10 cooler idea

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erigeration is one o the more energy-intensive tech-nologies used in domestic manuacturing operations– primarily ood plants. From an energy perspective,

the trend in many ood plants is toward processes that re-quire more, not less, rerigeration, thereby escalating overallenergy requirements. For example, ood companies are re-

sponding to consumer demand by increasing the sup-ply o rozen ready-to-cook entrees and sides. Tese

products oten require quick-reezing unit opera-tions to maintain high product quality. Quick reezing unit operations are among the mostenergy-intensive in ood plants, which increasesthe total energy required to produce a unit o

nished product.

Just as plant rerigeration processes are demandingmore energy, plant operations proessionals are pushing to re-duce overall energy consumption in their manuacturing op-erations. Why? Because indications are that the uture cost o energy will continue to rise. So it’s increasingly important orplants to improve operational eciency to maintain a competi-tive marketplace advantage. Tese trends put pressure on plantoperations sta to identiy eective measures to level or reducethe energy consumption in manuacturing.

Strategies or improving the eciency o industrial re-rigeration systems are available or all to adopt. Seemingly a simple order, improving energy eciency in these systems

is dicult or a number o reasons:

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• Industrial rerigeration systems are custom-engineeredto meet a plant’s specic process needs. Tis makes prescrip-tive energy conservation measures dicult to identiy be-cause each system is “one-o-a-kind.”

• Food production acilities oten require continuous re-rigeration. Energy eciency improvement measures thatrequire shutdown are dicult to implement.

• Because rerigeration uptime is crucial, plants’ opera-tions sta are oten risk-averse concerning any changes thatmight undermine their ability to provide “cold” to the pro-duction foor.

Consider these 10 ideas aimed at improving industrial rerig-eration system eciency. Each is proven to work. You just needto evaluate them or implementation in your plant’s context.

1. Floating head pressure controlMany plants operate their rerigeration systems with higherthan necessary head (condensing) pressures. Although theability to reduce a system’s head pressure is limited by ambi-ent conditions, many plants can operate with considerably lower minimum head pressures. I your ammonia-based re-rigeration system’s head pressure never alls below 125 psig, you might have an opportunity to improve system eciency.A useul guideline says you can expect the eciency o yoursystem’s compressors to improve by 1.3% or each degree F in lower saturated condensing temperature (1°F is about 3

psig or ammonia).

2. Raise suction pressure/temperatureI your plant uses evaporator pressure regulators on allo its loads, it might make sense to raise your system’ssuction pressure set point. You can expect your system’scompressor capacity to improve by 2.5% or each degreeF increase in saturated suction temperature. Eciency increases depend on the starting point o your suctionpressure increase, but improvements in the range o 2%or each degree F increase in saturated suction tempera-

ture are possible.

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2.4

2.2

2.0

1.8

1.66050 70 80 90 100

Fixed speed

Duty cycling

Variable speed

Tspace

=-20°F

Nevap

=7

TDdesign

=8.4°F

C o m p r e s s o r + e v a p o r a t

o r ( k W / t o n )

Evaporator part-load ratio

Load down, efciency up

Figure 1. The eciency advantage o a variable-speed

evaporator an increases as the cooling load decreases.

Table 1.Summary o industrial rerigeration energy eciency improvement opportunities

Strategy Energy eciency opportunityCapital cost Operational

risk Value For more inormation

New Retroft

1 Floating head pressure control L L L H EEG1 Section 4.1

2 Raise suction pressure n/a L-M M H EEG Section 5.1

3 VFD or evaporator ans L L-M L M Cold Front2 Vol. 4 No. 4, 2004

4 VFD or compressors L H L M Cold Front Vol. 4 No. 3, 2004

5VFD or evaporativecondenser ans

L L L L-MCold Front Vol. 4 No. 2, 2004

6 Heat recovery (oil cooling) M M L M Cold Front Vol. 6 No. 4, 2006

7Improved compressor sequenc-ing and capacity control

L L-M L MEEG Section 6.3, IRC TechNote: 2002

8Improve hot gas derostdwell period

L L M L-MEEG Section 5.4

9Convert liquid injection oilcooling to external oil cooling

M H M MEEG Section 6.2.2, Cold FrontVol. 3 No. 3, 2003

10 Reduce parasitic loads M M L L Cold Front Vol. 5 No. 3, 2005

L = low, M = medium and H = high.1 EEG is the Industrial Rerigeration Energy Eciency Guidebook, Reindl, D. T., Jekel, T. B., and Elleson, J. S., IRC, (2004).

2 Cold Front is the IRC’s quarterly newsletter available at www.irc.wisc.edu/?/newsletter.

September 2007 www.PLANTSERVICES.com 4

Plant operations professionalsare pushing to reduce overall

energy consumption.


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3. Variable-requency drives orevaporator ansBecause most evaporators don’t oper-ate at their design load 100% o thetime, their capacity needs to be variedto meet instantaneous thermal loads.

Evaporator eciency at part-loadconditions can be improved in mostsystems by using variable-requency drives (VFD) on evaporator ans. Tesavings attributable to this technol-ogy depends on a number o actorsincluding system suction pressure,evaporator part-load ratio, evaporatoran type, and ace velocity o air overthe evaporator coil.

