Two questions commonly asked by air conditioningand refrigeration service technicians are:

“What size vacuum pump should “Howlongshouldthepumpbe lefton the system to assure completebe used to properly evacuate a

system?” moisture removal?”

While it is important to realize thatmoisture in a refrigerant system isdirectly or indirectly the real cause ofmore problems and complaints thanall other causes combined, it is equal-ly important to learn why.

Basically, moisture can beclassified as visible and invisible. Oc-casionally, liquid water is found insystems, but this is rather unusual. In-visible moisture, or water vapor, isfound everywhere, in all solids, li-quids and gases. In the air, watervapor content is expressed in termsof relative humidity. It is this invisiblemoisture which causes the greatesttrouble in refrigeration and air condi-tioning systems.

MOISTURE TO ICE CRYSTALS

Ice crystals retard or stop the flowof the refrigerant, causing a reductionor complete loss of cooling. As theexpansion valve warms, due to thelack of refrigerant, the ice melts andpasses through the expansion valve.The refrigerant will then start againuntil the moisture returns to the ex-pansion valve and once more buildsa formation of ice crystals. The resultis intermittent cooling.

Whether a freeze-up’ actually oc-curs depends primarily upon theamount of water and the size of theice particles formed.

But a freeze-up’ is not the onlyproblem caused by moisture. It canalso cause corrosion, which can pre-sent serious trouble. Often the effectsof corrosion are not apparent until thereal damage has occurred.

____ taming chlorine, will slowly hydrolyzewith water and form hydrochloricacids. This acid greatly increases thecorrosion of metals and couldpossibly corrode copper plating.

Heat increases the rate of corro-sion due to acids because at highertemperatures the acid-forming pro-cess accelerates.

IURE REFRIGERANT ACID

MOISTURE TO ACIDSThis acid attacks all the materials it

contacts, the rate of corrosion of theindividual materials being determinedby their corrosion-resistant qualities.Steel will generally corrode at lowermoisture levels than copper or brass.

MOISTURE HEAT REFRIGERANT MORE ACID

A single drop of water may lookharmless, but to a refrigerant system,it is a monster, the number oneenemy of service technicians. Andwhat makes it so formidable anenemy is the fact that moisture entersa system easily and is hard toremove. Here is what it does to asystem.

First of all, it creates freeze-ups”in a refrigeration system. Moisturewill be picked up by the refrigerantand be transported through therefrigerant line in a fine mist fromwhich ice crystals form at the point ofexpansion (expansion valve).

MOISTURE FORMS CORROSION

For example, moisture alone in theform of water can cause corrosionafter a period of time. However,moisture mixed with refrigerantcreates much more corrosion trou-ble. Refrigerants such as R-1 2, con-

INCREASING A C/B DUE TO HEAT

Refrigerant oil presents anotherproblem caused by moisture.Refrigerant oil is an exception to therule that oil and water don’t mix.” Infact, refrigerant oil has an attractionfor moisture and will absorb it rapidlyif left open to the atmosphere. Waterformed acid mixes with refrigerantoil, the two forming a closely bondedmixture of fine globules. The effect iscalled “sludging” and greatlyreduces the lubricating ability of theoil.

Corrosion becomes troublesomefrom the operating standpoint when

MOISTURE IN A F

REFRIGERANTSYSTEM 0I

SINGLE DROP OF WA TER

ACID EFFECTS ON METALS

metallic surfaces are eaten away anda solid, detachable product is form-ed. This formation is also known as a‘sludge.” Sludge exists as slimy li-quids, fine powders, granular solidsor sticky solids and can cause avariety of problems. Sludge will plugfine strainers, expansion valves andcapillary tubes. And because it usual-ly contains acids, sludge will corrodewhatever it clings to, acceleratingsystem damage.

from the system and exhaustedthrough the vacuum pump.

The planet Earth is surrounded bymatter in the gaseous state compos-ed of about 78% nitrogen, 21 % oxy-gen and 1 % a mix of rare gases.Together they form our atmosphere,which extends approximately 600miles above the earth and is held tothe earth by gravity. Being a gas, theatmosphere has weight, and thatweight is measured, as in any fluidwhether liquid or gas, in pounds persquare inch (P.5.1.).

