Blower Selection for Wastewater Aeration

7/25/2019 Blower Selection for Wastewater Aeration

1/10

ROTATING EQUIPMENT/WASTEWATER TREATMENT

HYDROCARBON PROCESSING APRIL 2010 I

79

A

s the main electricity consumers in a wastewater treat-ment plant, aeration blowers greatly influence overall

plant operating cost. The most energy-efficient solutionmust be based on actual conditions that, in reality, are far fromstatic. In fact, energy efficiency alone may not be the governingfactor in optimized blower selection and neither are comparisonsmade at one hypothetical operating point only.

This article provides overview information that will help engi-neers understand the importance of more closely looking at themany factors that separate different designs and deserve far moredetailed evaluation to arrive at the best selection.

Economics and matching aeration blowers.In awastewater treatment plant, control design must interact withaeration demand. Subsystem selection and thoughtful integration

are of paramount importance, as is the overall facility operatingphilosophy. All of these influence total ownership cost and mustguide the selection process.

Many plants are oversized because projected population growthdid not materialize. As an example, a rotary-lobe blower packagesized originally for 3,000 cfm/8.5 psi might use 179 bhp whenequipped with a traditional belt drive and standard-efficiencymotor. Suppose this represented oversizing by 30%, in whichcase replacing the unit should certainly be considered. A moreefficient, modern rotary-lobe blower package with automaticbelt tensioning and a premium-efficiency motor would produce20% more cfm per hp than the old machine! Both energy andmaintenance outlays would be saved.

A normal operating point should be defined as the point atwhich usual operation is expected and optimum efficiency isdesired. This point is usually the certified point. In other words,if a plant is designed to operate at 80% of its peak, aerationblower performance should be optimized for that point. Optimi-zation is facilitated by machines designed with a plug-in-and-runconcept in mind. While this represents an efficient and attractiveproposition, keep in mind that flexibility is required to meetactual needs. For example, electricians prefer working on motorstarters and variable-frequency drive (VFD) controls (if needed)located in a clean and air-conditioned electrical room rather thanin a noisy machinery room. In general, blowers installed outdoors

eliminate the need for an enclosed locationa substantial costavoidance. Temperature control in a machine room must be

achieved without raising the operating costs for air conditioning;in addition to OSHA noise limits within the plant, continuousand transient noise** emissions may cause problems in closeproximity to residential areas. Initial installed equipment cost is apoor indicator of life cycle cost. Energy cost and expenditures fortypical maintenance, condition monitoring, spare parts, repairtime and overhead are key contributors and yet, there are otherfactors to weigh as well.

Unfortunately, the costs used to compare proposals are fre-quently based on hypothetical conditions. Some less-obviousaspects are sometimes completely neglected but are, nevertheless,important ingredients of the ownership cost. To mention a few:balanced and stable power supply, unused pressure reserves or

air flow, air filter maintenance/replacement frequency, numberof machines, etc.

Blower technologies compared.Four parameters: sitecondition, aeration system type, instantaneous air mass flow andrelated system variables both upstream and downstream, are ofobvious importance in blower selection. Available blower con-figurations are generally classified as either positive displacement

Blower selection

for wastewater aeration*

Use these guidelines to understand the many factorsthat differentiate different designs

Compiled by the staff of Aerzen USA, Coatesville, Pennsylvania

* Based on a comprehensive manuscript available from Aerzen USA, Coates-ville, Pennsylvania.

** Transient noise is generated by antisurge control valves and pressure safety orunloading valve opening.

X-ray view of a rotary-lobe blower stage.FIG. 1


2/10


80 IAPRIL 2010 HYDROCARBON PROCESSING

(rotary lobe, Fig. 1, also dry screw, Fig. 2) or dynamic (centrifugal)machines. Dynamic blowers incorporate either a rotor with step-up gearing (Fig. 3) or a driver that can generate the requisite highspeeds. Special-purpose, high-speed single-stage centrifugal blow-ers (Fig. 3) incorporate inlet guide vanes and outlet diffuser vanesto achieve wide turndown at constant operating speed. These

machines can be operated with a VFD to provide additional flex-ibility while maintaining highest efficiency. They are also offeredas a self-contained package (Fig. 4).

