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Supply Fan Control with IGV —

In most VAV units,

the supply fan static pressure is controlled by inlet guide vanes.
The inlet guide vanes operate independently from the micro-
processor. The supply static pressure is controlled by a differ-
ential pressure switch.

For example, assume that set point on supply fan differen-

tial switch is 1.9 in. wg. If pressure in supply duct goes above
1.9 in. wg, switch will make to the normally open contact and
energize inlet guide vane motor to drive inlet guide vanes to a
more closed position, thus reducing airflow and lowering duct
pressure. Once set point pressure is reached, switch will open
and deenergize inlet guide vane motor. If pressure in supply
duct is below 1.9 in. wg, the switch will make to the normally
closed contact and energize inlet guide vane motor to drive in-
let guide vane to a more open position; increasing airflow and
raising duct pressure. Once again, once desired pressure has
been reached, switch will open and deenergize inlet guide vane
motor. How far above or below the set point setting the switch
goes before energizing depends on setting of null span (null
span is pressure change that can be made without contacts
opening or closing). If null span is at maximum position, pres-
sure will vary from 0.17 in. wg to 0.31 in. wg depending on set
point (if set point is at minimum setting, null span will be
0.17 in. wg, while if it is at maximum position, the null span
will be 0.31 in. wg) before switch acts. If null span is adjusted
to a minimum setting, duct pressure will vary from 0.06 in. wg
to 0.11 in. wg (again depending on switch set point) before
switch acts. Setting null span to minimum position will result
in a smaller pressure fluctuation than if it is set at maximum
position.

Supply Fan Control with VFD —

When equipped

with the VFD option, the supply fan static pressure is con-
trolled by modulating the fan wheel speed. The VFD operates
independently from the microprocessor. A duct pressure trans-
ducer monitors duct static pressure. The transducer output (4 to
20 mA) is directed into the VFD. The VFD adjusts supply fan
motor speed (which changes wheel speed) as measured duct
pressure varies from set point as established at the VFD. The
VFD will modulate fan speed until the duct pressure set point is
achieved.
NOTE: The VFD will always provide the proper phase
sequence to the supply fan motor. This motor will operate in
proper rotation regardless of the phase sequence to the unit. If,
upon start-up, the outdoor fans operate backwards but the
indoor fan operates in the correct direction, reverse any two
leads to the main terminal block. All fans will then operate in
the correct direction.

Modulating Power Exhaust (48FK,JK and
50FK,JK Units Option or Accessory) —

The pow-

er exhaust assembly consists of two parallel and independent
belt-drive forward curve fans. The fans, motors, and drives are
located over the return air opening of the unit, in a plenum be-
neath the outside air intake plenum. The fans discharge air hor-
izontally out the back of the unit through individual barometric
backdraft dampers with hoods. (See Fig. 44 and 45.) Operation
is interlocked with economizer operation. Sheet metal installa-
tion is shown in Fig. 46 and 47.

Fan no. 1 is equipped with a variable position discharge

damper located in the outlet of the fan housing. This damper is
controlled by an actuator (PEDM), based on signals from the
building pressure differential pressure switch (DPS). Available
range on the DPS is -0.50 to +0.50 in. wg, adjustable. Building
pressure is sensed by a pick-up (field-supplied and -installed)
located in the occupied space.

Operation of the modulating power exhaust is a combina-

tion modulating/staged control, with fan no. 1 providing modu-
lating control from 0 to 50% of total exhaust capability, and fan
no. 2 being staged On/Off (for a step of 50% of total exhaust
capability) according to damper position on fan no. 1.

As the economizer actuator opens past 17% open, auxiliary

switch DMS1 closes, energizing fan contactor PEC1. Fan mo-
tor no. 1 starts and runs.

Capacity of fan no. 1 is controlled by the position of the out-

let damper. As building pressure increases above set point,
the DPS will close its contact and drive the power exhaust
damper motor (PEDM) open until set point is achieved. DPS
then opens its control contacts and PEDM maintains current
position.

When space demand moves PEDM to 90% of full-open po-

sition, auxiliary switch PEDMS closes, energizing fan contac-
tor PEC2. Fan motor no. 2 starts and runs. Increased exhaust
airflow will lower space pressure, causing DPS to drive PEDM
back towards its closed position, until the set point is achieved.

If space pressure decreases until PEDM position is reduced

to 10% of open position, PEDMS will open, deenergizing fan
contactor PEC2 and shutting off fan no. 2.

High Capacity Modulating Power Exhaust
(48FM and 50 FM,FS Units) —

The high-capacity

modulating power exhaust assembly consists of two parallel
and independent belt-drive large diameter forward curve fans.
The fans, motors, and drives are located in a cabinet extension
(not over the return air opening of the unit), in a plenum be-
neath the outside air intake plenum. The fans discharge hori-
zontally out the back of the unit through individual barometric
backdraft dampers with hoods. Operation is interlocked with
economizer operation. Space pressure is monitored by a
factory-installed differential pressure transducer. See Fig. 48
for component locations and sheet metal details.

The high-capacity modulating power exhaust is also avail-

able on horizontal 50FS units. Return duct opening is located
on the left-hand side of the unit; exhaust air exits the unit out
the back.

Control of the high-capacity modulating power exhaust sys-

tem is accomplished via a combination modulated capacity
fan/staged fan. Modulation is provided by a variable frequency
drive controlling the direct output to fan no. 1 and controlling
the ON/OFF status of fan no. 2.

Fan no. 1 is equipped with a Variable Frequency Drive,

matched to the motor size. VFD output is determined by the
VFD’s internal PID logic in response to actual space pressure
as monitored by the Building Pressure (BP) transducer. Set
point for BP control is established at the PE VFD. Available set
point range is –0.50 to +0.50 in. wg. Building Pressure is
sensed by a pick-up (field-supplied and -installed) located in
the occupied space and connected to the BP transducer by

-in. tubing (field-supplied and -installed).

Operation of the modulating power exhaust is a combina-

As the economizer actuator opens past 17% open, auxiliary

PEC1. Fan motor no. 1 starts and runs.

Capacity of fan no. 1 is controlled by the output level from

the BP VFD. As building pressure increases above set point,
the VFD logic will increase the output level to fan no. 1 until
set point is reachieved.

When space demand moves PE VFD output to 100%

(60 Hz), VFD internal relay closes, energizing fan contactor
PEC2. Fan motor no. 2 starts and runs. Increased exhaust air-
flow will lower space pressure, causing PE VFD to reduce its
output to fan no. 1 until set point is reachieved.

If space pressure decreases until PE VFD output is reduced

to 25% of maximum output (15 Hz), VFD internal relay will
open, deenergizing fan contactor PEC2 and shutting off fan
no. 2.

Unit Staging —

Compressor loading and unloading se-

quences are shown in Table 20.