If you were to take a square inchcolumn of the air extending six hun-dred miles above the earth, its weightand pressure exerted on the earth atsea level would be 14.7 pounds. Thisis called atmospheric pressure.Any pressure above atmosphericpressure is referred to as gaugepressure. Pressures below are refer-red to as vacuum.

This same square inch column ofair exerting 14.7 PSI. can support a

22~ 92’

The most effective way toeliminate moisture from a system iswith a good high vacuum pump

EFFECTSOF PRESSUREAND TEMPERATURESONTHE BOILING POINTSOF WATER

A high vacuum pump is capable ofremoving all moisture from ahermetic system by reducing internalsystem pressures to the boiling pointof water at normal temperatures. For

those being introduced here to highvacuum work, it should be stated thata vacuum pump does not “suck out”the liquid moisture,, but rather causesit to boil into a vapor state at whichtime it can be harmlessly removed

N-I—1 SO. IN.

MERCURY

BALANCING A TMOSPHERIC PRESSURE

one inch square column of mercury(Hg) 29.92 inches high. This conceptcan best be understood by compar-ing it to a teeter-totter. When a oneinch square column of Hg 29.92”high is placed on one end of theteeter-totter, and a 14.7 lb weight onthe other end, the board will bebalanced.

Atmospheric pressure decreasesat higher elevations. As stated, 600miles of atmosphere at sea level is

600 MILES

600 MILES flH h —1 SO. IN.

4~Ii —14.7 PSI

equivalent to 14.7 PSI. and/or 29.92inch column of mercury (hg). Goingabove sea level, to the summit of Mt.Whitney, for example, eliminatessome of the 600 miles of atmosphereand, consequently, some of thepressure.

Atmospheric pressure, therefore,governs the boiling point of water. Atsea level where atmosphericpressure is 14.7 P.5.1. (29.92” Hg),water boils at 2120 F, but on Mt.Whitney where atmospheric pressureis 8.32 PSI. (16.9” Hg), water boilsat only 1 84~ F. The lower the at-mospheric pressure is, the lower theboiling point of water. Therefore, ifwe can significantly reduce the at-mospheric pressure inside a sealedrefrigerant system, we can vaporize(or boil) moisture at even ~9QO F. Thisprinciple is illustrated in the chart

— below.

Three ways exist for eliminatingmoisture from a refrigerant system bythe boiling process:

1. Transport the system to a higherelevation where the ambienttemperature is sufficient to boil waterat the existing PSI.

2. Apply heat to the system caus-ing the moisture to boil.

3. Employ a high vacuum pump toreduce the pressureand boiling pointof water.

The first two choices are imprac-tical and must be discarded. Thus, ahigh vacuum pump is an essential

597.8 MILES8.32 PSI = 16.9 IN. HG 1840F

A TMOSPHERIC PRESSURE

MOISTURE TO SLUDGE

VAPOR

2’

WATER TO VAPOR

~‘ IEVFI 14.7 PSI

REDUCING A TMOSPHERIC PRESSURE

K.

CHART IBOILING TEMPERATURES OF WATER

AT CONVERTED PRESSURES

Temperaturein 0F.

Inches OfVacuum Microns*

Pounds Sq. In.(Pressure)

2120 0.00 759,968 14.6962050 4.92 535,000 12,2791940 9.23 525,526 10.1621760 15.94 355,092 6.8661580 20.72 233,680 4.5191400 24.04 149,352 2.8881220 26.28 92,456 1 .7881040 27.75 55,118 1.066

860 28.67 31,750 .614800 28.92 25,400 .491760 29.02 22,860 .442720 29.12 20,320 .393690 29.22 17,780 .344640 29.32 15,240 .295590 29.42 12,700 .246530 29.52 10,160 .196450 29.62 7,620 .147320 29.74 4,572 .088210 29.82 2,540 .049

60 29.87 1,270 .0245~24o 29.91 254 .0049

350 29.91 5 1 27 .00245~60o 29.91 9 25.4 .00049~700 29.9195 12.7 .00024~900 29.91 99 2.54 .000049

*Remaining pressure in system in microns1 .000 inch = 25,400 microns = 2.540 cm

.100 inch = 2,540 microns = .254 cm

.039 inch = 1,000 microns = .100 cm

service aid to every service techni-cian.