A standardized high-speed, single-s tage turbo blower isdepicted in Fig. 5. These recently introduced machines are direct-driven by a high-speed permanent magnet motor and alwaysrequire a VFD to accommodate varying air densities and/or air

pressure requirements. Impellers are either precision cast or fullymachined. No adjustable diffuser vanes are used and the machinesrely on a well-integrated, elaborate control system to accommo-date changing conditions.

Standardized, high-speed single-stage turbo blowers make useof the most current technology involving magnetic or air bearings.Continuous vibration monitoring is possible with magnetic bearingsbut not with air bearings. A packaged assembly is shown in Fig. 6.

Again, each of these blower technologies is further described ina comprehensive white paper available from the authors.

Site conditions.Site elevation above sea level determines theatmospheric pressure and, therefore, the inlet pressure and air

density. As a result, the compression ratio (defined as the ratioof the absolute discharge to the absolute inlet pressures) and thecompressed-air discharge temperature will vary accordingly.

Dry-screw compressor rotors with timing gears.FIG. 2

Centrifugal blower section with inlet guide vanes anddiffuser vanes (Source: Dresser Industries).

FIG. 3

Single-stage centrifugal blower package (Source: Siemens).FIG. 4

Turbo blower section with permanent magnet motor andairfoil bearings (Source: K-Turbo).

FIG. 5

Turbo blower package (Source: Neuros).FIG. 6


3/10


81

Considerable diligence must go into evaluating the full impactof site conditions, as seen in Fig. 7 and Table 1. Note the variationin oxygen content per unit of volume of ambient air. Going fromthe warmest and coldest conditions at constant relative humidity,the oxygen content in Miami is 14% vs. 31% in Denver.

Aeration system type and operating pressure.Please

recall that it is not the purpose of this article to discuss the mostefficient aeration system. Instead, the write-up is intended toprovide an overview of how aeration system components affectblower operation and to highlight the implicit role of competentmanufacturers in bringing all pertinent factors to the attention ofdiscerning operating facilities and buyers.

Aeration blowers must overcome diffuser submersion. Submer-sion depth determines hydrostatic head; submersion typicallyranges from 10 ft (3 m) to 26 ft (8 m) in municipal wastewatertreatment plants. In contrast, industrial systems may require 33 ft(10 m) to 66 ft (20 m). Municipal systems would be best servedby aeration blowers capable of pressures under 15 psig (~1,000mbar) while single-stage, oil-free screw or high-speed centrifugal

compressors should be considered for most industrial systems. Ineither case, the pressure losses indicated in Fig. 8 must be addedto the aeration depth.

It should be noted that piping and associated check valves,isolating valves, elbows or other piping components will engenderrestrictions that, at maximum flow, can easily reach, or even exceed,1.0 psi (70 mbar). Also, the head loss across the diffuser systemtypically 0.4 to 0.8 psi (30 to 60 mbar)needs to be considered.

It is appropriate to add a safety margin in the range of 0.5to 1.0 psi (35 to 70 mbar) to account for diffuser aging and/or fouling. Operating experience also demonstrates an elevatedstagnation pressure needs to be overcome for a short period.Moreover, condensate that may have appeared in the pipe must

be displaced.All these pressure losses and reserve margins must be consid-

ered for dependable and stable operation. While this pressurereserve margin can easily reach and even exceed 15% of the sub-mersion head at full flow, it will decrease to only a small amount atminimum flow with a clean system. Therefore, a plant can directlybenefit from reduced pressure losses if the blower power demanddecreases in direct proportion.

