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The circuiting within the evaporator is serpentined in such a manner as to guarantee a completely flooded
evaporator. Each evaporator is designed to insure adequate refrigerant velocity to eliminate the problem of oil
return to the crankcase.
4. Expansion Valves:
The thermostatic expansion valve (TXV) is used on Model D-320-A, and larger.
This expansion valve is controlled primarily by temperature and requires a hot gas bypass valve to
maintain pressure.
The temperature that is sensed by the (TXV) thermal bulb is the superheat temperature of the
refrigerant at the outlet of the evaporator. Superheat temperature is the temperature increase of the
refrigerant above its saturation temperature corresponding to the existing evaporator pressures.
5. Hot Gas Bypass Valve:
The hot gas bypass valve or capacity regulator permits medium to larger type refrigerated dryers to operate
through the range from zero load to 100% load while maintaining evaporator and suction line pressure and
temperature.
6. Low Pressure Switch:
The low pressure switch is a safety device that protects the refrigeration compressor from running with low
suction pressure that could result from a loss of refrigerant charge.
7. High Pressure Switch:
The high pressure switch is a safety device that limits the discharge pressure from the refrigerant compressor.
If the discharge pressure exceeds the factory set point, the high pressure switch opens the electrical circuit to
the compressor and the system is shut down.
High discharge pressures may result from conditions such as: plugged or dirty condensers, high ambient
temperature, high compressed air inlet temperatures, high compressed air flows, non-condensables (air inside
the refrigeration system) or fan motor failures.
8. Crankcase Heater (Where Required):
The crankcase heater is standard on models D-320-A to D-2000-A and D-400-W to D-1000-W. The crankcase
heater is a low wattage heater that is connected to or immersed in the refrigerant compressor crankcase.
The wattage of this heater is normally 50 to 65 watts. This heater is not controlled by the operational switch
of the dryer, but is factory wired directly to the incoming power.
The purpose of the crankcase heater is to prevent liquid refrigerant migration back to the crankcase while the
compressor is not in operation. This migration of the liquid refrigerant occurs when the compressor crankcase is
at a lower vapor pressure than the evaporator and other system components.
The crankcase heater must raise the temperature of the crankcase and oil to a temperature higher than the
evaporator and all of the interconnecting piping. To accomplish this, the crankcase heater must be energized
24 hours before the dryer is started. This is accomplished by turning the dryer operational switch to "OFF"
and switching on the main power. The dryer is then allowed to warm up for 24 hours after which time the
operational switch may be turned to the "ON" position.
9. Automatic Water Regulating Valve (Water Cooled only):
This control is found only in dryers equipped with water cooled condensers. The purpose of the valve is to
control the flow of water through the condenser at a rate that is factory set to maintain a constant condensing
pressure.