ASHRAE Journal - December 2011 - 13

the zone as cooling loads decrease (whereas the ACB system delivers a constant quantity of primary air to the zone), 2) the VAV system beneﬁts from system load diversity (whereas the constant-volume ACB system delivers design primary airﬂow to every zone all the time), and 3) the VAV fan energy use decreases exponentially as fan airﬂow is reduced. With regard to heating energy, the high-performance VAV system beneﬁted from the use of demand-controlled ventilation, which was not implemented for the ACB system. I disagree with the statement that, for ACB systems, primary airﬂow is “typically the minimum required for ventilation.” This might be the case for some applications, but in many cases the primary airﬂow required for either space dehumidiﬁcation or for space cooling (via induction) is often higher than the minimum ventilation requirement. This has been my experience from working with design engineers from Europe and Australia, where chilled beams have been used for decades. When an ACB system delivers primary air at a cool temperature (55°F in this example), it also uses “reheat” when the zone cooling load drops to the point at which the chilled-water valve on the chilled beam is closed and the cool primary air begins to overcool the space. Heat is either added by opening the hot-water valve in the ACB or by activating the separate heating system. If the primary air is instead

delivered at a “neutral” temperature (such as 70°F), the fan and pump energy use of the system increases signiﬁcantly, since the chilled beams are required to remove nearly the entire space cooling load. Finally, if the primary air system serving the active chilled beams is indeed comprised of 100% outdoor air, system ventilation efﬁciency is still not 100%. A 100% OA system also has ventilation inefﬁciency; it’s just not calculated in the Ventilation Rate Procedure of ASHRAE Standard 62.1. System ventilation efﬁciency is the sum of the outdoor airﬂows required in all breathing zones (Vou) divided by the outdoor airﬂow that enters the system-level intake (Vot). For a 100% OA system, the intake airﬂow is equal to the sum of peak zone outdoor airﬂows, each of which is based on peak zone population, with no allowance for system-wide population diversity. As a result, system-level intake airﬂow is based on all zones being at peak population simultaneously, even though this is typically not the case. For a more detailed comparison of ACB systems to VAV systems, I direct you to the newsletter titled “Understanding Chilled Beam Systems,” which can be downloaded from www. tinyurl.com/murphychilledbeam. John Murphy, Member ASHRAE, La Crosse, Wis.

