Chapter 7 – Maintenance
Page 7-12 TestEquity 1007H Temperature Chamber
Both the low- and high-stages each have a high/low pressure switch (14A, 14B respectively)
which turns off the entire refrigeration system in the event of an out of limit condition. The high-
stage compressor has a crankcase heater (CC1 or 8) to prevent refrigerant from condensing in the
oil when the compressor is off.
The F4T/F4 Controller cycles the low-stage liquid-line solenoid valve (SV1 or 18/19A) on/off
through cool relay CR1 to control the chamber temperature. When SV1 is on, liquid refrigerant
flows through the capillary tubes and evaporator to cool the chamber. When SV1 is off, the flow
stops. The hot gas regulator is adjusted to keep the suction pressure at 5 PSIG when SV1 is off.
This is also called “bypass mode”. Pilot Light PL2 provides an indication on the front panel
when the F4T/F4 Controller is turning SV1 is on.
The F4T/F4 Controller has internal logic to turn the compressors on if cooling is required to
maintain the temperature set point. The low-stage compressor turns on 30 seconds after the
high-stage turns on through Timing Module TM1. This reduces the system’s starting current,
while allowing the cascade condenser to get cool before the low-stage turns on.
Refrigeration System (controlled-humidity mode)
In controlled-humidity mode, cooling and dehumidification is accomplished by a single-stage
refrigeration system. Only the R-404A refrigeration system is used, and the R-508B system is
disabled. The Humidity mode is enabled through Event 2 (Digital Output 2) in the F4T/F4
Controller. Also, the chamber workspace must be between +7°C and +90°C. If the chamber is
outside this temperature range, the F4 Controller’s Alarm2 light will turn ON (there is no
indication on the F4T’s main display but it functions similarly) and the chamber will operate as
though the Humidity Enable was OFF (non-humidity mode).
Dry-bulb cooling is accomplished through the ambient cool thermostatic expansion valve (22)
and the finned ambient coil inside the chamber. The F4T/F4 Controller cycles the ambient cool
liquid-line solenoid valve (SV3) on/off through cool relay CR1 to control the chamber
temperature. Pilot Light PL2 provides an indication on the front panel when the F4T/F4
Controller is turning SV3 is on. A suction limit pressure switch (PS3) also cycles SV3 to limit
peak cooling demands to acceptable limits.
Dehumidification is accomplished through the finned dehumidify coil inside the chamber, which
uses a capillary tube as its metering device. The F4T/F4 Controller cycles the dehumidify coil
liquid-line solenoid valve (SV4) on/off through SSR3 to control the amount of dehumidification.
If the compressor is off because no dry-bulb cooling was required, then CR5 and TM2 enable the
refrigeration system upon demand for dehumidification.
An evaporator pressure regulator (EPR) valve keeps the ambient coil and dehumidify coil at 72
psig, thus preventing freezing temperatures. The hot gas regulator is adjusted to keep the suction
pressure at 10 PSIG when both S3 and SV4 are off. A suction line cooling thermostatic
expansion valve senses the suction line temperature and injects liquid refrigerant to cool the
suction return gas within safe limits.
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