Chapter 7 – Maintenance
TestEquity 3007C Temperature Chamber
Page 7-7
Theory of Operation
Overview
The chamber is heated by an open element nichrome heater. Cooling is accomplished by a
cascade refrigeration system. The air is circulated by two blowers. The heater, evaporator
(cooling coil), and blowers are located within an air plenum which is on the back wall of the
chamber interior.
The heater, compressor, and circulator fan motor operate directly from the 208/230 VAC input
line. All line branch circuits are individually fused. A stepdown transformer provides 115 VAC
for all instrumentation and control elements. 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 provides a time-proportioned output to the solid state
relays (SSR1, 2). This turns the heater on/off as required to maintain the temperature set point.
SSR1 is controlled by the Temperature Controller’s Heat Output. SSR2 is controlled by the
Temperature Controller’s Boost Heat Output. Relay CR5 disables the boost heat capability when
the refrigeration system is running in order to reduce the power consumption. Pilot light PL1
provides an indication on the front panel when the heater is on.
Fusible heat limiters (HL1, 2, 3) provide failsafe protection against a catastrophic failure by
opening the heater circuits 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, the safety relay (CR3), and the phase control relay (PCR1). 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. PCR1 will disable CR1 if the input power
phase-sequence is incorrect.
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, 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 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
