ASHRAE Journal - December 2009 - 23

Fact 2: IPLV uses only four operating points with weighting factors intended to indicate the percentage of time a single chiller, following an averaged load profile, will operate at different loads and with assumed entering condenser water temperature. AHRI Definition of IPLV4 A= kW/ton at 85°F at 100% Load 1% A 42% B 45% C 12% D B= kW/ton at 75°F at 75% Load C= kW/ton at 65°F at 50% Load D= kW/ton at 65°F at 25% Load Temperatures: Expected Entering Tower Water AHRI Conditions: Chilled Water: 54°F/44°F Condenser Water: 3 gpm/ton 1% Figure 1 indicates how the different 42% 57% weighting factors are used to calculate the 75°F IPLV value. Points A, B, C, and D are the kW/ton performance levels at each of the 65°F four loading points. Note that the calculation assumes that only 1% of the chiller operation occurs at high loads and high condenser water temperatures. It’s often argued that chillers simply do not Figure 1: AHRI definition of integrated part load value (IPLV/NPLV).4 operate at 100% load; therefore, the full load rating doesn’t matter. In a world where most chillers are oversized, 100 this may be true at design conditions in single chiller applications. 90 While some chiller plant sequence of operations turn a second 80 chiller on before the first chiller ever reaches full peak load, many 70 comfort cooling applications do not start another chiller until the 60 system supply-water temperature rises above the desired setpoint 50 for a defined period of time. In such cases, chillers do operate at 40 full load. For this discussion, it is assumed that chillers do not Chiller 1 30 Chiller 2 operate the majority of their time fully loaded. Does this validate Chiller 3 20 the IPLV/NPLV choice of 1%? Let’s take a look. 10 Figure 2 illustrates that a chiller plant with three chillers has a 0 very different load profile. Another industry rating system, seasonal 10 20 30 40 50 60 70 80 90 100 energy efficiency ratio (SEER), uses a larger number of operating System Load % points to more fully represent the entire range of equipment operaFigure 2: Three-chiller plant unloading profile. tion. Unfortunately, SEER is not used for large chiller rating, and the current rating system is limited to four distinct operating points. Specifically, the IPLV/NPLV calculation assumes that 57% of The IPLV/NPLV formula assumes a condenser water-relief the operating hours of the chiller are at 50% load or less. Figure schedule that results in corresponding chiller relief, or reduc2 shows that only one chiller will run at less than 50% load, tion in power, as lowering the condenser water temperature and this occurs only when the entire chiller plant is unloaded has a dramatic impact on chiller performance. Note that lower to less than 16.7% capacity. This point alone demonstrates that temperature tower water is not typically “free.” Specifically, this the IPLV formula is not an accurate evaluation tool to use for is due to the additional energy expended at the cooling tower central chiller plant performance. to create lower temperatures entering the condenser (if those Now, some control sequences will let all operating chillers temperatures are even possible), as the entering condenser water unload further than 50% before shutting them off to avoid the temperature is limited by the outdoor wet-bulb temperature. need to turn that chiller back on if the load increases slightly. In many climates of the world, the lower range of the con(When two chillers are running, both would be allowed to unload denser water temperatures can never be reached during the to about 45% capacity before turning off Chiller 2. The resulting cooling season. Further, even when the physics allow low load on Chiller 1 would require it to operate at 90% capacity.) cooling tower return water temperatures to be achieved, the The specific load profile must be examined to reach a definite energy required by the tower may increase the overall plant conclusion; however, it can be stated that a smaller portion of energy consumed. Plant controls should focus on balancing the the chiller’s operating hours will actually occur at full load. energy equations such that the chiller plus ancillary equipment minimize the plant’s overall energy consumption, not singularly Assumptions on Cooling Tower Temperatures is the Key focusing on one variable.6 Figure 3, p. 24, illustrates how the IPLV/NPLV formula More importantly, the assumed reduction of entering con“buckets” the operating criteria. Let’s take a look at the percent- denser water temperature changes coincidently as the load is age of load versus the entering condenser water temperatures.4,5 reduced. The IPLV/NPLV calculation assumes that the chiller December 2009 Chiller Load % 85°F ASHRAE Journal

