GENERAL DESCRIPTION
Page 2-25
Control System
REFRIGERATION (Refer to drawing: MCB-RF00 or MC3(H)-RF00 & MC3-EW00)
Mode: Normal cooling (Below 17°C)
When there is call for cooling the R-404A compressor turns on, 1-SOL is energized. After the stagger start
timer expires, approximately 1 minute, the R-508B compressor turns on.
When there is a call for cooling refrigerant (R-508B) ! ows through the solenoid (2-SOL) and the T.E.V. into
the evaporator and cools the chamber. When the chamber is near the set point the controller begins to cycle
2-SOL on and off. When 2-SOL is turned off, 5-SOL is energized allowing refrigerant to by-pass back to the
suction line.
Mode: Normal cooling (Above 17°C)
When there is a call for cooling the R-404A compressor turns on, 1-SOL and H1-SOL are energized. When
the chamber is at or near set-point H1-SOL begins to cycle. When H1-SOL is off, 4-SOL is energized allowing
refrigerant hot gas to by-pass back to the suction line.
Mode: Humidity
In humidity mode, the unit operates as a single stage R-404A refrigeration system.
When Dehumidi" cation is called for, Solenoid H-11 SOL is energized. The refrigerant passes through a
solenoid valve (item H27) and a Wet Coil capillary tube (item H23). Two phase refrigerant exits the Capillary
tube and enters the wet coil (item H25). The wet coil is cold enough to attract water from the chamber air,
but not cold enough to freeze the water on the coil. The temperature of the coil is regulated by a pressure
regulating E.P.R. valve (item H26). The E.P.R. valve is set at 68 psig/4.7 barG which corresponds to a 29°F/-
16°C wet coil temp. The warm chamber air keeps the moisture from freezing on the coil. After the E.P.R.
valve, there is a check valve (item H28) which keeps refrigerant from migrating to the wet coil when humidity
mode is inactive. After the refrigerant passes through the check valve, it enters the suction line and returns to
the compressor
When humidity is called for, Solenoid H-11 SOL is de-energized.
For Dry Bulb Temperature cooling in humidity mode, solenoid H1-SOL and 1-SOL are energized. This allows
refrigerant to ! ow through the R-404A Capillary tube into the evaporator, which cools the chamber. When HI-
SOL is off, 4-SOL is energized allowing refrigerant hot gas to by-pass back to the suction line.
For Dry Bulb Temperature heating, solenoid H1 SOL is de-energized, 4-SOL and 1-SOL are energized, acting
a by-pass for the refrigeration circuit. The heaters are activated to warm the chamber.
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