Chapter 7 – Maintenance
TestEquity 1007H Temperature Chamber Page 7-11
Refrigeration System (non-humidity mode)
In non-humidity mode, cooling is accomplished by a cascade refrigeration system. A cascade
refrigeration system consists of two interdependent refrigeration systems. The low-stage
provides cooling to the chamber interior through a finned evaporator coil (32), which is located
in the air plenum. The high-stage provides cooling to the cascade condenser (20). The cascade
condenser is a heat exchanger that has one circuit which is the evaporator of the high-stage, and
another circuit which is the condenser of the low-stage.
The high-stage uses refrigerant R-404A. High pressure liquid refrigerant is fed from the
condenser (part of 7) through the filter-drier (10), sight glass (11), solenoid valve (18/19B) to the
thermostatic expansion valve (17). The thermostatic expansion valve controls the feed of liquid
refrigerant to the evaporator circuit of the cascade condenser and, by means of an orifice, reduces
the pressure of the refrigerant to the evaporating or low side pressure. The reduction of pressure
on the liquid refrigerant causes it to boil or vaporize, absorbing heat which provides a cooling
effect. The refrigerant vapor travels through the suction line to the compressor (part of 7) 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 of the compressor
discharge valve and into the condenser. As the high pressure vapor passes through the condenser,
it is cooled by a fan, which blows ambient air across the finned condenser surface. The vapor
condenses into a liquid and the cycle is repeated. The hot gas regulator (9B) is adjusted to keep
the suction pressure at 10 PSIG during light loading conditions. A suction line cooling
thermostatic expansion valve (21B) senses the suction line temperature and injects liquid
refrigerant to cool the suction return gas within safe limits.
The Low-Stage uses refrigerant R-508B. High pressure liquid refrigerant is fed from the
condenser circuit of the cascade condenser (20), through the filter-drier (34), liquid-line solenoid
valve (18/19A), to the capillary tube/strainer assembly (35). The capillary tubes feed the finned
evaporator coil (32), 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 (1) 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 (2). As the high pressure vapor passes
through the desuperheater, it is air-cooled to remove some sensible heat. Next, the vapor goes
through the oil separator (4), 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.
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 (21A) 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 (28). If the discharge pressure exceeds 250 PSIG, the discharge pressure regulator valve
will “dump” refrigerant vapor into the expansion tank (5). This refrigerant is slowly returned
from the expansion tank to the suction line through a capillary tube (30). 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.
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