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9-4
SERVICE OPERATIONS
High-Low Pressure Safety Switch. The high-low pressure switch (HPS) (FIGURE 9-4) is a two
pole dual function switch. Located in the machine mounted to the frame near the compressor. It
protects the machine from possible damage due to abnormal pressure during operation.
The LOW pressure cut-in should be set at 40 psig and the cutout set at 20 psig for R-22
The LOW pressure cut-in should be set at 52 psig and the cutout set at 28 psig for R-404a.
After tripping at the cutout setting, the switch will reset automatically when the pressure rises to the
cut-in setting.
The HIGH pressure cutout should be set at 300 psig for R-22 and 350 psig for R-404a. After
tripping, reset the switch manually.
Differential Adjusting Screw
Set at 20 PSIG
(CW Raises Setting)
Reset Button
Vogt Part #: 12A2117D02
Low Pressure
Test Connection
If it becomes necessary to install a new high/low pressure switch, the following procedure is
recommended for its adjustment:
Turn the adjusting screws clockwise to raise the pressure setting. Turn counter-clockwise to lower
the setting. Adjust the switch to the indicated pressure settings and test with an accurate gage to be
sure the switch functions properly before installation.
Head Pressure. The head pressure should be maintained at 190-210 psig for R-22 and 230-250 psig
for R-404a during the freeze cycle. This pressure can be checked at the test connection in the high
pressure line near the high-low pressure switch.
Water-Cooled Units. A water regulating valve (FIGURE 9-5A) located in the condenser water
inlet line is used to control the water flow through the condenser. This valve should be adjusted to
maintain a head pressure of 195 psig for R-22 and 235 psig for R-404a. Increasing the water flow
lowers the head pressure and decreasing the water flow raises the head pressure. The valve is
adjusted during the factory test. The valve stem should not be opened as far as it will go or the valve
will not close fully when the head pressure drops below its setting.
4/2/14
When this switch causes the machine to stop, the cause should be
identified and corrected before resuming normal operation.
High Pressure
Test Connection
High-Low Pressure Switch
!
CAUTION
!
!
CAUTION
!
FIGURE 9-4
03TA Service Manual
Range Adjusting Screw
Set at 40 PSIG (R-22)
Set at 52 PSIG (R-404A)
(CW Raises Setting)
High Pressure Adjustment
Set at 300 PSIG (R-22)
Set at 350 PSIG (R-404A)
(CW Raises Setting)
Differential Factory Set
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