ASHRAE Journal - June 2012 - 70

Optimum staging for variable speed CW pumps was found to correlate very well to CW flow with 60% of the total design flow as the staging point, i.e., one pump should operate when the CWFR is below 60% and two pumps should operate when CWFR is above 60%, with a time delay to prevent short cycling. Optimum staging for constant speed pumps was found to vary with both CWRT-CHWST difference and with PLR, but with fairly weak correlations and relatively small energy impact regardless of logic. For simplicity, constant speed CW pumps should simply be staged with the chillers.

Speed 100% Two Chillers Surge Region Refrigerant Lift Two Chillers 60% 70% 90% One Chiller 80% One Chiller

Chiller Staging

Figure 9 (Page 68) shows the optimum number of chillers that should be run plotted against plant load for variable speed centrifugal chillers. The graph shows that it is often optimum to operate two chillers as low as 25% of overall plant load. This result may seem somewhat counterintuitive; conventional wisdom is to run as few chillers as possible. That is true for fixed speed chillers, but not for variable speed chillers, which are more efficient at low loads when condenser water temperatures are low. Figure 9 shows that staging chillers based on load alone will not optimize performance since there is a fairly wide range where either one or two chillers should be operated.

Load

Figure 10: Possible surge problem staging by load only. There is also another problem with staging based on load alone: it can cause the chillers to operate in surge. This can be seen in Figure 10, which schematically shows centrifugal chiller load vs. lift, defined as the difference between condenser and evaporator refrigerant temperature. If two chillers are operated when the refrigerant lift is high (red line), the chillers will operate in the surge region. To avoid

