ASHRAE Journal - December 2009 - 25

Figure 5: Global climates included in study. From a physics perspective, these compressors raise the pressure of the refrigerant by imparting velocity, or dynamic energy, using a rotating impeller, and convert it to pressure energy. In contrast a screw or scroll compressors are positive-displacement compressors where two mechanical pieces mesh together to trap refrigerant vapor, and reduce the volume of the refrigerant to a discharge point. This difference between dynamic and positive displacement compression leads to considerable difference in response to variations in speed (or hertz) that can be imparted by a variable-speed drive. By changing the speed of a positive displacement compressor, the amount of work, or load, handled is varied. Conversely, this impact is not seen with a centrifugal chiller. In trying to simply state the impact for a centrifugal chiller, reducing condenser water temperatures or increasing chilled water supply temperature will reduce the lift, or work, the compressor has to provide. It does not reduce the amount of load or refrigerant compression required. Thus, the dominant driver of any hertz variation, or speed change, of the compressor is the amount of lift the compressor can stably handle, not the load. Recall that IPLV/NPLV is dictated by four distinct operating buckets incorporating simultaneous load and lift reduction (Figure 4, p. 24). So, while variable-speed drives can offer significant savings in the right application for centrifugal chillDecember 2009 ers, they give the perception of efficiency gains when the IPLV/ NPLV formula is applied—a recipe for selling more drives, and not necessarily offering real energy savings. IPLV/NPLV Operating Buckets Fact 4: The intent of IPLV is to compare unloading characteristics, not to infer economic savings. Appendix D2 of ARI Standard 550/590 states: “it is best to use a comprehensive analysis that reflects the actual weather data, building load characteristics, operational hours, economizer capabilities and energy drawn by auxiliaries such as pumps and cooling towers, when calculating the chiller and system efficiency.”1 A detailed hour-by-hour energy analysis is needed to determine economic savings. Let’s take a look at a detailed analysis and compare it to IPLV. At the outset of this analysis (using TRACE 700), a high degree of importance was placed in offering a global perspective to demonstrate the many varying global climates that impact both the load and lift aspects of the chiller plants serving the building HVAC systems. The cities modeled are shown in Figure 5. The following are fixed and variable assumptions made for this analysis: • Typical commercial office space with corresponding diversity; • Two-chiller plant with dedicated towers per chiller; ASHRAE Journal 25

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
ASHRAE Journal - December 2009 - 71
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ASHRAE Journal - December 2009 - 84
ASHRAE Journal - December 2009 - 85
ASHRAE Journal - December 2009 - 86
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ASHRAE Journal - December 2009 - 88
ASHRAE Journal - December 2009 - 89
ASHRAE Journal - December 2009 - 90
ASHRAE Journal - December 2009 - 91
ASHRAE Journal - December 2009 - 92
ASHRAE Journal - December 2009 - 93
ASHRAE Journal - December 2009 - 94
ASHRAE Journal - December 2009 - 95
ASHRAE Journal - December 2009 - 96
ASHRAE Journal - December 2009 - 97
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ASHRAE Journal - December 2009 - 100
ASHRAE Journal - December 2009 - 101
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ASHRAE Journal - December 2009 - 103
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ASHRAE Journal - December 2009 - 105
ASHRAE Journal - December 2009 - 106
ASHRAE Journal - December 2009 - 107
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ASHRAE Journal - December 2009 - 109
ASHRAE Journal - December 2009 - 110
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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