Chapter 9 – Maintenance
Page 9-8 TestEquity 1007C Temperature Chamber
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. As the high pressure vapor passes through the desuperheater, it is cooled
by a fan, which blows ambient air across the finned surface to remove 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 capillary
tubes and evaporator to cool the chamber. When SV1 is OFF, the flow stops. The R-508B hot
gas regulator is adjusted to keep the suction pressure at 5 PSIG when SV1 is OFF. Pilot Light
PL2 provides an indication on the front panel when SV1 is ON.
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 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 250 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.
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