable for special, direct, indirect, incidental orconsequential damages, including, but not limited to, lossof use or profits or to interruption of business activity.

© 1995 Armstrong International, Inc.

Armstrong humidifiers, steam traps, strainers and trapparts are stocked locally by Armstrong Representativesand by many leading industrial distributors.

Application Engineering Service

®