Engineered Systems - April 2008 - (Page 40)

This Time, Water Cooled Was All Wet was understood that the non-critical occupied areas would have to be addressed with either a low-temperature chilled water system operating at a more traditional temperature, or with heat pump AHUs using the higher chilled water temperature loop as a condenser medium. Since the building required N+1 redundancy, it was decided that we would study two plants providing 1,000 tons each. It was also assumed that the secondary pumping energy would be equal in both cases studied. And since I didn’t have enough options to study, I was asked to consider two locations, one in Pennsylvania and the other in North Carolina. THE PLANTS COMPARED Old school first: A straightforward variable primary chilled water plant was modeled with three nominal 500-ton centrifugal VFD chillers in parallel, three equally sized cooling towers in parallel, one nominal 1,000 ton plate-and-frame heat exchanger, and three condenser pumps running at about .07kW/ton. The model reflected the following sequence: When the outdoor wetbulb temperature was above 47°, the chillers would handle the entire load. Below 47° wetbulb, the chillers would be shut down and essentially free cooling would be provided via the plate-and-frame heat exchanger. We recognized that in actual operation it would not be possible to make the transition from chillers to free cooling every hour the temperature drops below the wetbulb threshold. But to keep our analysis as objective as possible (and simple), each hour was counted toward the free cooling total. In turn, the energy usage estimated may have been optimistically low, but in our opinion, not enough to unfairly skew the results. Our elegant solution consisted of two plants with three nominal When the outdoor drybulb When the outdoor drybulb y temperature was 57º, the air cooled p t 57º th i ld chillers wou chillers would handle the entire ille ould handle the enti andl tire load Between pp oximately 47º load. Between approximately 47º and 57º drybulb, and 57 drybulb, a portion of the y , ortion of the load would be handled y the dry load would be handled by the dry y , , y , coolers, which, as you may recall, would be in series with the air ou in seri ries wit the a ith cooled chillers. As the drybulb y temperatures drops the p portion temperatures drops, the proportion p p of essentially free cooling to cooling essentially free cooling y cooling g requiring the compressors would requiring the compressors would q p increase Below 47º drybulb increase. Below 47 drybulb, the th dry coolers would handle the l ld dl th entire load. compressor efficiencies. But more importantly, it would increase the number of available free cooling hours. Ultimately, the chilled water parameters used in the study were 57° supply with a 12° differential. You and I both know that you couldn’t maintain acceptable occupied space conditions with an entering water temperature that high. So it FREE INFO: 183 40 En gi neer ed S y stem s April 2008 http://www.emon.com/es4.asp http://www.emon.com/es4.asp

Table of Contents Feed for the Digital Edition of Engineered Systems - April 2008

Engineered Systems - April 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips This Time, Water Cooled Was All Wet Cleanroom Design In 10 Easy Steps It's Not The Heat, It's The Humidity Issues & Events Computers and Software Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - April 2008

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