Chapter 8 – Maintenance
Page 8-8
TestEquity 123C Temperature Chamber
During a high temperature pull down or a continuous bypass condition, it is possible for
excessive hot gas to return to the compressor. The suction line cooling thermostatic expansion
valve (10) senses the suction line temperature and injects liquid refrigerant to cool the hot gas
within safe limits.
The low-stage discharge pressure is kept within safe limits with a bypass pressure switch (14). If
the discharge pressure exceeds 280 PSIG, the bypass pressure switch will energize the bypass
solenoid valve (SV5 or 18B/19B). This will “dump” refrigerant vapor into the expansion tanks
(11). This refrigerant is slowly returned from the expansion tanks to the suction line through a
strainer /capillary tube (22/23). The expansion tank also provides sufficient volume in the system
to keep the “standby pressure” (also known as static or balance pressure), when the system is off,
within safe limits.
Both the low- and high-stages each have a high pressure switch (8A, 8B respectively) which
turns off the entire refrigeration system in the event of an out of limit condition.
The Temperature Controller cycles the low-stage liquid-line solenoid valve (SV1 or 18A/19A)
on/off to control the chamber temperature. When SV1 is on, liquid refrigerant flows through the
capillary tube and evaporator to cool the chamber. When SV1 is off, the flow stops. The hot gas
regulator (9A) is adjusted to keep the suction pressure at 5 to 8 PSIG when SV1 is off. This is
also called “bypass mode”. Pilot Light PL2 provides an indication on the front panel when the F4
Controller is turning SV1 is on.
The Temperature Controller has internal logic to turn the compressors on if cooling is required to
maintain the temperature set point. The low-stage compressor turns on 30 seconds after the
high-stage turns on through Timing Module TM1. This reduces the system’s starting current,
while allowing the cascade condenser to get cool before the low-stage turns on.