TABLE 1.Example of the effects of location oncompression ratio and air flow

Example: If the system pressure to overcome is 8 psi (550 mbar), considering a

constant relative humidity of 36% (data source: weatherbase.com)

Location examples Miami, Florida Denver, Colorado

Atmospheric pressure 14.7 psia 12.2 psia

(1.013 bar abs) (0.84 bar abs)Discharge pressure 22.7 psia 20.2 psia

(1.56 bar abs) (1.39 bar abs)

Compression ratio @ 8 psid (550 mbar) 1.544 1.655

Average temperature 76F (24C) 55F (13C)

Lowest temperature 30F (1C) 17F (27C)

Highest temperature 98F (36C) 115F (46C)

Air density at lowest temperature 0.081 lb/ft3 0.076 lb/ft3

(1.297 kg/m3) (1.225 kg/m3)

Air density at average temperature 0.074 lb/ft3 0.065 lb/ft3

(1.184 kg/m3) (1.046 kg/m3)

Air density at highest temperature 0.071 lb/ft3 0.059 lb/ft3

(1.134 kg/m3

) (0.937 kg/m3

)Oxygen content variationper unit 14% 31%

of volume of ambient air between the

warmest and coldest conditions at

constant relative humidity

The effect of the relative humidity must also be taken into account; a rise in

humidity results in a lower air density, all other conditions being equal. Example:

at the maximum temperature, air with 100% RH will have a 3% lower density in

Miami, and 2% in Denver, than 36% RH air.

Impact of geographic location and altitude on theoperating conditions.

FIG. 7

Select 175 at www.HydrocarbonProcessing.com/RS


4/1082

ROTATING EQUIPMENT

Blower control schemes and turndown capability.The system turndown capability is determined by the numberof blowers, their individual turndown capability and the operat-ing mode (variable flow or on/off cycling operation). Turndowncapability is largely a function of the blower and its associateddrive technology. For an entire system, a wide turndown rangeis often required to meet varying oxygen demand. With overallsystems often demanding turndown capabilities from 4:1 to 10:1,multiple blowers and/or well-thought-out blower control schemes

are required.The purpose of the control system is to match instantaneous

air output to the actual air demand in the most efficient manner.This, of course, implies running the least number of machines andoperating them at or near their respective best efficiencies.

A large turndown means that the system will be able to meetthe lowest air requirements without wasting energy by idling orbleeding off excess discharge air to the atmosphere. Large turn-down also provides additional flexibility and enables the blowersystem to meet the air requirements in a step-less manner with theminimum number of machines and lowest number of frequencyinverters or VFDs. The turndown of each machine must allow forsome overlapping (preferably 5% of the flow of an individual

machine). Therefore, a 55% turndown is required for a stablecontrol system, avoiding step-like output strategies or wasting

TABLE 3.Drive systems for the various blower types

Special-purpose, high-speed Rotary-lobe blowers centrifugal Standardized high-

Screw compressors blowers speed turbo blowers

Motor type Standard induction Standard induction Permanent magnet

Drive Belt drive Gear Direct

VFD VFD optional VFD optional VFD always required

Not the length is important

but the technique Abstain

from drive shafts being long and susceptible to troubles Spare

needless shaft and guide bearings

Forget

complex and cost-intensive seal technology

Require

best available technology for a long service life

and high availability

HERMETIC-Pumpen GmbHP.O. Box 1220D-79191 Gundelfingen

[email protected]

support and

cable pipe

pressure /

discharge line

extremely short

drive shaft

if 2 or 20 meters we are flexible

H E R M E T I C A L L Y S E A L E D

S U B M E R S I B L E P U M P S

TABLE 2.Air mover typical rotor tip speed andrpm range

Typical maximum High-speed Rotary-lobe Oil-free

tip speeds centrifugal/turbo blowers screw machines

ft/sec 1,150 140 400

m/sec 350 45 120

Typical operating 20,000 to 65,000 1,000 to 5,000 3,000 to 15,000

speeds, rpm

Typical system pressure curve

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10 20 30 40 50 60 70 80 90

Total air flow, %

Systemp

ressure,

psi

Submersion head

Piping losses

Diffuser head

Aging, fouling and stagnation margin

Typical system pressure diagram in relation to air flow andto aeration system condition.

