Air vs. Water Cooled Operating Costs

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Conventional thinking has been that water cooled chillers are more efficient than air cooled chillers. If we only look at compressor costs, this may be true. However, using state-of-the-art technology with centrifugal compressors and variable speed control, air-cooled chillers are often the better choice. When taking a deeper look into air cooled chillers versus water cooled chillers, it is recommended to consider the operating costs associated with each chiller system.

Thermal Care R-134A Air Cooled Centrifugal Chiller
System Loading
(%)
System Cooling Load
(Tons)
Operating Profile
(%)
Operating Time
(Hrs/Yr)
Total Unit Energy Consumption
(kW/Ton)
Total Chiller Energy Cost
($/Yr)
100% 140 1% 60 0.62 $364
75% 105 42% 2,520 0.482 $8,934
50% 70 45% 2,700 0.435 $5,762
25% 35 12% 720 0.435 $768
    100%     $15,828
           
R-134A Water Cooled Rotary Screw Chiller
System Loading
(%)
System Cooling Load
(Tons)
Operating Profile
(%)
Operating Time
(Hrs/Yr)
Total Unit Energy Consumption
(kW/Ton)
Total Chiller Energy Cost
($/Yr)
100% 140 1% 60 0.614 $361
75% 105 42% 2,520 0.591 $10,940
50% 70 45% 2,700 0.619 $8,192
25% 35 12% 720 0.619 $1,092
          $20,585
           
Cooling tower system water cost (evaporation) $4,699
Cooling tower system water cost (bleed off) $940
Cooling tower system chemical treatment cost $2,268
Cooling tower system pump energy (process & recirculation pumps) $8,428
Cooling tower system cooling tower fan energy cost $1,094
Cooling tower operating cost total $17,429
   
Typical cooling tower annual operating cost $17,429
Typical R-134A water cooled rotary screw chiller annual energy cost $20,585
Total for Typical R-134A water cooled rotary screw compressor $38,014
   
Typical R-134A water cooled rotary screw chiller annual operating cost $38,014
Thermal Care R-134A air cooled centrifugal chiller annual operating cost $15,828
58% total annual savings $22,186

It is important to look at the total operating costs involved with the chillers, not just the compressor costs. As the graphic indicates, cooling tower operating costs should be added to the operating costs of a water cooled chiller. A cooling tower system's costs include the tower fan, water and sewer costs, chemical costs and pumping costs. In process applications, tower systems generally have process pumps and recirculation pumps which can add a significant cost. The illustration below compares the operating cost of an air cooled variable speed centrifugal chiller versus a water cooled screw chiller. This comparison is based upon a 140 ton load, Chicago weather data, $.07/kwh electrical costs, $5.00/1000 gallon water and sewer costs and 6,000 hrs/year operation.

Because the air cooled chiller uses floating head pressure control, the compressor energy costs are actually less than the water cooled chiller's compressor costs-a $4,757 difference ($20,585-$15,585). Adding the tower system's operating cost of $17,429 results in a $22,186/year operating cost savings. A more efficient variable speed water cooled centrifugal chiller reduces the cost difference to $16,725-still a significant annual cost penalty.

WaterAirComparisonGraphic

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