www.info.hotims.com/37993-3

ASHRAE Journal - December 2011

Table of Contents for the Digital Edition of ASHRAE Journal - December 2011

ASHRAE Journal - December 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Testing for Leaks in Underfloor Plenums
Optimizing Design & Control of Chilled Water Plants, Part 3: Pipe Sizing and Optimizing ΔT
Performance of Combination Hydronic Systems
Data Center Environments: ASHRAE’s Evolving Thermal Guidelines
Technology Award Case Studies:
Beyond the Envelope
Medical Center Rx
Special Section
New Product Preview
Standing Columns
Emerging Technologies
IAQ Applications
Washington Report
People
Products
2011 ASHRAE Journal Indices
Classified Advertising
Advertisers Index
ASHRAE Journal - December 2011 - ASHRAE Journal - December 2011
ASHRAE Journal - December 2011 - Cover2
ASHRAE Journal - December 2011 - 1
ASHRAE Journal - December 2011 - 2
ASHRAE Journal - December 2011 - Contents
ASHRAE Journal - December 2011 - Commentary
ASHRAE Journal - December 2011 - 5
ASHRAE Journal - December 2011 - Industry News
ASHRAE Journal - December 2011 - 7
ASHRAE Journal - December 2011 - 8
ASHRAE Journal - December 2011 - Letters
ASHRAE Journal - December 2011 - 10
ASHRAE Journal - December 2011 - 11
ASHRAE Journal - December 2011 - 12
ASHRAE Journal - December 2011 - 13
ASHRAE Journal - December 2011 - Meetings and Shows
ASHRAE Journal - December 2011 - 15
ASHRAE Journal - December 2011 - Testing for Leaks in Underfloor Plenums
ASHRAE Journal - December 2011 - 17
ASHRAE Journal - December 2011 - 18
ASHRAE Journal - December 2011 - 19
ASHRAE Journal - December 2011 - 20
ASHRAE Journal - December 2011 - 21
ASHRAE Journal - December 2011 - Optimizing Design & Control of Chilled Water Plants, Part 3: Pipe Sizing and Optimizing ΔT
ASHRAE Journal - December 2011 - 23
ASHRAE Journal - December 2011 - 24
ASHRAE Journal - December 2011 - 25
ASHRAE Journal - December 2011 - 26
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ASHRAE Journal - December 2011 - 32a
ASHRAE Journal - December 2011 - 32b
ASHRAE Journal - December 2011 - 33
ASHRAE Journal - December 2011 - 34
ASHRAE Journal - December 2011 - 35
ASHRAE Journal - December 2011 - Performance of Combination Hydronic Systems
ASHRAE Journal - December 2011 - 37
ASHRAE Journal - December 2011 - 38
ASHRAE Journal - December 2011 - 39
ASHRAE Journal - December 2011 - 40
ASHRAE Journal - December 2011 - 41
ASHRAE Journal - December 2011 - Data Center Environments: ASHRAE’s Evolving Thermal Guidelines
ASHRAE Journal - December 2011 - 43
ASHRAE Journal - December 2011 - 44
ASHRAE Journal - December 2011 - 45
ASHRAE Journal - December 2011 - 46
ASHRAE Journal - December 2011 - 47
ASHRAE Journal - December 2011 - 48
ASHRAE Journal - December 2011 - 49
ASHRAE Journal - December 2011 - Beyond the Envelope
ASHRAE Journal - December 2011 - 51
ASHRAE Journal - December 2011 - 52
ASHRAE Journal - December 2011 - 53
ASHRAE Journal - December 2011 - 54
ASHRAE Journal - December 2011 - 55
ASHRAE Journal - December 2011 - Medical Center Rx
ASHRAE Journal - December 2011 - 57
ASHRAE Journal - December 2011 - 58
ASHRAE Journal - December 2011 - 59
ASHRAE Journal - December 2011 - 60
ASHRAE Journal - December 2011 - New Product Preview
ASHRAE Journal - December 2011 - 62
ASHRAE Journal - December 2011 - 63
ASHRAE Journal - December 2011 - 64
ASHRAE Journal - December 2011 - 65
ASHRAE Journal - December 2011 - 66
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ASHRAE Journal - December 2011 - 96
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ASHRAE Journal - December 2011 - 115
ASHRAE Journal - December 2011 - 116
ASHRAE Journal - December 2011 - 117
ASHRAE Journal - December 2011 - Emerging Technologies
ASHRAE Journal - December 2011 - 119
ASHRAE Journal - December 2011 - 120
ASHRAE Journal - December 2011 - IAQ Applications
ASHRAE Journal - December 2011 - 122
ASHRAE Journal - December 2011 - 123
ASHRAE Journal - December 2011 - 124
ASHRAE Journal - December 2011 - 125
ASHRAE Journal - December 2011 - Washington Report
ASHRAE Journal - December 2011 - 127
ASHRAE Journal - December 2011 - People
ASHRAE Journal - December 2011 - Products
ASHRAE Journal - December 2011 - 2011 ASHRAE Journal Indices
ASHRAE Journal - December 2011 - 131
ASHRAE Journal - December 2011 - 132
ASHRAE Journal - December 2011 - 133
ASHRAE Journal - December 2011 - Classified Advertising
ASHRAE Journal - December 2011 - 135
ASHRAE Journal - December 2011 - Advertisers Index
ASHRAE Journal - December 2011 - Cover3
ASHRAE Journal - December 2011 - Cover4
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