Chapter 8 – Maintenance
TestEquity 1007S Temperature Chamber
Page 8-7
Theory of Operation
The chamber is heated by an open element nichrome heater. Cooling is accomplished by a
single-stage refrigeration system. 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.
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.
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.
Refrigeration System
Cooling is accomplished by a single-stage refrigeration system. The refrigeration system
provides cooling to the chamber interior through a finned evaporator coil, which is located in the
air plenum.
The system 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 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 thermostatic
expansion valve controls the feed of liquid refrigerant to the evaporator 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. 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.