ASHRAE Journal - December 2009

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

ASHRAE Journal - December 2009
Contents
Commentary
Industry News
Letters
Meetings and Shows
ASHRAE Building EQ Program
Feature Articles
A Closer Look at Chiller Ratings
Cooling With Less Air: Using Underfloor Air Distribution and Chilled Beams
Cooling Concrete: Containerized Water Chilling Plant
Geothermal for Community Center
Anniversary Feature: Air-Recovery System Versus Conventional Air Conditioning
Technical Topics
New Product Preview
Products
Emerging Technologies
Washington Report
People
Special Products
2009 Feature Articles Indices
Classified Advertising
Advertisers Index
ASHRAE Journal - December 2009 - ASHRAE Journal - December 2009
ASHRAE Journal - December 2009 - Cover2
ASHRAE Journal - December 2009 - 1
ASHRAE Journal - December 2009 - 2
ASHRAE Journal - December 2009 - Contents
ASHRAE Journal - December 2009 - 4
ASHRAE Journal - December 2009 - Commentary
ASHRAE Journal - December 2009 - Industry News
ASHRAE Journal - December 2009 - 7
ASHRAE Journal - December 2009 - 8
ASHRAE Journal - December 2009 - 9
ASHRAE Journal - December 2009 - 10
ASHRAE Journal - December 2009 - Letters
ASHRAE Journal - December 2009 - 12
ASHRAE Journal - December 2009 - 13
ASHRAE Journal - December 2009 - 14
ASHRAE Journal - December 2009 - 15
ASHRAE Journal - December 2009 - Meetings and Shows
ASHRAE Journal - December 2009 - 17
ASHRAE Journal - December 2009 - Feature Articles
ASHRAE Journal - December 2009 - 19
ASHRAE Journal - December 2009 - 20
ASHRAE Journal - December 2009 - 21
ASHRAE Journal - December 2009 - A Closer Look at Chiller Ratings
ASHRAE Journal - December 2009 - 23
ASHRAE Journal - December 2009 - 24
ASHRAE Journal - December 2009 - 25
ASHRAE Journal - December 2009 - 26
ASHRAE Journal - December 2009 - 27
ASHRAE Journal - December 2009 - 28
ASHRAE Journal - December 2009 - 29
ASHRAE Journal - December 2009 - 30
ASHRAE Journal - December 2009 - 31
ASHRAE Journal - December 2009 - 32
ASHRAE Journal - December 2009 - 33
ASHRAE Journal - December 2009 - Cooling With Less Air: Using Underfloor Air Distribution and Chilled Beams
ASHRAE Journal - December 2009 - 35
ASHRAE Journal - December 2009 - 36
ASHRAE Journal - December 2009 - 37
ASHRAE Journal - December 2009 - 38
ASHRAE Journal - December 2009 - 39
ASHRAE Journal - December 2009 - 40
ASHRAE Journal - December 2009 - 41
ASHRAE Journal - December 2009 - Cooling Concrete: Containerized Water Chilling Plant
ASHRAE Journal - December 2009 - 43
ASHRAE Journal - December 2009 - 44
ASHRAE Journal - December 2009 - 45
ASHRAE Journal - December 2009 - 46
ASHRAE Journal - December 2009 - 47
ASHRAE Journal - December 2009 - Geothermal for Community Center
ASHRAE Journal - December 2009 - 49
ASHRAE Journal - December 2009 - 50
ASHRAE Journal - December 2009 - 51
ASHRAE Journal - December 2009 - Anniversary Feature: Air-Recovery System Versus Conventional Air Conditioning
ASHRAE Journal - December 2009 - 53
ASHRAE Journal - December 2009 - 54
ASHRAE Journal - December 2009 - 55
ASHRAE Journal - December 2009 - 56
ASHRAE Journal - December 2009 - 57
ASHRAE Journal - December 2009 - 58
ASHRAE Journal - December 2009 - 59
ASHRAE Journal - December 2009 - 60
ASHRAE Journal - December 2009 - 61
ASHRAE Journal - December 2009 - Technical Topics
ASHRAE Journal - December 2009 - 63
ASHRAE Journal - December 2009 - 64
ASHRAE Journal - December 2009 - New Product Preview
ASHRAE Journal - December 2009 - 66
ASHRAE Journal - December 2009 - 67
ASHRAE Journal - December 2009 - 68
ASHRAE Journal - December 2009 - 69
ASHRAE Journal - December 2009 - 70
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ASHRAE Journal - December 2009 - 111
ASHRAE Journal - December 2009 - 112
ASHRAE Journal - December 2009 - 113
ASHRAE Journal - December 2009 - Products
ASHRAE Journal - December 2009 - 115
ASHRAE Journal - December 2009 - 116
ASHRAE Journal - December 2009 - Emerging Technologies
ASHRAE Journal - December 2009 - Washington Report
ASHRAE Journal - December 2009 - 119
ASHRAE Journal - December 2009 - People
ASHRAE Journal - December 2009 - Special Products
ASHRAE Journal - December 2009 - 2009 Feature Articles Indices
ASHRAE Journal - December 2009 - 123
ASHRAE Journal - December 2009 - 124
ASHRAE Journal - December 2009 - 125
ASHRAE Journal - December 2009 - Classified Advertising
ASHRAE Journal - December 2009 - 127
ASHRAE Journal - December 2009 - Advertisers Index
ASHRAE Journal - December 2009 - Cover3
ASHRAE Journal - December 2009 - Cover4
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