As illustrated above, the purposeof a vacuum pump is to reduce the in-ternal system pressure of a refrigera-tion/air conditioning system somoisture and other contaminants canbe removed. Before we explore thedifferent types of vacuum pumpsavailable for service work, it may behelpful to clarify the concepts of“high vacuum” and “deep vacuum.”The following chart shows the rela-tionship between these two con-cepts.

Remember that:PERFECT VACUUM = 29.92” Hg

at sea level (A perfect vacuum existsonly in theory)

PERFECT VACUUM = 0 MICRONSHIGH VACUUM = LOW MICRONSDEEP VACUUM = LOW MICRONS

The term “high vacuum” describes acondition Where the internal systempressure is extremely low, or close toa perfect vacuum. The higher the

— 25.40 mm- 2.54 mm- 1.00mm

vacuum is in a system, the closer themicron reading is to 0 microns. Deepvacuum can be thought of in thesame way. The deeper a vacuum is,the closer the micron reading is to 0microns.

“High vacuum” and “deepvacuum” actually describe the samecondition inside a closed system. Forrefrigeration/air conditioning serviceapplications, we will think in terms ofhigh vacuum = good vacuum, ora low micron reading on the system.

SELECTING HIGHVACUUM PUMPS

To make the following discussioneasier to understand, it would be bestfrom this point on to think of pressure(PSI.) as the amount of mercury itwill support; e.g., atmosphericpressure at 29.92 inches Hg insteadof 14.7 PSI. This will permit us to

use Chart II as a visual aid whendetermining the vacuum which mustbe attained to boil water undervarious ambient temperatures.

CHART IIPRESSURE IN

TEMP. IN IN. OFDEGREES F. MERCURY

—2120F 29.92—

~205O 25—

~194O 20—

15—

1.0 —.9 —.9 —.7 —.5 —.5 —.4 —.3 —.18—.1—

~900

—lao~72o

590

540

— 590530

450

~320~21o

Chart II shows a vacuum pumpcapable of eliminating all butone inchof mercury. It is able to removemoisture at an ambient temperatureof 800 F or over. While any pump pull-ing within one inch of atmosphericpressure can eliminate moisture, itmust also be capable of holding thatvacuum throughout the dehydrationprocess. In addition, it must pull thatvacuum on the entire system and notsimply at the intake of the pump.

Before considering the variablesaffecting a high vacuum pump’s per-formance, we should first reviewsome general classifications ofvacuum pumps relative to their abilityto remove moisture by the boilingprocess

VACUUMPUMPS—AIR COMPRESSORTYPE

Air compressors were designed toremove large volumes of air ratherthan to remove all but a small amountof pressure. This type of vacuumpump will not eliminate any moistureby theboiling method. At best, it can-not pull more than 28 inches of mer-cury and consequently should not beconsidered suitable for high vacuumwork.

VACUUM PUMPS—COMPRESSORTYPE

For clarification, compressor typevacuum pumps should be separatedinto two categories: piston type androtary vane type.

Under ideal conditions, a pistontype compressor might pull avacuum of 29 inches of mercurywhich will boil water at an ambienttemperature of 800 F or over. But it isquestionable whether or not it canhold this vacuum once moisture con-denses into its oil. Quite a number ofthese vacuum pumps have been soldas evacuators and are used in con-junction with a true high vacuumpump. By themselves, they cannotremove moisture by the boilingmethod under normal ambient condi-tions.

