Chapter 8 – Maintenance
Page 8-6
TestEquity 115A-EX Temperature Chamber
desuperheater removes some of the heat of compression. Next, the vapor goes through the oil
separator, which returns any entrained oil back to the compressor’s crankcase. The vapor flows
through the condenser circuit of the cascade condenser, where it is condensed back into a liquid.
The temperature controller cycles the low-stage liquid-line solenoid valve SV2 on/off to control
the chamber temperature. When SV2 is on, liquid refrigerant flows through the capillary tube
and evaporator to cool the chamber. When SV2 is off, the flow stops. The hot gas regulator (19)
is adjusted to keep the suction pressure at 5 PSIG when SV2 is off. This is also called “bypass
mode”. Pilot Light PL2 provides an indication on the front panel when the F4 Controller is
turning SV2 is on.
The low-stage discharge pressure is kept within safe limits with the discharge pressure switch
(PS1). If the discharge pressure exceeds 280 PSIG, the PS1 will energize solenoid valve SV1 to
“dump” refrigerant into the expansion tank. 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.
During a high temperature pull down, it is possible for excessive hot gas to return to the
compressor. The suction line cooling thermostatic expansion valve senses the suction line
temperature and injects liquid refrigerant to cool the hot gas within safe limits.
The high-stage has a high pressure switch (PS2) which turns off both compressors if the high-
stage discharge pressure exceeds safe limits (approximately 350 psig).
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.
