planning air system upgrades

8/6/2019 Planning Air System Upgrades

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ost compressed air systems are relatively mod-est at rst, but grow and develop as production,inappropriate uses and leaks increase over time.Even assuming that air leaks and inappropriate

uses have been investigated and reduced, adding productionequipment raises demand for compressed air. Some equipment

may need a different operating pressure, and the required airquality may change. ese modications represent potentialproblems for the plant engineer who wonders:• What is the required volumetric ow rate and the pressure

of compressed air, and what size of air compressor shouldbe added?

• What type of compressor and control system offers greatestreliability and lowest life-cycle operating costs?

• Where should the compressor be located?• Is sufficient power, ventilation and cooling capacity available?• What type of system capacity controls would be best?• Is primary compressed air storage sufficient?

• Is distribution piping adequate?• Is secondary compressed air storage sufficient?

• Is the current compressed air quality satisfactory?• Does this project require professional help?

Compressor size e current average and peak compressed air ow rates, incubic feet per minute (cfm), should have been established

before considering any proposed additions. e rated outputof the existing compressor(s) also should have been estab-lished. e specications for the proposed equipment thatneeds additional compressed air should state the requiredow rate, pressure and air quality. is information providesthe new total ow rate. Deducting the rated output of theexisting compressor(s) gives the additional air ow requiredfrom a new compressor.

is calculation ignores potential leakage and increasedinappropriate compressed air use. Also, it doesn’t accountfor differences in the frequency of operation of each piece of production machinery. Individual consumption peaks might

not occur simultaneously. Nevertheless, you’ll need to deter-mine the average and peak ow rates (Table 1).

RELIABILITY Compressors


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Don’t add a udge actor to therequired air low when calculatingthe rated capacity or the additionalair compressor(s) because it couldresult in the new unit(s) operatingat less than u ll capacity most o the

time, robbing e iciency. Should youdecide to oversize the additionalcompressor by 20% or more, select

the compressor that combines thebest u ll-load and part-load economy to minimize t he operating cost overthe ul l range o the compressed airrequirements.

Another important consideration is

standby capacity in case o compres-sor mal unction or needed repair. Tismay require at least one additional

compressor. Conventional wisdomsays that three 50%-capacity com-pressors are better than two 100%compressors, because this providesmore lexibility without sacri icingsystem reliability, particularly during

periods o reduced consumption. It’salso bene cial to operate the smallesttotal compressor horsepower, particu-larly or periods o reduced capacity requirements, such as a second- orthird-shi t operation. Tese actors,combined with reliable compressorservice, are keys to maintaining energy and production efciencies and pro t-able outcomes.

Compressor typeEach type o compressor has itsadvantages, disadvantages and pre-

erred range o capacity and pressure. able 2 provides a simple method

or comparing di erent compressortypes. Li e-cycle cost analysis alwaysis recommended, and should includespeci ed maintenance.

Compressor cooling is a major con-sideration. I water-cooled, the impor-tant issues include availability andquality o cooling water, disposal orrecirculation, possible treatment and

overall cost. I compressors are air-cooled, adequate room ventilation isessential. Heat recovery also is a poten-tial opportunity.

Compressor size and type deter-mines the electrical power require-ments. Additional ancilla ry equip-ment may require a di erent voltageand current. Consider, too, theavailabilit y o the required electri-cal supply and its support equip-ment. Ensure that proper circuit

protection is provided or the addedelectrical load.

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Don’t add a udgeactor to the required

air ow whencalculating therated capacity orthe additional air

compressor.


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Location, location, locationMany plants have a compressor room and, in some cases, thatroom is shared with other equipment. Several actors mustbe considered i a compressor is to be added. Tese includethe need or a oundation, space or maintenance activities,space or drying and ltration equipment, room ventilationto handle the added heat release and the sound level.

It might make sense to instal l the new compressor in a di -erent location, perhaps closer to the point o greatest demand

or at the application requiring the highest pressure.

Capacity controls Tere are several types o capacity control or individual com-pressors, sequencing controls or multiple compressors andpressure and ow controls or compressed air systems. Tecorrect selection o each determines system efciency overthe anticipated operating ranges.

wo rules or achieving optimum efciency are (1) only the number o compressors needed to maintain the requiredsystem pressure should be in operation at any given time, and(2) all but one, a trim compressor, should be running at ullcapacity and pressure. Te trim compressor should have an

efcient capacity-control mode. I it’s a reciprocating com-pressor, this could be unloading in a series o capacity steps.

For a rotary compressor, variable-speed control or variabledisplacement is most efcient.

Storage and piping Te size and location o the primary air receiver a ects the


Table 1. Basic demand worksheet

End useridentity

Minimumow (c m)

Averageow (c m)

Peak ow(c m)

Cycle time(seconds)

#1

#2

#3#4

#5

#6

Totals

Note: In some cases, the minimum ow rate may be very low orzero (cycle time - o ) until an intermittent operation (demandevent) occurs, when there is a large demand (peak ow rate) or atime (cycle time – on). The combination o these determines theaverage rate o ow. End users having a constant demand shouldbe tabulated by the average ow rate. Peak ow events may requireadditional primary storage and secondary storage.

