Evaporative-Cooled vs. Air-Cooled Chillers: Kirtland AFB Case Study

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DISCUSSION TOPICS

Brief Overview of Air-Cooled and Evaporative-Cooled Condenser Technologies

Pros and Cons

Scope and Results of Kirtland AFB Study

Conclusions and Recommendations

Q&A

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AIR-COOLED CONDENSER BASICS

Refrigerant hot gas is cooled and condensed in Air-to-Refrigerant Heat Exchanger (coil).

Air-Cooled Condenser efficiency depends on ambient air dry bulb temperature, i.e. the higher OAT is, the more power is required to compress hot gas in order to condense it into liquid.

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EVAPORATIVE-COOLED CONDENSER BASICS

Treated water is sprayed over condenser coil. Part of water is evaporated thus lowering surface temperature of the condenser and the air drawn across it.

Remaining water is collected in drain pan and recirculated back to sprayer.

Make-up city water is added to replace evaporated water.

Refrigeration system efficiency is increased by 25-50% compared to air-cooled due to lower vapor compression pressure, thus reducing compressor‘s energy.

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AIR-COOLED AND EVAPORATIVE-COOLED CONDENSER COMPARISON

PROS OF AIR-COOLED vs. EVAPORATIVE-COOLED:Smaller cabinetLighter weightNo condenser water piping and treatmentLower equipment costLower maintenance costNo water consumption for condenser operation

PROS OF EVAPORATIVE-COOLED vs. AIR-COOLED:20-40% lower electrical consumption of the chillerQuieter operation

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KIRTLAND AFB ENERGY STUDY

OBJECTIVE: Increase the energy efficiency of existing facilities on various Air Force Bases in order to reduce the building annual operating cost.

STUDY SCOPE:Evaluate life cycle costs of replacement of existing Air-Cooled Chillers with Evaporative-Cooled Chillers at seven (7) buildings.Life Cycle Cost Analysis compared costs associated with Evaporative-Cooled Chillers against Status Quo (air-cooled chillers), and included:

Capital Cost Estimate (material and labor) at +/-30% accuracy.Maintenance Cost analysis.Operating Cost Analysis (electricity and water).

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KIRTLAND AFB ENERGY STUDY RESULTS

BUILDING NO.

CHILLER TONNAGE

EVAPORATIVE-COOLED CHILLERS

AIR-COOLED CHILLERS

1005 60 $178,000 $74,000

20140 60 $178,000 $74,000

322 70 $204,000 $84,0001017 90 $262,000 $113,00020222 100 $279,000 $130,00020361 165 $389,000 $176,000

472 250 $471,500 $235,000

CAPITAL COST COMPARISON FOR EVAPORATIVE AND AIR-COOLED CHILLERS

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KIRTLAND AFB ENERGY STUDY RESULTS

CHILLER TONS

ELECTRICITYKWh x 1,000 / COST

(chiller only)

CITY WATER AND SEWER

(chiller only)

MAINTENANCE(chiller only)

AIR-COOLED EVAP-COOLED

AIR-COOLED

EVAP-COOLED

AIR-COOLED

EVAP-COOLED

60 49 / $3,920 28 / $2,240 $0 $323 $1,600 $2,000

60 49 / $3,920 28 / $2,240 $0 $323 $1,600 $2,000

70 56 / $4,480 30 / $2,400 $0 $353 $1,600 $2,000

90 34 / $2,720 16 / $1,280 $0 $196 $2,400 $3,200

100 66 / $5,280 39 / $3,120 $0 $463 $2,400 $3,200165 102 / $8,160 56 / $4,480 $0 $665 $2,400 $3,200

250 200 / $16,000 108 / $8,640 $0 $1,260 $2,400 $3,200

ANNUAL OPERATING COST COMPARISON FOR EVAPORATIVE AND AIR-COOLED CHILLERS

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KIRTLAND AFB ENERGY STUDY RESULTS

10- YEAR LIFE CYCLE COST COMPARISON FOR EVAPORATIVE AND AIR-COOLED CHILLERS

BLDG NO.

