ASHRAE Journal - December 2009 - 30

The Impact of Chiller Count In this analysis we also focused on multiple chillers versus 10 location-dependent weather and yielded the same conclusion: 8 the highest load and lift bucket remains the most prominent. 6 Figure 9 shows a variation in the number of chillers in Chicago where condenser water relief is certainly attainable, as the vastly 4 contrasting seasons offer many shoulder months of operational 2 hours with cool temperatures. Again, each plot point represents 0 one hour of chiller operation during the year. Conductive Loads Figure 9 shows that as the number of chillers in a plant grows, –2 Ventilation Loads Internal Loads so does the importance of the higher bucket. Even a single Supply Fan Heat –4 Exhaust Heat chiller plant analysis in Chicago where relief is imminent, the Solar Load –6 number does not even get close to 1%. J F M A M J J A S O N D Month Table 2 provides a summary of the multiple chillers plants for the geographic region. Once again, it clearly shows that Figure 8: Twenty-story office building in St. Louis; monthly load a holistic approach to selecting chillers should be standard component profile.5 practice, rather than reliance on a single number. In this particular analysis, the chiller plant is a solid candidate for a single variable-frequency drive on a single centrifugal chiller, as it has both a significant number of hours at low operating tons (part load) and low condenser water temperatures (part lift). Conversely, as chillers are added, the plant becomes increasingly less suitable as a candidate for variable-frequency drives. A better option might be to invest in more heat exchanger surface to maximize operational performance at high loads and high condenser water temperatures, or possibly provide a single drive on Figure 9: Impact of multiple chillers in Chicago. only one chiller for the low-load, 100% Load with 75% Load with 50% Load with 25% Load with low-lift conditions. Bottom line: 85°F Entering 75°F Entering 65°F Entering 65°F Entering the right design can be speculaCondenser Condenser Condenser Condenser tive, but a detailed analysis is the Water Temp. Water Temp. Water Temp. Water Temp. only prudent method to determine AHRI Percentage 1 42 45 12 the most economic and energy1 Chiller Plant 23 33 4 40 efficient plant. 2 Chiller Plant Btu/h (millions) 41 53 29 26 9 9 21 12 Conclusions IPLV/NPLV should not be used 4 Chiller Plant 62 24 7 8 as an efficiency standard. When Average 44.8 28.0 7.3 20.3 looking at the real operating points of a chiller, it becomes inTable 2: Average performance weighting in Chicago. creasingly obvious that high-load operating points are extremely important. It also becomes efficiency standards (Figure 10, p. 32). These standards require obvious that using the ARI 550/590 Standard can lead to a the user to meet both full-load and a part-load performance misrepresentation of where two chillers really operate. IPLV/ measures. Good full-load performance is critical to minimize NPLV methodology makes sense when determining minimum peak energy consumption, which impacts building owners efficiency requirements such as in the Standard 90.1 or IECC around the world as these peak charges establish demand 30 ASHRAE Journal ashrae.org December 2009 3 Chiller Plant

ASHRAE Journal - December 2009

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