ASHRAE Journal - August 2013 - 14

letters

VAV Reheat Versus Active
Chilled Beams & DOAS

(BHP) of 17.5, which is at 40% of the design condition, not
20% (8.8 BHP) as Figure 10 implies.
The following are additional factors that we believe bias
The AHRI Chilled Beams Section has some serious conthe comparison.
cerns regarding May 2013’s “VAV Reheat Versus Active
• The ACB system is designed for primary airflow rates
Chilled Beams & DOAS.”
that are almost double those typically required in office
In particular, the article did not evaluate comparable
building applications.
systems. The active chilled beam (ACB) system described
• The ACB system uses a dedicated outdoor air system
in the article is a conservative, non-optimized design. It is air-handling unit (AHU) (not usually applicable to office
compared by the authors to an all-air variable air volume buildings) with no heat recovery, which compounds the
reheat (VAVR) system whose suggested performance is
energy effects of the unnecessarily high primary airflow
unachievable under real-life conditions and at best mar- rates, resulting in extremely high ventilation air-condiginally code compliant. Neither design is representative of tioning costs.
a “typical” system.
• Main air duct velocities in the ACB system are limited
Because this article does not provide a fair comparison
to 900 fpm, while the VAVR system uses 2,000 fpm and
between the two systems, it may lead some readers to
higher. This unbalanced comparison minimizes the
incorrect conclusions regarding the relative merits of both potential savings included in the ductwork and insulatypes of systems.
tion of the ACB system.
The article provides little documentation on the ACB
• The number of beams used in the example is excessive.
system with respect to temperature control, thermal
For example, the ACB system uses 12 (seemingly 8 ft long)
zoning, and condensation prevention, all of which
beams in the open office area to deliver 340 cfm, equatcan significantly affect its first cost. The VAVR system
ing 3.5 cfm/ft of beam. The information we have from our
to which the ACB system is compared is unrealistic,
members that manufacture such systems is that only four
because the part-load operation that is described—and
beams would be needed to service this area.
which largely determines its energy performance—is not
• While the VAVR system’s primary air temperature is
achievable for the following reasons:
reset according to outside temperature and space sensi• The article suggests that certain zones served by
ble loads, no such provision was included in the ACB systhe VAVR system can have little or no direct air suptem. Had the beam system been provided with such flexply and still maintain acceptable thermal comfort
ibility, it is likely that the primary air for the described
and IAQ conditions. We are unsure how this would
design could have been delivered at room temperature
be possible without raising code compliance issues/ most of the time, significantly reducing the AHU cooling
questions.
energy.
• The supply fan power relationships in Figure 10
In summary, the conclusions drawn in the article are
suggest the VAVR system’s horsepower is reduced in
inconsistent with the predicted and measured perforproportion to the cube of the airflow reduction. The
mance of typical ACB or all-air VAV systems. We find the
only way this can be achieved is for the fan to “ride the information presented in the article to be misleading,
system curve,” meaning no dampers ever move. This
and are disappointed that it was presented in such a
would never happen in an actual system in a building. highly regarded industry publication as ASHRAE Journal.
If the overall fan airflow reduction were 40%, yet one
We welcome further discussion on the topic.
Ken Loudermilk, P.E., Member ASHRAE, Chairman, AHRI Chilled Beams Section
zone still required 80% of its design airflow, the static
pressure requirement of that zone would limit the sysThe Author Responds
tem static pressure reset.
We agree that the ACB/DOAS system was not optiApplying the fan laws to that specific zone, the static
mum, but it was submitted by an experienced firm that
pressure requirement for that terminal could only be
reduced by 36%. Using Table 1 data, a 40% system airflow has designed several ACB/DOAS systems with the same
approach in the past. The hybrid design fixed many of
reduction combined with a 36% reduction in the fan
the design deficiencies of the ACB/DOAS design, yet it too
static pressure would result in a fan break horse power

