ASHRAE Journal - March 2020 - 57

TECHNICAL FEATURE

This solution mimics the older
cleanroom designs, establishing a
miniature pressurized plenum in
the duct between the terminal unit
and the fan ﬁlter unit. Because the
terminal unit is no longer concerned
with satisfying airﬂow but rather
maintaining a constant static pressure, the fan ﬁlter unit can increase
its own motor speed only as far as it
needs to maintain proper airﬂow.
This once again allows for ﬁlter loading to be compensated for by the fan
ﬁlter unit, and eliminates noise or
shutdown issues caused by the two
control points.

Implementation of Constant Air
Pressure Control Scheme
To implement this control scheme,
a static pressure probe must be
inserted in the duct downstream
of the terminal unit. This static
pressure tap is to be connected to
the high side of the terminal unit
controller, and the low side of the
controller is to be left open to atmosphere so a single connection is
made (Figure 6). The fan ﬁlter unit
should be kept off until the connection has been made and the controller has been programmed.
After the tube connection has been
made and power has been brought
to the terminal unit controller, the
conversion from air volume ﬂow to
static pressure must be made. While
controllers typically rely on velocity
pressure and a K-factor to read air
volume ﬂow, here the unit is instead
seeking a desired static pressure. For
this, you must look to the calibration
chart that likely came with the terminal unit.
Using Figure 7 as an example, to
ﬁnd the correct airﬂow value to program into the controller, read from

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MARCH 2020

ashrae.org

ASHRAE JOURNAL

57

http://www.nortekair.com/contact-cleanroom http://www.ashrae.org

ASHRAE Journal - March 2020

Table of Contents for the Digital Edition of ASHRAE Journal - March 2020

Contents
ASHRAE Journal - March 2020 - Intro
ASHRAE Journal - March 2020 - Cover1
ASHRAE Journal - March 2020 - Cover2
ASHRAE Journal - March 2020 - 1
ASHRAE Journal - March 2020 - Contents
ASHRAE Journal - March 2020 - 3
ASHRAE Journal - March 2020 - 4
ASHRAE Journal - March 2020 - 5
ASHRAE Journal - March 2020 - 6
ASHRAE Journal - March 2020 - 7
ASHRAE Journal - March 2020 - 8
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ASHRAE Journal - March 2020 - Cover3
ASHRAE Journal - March 2020 - Cover4
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