Engineered Systems - December 2008 - (Page 52)

Basics For Refrigerant Chillers ity about 1% of the time, 75% capacity about 42% of the time, 50% capacity about 45% of the time, and 25% capacity about 12% of the time (See graph 1 for kW/ton vs. percent of part load for typical scroll, screw, and centrifugal chillers). These values were incorporated into IPLV equation, which is: IPLV = 0.01A + 0.42B + 0.45C + 0.12D where: A = kW/ton at 100% capacity (condenser water at 85°) B = kW/ton at 75% capacity (condenser water at 75°) C = kW/ton at 50% capacity (condenser water at 65°) D = kW/ton at 25% capacity (condenser water at 65°) When evaluating different chiller energy usage, the IPLV provides the most accurate average chiller energy usage. When the known parameters are different than prescribed above, the part-load performance becomes NPLV, which is described using the same equation. Ultimately, the chiller’s energy usage is primarily based upon the “lift” or temperature difference between the chilled water leaving temperature and condenser water leaving temperature. Lowering the condenser water leaving temperature or raising the chilled water leaving temperature will reduce lift and energy usage for chillers but not necessarily the chilled water system. Raising the condenser water leaving temperature or lowering the chilled water leaving temperature will increase lift and energy usage for chillers but not necessarily the chilled water system. Varying a number of chiller variables and the resulting impact on chiller capacity and energy usage will be discussed. CHILLER OPERATING PERFORMANCE Most chillers do not operate at the standard operating conditions noted above. As noted in the introduction, there are many variables that affect chiller operation. Below, this article investigates the effect of modifying three chiller variables (condenser water temperature, chilled water temperature difference, chilled water leaving temperature) on chiller operations. Note the graphs are illustrative only. Always use the particular chiller performance data when evaluating a chiller. Because centrifugal chillers are custom-made, data were not available for all variables being discussed. Effect of condenser water temperature changes. The standard chiller condenser water temperature is 85°. Looking at graph 2, chiller capacity increases as the condenser water temperature decreases. At a condenser water temperature of 75°, scroll chiller capacity increased 4.9%, screw chiller capacity increased 5.6%, and centrifugal chiller capacity increased 10% as compared to condenser water at 85°. Conversely, chiller capacity decreases as the condenser water temperature increases. At a condenser water temperature of 95°, scroll-chiller capacity decreased 4.4%, screw chiller capacity decreased 6.1%, and centrifugal chiller capacity decreased 10% as compared to condenser water at 85°. Looking at energy usage, the chiller’s energy usage reduces as the condenser water temperature decreases. At a condenser water temperature of 75°, screw chiller energy usage decreased 18% and centrifugal chiller energy usage decreased 11% as compared to Tested at Wiss Janey Elstner Labs ©2008 The Metraflex Company 312-738-3800 Input 180 at www.esmagazine.com/instantproductinfo 52 En g i neer ed Sy stem s December 2008 http://www.metraflex.com http://www.metraflex.com http://www.esmagazine.com/instantproductinfo

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

Engineered Systems - December 2008 Contents Editor's Note HVAC Challenge Back2Basics Case In Point Commissioning Building Automation HVACR Designer Tips Applications Checklist Lakefront Library: Radiant Systems Meet Natural Ventilation Before (And After) The Flood Basics For Refrigerant Chillers Wire-To-Shaft Efficiency For HVAC Pumps Products Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - December 2008

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