FIG. 8



5/10


HYDROCARBON PROCESSING APRIL 2010 I

83

power by blowing-off air to which pressure (energy) has alreadybeen imparted.

The number of blowers required depends on the turndowncapabilities of each machine and, if a system requires a 4:1 turn-down ratio, this can be accomplished with two machines eachincorporating only a 2:1 turndown. For ease of control, eachmachine should be capable of a flow range from 45% to 100%

under the most severe conditions, i.e., highest pressure ratio andlowest air density combined.

Example:A 10:1 system turndown can be achieved in twoways (Fig. 9): three blowers, of which at least two have an operat-ing range from 30% to 100% while one can have a constant flow,or five blowers would be required, of which at least two must havea safe operating range from ~ 45% to 100% while three couldhave constant flow.

In wastewater aeration applications, depending on the aera-tion control system, the operating pressure will drop slightly asairflow decreases. The main portion of this operating pressurecorresponds to the hydrostatic pressure, which remains as con-stant as the water level. As the machine flow is regulated down,

its efficiency will vary. At pressure ratios of 1.5 to 1.7 that arefrequently encountered in wastewater aeration, efficient rotary-lobe blowers and oil-free screw machines can offer turndowncapabilities up to 4:1 of the machines flow capability, whilespecial-purpose, high-speed centrifugal blowers offer a turndownup to about 2.2:1 at constant pressure. Standardized high-speedturbo blowers may have a turndown as high as 2.2:1 at constantpressure, but this largely depends on the impeller characteristics,the size fit and the pressure ratio. For all the machines includedin our comparison study, the higher the pressure ratio, the smallerthe turndown capability.

In virtually all cases, aeration requirements vary in the courseof a year (Fig. 10) and turndown capability must be considered

in the selection process.

Other system variables: upstream and downstream.Inlet air filtration and filter location.Filter cleanliness hasan important impact on energy consumption. Likewise, filtermaintenance frequency affects the maintenance budget. To pro-tect both blower rotor and downstream dif-fuser system, the filter must be fine. Butfine filters require a large filtration area and/or more frequent cleaning or replacement.Machines with high tip speeds, such as high-speed centrifugal blowers and turbo blowers(Table 2), are particularly sensitive to par-

ticles and droplets in the air stream; they,therefore, require very fine filtration.

Although the pressure loss of a clean fil-ter may be negligible, dirty filters can easilycause an additional 0.5-psi (35-mbar) drop.In that instance, and at sea level, the result-ing increase in compression ratio would bein the vicinity of 3%. There would also bea similar increase in energy demand. Inletpressure losses have a much more important

impact on power usage and actual flow than the same pressuredrop on the discharge side.

It is preferable that the filter be the last element contacted bythe inlet air before entering the blower. This is most importantin the case of very-high-speed machines such as the turbo andcentrifugal blowers. The authors found that most suppliers do notpay much attention to this detail. Yet, there is danger that loose

particles from the silencers (or, in the case of standardized turbo

TABLE 4.Quick selection guide

Selection criteria based on standardconditions at blower inlet and flowper individual machine:T = Standardized turbo blowerC = Special-purpose centrifugal blower

L = Rotary-lobe blowerS = Dry-screw compressor

Flow per machine < 1,000 cfm/30 m3/min L LS LST ST ST

Flow per machine 1,000 to 8,500 cfm L L TS TSC TSC

(30 to 250 m3/min.) with significant

pressure reduction at partial load

Flow per machine 1,000 to 8,500 cfm L TS TS TSC SC

(30 to 250 m3/min.) with narrow discharge

pressure band

Low operating hours/intermittent operation L L L L S

Continuous operation L LST TSC TSC TSC

Turndown capability for each machine > 55% L LS LS S S

Turndown capability for each machine 55% L LS TLSC TSC TSC

Simplicity of controls L LS LS S S

1

5

psid

*** Used for comparison: Aerzen VML and GMblower packages (provided by manufacturer);

K-Turbo and Siemens Turbo (based on Websiteinformation).

