Chapter 7 – Maintenance
Page 7-8
TestEquity 3007C Temperature Chamber
across the finned condenser surface. The vapor condenses into a liquid and the cycle is repeated.
During heavy load conditions as determined by the discharge pressure of the low-stage, pressure
control PS3 energizes solenoid valve SV3 which allows an additional expansion valve to feed the
cascade condenser inlet for increased capacity.
The Low-Stage uses refrigerant R-508B. High pressure liquid refrigerant is fed from the
condenser circuit of the cascade condenser, through the filter-drier and liquid-line solenoid valve
to the thermostatic expansion valve. The thermostatic expansion valve feeds the finned
evaporator coil, which is located in the air plenum where heat is absorbed to provide a cooling
effect within the chamber. The refrigerant vapor travels through the suction line to the
compressor suction inlet. The compressor takes the low pressure vapor and compresses it,
increasing both the pressure and the temperature. The hot, high pressure vapor is forced out the
compressor discharge valve and into the desuperheater. The 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 (SV1) ON/OFF to
control the chamber temperature. When SV1 is ON, liquid refrigerant flows through the
thermostatic expansion valve and evaporator to cool the chamber. When SV1 is OFF, the flow
stops. The hot gas bypass solenoid valve (SV2) is ON whenever SV1 is OFF. SV2 feeds high
pressure vapor to the hot gas regulator, which meters a precise amount into the suction line to
maintain a minimum load on the system and keep it out of a vacuum. The hot gas regulator is
adjusted to keep the suction pressure at 8 PSIG when SV1 is OFF. This is also called “bypass
mode”. The temperature controller alternately cycles SV2 and SV1 through cool relay CR1. Pilot
Light PL2 provides an indication on the front panel when SV1 is ON.
During a high temperature pulldown or a continuous bypass condition, it is possible for
excessive hot gas to return to the compressor. A suction line cooling thermostatic expansion
valve (on both high- and low-stages) 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 the discharge pressure regulator
valve. If the discharge pressure exceeds 285 PSIG, the discharge pressure regulator valve will
“dump” refrigerant into the expansion tank. This refrigerant is slowly returned from the
expansion tank to the suction line through a capillary tube. 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 high- and low-stages each have a high/low pressure switch which turns off the entire
refrigeration system in the event of an out of limit condition. The high-stage compressor has a
crankcase heater to prevent refrigerant from condensing in the oil when the compressor is off.
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.