Standby capacity in case o compressor mal unction or needed

repair may require anadditional compressor.


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efciency o your capacity control. Efcient system controlmay require changes in air storage volume, but adding receiver volume won’t compensate or insufcient compressor capacity or inadequate distribution piping.

Many compressed air distribution systems originate ata primary air receiver in the compressor room where dis-

tribution piping moves the air throughout the plant. Asproduction increases, another one or more buildings mightbe erected, but the supply o compressed air still passesthrough the origina l distribution piping. I the piping isn’tadequate or the increased demand, the result can be exces-sive pressure losses and increased energy consumption.

Intermittent high-volume demand can cause severedynamic pressure uctuations in the entire compressed airsystem that can upset manu acturing processes. Many uc-tuations can be so tened with an appropriately sized andlocated secondary air receiver that can provide enough air tosatis y the intermittent demand without compromising thepressure in the main system.

Air qualityEach piece o production equipment requires compressedair at a given ow rate, pressure and air quality. Tese con-siderations may vary signi cantly. A cardinal rule is to avoiddrying and ltering compressed air any more than is needed

or the speci c application. Going overboard can result inincreased pressure losses and energy consumption. Considerthe idea o satis ying the major compressed air requirements

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A typical compressed air system

Figure 1. Compressor e ciency is a unction o pipegeometry, leaks, operating pressure and other actors.

The size and location o the primaryair receiver a ects the efciency o

your capacity control.


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centrally and supplementing these requirements locally, where needed.

Many industrial applications can be served well with apressure dewpoint o 35°F to 38°F, which can be achieved with a re rigerated dryer. Standard regenerative desiccantdryers can drop the pressure dewpoint to –40°F, and morespecialized dryers can bring it down to –100°F. Dry the aironly to the requirements o the end users or to meet localambient conditions.

Improving your air quality also requires lters to removeparticulates and might require coalescing and adsorption l-ters to remove liquids and other contaminants. Tese added

lters will result in increased pressure losses and maintenancerequirements.

Outside help

In most cases, seeking pro essional help is a good idea. Equip-ment distributors with good local service capabilities can be

help ul. An alternative approach is hiring an independentcompressed air consultant to provide a product-neutral opin-ion or solution. p

Bill Scales, P.E. owns Scales Air Compressor Corp. in Carle Place,N.Y. Contact h im at [email protected] and (516) 248-9096ext. 611. David M. McCulloch owns Mac Consulting Services inShalimar, Fla. Contact him at [email protected] and (850)651-4540.

The Compressed Air Challenge (CAC) is a national collabora-tive o public and p rivate organizations dedicated to increasingthe understanding and improving e iciency o compressed airsystems within U.S. industry. The CAC o ers Fundamentals o Compressed Air Systems and Advanced Management o Com-pressed Air Systems training, and in cooperation with the U.S.Department o Energy, the Compressed Air Systems Sourcebook

or Industry as well as the Quali ied AIRMaster+ Specialist train-ing. CAC has built a reputation or being a reliable resource orcost-e ective solutions and unbiased in ormation, including therecent publication Best Practices or Compressed Air Systems, acomprehensive and detailed re erence or plant personnel. Formore in ormation about CAC training and publications call (301)751-0115 or visit www.compressedairchallenge.org. The authors

o this article, David McCulloch and Bill Scales were also theauthors o Best Practices or Compressed Air Systems .


Table 2. Simple matrix for comparing compressor types 1

Compressorcharacteristic 2

Compressor type

Two-stage, double-acting reciprocating Lubricant-injected screw Lubricant- ree screw Centri ugal

Size and weight 3 1 2 2

Compact size andcomplete package 3 1 1-2 1-2

Can be located closeto points o use 4 2-3 2-3 3

Maintenance cost 3 2 2 1

Foundation requirements 4 1 1 1-2

Reducedcapacity e ciency 3 1-2 1-4 1-3 1-3

Lubricant- ree air delivery- lube/lube- ree 4/1 2 1 1

Lubricant carryover- lube/lube- ree 4/1 3 1 1

Lubricant changes ormakeup - lube/lube- ree 4/1 3 1 1

First cost,including installation 4 1 2 2

Full-load operatingcost, kW/100 c m 4 15 to 16 16 to 19 18 to 22 16 to 20

1 These evaluations are general in nature and might not cover specifc eatures o a given compressor type or manu acturer. They’re intendedto provide a general guide or comparing compressors. It’s important to evaluate each point in any comparison o quoted equipment. Other

actors to be considered include relative size and cost, warranty and service.2Each compressor type is rated rom 1 to 4. Key: 1 = very good; 2 = good; 3 = air; 4 = poor.3Re er to the section on compressor controls. It’s important to compare kW/100 c m at all reduced capacities.4Operating costs are based on ull capacity at a discharge pressure o 100 psig; a ull-load motor e ciency o 92% and 0.746 kW/bhp.

Each piece o productionequipment requires compressed air

at a given ow rate, pressureand air quality.

planning air system upgrades

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