Chapter 8 – Maintenance
Page 8-6
TestEquity 1016S and 1027S Temperature Chamber
Theory of Operation
The chamber is heated by an open element nichrome heater. Cooling is accomplished by a
cascade 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, 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.
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.
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.
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.
