Standard Controls; High Pressure Switches; Low Pressure Switches; Compressor Motor Protector - McQuay SWP 018 Installation And Maintenance Manual

Standard Controls

High Pressure Switches

The high pressure switch (HP1-HP6) is a single pole pres-
sure activated device that opens on a pressure rise. When the
switch opens it de-energizes the compressor circuit, shutting
down the compressor. The MicroTech II controller will dis-
play an alarm condition. Once the cause of the fault has been
identified and corrected, the unit may be manually reset
through the MicroTech II keypad/display interface. The con-
trol is attached to a Shrader fitting and is located at the com-
pressor. To check the control, shut off water flow to the
condensers and observe the cutout point on a high pressure
gauge. The high pressure control should open at 360 psig
and close at 300 psig. After testing the high pressure control,
check the pressure relief device for leaks.

Low Pressure Switches

The low pressure switch (LP1-LP6) is a single pole pressure
activated device which closes on a pressure rise. It senses
evaporator pressure and is factory set to close at 60 psig and
open at 35 psig. Compressor operation is not allowed until
the switch closes. The low pressure switch is an automatic
reset control. If the condition occurs on any one compressor
three times in a 24-hour period, the alarm will have to be
manually reset through the MicroTech II keypad/display
interface to restart the compressor. The low pressure switch
is attached to a Shrader fitting and is located at the compres-
sor.

Compressor Motor Protector

All compressors are thermally protected. All 13 horsepower
and larger compressors use a solid state protection device
(MP1 - MP6) located in the compressor junction box. When-
ever the protection system opens the compressor is shut
down for a period of 36 minutes and an alarm indication is
made at the MicroTech II controller.
All 6 and 10 horsepower compressors have in-line protec-
tion. The control automatically resets when the alarm condi-
tion is removed and the time delay is satisfied.
If the condition occurs on any one compressor three times in
a 24-hour period, the alarm will have to be manually reset
through the MicroTech II keypad/display interface to restart
the compressor.
IM708

Proof of Airflow Switch

A positive proof of airflow switch (PC7) is provided with all
units. The switch is factory set to close at 0.2 inches of water
column. The switch has a field adjustable set point range of
0.17 to 5.0 inches of water column. Turn adjustment screw
clockwise to decrease differential pressure setting. Turn
adjustment screw counterclockwise to increase differential
pressure setting. In a constant volume system, if the fan sys-
tem is energized and the minimum pressure setting of the
switch has not been reached, the unit will be shut down and a
loss of airflow alarm indicated at the MicroTech II controller.
For variable air volume units, the unit will shutdown due to
loss of airflow only if the airflow switch is open AND the
duct static pressure is less than half the duct static pressure
setpoint. Once the reason for the fault has been corrected, the
unit can be manually reset through the MicroTech II key-
pad/display interface. PC7 is located in the fan section on the
motor side.

Frost Protection Switches

A frost protection switch (FP1-FP6) is used on each refriger-
ant circuit to protect against evaporator coil freeze up. The
frost protection switches are normally closed and open on a
drop in temperature. When a frosting condition is sensed the
compressor circuit is shutdown until the condition has been
removed. The frost protection control is an automatic reset
control. If the condition occurs on any one compressor three
times in a 24 hour period, the alarm will have to be manually
reset through the MicroTech II keypad/display interface to
restart the compressor. The MicroTech II control will indi-
cate a warning when a frost condition exists. The tempera-
ture sensors are located on a return bend for each refrigerant
circuit.

Clogged Filter Switch

A clogged filter switch (PC5) is provided to indicate when
unit filters are to be changed. The switch is factory set to
close at 0.6 inches of H O. The switch has a field adjustable
2
set point range of 0.17 to 5.0 inches of H
screw clockwise to decrease differential pressure setting.
Turn adjustment screw counterclockwise to increase differ-
ential pressure setting. When the filter pressure differential
exceeds the switch setpoint, a clogged filter indication is
made at the MicroTech II controller. The unit is allowed to
continue operation. PC5 is located in the fan section on the
motor side.
O. Turn adjustment
2
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