A rotary-type compressor can pulla vacuum as high as 29.63”. Thisvacuum will remove moisture by theboiling method. However, because oftheir limited CFM capacity, thesepumps should be limited torefrigerator and freezer service work.They are also used in the automotiveair conditioning service field

HIGH VACUUM PUMPS—SINGLE STAGE

Single stage high vacuum pumpsare desirable for several reasons.First, they are almost always smallerand lighter in weight than two stagepumps of equal capacity in CFM. Se-cond, single stage pumps aregenerally less expensive than twostage pumps. These advantagesmake single stage pumps a goodchoice for students and techniciansjust beginning refrigeration/air condi-tioning service work. A single stagehigh vacuum pump is also a goodchoice for a second pump amongmore experienced servicemen.

INTAKE

SINGLE STAGE VACUUM PUMP

There are single stage vacuumpumps available which can pull downto about 50 microns under ideallaboratory conditions. Single stagepumps discharge directly into the at-mosphere, and due to the higherdischarge pressure, higher vacuumsare not possible at the intake of thepump. A gas ballast feature will helpsingle stage pumps to keep the oilfree of moisture and other con-taminants for a longer period of timethan similar units without. Most singlestage pumps with the gas ballastopen will pull down to about 1000microns. There are, however, singlestage pumps with gas ballast andhigh internal temperatures thatreduce oil contamination and aretherefore, able to pull a system to arespectable micron level for mostservice applications.

HIGH VACUUMPUMPS—TWOSTAGE

The majority of refrigeration/airconditioning service technicians usea two stage high vacuum pump forthe bulk of their service jobs. A twostage pump is usually larger andweighs a little more than a singlestage pump of equal CFM capacity.

A two stage pump features a se-cond pumping chamber to enable thepump to reach a higher ultimatevacuum than a single stage model. Ina two stage pump, the exhaust of thefirst pumping stage is discharged intothe intake of the second pumpingstage, rather than to atmosphericpressure. The second stage beginspumping at a lower pressure andtherefore pulls a higher vacuum onthe system than the first stage iscapable of on its own.

EXHA US INTAKEBSECOND FIRST

TWO STAGE VACUUM PUMP

Compound two stage vacuumpumps are capable of pulling down toan extremely high vacuum such as1 .0 micron. While they will seldompull down to this point under fieldconditions, two stage high vacuumpumps can continuously pull down to20 microns for prolonged periods of

time. For this reason a two stage highvacuum pump is well suited forrefrigeration/air conditioning service.

The higher ultimate vacuumcapability of a two stage vacuumpump will help to ensure that allmoisture and non-condensables areremoved from a system. It is impor-tant to achieve the highest vacuumpossible before recharging a systemwith refrigerant

GAS BALLAST ORVENTEDEXHAUST

The gas ballast or vented exhaustfeature is a valving arrangementwhich permits relatively dry air fromthe atmosphere to enter the secondstage of the pump. This air reducesthe compression in the final stage,which helps to prevent the moisturefrom condensing into a liquid andmixing with the vacuum pump oil.

This may sound complex, but ac-tually it is relatively simple. A com-parison should help explain it. Im-agine a damp towel being twisted un-til water drops out. This can be com-

000

+ DRY

MIXING WETAND DRYAIR

‘~u1z~0

pared to a high vacuum pump whichis not equipped with a gas ballast.Moisture being pulled from a wetrefrigerant system is compressed in-ternally in thevacuum pump and con-denses into a liquid. Now imaginethat same damp towel entwined witha dry towel and then twisted. It wouldtake a considerable amount oftwisting before any water would dropout. Thus the process of the gasballast arrangement. It permits themoisture laden air passing throughthe pump to mix with relatively dry air

EXHAUST

L

to such a degree that compressiondoes not cause condensation.

The damp towel comparison alsoillustrates why this valving featurecannot handle large amounts ofmoisture. If the towel is almostsaturated with water, even the in-troduction of the completely drytowel will not prevent some of thewater from dropping out when thetwo are compressed. Because ofthis, some pumps are designed torun with both high internaltemperatures (to reduce moisturecondensation in the oil) and a gasballast.

Due to the “severe” application ofa vacuum pump when used to “boilwater”, it is necessary to select aquality two stage model equippedwith gas ballast and high internal run-ning temperatures to achieve ade-quate performance over a longperiod of time. It should be emphasiz-ed, however, that even with the bestpump available, regular maintenancehas to be performed.