CHILLER TONNAGE

LCC OF EVAPORATIVE-

COOLED CHILLERS

LCC OF AIR-COOLED

CHILLERS

COST RATIO OF AIR-COOLED / EVAP-COOLED

CHILLERS1005 60 $301,000 $201,000 67%

20140 60 $301,000 $201,000 67%

322 70 $350,000 $233,000 62%

1017 90 $462,000 $293,000 64%

20222 100 $513,000 $335,000 65%

20361 165 $757,000 $557,000 74%

472 250 $1,386,000 $1,191,000 86%

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KIRTLAND AFB ENERGY STUDY RESULTS

10- YEAR LIFE CYCLE COST COMPARISON FOR EVAPORATIVE AND AIR-COOLED CHILLERS

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CONCLUSIONS

Mid-range (50-150 tons) Air-Cooled Chillers may cost less than Half of Evaporative-Cooled Chillers, which require: Special corrosion resistant coatings on condenser coils.Water spraying equipment. Larger cabinet size to accommodate water spraying equipment.Water treatment equipment, water and sewer piping.

Air-Cooled Chillers are more available and its pricing is more competitive than Evaporative-Cooled Chillers..

Evaporative-Cooled Chillers require a larger foundation, water and drain piping, and water treatment equipment.

EVAPORATIVE-COOLED CHILLERS MAY NOT BE THE BEST OPTION, BECAUSE:

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CONCLUSIONS (continued)

Evaporative-Cooled Chillers consume a significant amount of Potable Make-Up Water, which is in short supply and is costly in high-desert dry and arid areas.

Additional Maintenance Cost to treat Make-Up Water and clean condenser coils, which partially offsets energy savings.

Energy Consumption of mid-range chillers is only 10-15% of the Total building energy consumption; therefore the 25-40% Energy Savings for Evaporative-Cooled Chillers amount to only 5-7% of the Total Energy bill reduction

EVAPORATIVE-COOLED CHILLERS MAY NOT BE THE BEST OPTION, BECAUSE:

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RECOMMENDATIONS

Review and Evaluate All Costs related to installation and operation of Evaporative-Cooled Chillers before making a decision.Capital EnergyWaterMaintenance

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QUESTIONS AND ANSWERS

THANK YOU!Corry Freeman, CEM

Staff Mechanical EngineerAMEC

corry.freeman@amec.com

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QUESTIONS AND ANSWERS

BACK-UP SLIDES

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BRIEF HISTORY OF EVAPORATIVE COOLING

Records of Evaporation Cooling use for human comfort trace back to 2,500 BC.

In 16th Century, Leonardo Da Vinci is known to be first to design a Mechanical Evaporative Cooler – a hollow wheel moving through a water bath.

In early 20th Century, Willis Carrier developed a psychrometric chart similar to ones in use today along with the development of a formula that linked the transformation of sensible heat into latent heat during the adiabatic (no external heat input or output) saturation of air.

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TYPES OF EVAPORATION COOLING EQUIPMENT Direct Evaporative Coolers Simply draw air through a moistened wick material and delivers the cooled

(but more humid) air to the space to be conditioned. Supply air temperature is limited by outside air wet-bulb temperature.

Indirect Evaporative Coolers Operate similarly to a direct evaporative cooler but uses a heat exchanger. Stream of outdoor air passes through the heat exchanger, gets cooled by the

heat exchanger cold surfaces, and then is delivered to the space. The benefit of this is that the cooled air does not pick up any humidity as it

does in the direct evaporative process. The drawback is the delivered air does not get as cool as in a direct evaporative process because it is limited by the ambient wet bulb temperature and the heat exchanger efficiency.

Mechanical refrigeration Uses Vapor Compression Cycle consisting of Compression, Condensing,

Expansion, and Evaporation stages. Water is sprayed on condenser coil to increase refrigerant condensing rate,

thus saving AC system energy.