©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 2, February 2014.
By Frank Morrison, Member ASHRAE
About the Authors
Frank Morrison is manager, global strategy at Baltimore Aircoil Company in Jessup, Md. He is chair of ASHRAE TC 3.6, Water Treatment, and a voting member of ASHRAE SSPC 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings.
Water-cooled systems offer lower energy use than air-cooled alternatives. Many years ago, the first water-cooled systems used potable water directly in the condenser to provide heat rejection with the cooling water wasted to a drain. Cooling towers were developed to recycle more than 98% of this water, resulting in tremendous reductions in water and energy use as these systems grew in both size and popularity.
Since then, water-cooled systems have steadily improved their performance. For instance, the efficiency of a 500 ton (1757 kW) water-cooled centrifugal chiller has improved by over 50% since 1975 as indicated by the requirements of ASHRAE/IES Standard 90.1 (hereafter referred to as Standard 90.1). Cooling towers have also evolved from centrifugal fan units to much more energy-efficient axial fan designs with improved heat transfer surfaces, known as fill. In addition, independent certification of thermal performance for open circuit cooling towers per the Cooling Technology Institute’s Standard 201 has become widely accepted in the marketplace and became required by Standard 90.1 in the 2007 edition.
While the efficiency improvements of individual system components have certainly lowered overall energy use, even greater improvements are possible by optimizing the way cooling systems are designed and operated. For instance, the full load energy use in a 500 ton (1757 kW) water-cooled chiller system, based on Standard 90.1-2013 minimum efficiencies, is roughly broken down as follows: chiller – 77%, cooling tower – 8%, condenser pump – 7%, and chilled water pump – 8%. With the chiller accounting for the majority of the energy use, many contend that it makes sense to operate the cooling tower fan and condenser pump such that compressor energy use is reduced—since it is by far the largest motor in the system.
For instance, to lower chiller energy, the cooling tower is often operated at full fan speed and flow until ambient conditions allow the minimum condenser water temperature limit to be reached. Below this level, the fan speed of the cooling tower is modulated, typically by a variable speed drive (VSD), to maintain the setpoint. This is essentially the operating sequence for the water-cooled baseline buildings found in Appendix G of Standard 90.1-2013, which uses 70°F (21.1°C) as the lower condenser water setpoint (though this value is above the low limit for almost all chillers). In practice, this lower limit varies and is dependent on the type of chiller. The closer to full load the system runs, the greater the energy savings from such strategies. However, most chiller systems operate at less than full load for the majority of time.
While it may seem counterintuitive, many designers and operators have found that using less cooling tower energy reduces overall system energy at many off-design conditions. At such conditions, ancillary equipment (condenser pumps and cooling tower fans) operating at full design speed becomes a larger portion of the system energy use, especially when variable speed chillers are used. Reducing cooling tower fan speed can reduce associated fan power significantly while increasing chiller power only marginally. For example, slowing the tower fan speed to 80% of design reduces tower fan power by about half, while only raising the cooling tower leaving water temperature about 3 °F (1.7°C). Depending on the specific load point, the increase in chiller energy consumption from the higher condenser water temperature may or may not be less than the reduction in cooling tower energy. The key is to balance the performance of the system components so overall performance is optimized. Articles such as Taylor’s excellent series on chilled water system design provide more details on such strategies.
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