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ASHRAE Journal - June 2012

Table of Contents for the Digital Edition of ASHRAE Journal - June 2012

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Minimum Outdoor Airflow Using the IAQ Procedure
Basics of Atrium Smoke Control
 Long-Term Commercial GSHP Performance Part 1: Project Overview and Loop Circuit Types
Optimizing Design & Control of Chilled Water Plants Part 5: Optimized Control Sequences
Technology Award Case Studies:
Retail in Cold Climate
Rethinking Hospital Design
Standing Columns
Refrigeration Applications
Emerging Technologies
Engineering Applications
Washington Report
Products
IAQ Applications
Special Products
Classified Advertising
Advertisers Index
ASHRAE Journal - June 2012 - Intro
ASHRAE Journal - June 2012 - Cover1
ASHRAE Journal - June 2012 - Cover2
ASHRAE Journal - June 2012 - 1
ASHRAE Journal - June 2012 - 2
ASHRAE Journal - June 2012 - Contents
ASHRAE Journal - June 2012 - Commentary
ASHRAE Journal - June 2012 - 5
ASHRAE Journal - June 2012 - Industry News
ASHRAE Journal - June 2012 - 7
ASHRAE Journal - June 2012 - 8
ASHRAE Journal - June 2012 - 9
ASHRAE Journal - June 2012 - 10
ASHRAE Journal - June 2012 - 11
ASHRAE Journal - June 2012 - 12
ASHRAE Journal - June 2012 - 13
ASHRAE Journal - June 2012 - Letters
ASHRAE Journal - June 2012 - 15
ASHRAE Journal - June 2012 - 16
ASHRAE Journal - June 2012 - 17
ASHRAE Journal - June 2012 - 18
ASHRAE Journal - June 2012 - 19
ASHRAE Journal - June 2012 - 20
ASHRAE Journal - June 2012 - 21
ASHRAE Journal - June 2012 - 22
ASHRAE Journal - June 2012 - 23
ASHRAE Journal - June 2012 - Meetings and Shows
ASHRAE Journal - June 2012 - 25
ASHRAE Journal - June 2012 - Minimum Outdoor Airflow Using the IAQ Procedure
ASHRAE Journal - June 2012 - 27
ASHRAE Journal - June 2012 - 28
ASHRAE Journal - June 2012 - 29
ASHRAE Journal - June 2012 - 30
ASHRAE Journal - June 2012 - 31
ASHRAE Journal - June 2012 - 32
ASHRAE Journal - June 2012 - 33
ASHRAE Journal - June 2012 - 34
ASHRAE Journal - June 2012 - 35
ASHRAE Journal - June 2012 - Basics of Atrium Smoke Control
ASHRAE Journal - June 2012 - 37
ASHRAE Journal - June 2012 - 38
ASHRAE Journal - June 2012 - 39
ASHRAE Journal - June 2012 - 40
ASHRAE Journal - June 2012 - 41
ASHRAE Journal - June 2012 - 42
ASHRAE Journal - June 2012 - 43
ASHRAE Journal - June 2012 - 44
ASHRAE Journal - June 2012 - 45
ASHRAE Journal - June 2012 - 46
ASHRAE Journal - June 2012 - 47
ASHRAE Journal - June 2012 -  Long-Term Commercial GSHP Performance Part 1: Project Overview and Loop Circuit Types
ASHRAE Journal - June 2012 - 49
ASHRAE Journal - June 2012 - 50
ASHRAE Journal - June 2012 - 51
ASHRAE Journal - June 2012 - 52
ASHRAE Journal - June 2012 - 53
ASHRAE Journal - June 2012 - 54
ASHRAE Journal - June 2012 - 55
ASHRAE Journal - June 2012 - Optimizing Design & Control of Chilled Water Plants Part 5: Optimized Control Sequences
ASHRAE Journal - June 2012 - 57
ASHRAE Journal - June 2012 - 58
ASHRAE Journal - June 2012 - 59
ASHRAE Journal - June 2012 - 60
ASHRAE Journal - June 2012 - 61
ASHRAE Journal - June 2012 - 62
ASHRAE Journal - June 2012 - 63
ASHRAE Journal - June 2012 - 64
ASHRAE Journal - June 2012 - 65
ASHRAE Journal - June 2012 - 66
ASHRAE Journal - June 2012 - 67
ASHRAE Journal - June 2012 - 68
ASHRAE Journal - June 2012 - 69
ASHRAE Journal - June 2012 - 70
ASHRAE Journal - June 2012 - 71
ASHRAE Journal - June 2012 - 72
ASHRAE Journal - June 2012 - 73
ASHRAE Journal - June 2012 - 74
ASHRAE Journal - June 2012 - 75
ASHRAE Journal - June 2012 - Retail in Cold Climate
ASHRAE Journal - June 2012 - 77
ASHRAE Journal - June 2012 - 78
ASHRAE Journal - June 2012 - 79
ASHRAE Journal - June 2012 - 80
ASHRAE Journal - June 2012 - 81
ASHRAE Journal - June 2012 - 82
ASHRAE Journal - June 2012 - 83
ASHRAE Journal - June 2012 - Rethinking Hospital Design
ASHRAE Journal - June 2012 - 85
ASHRAE Journal - June 2012 - 86
ASHRAE Journal - June 2012 - 87
ASHRAE Journal - June 2012 - 88
ASHRAE Journal - June 2012 - 89
ASHRAE Journal - June 2012 - 90
ASHRAE Journal - June 2012 - 91
ASHRAE Journal - June 2012 - 92
ASHRAE Journal - June 2012 - 93
ASHRAE Journal - June 2012 - Refrigeration Applications
ASHRAE Journal - June 2012 - 95
ASHRAE Journal - June 2012 - Emerging Technologies
ASHRAE Journal - June 2012 - 97
ASHRAE Journal - June 2012 - 98
ASHRAE Journal - June 2012 - 99
ASHRAE Journal - June 2012 - Engineering Applications
ASHRAE Journal - June 2012 - 101
ASHRAE Journal - June 2012 - 102
ASHRAE Journal - June 2012 - Washington Report
ASHRAE Journal - June 2012 - Products
ASHRAE Journal - June 2012 - 105
ASHRAE Journal - June 2012 - IAQ Applications
ASHRAE Journal - June 2012 - 107
ASHRAE Journal - June 2012 - 108
ASHRAE Journal - June 2012 - Special Products
ASHRAE Journal - June 2012 - 110
ASHRAE Journal - June 2012 - Classified Advertising
ASHRAE Journal - June 2012 - Advertisers Index
ASHRAE Journal - June 2012 - Cover3
ASHRAE Journal - June 2012 - Cover4
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