Frequent oil changes should be an-ticipated and considered as thesingle most important factor in apreventative maintenance program.As stated previously, even a pumpequipped with gas ballast cannothandle large amounts of moisturewithout some being condensed intothe oil. If allowed to remain inside thepump, this moisture will attack themetal components and result in lockups or loss of efficiency and/orcapacity. Normally, oil changes willnot be required during a singledehydration job. But it would be wellto change oil after each major pumpdown. This is especially critical whenpumping down a system known to bewet or the result of a compressorburn-out.

FACTORSAFFECTINGTHE SPEEDAT WHICHA PUMP CANDEHYDRATE AREFRIGERANTSYSTEM

Several factors influence the “pum-ping speed” of a high vacuum pump,and thus the time required to removeall moisture from a refrigerantsystem. Some of the most importantare: the cubic feet capacity of thesystem itself; the amount of moisture(both visible and invisible) containedwithin the system; the ambienttemperature present; internal restric-

tions within the system; externalrestrictions between the system andthe vacuum source; and the size ofthe pump.

The original equipment manufac-turers determine the cubic feetcapacity of thesystem and their inter-nal restrictions. Malfunctions and in-efficiencies result from the presenceof moisture. And Mother Nature con-trols the ambient temperature of theday. Consequently, the only factorsunder the control of the servicetechnician are the external restric-tions between the system and thevacuum pump. Nevertheless, thisone factor is very important and war-rants more discussion.

Let us assume that all othervariable factors affecting pump downtime are equal. Also for the sake ofsimplicity, weshall use the illustrationabove to show a pump hookup to asystem.

Within a system, variablepressures attempt to equalize oneanother. In this equalization, thehigher pressure “flows” toward thelower pressure. The abovehypothetical illustration shows thevariable pressure creating a vacuumof 1 00 microns and the system at at-mospheric pressure. The higherpressure in the system will flowtoward the vacuum pump until it isreduced or equal to the 100 micronsof pressure. The speed at which itwill flow is controlled by the ID. andlength of the connecting line.Laboratory tests show that pump-down time can be significantly reduc-ed by use of larger diameter hoses.For optimum pumping speed keepaccess lines as short in length and aslarge in diameter as possible.

All of us have released an inflatedballoon, allowing the escaping airpressure to propel it on a wild course.At other times you have inflated aballoon and stretched its stem, allow-ing the air to escape with a squealingsound. You controlled thepitch of thesqueal and its duration with theamount of stretch on the stem. In ef-

fect, you were controlling the con-necting ID. as in the above illustra-tion and, in turn, controlling the time ittook to release all of its pressure. Theprincipal is identical: the higherpressure is trying to move toward thelower pressure.

HIGHER PRESSURE MOVESTOWARD LOWER PRESSURE

14.7 PSI

The illustrations stress the impor-tance of eliminating all externalrestrictions whenever possible. It isrealized that this is almost impossiblebecause of the size of valves,manifolds and lines commonly usedin our industry today.

It is perfectly acceptable to use a4.5 CFM or larger vacuum pump onsmall systems. Using too small of apump on a large system, e.g., a 1 .1CFM vacuum pump on a 40 ton unit,could cause the pump to operate in“free air” condition for an extendedperiod of time, thus risking prematurepump wear.

When determining the size ofpump needed, use the formula CFMx 7 = Maximum System Size (Tons).For example, a 4.5 CFM vacuumpump, when applied to the formula:4.5 CFM x 7 = 31 .5 ton system max-imum, which states as a general rulea 4.5 CFM vacuum pump should onlybe used on a system smaller than31 .5 tons. On systems larger than31 .5 tons, it is recommended to useeither a larger pump or two or more4.5 CFM pumps.

SIZE OF PUMP—The followingtable will give you a reasonableIdea of what capacity pump youneed for various applications.These are suggested minimumCFM’s. Larger capacity highvacuum pumps can easily be us-ed on smaller systems.

