Chapter 9 – Maintenance
TestEquity 1007C Temperature Chamber Page 9-7
Theory of Operation
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 back wall of the chamber interior.
The heater, compressors, and circulator fan motor operate directly from the 208 or 230 VAC
input line. All line branch circuits are individually fused. A step-down transformer provides 115
VAC for all instrumentation and control elements.
Refer to the electrical and refrigeration drawings to identify the referenced items described
below.
The chamber is heated by an open-element nichrome heater (HT1). The heater is located in the
air plenum. The temperature controller 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. 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 Master Switch
and safety relay (CR3). CR3 is controlled by the temperature limit controller (TCR2). If either
the high or low temperature safety limits are exceeded, TCR2 turns off CR3, which turns off C1.
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, and sight glass to the thermostatic expansion valve.
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 expansion valve
regulates the flow as necessary to maintain a preset temperature difference or superheat between
the evaporating refrigerant and the vapor leaving the evaporator circuit of the cascade condenser.
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 liquid-line solenoid valve
to the capillary tube/strainer assembly. The capillary tubes feed the finned evaporator coil, which
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