Chapter 8 – Maintenance
TestEquity 123C Temperature Chamber
Page 8-7
Theory of Operation
Overview
Refer to the electrical and refrigeration drawings to identify the referenced items described
below. The electrical items are referenced with a letter(s) and number. The refrigeration items
are referenced with a number only or number followed by a single letter.
Air Heating System
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. Relay CR4, High Heat
Interlock, provides staging for the available heat. When the compressors are off, full heat of
1,000 Watts is available. Then the compressors are on, half heat of 500 Watts is available. A
fusible heat limiter (HL) provides failsafe protection against a catastrophic failure by opening the
heater circuit at +240°C.
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 (26), which is located in the air plenum. The
high-stage provides cooling to the cascade condenser (21). 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 5) through the filter-drier (6), sight glass (7) to the thermostatic expansion
valve (20). The thermostatic expansion valve controls the feed of liquid refrigerant to the
evaporator circuit of the cascade condenser and reduces the pressure of the refrigerant to the
evaporating pressure. The refrigerant vapor travels through the suction line to the compressor
(part of 5) 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 5 to 8 PSIG during light load conditions.
The Low-Stage uses refrigerant R-508B. High pressure liquid refrigerant is fed from the
condenser circuit of the cascade condenser (21), through the filter-drier (34), liquid-line solenoid
valve (18A /19A), to the capillary tube/strainer assembly (24 and 25). The capillary tube feeds
the finned evaporator coil (26), 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 (3), 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.