System Sizeup to 10 tonsPassenger carDomestic_refrigerationup to 30 tonsPanel trucks, RV’sResidential A/Cup to 50 tonsTractor/trailers, busesRooftop A/C systems

up to 70 tons

CHART IIISuggested High

vacuum Pump Size

1.2 - 1.5 GFM

3-4 CFM

5-6 CFM

8-10 CFM

HOW VACUUMCANBE MEASURED

In the refrigeration industry,vacuum is measured with a standardcompound gauge, closed endmanometer or an electronic ther-mistor vacuum gauge. Keeping inmind that it is a vacuum and its rela-tionship to the boiling points of waterwe are attempting to measure, wecan refer back to Chart I to see therelationship between pressure interms of inches of Hg, millimetersand microns.

A standard bourdon tube com-pound gauge is a rugged typedesigned to read low pressures in in-ches of vacuum. It is suitable forreading vacuums, e.g., 28” Hg.However, it cannot be expected toread millimeters or microns, and,because of this, is not suitable for usewith high vacuum pumps.

A closed end U tube mercurymanometer can be read with goodaccuracy in millimeters. With onemillimeter equal to 1000 microns, it ispossible to read 500 microns withthis gauge. It is, however, a fairlydelicate instrument, making it moresuitable for laboratory or shop workrather than for field service.

Electronic thermistor vacuumgauges are specifically designed foruse with high vacuum pumps andcan be accurately read as low as 1 .0micron. Through the use of a sensing

tube mounted at some point in thevacuum line, an electronic circuit pro-vides an output calibrated in microns.This output can be an analog meterscale, a digital display or a LED se-quence display. One positive effectfrom a thermistor gauge is that thetube reads total vapor pressure as afunction of the thermal conductivity ofa gas. It is sensitive to water vaporand other condensablesand can givea good indication of the actualvacuum level within a system. A ther-mistor vacuum gauge is consideredessential as a companion instrumentfor high vacuum dehydration of arefrigerant system. Althoughdeveloped as a laboratory instru-ment, there are electronic thermistorvacuum gauges on the market rug-ged and reliable enough for field ser-vice work.

When reading vacuum, you shouldremember that the location of thevacuum gauge tube will affect thereading. The closer to the vacuumsource, the lower will be the reading.

When reading thevacuum createdin a refrigerant system, you shouldisolate thevacuum pump with a goodvacuum valve and allow the pressurein the system to equalize before tak-ing a final reading.

If thepressure will not equalize, it isan indication of a leak. If it doesequalize at a pressure which is toohigh, it is an indication of moisture,and more pumping time is required.

14830Solid State thermistor vacuum gaugeOperating Temp. 00 F to 1200 FStorage Temp. 00 F to 1580 FThis rugged thermistor vacuum gauge features solid state circuitry and 10 in-dividual light emitting diodes to indicate vacuum levels from 25,000 to 50microns. Battery operated (not included) and furnished with durable plasticcase that can be hung from a hook. The 14830 is the ultimate in convenienceand economy for vacuum measurement. Uses standard 44416 replacementvacuum gauge tube.3D Batteries power the circuitry—40 hrs.3AA Batteries power the LED display—shelf life.

I-

10 CFMModel15120

Rated at a full 10 OEM, this large capacity 15120 pump wasdesigned for systems up to 70 tons. Factory rated at 20microns.

1.2 CFMModel15200

IMP

Weighing just 10 pounds, the single stage 15200 is ideal forsmall service applications up to 7 tons, especially domesticair conditioning. Factory rated at 50 microns.

VacuumPump OilThere’s no better oil for yourvacuum pump than Robinair’sPremium High Vacuum PumpOil. Chemically engineeredto maintain maximumviscosity at high runningtemperatures, it also helps improve cold weathei ~iL~irts.

Robinair pump oil is available in convenient plastic bottles13203 quart and 13204 gallon.

ServiceLineA toll-free number to call for assistance in usingand servicing Robinair equipment.

1 =8OO-822~5561in the Continental U.S.

High performance pumps designed to handle systems up to30 tons for model 15400 and up to 50 tons for model 15600.Factory rated at 20 microns.

S