Adjustable Frequency Drive; Hot Gas Bypass; Freezestat - McQuay SWP Series Installation And Maintenance Manual

Unit Options

Adjustable Frequency Drive

As an option an adjustable frequency drive (AFD) is available
for airflow modulation. A manually activated bypass contactor
is provided to allow system operation in the event drive
servicing is required.
Static pressure is controlled by the unit-mounted MicroTech III
controller. Indication of current fan capacity is available at the
MicroTech III controller. Static pressure is sensed by one or
two factory-mounted duct sensors. The installer provides and
installs the sensor tubing from unit mounted sensor(s) to duct
location(s). The static pressure set point is keypad adjustable
through the MicroTech III DDC controller.
All variable air volume units include a field-adjustable, duct
high limit safety control to protect ductwork from excessive
duct pressure.

Hot Gas Bypass

Hot gas bypass is a system for maintaining evaporator pressure
at or above a minimum value. The purpose for regulating the
hot gas into the distributor is to keep the velocity of the
refrigerant as it passes through the evaporator high enough for
proper oil return to the compressor when cooling load
conditions are light.
The system consists of a pressure regulating valve as shown in
Figure 34. The solenoid valves are factory wired to open
whenever the controller calls for the first stage of cooling. The
pressure regulating valve starts to modulate open at 57 psig
(393 kPa).
34
Figure 34: Hot Gas Bypass System
Suction
header
To evaporator
distributor tubes
Thermostatic
expansion valve
To determine the regulating valve opening point, slowly
reduce the system load or reduce the required discharge air
temperature setting while observing the suction pressure.
When the bypass valve starts to open, the refrigerant line on
the evaporator side of the valve begins to feel warm to the
touch.
Do not touch gas line during valve checkout. The hot gas line
can become hot enough in a short time to cause personal
injury.

Freezestat

A non-averaging type freezestat (FS1) is provided to protect
hydronic coils from subfreezing temperatures. If the unit has
an economizer coil, the control is mounted on the entering face
of the economizer coil. If the unit does not have an economizer
coil, the control is mounted on the leaving face of the hot water
coil. Upon sensing a temperature above specification, the unit
shuts down, opens the hydronic control valves, and sends an
alarm indication via the MicroTech III controller. The
freezestat has a field-adjustable set point range of 35°F to
45°F. To change the set point, turn the adjustment screw until
the pointer is opposite the desired cutout point. The adjustment
screw is accessible at the bottom of the control or at the top
when the cover is removed.
Hot gas Hot gas
bypass bypass
Suction
line
CAUTION
McQuay IM 937
Liquid lilne