ASHRAE Journal - December 2009 - 32

Lift = Entering Condenser Water Temperature – Leaving Chilled Water Temperature Condenser DT = Leaving Condenser Water Temperature – Entering Condenser Water Temperature COPadj = Kadj × COPstd Kadj = 6.1507 – 0.30244(x) + 0.0062692(X)2 – 0.000045595(X)3 kW/tonadj = 1/(COPadj × 0.2845) NPLV Value at Conditions of 3 gpm/ton and 41°F Lift is IPLV Figure 10: kW/ton and NPLVs for nonstandard centrifugal chillers with and without variable speed drives. 7 (Centrifugal chillers >300 tons.) charges and ratchets within utility bill clauses. Good part-load performance is critical to ensure a chiller will properly reduce energy consumption as the lift and load is reduced. The bottom line is that you need to do the right thing. Using single number evaluations cannot accurately represent a chiller’s energy use in a system. It also cannot predict the savings that can be associated with the additional investment of a variable-speed drive, and can in no way be indicative of a financial payback. Perhaps if a simplified “IPLV-type” methodology is required, using the weighting guidelines found in Table 1 or generating a customer part-load value, may provide a more realistic guideline for predicting chiller performance. Meanwhile, today’s marketplace offers a myriad of computer simulation programs with the capability of modeling a surprising number of critical variables including hourby-hour energy analysis, variable electrical rates, diverse building types and operating profiles, chiller plants, and performance characteristics such as variable-speed drives and high-efficiency chillers. Additionally, drastic improvements in user interfaces of available simulation programs offer operator effectiveness and efficiency shortening startto-end completion times never before possible, eliminating the need for short cuts. 32 ASHRAE Journal The desire to find the “easy” answer is understandable. However, as Standard 550/590, Appendix D, suggests, careful analysis is the only real means to accurately determine a building’s energy usage with corresponding economic impacts to determine fiscally responsible conclusions. The bottom line is that there is no easy answer. It’s our responsibility as an industry to use the tools and technology available to practice due diligence and offer our clients viable, sustainable and proven solutions. References 1. Air Conditioning, Heating, and Refrigeration Institute. AHRI 550/590, Standard for Water Chilling Packages Using the Vapor Compression Cycle, Appendix D, D2.1. 2. McGraw-Hill Construction Network. 3. Trane Service and Order Records. 4. Air Conditioning and Refrigeration Institute. 2003. ARI Standard 550/590, Standard for Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle. 5. Trane Air Conditioning and Economics (TRACE™ 700). 6. Schwedler, M., and B. Bradley. 1995. “Tower water temperature – control it how???” Engineers Newsletter 24(1) LaCrosse, Wis.: Trane. 7. ANSI/ASHRAE/IESNA Standard 90.1-2004, Section 6.2.1, Mechanical Equipment Efficiency. ashrae.org December 2009

ASHRAE Journal - December 2009

Table of Contents for the Digital Edition of ASHRAE Journal - December 2009