Flow, %

Power

100%10%

1xVFD

2xVFD

+1 x VFD driven+1 x fixed rpm

+2 x VFD driven+1 x fixed rpm

Example of a 10:1 turndown with three rotary-lobeblowers, two of which are VFD-driven and one driven atfixed speed.

FIG. 9

Typical aeration airflow requirements

0

1,0002,000

3,000

4,000

5,000

6,000

1 2 3 4 5 6 7 8 9 10 11 12

Month

scfm

Min.

Max.Average

Typical wastewater treatment plant air flow variationsover a year.

FIG. 10


6/10


84 IAPRIL 2010 HYDROCARBON PROCESSING

blowers, from the acoustic enclosure) could enter the blower.Cleanliness is particularly crucial for high-speed machines and

machines with high tip speeds. A few particles can damage an airbearing or cause damage when coming into contact with impellersrotating at tip speeds of 1,000 ft/sec and more. If the filter elementis such that dust particles can fall off during the filter change, itis recommended to pay particular attention to dust removal prior

to installing the clean filter element. Particles that find their wayinto the downstream piping will ultimately restrict the air flow infine bubble diffusers.

Inlet piping and air preheating.For various reasons, engi-neers often prefer to manifold multiple blower external air inletpiping. Air is thus pulled in from outside a blower room and theincremental inlet pipe pressure losses need to be accounted for inloss calculations. Moreover, inlet air noise abatement may requireadditional acoustical treatment.

TABLE 5.Energy costs comparison at constant operating pressure

14 psig 11 psig 8 psig

W2W, kW Yearly W2W,kW Yearly W2W, kW YearlyAir flow, icfm 3,300 2,640 1,980 energy, 3,300 2,640 1,980 energy 3,300 2,640 1,980 energy,

% operating time 10% 60% 30% kWh/yr 10% 60% 30% kWh/yr 10% 60% 30% kWh/yr

Low-pressure screw 164 129 99 1,072,905 141 107 80 891,765 116 87 64 723,862

Rotary-lobe blower 212 176 140 1,468,798 165 135 107 1,130,507 119 97 76 805,864

Standardized turbo 167 134 102 1,108,135 136 112 89 938,428 104 83 65 693,502

(optimized)

W2W includes VFD losses and motor losses for all machines as well as transmission losses for the rotary-lobe blower and the low-pressure screw compressor.

0

10

20

30

40

50

60

70

80

90

100

20 30 40 50 60 70

Flow, %80 90 100 110 120 130

Performance curves: positive-displacement vs.centrifugal-turbo-type blowers

Press

ure,

%

turbo @ .%of full speed

PD @ ...% ofmaximum speed

40% 100%90%80%70%

100%

90%80%

70%

Centrifugal vs. positive displacementcomparativediagrams of pressure vs. flow characteristics.

FIG. 11

Platinum Sponsor:

Silver Sponsors:

Dont miss . . .

the largest womens event in the energy industry.

Get ready . . .to discuss key environmental, economic, professional development and

human capital issues in one setting.

Learn more . . .details at www.WGLNetwork.com!

For more information, [email protected] or call +1 (713) 520-4475.

SAVE THE DATE

Media Sponsor:

2728 Octobe r 2010 The Wes t i n Ga l l e r i a Hous ton , Te xas USA

GULFPUBL ISH ING COMPANY



7/10

In a single process step, BASFcombines a new, advanced FCCcatalyst matrix with our provenzeolite technology, in an ultra-lowsodium, intimately dispersed, proximalstructure. The sum ... Prox-SMZ(Proximal Stable Matrix and Zeolite)a proprietary technology platformproviding:

Leading hydrothermal stability

Deep bottoms conversion and bestbottoms to coke ratio

Low dry gas and coke production

Suitable for all FCC feedstocksincluding highly contaminated resids

HDXtra, the first catalyst derived fromthe innovative Prox-SMZ technology

platform, optimizes the zeolite-to-matrixratio to maximize bottoms-to-LCOconversion to meet growingdiesel demand.

