Chapter 8 – Maintenance
TestEquity 115A-EX Temperature Chamber
Page 8-5
Theory of Operation
Overview
The chamber is heated by an open element nichrome heater. Cooling is accomplished by a
cascade refrigeration system, which consists of two compressors. The air is circulated by a
propeller fan. The heater, evaporator (cooling coil), and fan are located within an air plenum
which is on the top of the chamber interior.
Refer to the electrical and refrigeration drawings to identify the referenced items described
below.
Heating System
The chamber is heated by an open-element nichrome heater (HT1). The heater is located in the
air plenum. The temperature controller (TCR1) provides a time-proportioned output to a solid
state relay (SSR1). This turns the heater on/off as required to maintain the temperature set point.
The heat pilot light (PL1) provides an indication on the front panel when the heater is on.
A fusible heat limiter (HL) provides failsafe protection against a catastrophic failure by opening
the heater circuit at +240°C. The master heat contactor (C1) provides a power interlock for the
heaters, circulator fan motor, and the control system. C1 is controlled by both the power switch
(SW1), temp switch (SW2), and temperature limit controller (TCR2).
Refrigeration System
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, which is located in the air plenum. The high-
stage provides cooling to the cascade condenser. 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 through the liquid line filter-drier, to a capillary tube with feeds the evaporator circuit
of the cascade condenser. The capillary tube 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 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 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 capillary tube. The
capillary tube 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 finned coil