Figure 1 illustrates the potential ina reezer application by showing thecombined compressor and evaporatoroperating eciency serving a -20°F space with an evaporator temperaturedierence o 8.4°F. Note the increas-ing advantage o the VFD an optionover the xed-speed an option as theevaporator’s part-load ratio decreases.Low-capacity operation with xedspeed ans is costly.

4. Variable-speed compressorsApproach variable–speed rerigera-tion compressors with caution. Com-pressors in these systems tend to be

driven by a large-rame motor, whichmakes the VFD option expensive. Atmost, consider having only one vari-able-speed compressor per suctionpressure level in the plant. Use theVFD compressor as a trim machineor capacity modulation.

5. Variable-speed condenser ansIn many cases, VFD condenser anscan yield operating costs savings o 2% to 3%, depending on a number

o actors including: relationship o

heat rejection capacity available tothat required, minimum head-pres-sure set point, condenser an type,and others. I you pursue a VFDproject or condenser ans, installVFDs on every condenser an and

modulate their capacity equally tomaximize energy savings and avoid

liquid management problems on thesystem’s high side.

6. Heat recovery at oil coolersIt’s possible to recover heat rom thedischarge gas on high-stage compres-

sors. However, a more eective optionis to recover heat rom oil-cooling

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Approachvariable-speed

drives for refrigerationcompressors with

caution.



Reerences:

Manske, K., “Perormance Optimization o In-dustrial Rerigeration Systems,” M.S. Thesis,University o Wisconsin-Madison, (2000).

Reindl, D.T., et. al., Industrial RerigerationEnergy Eciency Guidebook, Industrial Re-rigeration Consortium, www.irc.wisc.edu/?/order (2004).

September 2007 www.PLANTSERVICES.com 51

heat exchangers on screw compressorpackages. Te heat available rom oilcooling heat exchangers is available inreasonable quantities and at a highertemperature when compared to theheat available or recovery rom the

discharge gas stream.

7. Compressor sequencingand controlControls are required to match com-pressor capacity to system demand. Temost widely used compressor technol-ogy in industrial rerigeration systemsis the screw compressor. Unortunately,screw compressor eciency decreases

as it unloads in response to decreasingdemand. For example, a typical screw compressor operating at -20°F suctionand 90°F condensing will have a ull-load eciency o about 2.2 BHP/ton. When unloaded to its minimum capaci-ty (10% in this case), the horsepower perton requirement increases to about 8.8BHP/ton. Review your sequence o op-eration and minimize the time intervalsat which individual machines operate atpart-load ratios less than 70%.

8. Improve derost sequencesAir-cooling evaporators that operateat low temperatures will accumulate

rost. As the coil ices up, its capac-ity decreases, which decreases systemeciency. Manske (2000) estimated

that poor hot-gas derost sequencesand controls accounted or 13% o the electrical energy consumption ina cold storage warehouse. Establisha derost sequence that avoids hotgas dwell times in excess o 15 min-

utes duration and derost individualevaporators only on an as-needed basis

rather than derosting on the basis o a time-clock.

9. Convert rom liquid-injectedoil cooling to external coolingScrew compressors require some means

o oil cooling. Using high-pressure liq-uid rerigerant or oil cooling is common

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Establish a defrostsequence that avoidshot gas dwell times inexcess of 15 minutes

duration.


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in a number o systems. Liquid-injection oil cooling conspiresto reduce the system’s eciency because it increases compres-sor power requirements and reduces capacity. Converting romliquid injection to external (thermosiphon or fuid-cooled) oilcoolers can yield savings in the range o 3% to 10%.

10. Reduce parasitic loadsLook or opportunities to eliminate the heat leaks into yoursystem. Attending to ailed insulation, inadequate doorseals, open doors and oversized conveyor openings are ex-amples o easy xes that reduce the heat gains that rob yoursystem o both capacity and eciency. Visual inspectionsand more sophisticated thermal imaging can pinpoint thesehot spots. Find and x them.

O course, you’ll need to evaluate each o these sugges-tions in the context o an individual system. able 1 ratesthe 10 opportunities qualitatively (low-medium-high) andby their capital cost (both new and retrot), operational risk and overall value.

Get more inormationIn 2004, the Industrial Rerigeration Consortium (IRC)published the Industrial Rerigeration Energy Eciency Guidebook that covers these and other energy-eciency improvement strategies or industrial rerigeration systems.In addition, the IRC has published several energy-eciency improvement-related articles in its quarterly newsletter, Cold

Front . Past issues o the Cold Front are available or down-load at: www.irc.wisc.edu/?/newsletter. Seeking out, evalu-ating and implementing energy eciency improvements at your plant isn’t only enjoyable, it’s rewarding.

Douglas T. Reindl, Ph.D., P.E., is proessor and director o the Indus-

trial Rerigeration Consortium at the University o Wisconsin-Madi-

son. Contact h im at [email protected] and (608) 262-6381.

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12 point checklist – “The path to superior chiller perormance”Chiller selection – “Evaluating chillers”Starter controls – “Choosing a screw chiller or all occasions”Ammonia rerigerants – “The case or ammonia rerigeration”Chiller tips – “10 tips or improving chiller eciency”Ammonia management – “Chill out and be sae”Speaking fnance – “Selling your HVAC project”

For more, search www.PlantServices.com using the keywords con-denser, evaporator and rerigeration.

More resources at www.PlantServices.com/thismonth

Converting from liquid injection to

external oil coolers can yield savingsin the range of 3% to 10%.

10 cooler idea

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