Chapter 7 – Maintenance
Page 7-12
TestEquity 1016H-2 Temperature/Humidity Chamber
The low-stage discharge pressure is kept within safe limits with the discharge pressure regulator
valve. If the discharge pressure exceeds 285 PSIG, the discharge pressure regulator valve will
“dump” refrigerant into the expansion tank. This refrigerant is slowly returned from the
expansion tank to the suction line through a capillary tube. The expansion tank also provides
sufficient volume in the system to keep the “standby pressure” (also known as static or balance
pressure), when the system is off, within safe limits.
Both the high- and low-stages each have a high/low pressure switch which turns off the entire
refrigeration system in the event of an out of limit condition. The high-stage compressor has a
crankcase heater to prevent refrigerant from condensing in the oil when the compressor is off.
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 high-stage refrigeration system is used, and the low-stage 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 b7
°
C and +90
°
C. If the chamber is
outside this temperature range, the F4T/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 (9) and
the finned ambient coil (31) inside the chamber. The F4T/F4 Controller cycles the ambient cool
liquid-line solenoid valve (SV4) on/off through cool relay CR3 to control the chamber
temperature. Pilot Light PL2 provides an indication on the front panel when the F4 Controller is
turning SV4 is on.
Dehumidification is accomplished through the finned dehumidify coil (32) inside the chamber,
which uses a capillary tube (35) as its metering device. The F4T/F4 Controller cycles the
dehumidify coil liquid-line solenoid valve (SV5) on/off through SSR4 to control the amount of
dehumidification. If the compressor is off because no dry-bulb cooling was required, then TM2
enables the refrigeration system upon demand for dehumidification.
An evaporator pressure regulator (34) valve keeps the ambient coil and dehumidify coil at
68 PSIG, thus preventing freezing temperatures. The hot gas regulator (10) is adjusted to keep
the suction pressure at 10 PSIG when both SV4 and SV5 are off. A suction line cooling
thermostatic expansion valve (25) senses the suction line temperature and injects liquid
refrigerant to cool the suction return gas within safe limits.
