Propeller Exhaust Fan Control - McQuay RPS Installation And Maintenance Manual

Propeller Exhaust Fan Control

Exhaust Fan On/Off Control
The Exhaust fans are turned on and off based on building static
pressure, outdoor air damper position, and discharge fan
capacity. Exhaust fans do not have to always run while the
supply fan is on, as the does a return fan. They are turned on
and off through DO # 2 on the Mod DCU as described below.
The Exhaust Fans are turned on when:
• All of the following are true for longer than the Min Start Time
(Default Time = 120 Seconds). :
• OA Damper Capacity exceeds Min OA Dmpr Pos
• Either the Discharge Fan Capacity exceeds the Min DF Cap or
the Discharge Fan Control Type is Constant Volume. This two
part condition may be able to be reduced to one condition if
there is one attribute that is set to 100% when constant volume
units are on and to the current fan speed or actuator position for
variable air volume units.
• The type of control (RF/EF Ctrl) is set to Building Pressure
• The building static pressure is above the building static pres-
sure setpoint by more than the deadband/2.
OR
• All of the following are true for longer than the Min Start Time
(Default Time = 120 Seconds).
• OA Damper Capacity exceeds Min OA Dmpr Pos
• The Discharge Fan Capacity exceeds the Min DF Cap or the
Discharge Fan Control Type is Constant Volume. This two part
condition may be able to be reduced to one condition if there is
one attribute that is set to 100% when constant volume units are
on and to the current fan speed or actuator position for variable
air volume units.
• The type of control (RF/EF Ctrl) is set to Position
• The Exhaust Fan VFD capacity (Rem RF/EF Cap) is set to a
value above the Min Exh Fan Cap (Default = 25%) .
The Exhaust Fans are turned off when:
• All of the following are true:
• The type of control (RF/EF Ctrl) is set to Building Pressure
• The building static pressure is below the building static pres-
sure setpoint by more than the deadband/2
• The Exhaust Fan capacity is at the Min Exh Fan Cap (Default =
25%) or lower for longer than the Min Stop Time (Default
Time = 120 Seconds).
OR
• Both of the following are true:
• The type of control (RF/EF Ctrl) is set to Position
• The Remote Exhaust Fan capacity setpoint (Rem RF/EF Cap)
is set below the Min Exh Fan Cap (Default = 25%) for longer
than the Min Stop Time (Default Time = 120 Seconds).
OR
• The unit is off
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Exhaust Fan Speed Control
The exhaust fan variable frequency drive can be controlled
by two different methods that maintain building static pres-
sure. Fan tracking is not provided with exhaust fans. The
Exhaust Fan VFD speed is increased through BO # 16 and
decreased through BO # 15 on the Mod-DCU just like the
Return Fan.
The minimum speed of the Exhaust Fan will be set to it's Min
Exh Fan Cap (Default = 25%) through the keypad on the
VFD. This same minimum speed value is to be entered
through the MicroTech II keypad to provide the capability to
turn on and off the Exhaust Fans as described above.
When Building Static pressure is selected, the exhaust vari-
able frequency drive is controlled to maintain building static
pressure directly. A minimum speed is maintained by the
VFD, not by MicroTech II.
The outputs, BO # 15 and BO # 16, will be controlled by a
Proportional/Integral (PI) algorithm that increases the speed
of the VFD in order to lower the building static pressure and
decreases the speed of the VFD in order to raise the building
static pressure. The building static pressure will be provided
as it is now through AI # 14 with a range of - .25 IWC to +
.25 IWC. This control algorithm is identical to that currently
used to control building static pressure using a return fan
VFD except that the minimum speed (Default = 25%) is not
maintained by MicroTech II.
When position control is selected, the speed of the exhaust fan
variable frequency drive is controlled to the Remote RF/EF
setting. BO # 15 and BO # 16 are controlled to maintain a
fixed speed provided through a connected network or the key-
pad. This control algorithm similar to that currently used to
control a return fan VFD to maintain a fixed speed except that
the minimum speed (Default = 25%) is not maintained by
MicroTech II. This minimum speed is maintained by the VFD.
When the unit is in an off state, the variable frequency drive
is turned off by turning off BO #2. At the same time, BO #15
is turned on, and BO #16 is turned off.
Keypad Entries
Add a new table menu, Exhaust Fan Setup, after Bldg Static P
Setup and before Zone Temp Setup with the following entries.
Exhaust Fan Setup
Min Exh Fan Cap = 25% ...................(1 - 99%)
Min OA Dmpr Pos= 5% ...................(1 - 99%)
Min DF Cap = 10%............................(1 - 99%)
Min Start Time=120 s ........................(10 - 240 sec)
Min Stop Time=120 s ........................(10 - 240 sec)
IM-738