TestEquity 123HS, Operation And Service Manual

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Chapter 7 – Maintenance
Page 7-10 TestEquity 123HS Temperature/Humidity Chamber
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 F4/F4T Controller provides a time-proportioned output to 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.
Humidity System
Water is heated by an immersion-type heater (HT2) to generate vapor. The heater is located in
the humidifier tank. The F4/F4T Controller provides a time-proportioned output to a solid state
relay (SSR2). This turns the humidifier heater on/off as required to maintain the humidity set
point. Pilot light PL4 provides an indication on the front panel when the humidifier heater is on.
Water level in the humidifier is maintained by float a float valve assembly which is external to
the humidifier tank.
The immersion heater has an embedded thermocouple which is connected to Input 3 of the
F4/F4T Controller to control an Alarm output. This provides failsafe protection against a heater
failure in the event of a low water condition by opening the humidifier heater contactor C4 if the
heater r125°C. The alarm will self-reset when the heater r100°C.
Refrigeration System
The F4/F4T Controller turns the compressor ON based on the demand for cooling. Dry-bulb
cooling is accomplished through the cool expansion valve (30) and the finned evaporator coil
(15) inside the chamber. The F4/F4T Controller cycles the liquid-line solenoid valve (SV1 or
18/19A) ON/OFF as required to maintain the temperature set point. Pilot Light PL2 provides an
indication on the front panel when the F4/F4T Controller is turning SV1 is on.
Dehumidification is accomplished through expansion valve (11B) and the finned dehumidify coil
inside the chamber. The F4/F4T Controller cycles the dehumidify coil liquid-line solenoid valve
(SV2, 18/19D) ON/OFF through SSR3 to control the amount of dehumidification. If the
compressor is off because no dry-bulb cooling was required, then TM1 enable the refrigeration
system upon demand for dehumidification.
The hot gas regulator (9) is adjusted to keep the suction pressure at 2 PSIG when both S1 and
SV2 are off. A suction line cooling thermostatic expansion valve (11A) senses the suction line
temperature and injects liquid refrigerant to cool the suction return gas within safe limits.