14

ASHRAE JouRnAl

ashrae.org

Au g ust 2013



ASHRAE Journal - August 2013

Table of Contents for the Digital Edition of ASHRAE Journal - August 2013

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Presidential Address: Shaping the Next
Technical vs. Process Commissioning: Owner’s Project Requirements
Energy Codes & Standards, Part 2
Shaping the Next: Indoor Air Quality
Cutting-Edge Renovation
System Efficiency for Natural Refrigerants
Standing Columns
Building Sciences
InfoCenter
Data Centers
Emerging Technologies
HVAC Applications
Energy Modeling
Refrigeration Applications
Product Showplace
People
Classified Advertising
Advertisers Index
ASHRAE Journal - August 2013 - Cover1
ASHRAE Journal - August 2013 - Cover2
ASHRAE Journal - August 2013 - 1
ASHRAE Journal - August 2013 - 2
ASHRAE Journal - August 2013 - Contents
ASHRAE Journal - August 2013 - Commentary
ASHRAE Journal - August 2013 - 5
ASHRAE Journal - August 2013 - Industry News
ASHRAE Journal - August 2013 - 7
ASHRAE Journal - August 2013 - 8
ASHRAE Journal - August 2013 - 9
ASHRAE Journal - August 2013 - 10
ASHRAE Journal - August 2013 - 11
ASHRAE Journal - August 2013 - 12
ASHRAE Journal - August 2013 - 13
ASHRAE Journal - August 2013 - Letters
ASHRAE Journal - August 2013 - 15
ASHRAE Journal - August 2013 - 16
ASHRAE Journal - August 2013 - 17
ASHRAE Journal - August 2013 - 18
ASHRAE Journal - August 2013 - 19
ASHRAE Journal - August 2013 - Meetings and Shows
ASHRAE Journal - August 2013 - 21
ASHRAE Journal - August 2013 - Presidential Address: Shaping the Next
ASHRAE Journal - August 2013 - 23
ASHRAE Journal - August 2013 - 24
ASHRAE Journal - August 2013 - 25
ASHRAE Journal - August 2013 - 26
ASHRAE Journal - August 2013 - 27
ASHRAE Journal - August 2013 - 28
ASHRAE Journal - August 2013 - 29
ASHRAE Journal - August 2013 - 30
ASHRAE Journal - August 2013 - 31
ASHRAE Journal - August 2013 - Technical vs. Process Commissioning: Owner’s Project Requirements
ASHRAE Journal - August 2013 - 33
ASHRAE Journal - August 2013 - 34
ASHRAE Journal - August 2013 - 35
ASHRAE Journal - August 2013 - 36
ASHRAE Journal - August 2013 - 37
ASHRAE Journal - August 2013 - 38
ASHRAE Journal - August 2013 - 39
ASHRAE Journal - August 2013 - Energy Codes & Standards, Part 2
ASHRAE Journal - August 2013 - 41
ASHRAE Journal - August 2013 - 42
ASHRAE Journal - August 2013 - 43
ASHRAE Journal - August 2013 - 44
ASHRAE Journal - August 2013 - 45
ASHRAE Journal - August 2013 - 46
ASHRAE Journal - August 2013 - 47
ASHRAE Journal - August 2013 - 48
ASHRAE Journal - August 2013 - 49
ASHRAE Journal - August 2013 - Shaping the Next: Indoor Air Quality
ASHRAE Journal - August 2013 - 51
ASHRAE Journal - August 2013 - 52
ASHRAE Journal - August 2013 - 53
ASHRAE Journal - August 2013 - 54
ASHRAE Journal - August 2013 - 55
ASHRAE Journal - August 2013 - 56
ASHRAE Journal - August 2013 - 57
ASHRAE Journal - August 2013 - 58
ASHRAE Journal - August 2013 - 59
ASHRAE Journal - August 2013 - Cutting-Edge Renovation
ASHRAE Journal - August 2013 - 61
ASHRAE Journal - August 2013 - 62
ASHRAE Journal - August 2013 - 63
ASHRAE Journal - August 2013 - 64
ASHRAE Journal - August 2013 - 65
ASHRAE Journal - August 2013 - System Efficiency for Natural Refrigerants
ASHRAE Journal - August 2013 - 67
ASHRAE Journal - August 2013 - 68
ASHRAE Journal - August 2013 - 69
ASHRAE Journal - August 2013 - 70
ASHRAE Journal - August 2013 - 71
ASHRAE Journal - August 2013 - Building Sciences
ASHRAE Journal - August 2013 - 73
ASHRAE Journal - August 2013 - 74
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ASHRAE Journal - August 2013 - 76
ASHRAE Journal - August 2013 - 77
ASHRAE Journal - August 2013 - 78
ASHRAE Journal - August 2013 - 79
ASHRAE Journal - August 2013 - 80
ASHRAE Journal - August 2013 - 81
ASHRAE Journal - August 2013 - InfoCenter
ASHRAE Journal - August 2013 - 83
ASHRAE Journal - August 2013 - 84
ASHRAE Journal - August 2013 - 85
ASHRAE Journal - August 2013 - 86
ASHRAE Journal - August 2013 - 87
ASHRAE Journal - August 2013 - Data Centers
ASHRAE Journal - August 2013 - 89
ASHRAE Journal - August 2013 - 90
ASHRAE Journal - August 2013 - 91
ASHRAE Journal - August 2013 - Emerging Technologies
ASHRAE Journal - August 2013 - 93
ASHRAE Journal - August 2013 - 94
ASHRAE Journal - August 2013 - 95
ASHRAE Journal - August 2013 - HVAC Applications
ASHRAE Journal - August 2013 - 97
ASHRAE Journal - August 2013 - Energy Modeling
ASHRAE Journal - August 2013 - Refrigeration Applications
ASHRAE Journal - August 2013 - Product Showplace
ASHRAE Journal - August 2013 - 101
ASHRAE Journal - August 2013 - People
ASHRAE Journal - August 2013 - Classified Advertising
ASHRAE Journal - August 2013 - Advertisers Index
ASHRAE Journal - August 2013 - Cover3
ASHRAE Journal - August 2013 - Cover4
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