Staminacombines selective crackingwith proven metals tolerance featuresfor resid feeds.

When the right reaction matters, trustBASF for bottoms upgrading anddistillate maximization.

1 + 1 =#

1

For more information, please visit

www.catalysts.basf.com/prox-smz



8/10


86

Some blower packages use the intake air to cool the electric-drivemotor and some of the power electronics. The result is a higheramount of energy being used for the same amount of oxygen; thehigher air inlet temperature results in lower air density and propor-tionally lower oxygen mass per unit of air volume. For example: a20F increase in inlet temperature results in about 4% lower oxygenmass per cubic foot of air, a 4% lower density, higher discharge

temperature, higher air velocity and higher pressure losses.Discharge check valve and additional flow control valve.

While nearly always required, the discharge check valve is rarelyincluded in a blower package. Whether the pressure loss causedby the check valve has been accounted for is not always clear. The

check valve pressure loss should be less than 0.15 psi (10 mbar).Check valves must have low opening pressure and operate withoutchatter at reduced flows.

Some control systemsin particular ones used with centrifugalor turbo blowersmake use of a discharge pressure control valve.This valve receives its signal from the dissolved oxygen (DO) con-trol. Should less oxygen be required, the control valve will restrict

total airflow, thereby increasing its upstream pressure. The blowerflow control, being set to maintain a constant discharge pressure,will reduce blower flow until the set pressure is reached again. Thepressure drop across such a control valve is not negligible (0.3 to0.5 psi or 20 to 35 mbar). Also, with such a control system, thepressure generated by the blowers remains constant. The processmay thus be unable to take advantage of any drop in system pres-sure at partial flow.

Performance characteristics.Fig. 11 shows how dynamiccompressing machines depend on speed to produce pressure andhow pressure relates to the capability of controlling flow. As flowis lowered, the resulting pressure rise becomes progressively less.

In other words, near the left extreme of a flow-versus-pressure-rise curve, small changes in pressure might indicate significantchanges in flow.

The graph also illustrates how, while there is a pronouncedrelationship between speed and flow in positive-displacementmachines, the influence that speed has on discharge pressurecapability is relatively minor.

Efficiency diagram.***Fig. 12 shows the efficiency vs. flowdiagrams at a constant 10 psig pressure for the various types ofblowers compared in this article.

Efficiency range for various manufacturers of the sameequipment type.There is a relatively wide spread of energy effi-

ciency valuesas high as 15%among products of the same typebut from different manufacturers.

Positive-displacement machine efficiency depends largely onthe operating clearances between rotors and between rotors andhousing. Screw compressors that are designed for higher pressureswill also work at low pressure but they will not be as efficientand their vibration level will be higher than screw compressorsdesigned specifically for low pressure.

There are also differences in the efficiency of the standardizedturbo blowers, depending on the specific fit of a standardized

Typical overall efficiency graphs for variousblower types at constant pressure

30 40 50 60 70 80 90 100

Full volume flow

Efficiency,

%

Standardized turbo Low-pressure screwRotary-lobe blower Special-purpose centrifugal

70

80

55

50

45

40

60

65

Efficiency vs. flow at constant pressure for the variousblower types.

