TAB Journal Spring 2018 - 9

Precision Test & Balance was recently contracted to perform
air balancing at a healthcare research facility. The HVAC
system design utilized Active Chilled Beams (ACB) divided
into zones. Outside Air (O.A.) was delivered to each chilled
beam by a Dedicated Outdoor Air System (DOAS) via
Constant Volume Supply (CVS) boxes.

T

ypically, the O.A. delivered to a chilled beam is measured via plenum pressure and
calculated by comparing those results with either a Flow Curve or a single design
pressure which can be converted using the affinity laws. This project appeared to be
cut and dry, but upon an initial "preliminary" balance meeting, the construction and design
team had expressed concerns about the accuracy of the pressure reading which was based
on a previous typical project, using the same overall design concept and same chilled beam
manufacturer. The GC, Design Engineer, control contractor and commissioning agent were
all part of the "previous typical" job and had stated that there was no way to ascertain a
proper CFM reading by taking a plenum pressure, that the converted CFM was artificially
low. They were unable to provide a proper response beyond that. During the meeting, they
outlined the new/altered procedure for balancing the chilled beams and CVS boxes. The
procedure was as follows:

1. Calibrate and adjust the CVS box by means of duct traverse
2. Read out the chilled beam plenum pressure
3. Balance the chilled beams proportionally by comparing the plenum pressure. Ignore the
actual CFM derived.
For obvious reasons, Precision Test & Balance found this concerning and possibly incorrect.
On day 1 of balancing a strong effort was made to diagnose and help explain why a simple
manufacturer's conversion which had been successful for so many years on so many products
was not acceptable on this project.
The initial plan was to attempt to balance the chilled beams as had originally been intended and
examine those results (see CVS-39-2). CVS-39-2 is rated for a design air flow rate of 1380 CFM
and services (4) chilled beams, each rated for 345 CFM at a 1.38" plenum pressure. Technicians
began by commanding the CVS to design and perform a traverse at the inlet of the box, calibrate
and adjust until within design. After confirming correct total flow, a readout was taken of the (4)
chilled beams. The following results were recorded:

IDENTIFICATION

DESIGN
PRESSURE
in wc

ACTUAL
PRESSURE
in wc

DESGN CFM

ACTUAL CFM

CVS-39-2

DUCT

TRAVERSE

1380

1390
(TRAVERSE)

ACB-37

1.38"

.74"

345

253

ACB-38

1.38"

.79"

345

261

ACB-39

1.38"

.71"

345

247

ACB-40

1.38"

.8"

345

263

1,380

1,024

ACB TOTAL

TAB Journal Spring 2018

9



Table of Contents for the Digital Edition of TAB Journal Spring 2018

TAB Journal Spring 2018 - Cover1
TAB Journal Spring 2018 - Cover2
TAB Journal Spring 2018 - 1
TAB Journal Spring 2018 - 2
TAB Journal Spring 2018 - 3
TAB Journal Spring 2018 - 4
TAB Journal Spring 2018 - 5
TAB Journal Spring 2018 - 6
TAB Journal Spring 2018 - 7
TAB Journal Spring 2018 - 8
TAB Journal Spring 2018 - 9
TAB Journal Spring 2018 - 10
TAB Journal Spring 2018 - 11
TAB Journal Spring 2018 - 12
TAB Journal Spring 2018 - 13
TAB Journal Spring 2018 - 14
TAB Journal Spring 2018 - 15
TAB Journal Spring 2018 - 16
TAB Journal Spring 2018 - 17
TAB Journal Spring 2018 - 18
TAB Journal Spring 2018 - 19
TAB Journal Spring 2018 - 20
TAB Journal Spring 2018 - Cover3
TAB Journal Spring 2018 - Cover4
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http://www.nxtbook.com/ygsreprints/AABC/g94804_tab_spring2018
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