TAB Journal Fall 2020 - 3

performance from the submittal is shown in the table and
Figure 1 on pg. 4.
The process air was traversed at the discharge duct connecting
to the surgery AHU. The design of 4,000 cubic feet per
minute (CFM) was easily set and confirmed using the traverse
and variable frequency drive. The regen air was more difficult
to confirm because of the configuration of the unit. The intake
had a small rain hood, the discharge was very turbulent with
only louvers on the side of the unit, and the filter bank was
close to the fan discharge. Comparing velocity readings taken
at both the filter bank and in windy conditions at the regen
intake hood yielded airflows ranging from 1,750 - 3,500 CFM.
The humidity leaving the desiccant wheel (C on Figure 1)
was not reaching design, achieving only 42 percent relative
humidity (RH), and thus leaving the process side at over 60
percent. The regen temperature was not at design either; the
regen steam coil was only measuring 195°F. The hospital and
the engineer were convinced the regen airflow was too high.
The desiccant wheel manufacturer's representative suggested
using the pressure drop across the wheel to calculate the airflow
of the regen air. This is the method they use at the factory

to test airflow prior to shipping. The process air measured
pressure drop matched the submittal at 0.72 in. water gauge
(w.g.) and confirmed the duct traverse at 4,000 CFM. The regen
air pressure drop measured 0.28 in. w.g. Per the submittal, the
pressure drop should be 0.55 in. w.g. At design flow, the in. w.g.
calculated to 1,800 CFM, confirming the regen airflow was, in
fact, low. The hospital admitted they had changed the motor
and fan sheaves a few months before to increase the regen
temperature by decreasing the suspected high regen airflow.
The original sheaves were found and replaced. The wheel
pressure drop was retested and now confirmed back up to
0.55 in. w.g. The regen airflow now confirmed at 2,500 CFM.
The regen temperature was still only 188°F versus a design of
213°F and not taking the moisture off the new desiccant wheel.
The steam control valve was removed and found to be only
partially opening. The steam control valve was replaced and
220°F was achieved. The process air was now operating and
testing as designed. The operating rooms quickly recovered by
recording room conditions of 62°F at 50 percent humidity.
Reflecting back on the steps taken to get the desiccant
dehumidification unit back to design conditions; the problem

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TAB Journal Fall 2020

Table of Contents for the Digital Edition of TAB Journal Fall 2020

TAB Journal Fall 2020 - Cover1
TAB Journal Fall 2020 - Cover2
TAB Journal Fall 2020 - 1
TAB Journal Fall 2020 - 2
TAB Journal Fall 2020 - 3
TAB Journal Fall 2020 - 4
TAB Journal Fall 2020 - 5
TAB Journal Fall 2020 - 6
TAB Journal Fall 2020 - 7
TAB Journal Fall 2020 - 8
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TAB Journal Fall 2020 - Cover3
TAB Journal Fall 2020 - Cover4