FIG. 12

TABLE 6.Data points for the evaluation

Data Total flow, Flow per Pressure, Inlet Timepoints scfm machine, scfm psig conditions operating

1 5,100 (maximum) 2,550 (2 units) 9.43 99F and 10%

78% RH

2 1,900 (minimum) 1,900 (1 unit) 7.72 -5F and 10%

0% RH

3 4,080 (80%) 2,040 (2 units) 9.43 66F and 20%

78% RH

4 4,080 (80%) 2,040 (2 units) 7.72 47F and 20%

0% RH

5 3,060 (60%) 1,530 (2 units) 9.43 66F and 20%

78% RH

6 3,060 (60%) 1,530 (2 units) 7.72 47F and 20%

0% RH

VAPORPRESSUREPROCESS ANALYZER

MINIVAP ON-LINE

VP of Gasoline, Crude

Oil and LPG

ASTM D5191, 5188, 6377,

6378, 6897, EN 13016-1+2,

IP394, 409, 481

Highest Accuracy for

Best Possible Blending

to Official Limits

Up to 2 Sample Streams

Automatic Calibration

Fast & Easy Maintenance

Automatic Lubrication NEW!



9/10


87

impeller to a given set of conditions. The package design alsogreatly influences energy usage. In some cases, the intake air ispreheated by the heat rejected by the motor or even the entirepackage, resulting in a drop in performance. Belt-drive tensioncan be maintained by manual, partially automatic or fully auto-matic means. Fully automatic belt tensioners are the only onesthat will maintain peak equipment efficiency without frequent

maintenance intervention.Regardless of blower technology, the engineer must know that

not every manufacturer includes all pressure losses across the variousaccessories. While it should be evident that mechanical and/or elec-trical drive losses should be reflected in the stipulated performancedata, some manufacturers forget to account for these losses.

Drive system.In addition to the machine thermodynamiccharacteristics, the drive system needs to be considered.

Drive efficiencies.Much is said about drives. Here are somefacts:

Narrow, cogged V-belt drives:a. 98% to 97% efficient when optimally sized, constantly

properly aligned and tensionedb. Worst case: 90% efficient, resulting from over-sizing, poor

design, lack of tension or poor alignmentc. Advantage: speed selection and motor sizing flexibility for

best efficiency Speed-increasing gears:a. 97 to 99% efficientb. Advantage: speed selection and motor sizing flexibility for

best efficiency; reliable and low maintenance Frequency inverter:a. 95% to 98% efficient average; however, the efficiency is not

constant over the entire operation range. The total drive efficiencywill vary with speed and load.

b. Moreover, the inverter and the motor influence each other,as mentioned in research papers.

c. Some VFD types and applications may limit the distancebetween the VFD and the motor.

Asynchronous induction motors:a. > 95% for premium efficiency motors at 100 hp and

aboveb. Efficiency drops as the load dropsc. Power factor drops as the load drops, however, the power

factor is corrected with the use of a frequency inverter. Permanent-magnet motors:a. Permanent-magnet motors used on high-speed turbo blow-

ers are custom. Their efficiency is slightly better than that of

premium-efficiency asynchronous motors. Only little informationis available on their performance.

b. Magnetism may be affected at higher temperatures andexposure to magnetic fields; sufficient cooling is critical: Somemanufacturers require air conditioning of the blower enclosure;some require motor water cooling above a certain power rating.

c. Distance between VFD and motor is limited.

Power-factor issues and other points to consider.For induction motors, power factor drops with decreasing loadand speed; synchronous motors (for example with permanentmagnets) operate at improved power factors. An electric utilitymay assess a power-factor penalty if the plant operates at a power

factor that is less than some predefined limit. The power-factorpenalty is usually billed as an additional demand charge. If penal-

ties become high, plants usually consider adding capacitors tocorrect (i.e., increase) the power factor. VFDs allow power-factorcorrections approaching unity.

Single-stage, high-speed centrifugal blowers feature a speed-increasing gear with their high-speed shafts on hydrodynamicbearings. These require pressure lubrication. The oil pump powerneeds to be added to the blower power.

