ASHRAE Journal - March 2010 - 12

Re: Chiller Ratings “A Closer Look at Chiller Ratings” in the December 2009 issue offers an interesting perspective on the relevance of the AHRI IPLV/NPLV formula. The authors suggest that the use of the IPLV/NPLV ratings may lead the HVAC designer to select a chiller or to apply technology, such as a variable frequency drive, that cannot be justified by energy and economic considerations. In an ideal world, all project budgets and schedules would permit a thorough system energy analysis of the type recommended by the authors. In reality, this is not always possible. The IPLV/NPLV chiller rating allows for energy codes and standards to prescribe a meaningful minimum chiller efficiency standard for the many cases where a thorough system energy analysis is not feasible. Use of even an imperfect IPLV/NPLV rating results in real and significant energy savings for a large number of projects. It is one thing to suggest that a full hour-by-hour energy analysis is the preferred method for evaluating the energy performance of a chilled water plant. It is quite another to leap to the conclusion that these authors assert when they state, “IPLV/NPLV should not be used as an efficiency standard.” Most energy codes and standards already provide mechanisms that encourage the designer to use additional analysis methods beyond the IPLV/NPLV ratings to demonstrate energy savings exceeding the prescribed minimums. Are the authors trying to solve a problem that does not exist? The IPLV/NPLV rating provides a great benefit to the HVAC design community as a simplified means of comparing the energy performance of different chiller options. It can be compared to the EPA miles per gallon ratings for cars. No one expects to get the exact mileage posted on the window of a new car. However, for most common situations, the car with the higher mileage rating will provide the best fuel economy. Just as a car’s miles per gallon rating does not describe how it handles, the IPLV/NPLV 12 ASHRAE Journal Authors Respond Mr. Good, we appreciate your comments on the article. Thank you for delivering a different vantage point on the subject. We agree in many ways with your comments. IPLV/NPLV is widely used in our industry, and will continue to be used for years to come. Codes and energy standards are important, and the industry needs a simple methodology to ensure that a minimum standard of performance is met for buildings. This position is not challenged. Today’s marketplace has many green building programs emerging (LEED, Green Globes, and ENERGy STAR) that require minimum efficiency levels for full- and part-load values, but also require the overall building performance to be improved. This is a much more robust method of ensuring efficiency improvements, and often results in full building energy analysis. Further, after construction and building commissioning, the design energy consumption is to be validated, underpinning the importance of more robust modeling efforts and the reduction in rules of thumb and singular numbers for evaluation of all contributing components to energy consumption. As pointed out in the article, the energy analysis tool used to generate the analysis and findings was TRACE 700. For over 28 years, it has been a well-respected tool within the industry, and analysis completed using the tool is accepted by USGBC. In fact, many LEED-certified buildings have used TRACE as the evaluation tool. During the analysis for the article, effort was taken to ensure an “average” performance example is given. This analysis is not intended as a substitute for independent energy studies. Nor does it offer new values for use into the IPLV/NPLV standard. Rather, it gives an illustration of a better way to look at the issue. See Authors Respond, Page 14 March 2010 does not explain all of the performance complexities inherent in a complete chilled water plant. Ultimately, it is the skill and experience of the designer that will determine the chiller plant performance. The IPLV/NPLV rating is one of the many tools available to assist the designer in this effort. To discredit the IPLV/NPLV rating, the authors bluntly attack the weightings in the formula and state “It quickly becomes evident that the two-chiller plant operation does not align with the values of IPLV/NPLV,” and further, “The global analysis figures show how the AHRI loading points misrepresent a typical chiller plant.” To support this case they offer extensive graphs and tables of data generated by a proprietary energy analysis tool. When presenting the results of their analysis, they appear to combine operating hours for ranges of chiller operation that are not the same bin ranges used in the original derivation of the IPLV/NPLV formula (reference: AHRI Standard 550/590 Appendix D). For instance, the percentage of operating hours at high load and high lift are misrepresented by the claim “as stated by the AHRI IPLV definition, all operating points above 75% load and warmer than 75°F water are assumed to occur only 1% of the time.”Appendix D shows that the bin range corresponding to 1% of the operating ton-hours is for loads of 93% and greater, which correspond to 82.2°F See Chiller Ratings, Page 14 ashrae.org

ASHRAE Journal - March 2010

Table of Contents for the Digital Edition of ASHRAE Journal - March 2010