The power required to drive cooling fans is not included,but needs to be added. Some manufacturers do not include acheck valve in their standard package. Its pressure loss must

TABLE 7.Power required for each blower type foreach data point

BHP per Machine

Screw StandardizedData points PD blower compressor turbo blower

1 155 137 132

2 71 68 67

3 111 100 98

4 85 81 80

5 85 76 75

6 65 61 62

TABLE 8.Energy costs comparison

Rotary-lobe blower Screw compressor Standardized turbo blower

$207,569 $184,117 $181,493

BOLTING PROBLEMS?

1-412-279-11491-800-345-BOLT (USA)

SuperboltMulti-Jackbolt

Tensioners eliminate

unsafe and time consuming

bolting methods. Only

hand/air tools required for

any size tensioner!

Fast, safe,and easybolting solution!

Request FREE DVDor catalog today!

www.superbolt.com

Meet the solution:



10/1088

ROTATING EQUIPMENT

be included. The power loss of integral-gear drives are usuallyincluded in the blower performance data, while belt-drive lossesoften need to be added.

Finally, the applicable testing codes and procedures differ forthe various blower types and invoking the wrong one has intro-duced serious inaccuracies. Working with a competent blowermanufacturer will prove of great value in avoiding inaccuracies

and misunderstandings.Larger flow per machine > 8,500 cfm/250 m3/min can best

be compressed with a special-purpose centrifugal blower or, if thepressure is low or the turndown requirement is low, a multistagecentrifugal blower.

Energy usage example for three blower types.Table5 compares energy costs at constant operating pressure.

Evaluation in a typical application with multiplemachines.

Location: St. Louis, Missouri

Normal atmospheric pressure: 14.32 psiaNormal average temperatures over a year: max. 66F/min. 47FTemperature max./min.: + 99F/5FMaximum recorded temperature: +115FRH: yearly average: day 60%/night 78%

SystemSubmersion: 15 ft (6.63 psi)Piping pressure loss: 0.5 psiDiffuser head loss: 0.5 psiAllowance (diffuser fouling and reservefor emergencies): 0.75 psiInlet filter pressure loss clean/contaminated: 0.1/0.75 psi

Allowance for stagnation 0.75 psiDischarge check valve pressure loss 0.2 psiDesign pressure: 9.43 psig

Discharge control valve pressure loss 0.5 psiDesign compression ratio: 1.73Operating pressure new/clean system: 7.72 psigOperating compression ratio new/clean system: 1.55Lowest normal air density (99F/60% RH): 0.069 lb/ft3

Extreme low air density (115F/36% RH): 0.068 lb/ft3

Highest normal air density (5 F/36% RH): 0.086 lb/ft3

Maximum annual airflow requirement: 5,100 scfmMinimum annual airflow requirement: 1,900 scfm

Two operating units and one standby unit are desired.Each blower unit should be designed to handle 2,550 scfm at

9.43 psig during the worst-case site ambient conditions.Annual energy costsAssumptions: Electricity cost = $ 0.12 per kW-hour Motor efficiency = 95% for all machines VFD efficiency = 97% for all machines Belt-drive efficiency = 97% for the PD blower

Notes.The Office of Industrial Technologies of the US Depart-ment of Energy has published a useful guide for calculating thelife cycle costs of pumps (http://www.eere.energy.gov/industry).

We believe that the guide can be adapted to aeration blowersas well. HP

7thBiennial Valve World Conference & Expo

Dsseldorf, Germany

Nov. 30 Dec. 2, 2010

Turn it on now!Turn it on now!

w w w . v a l v e w o r l d e x p o . c o m

Now get all the congress and trade fair

information daily on your cell phone!

Simply use the QR-Code Reader of the

camera in your phone.

The Valve World Expo presents continual growth,

outstanding innovations and the highest level of

technology now in its new home in Dsseldorf. Valves

and the complete range of accessories as well as valve-

related technologies are the focus. As the most important

event for the industry, the Valve World Conference

analyses the future of markets against a backdrop of

fascinating developments and scientific evaluations.

Dsseldorf turns it on!


Blower Selection for Wastewater Aeration

Documents

Transcript of Blower Selection for Wastewater Aeration