Brochure

Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
Catalog CIC-2003-1/US
Thermostatic and Constant Pressure (Automatic) Expansion Valves
184
Thermal Electric ValvesApplications
Chillers
This chiller design uses the thermistor as a liquid
level control and the electric valve as a normal
expansion device.
Constant Pressure Control
When combined with a pressure or temperature
switch, the electric valve can be used to control
evaporator pressure. The switch, which senses
evaporator pressure or temperature, closes when
the evaporator drops below the set point. The
closing of the switch energizes the electric valve.
The resulting increase in refrigerant flow raises
evaporator pressure. The pressure switch then
opens and the valve modulates, maintaining the
set point.
Pressure switches can be used to limit both
maximum and minimum system pressure. These
switches can be placed anywhere in the system
to close the electric valve or limit its capacity when
desired levels of pressure or temperature are
reached.
Pressure transducers and certain proportional
controllers can also be used to provide closer
pressure control.
High and Low Pressure Limit
Combine the thermistor with a pressure switch to
automatically convert the electric valve to a pres-
sure limiting control. A high pressure limit switch
prevents further increase when its pressure setting
is reached. A low pressure limit switch prevents fur-
ther decrease when its pressure setting is reached.
A low pressure limit switch is wired in parallel with
the thermistor. This arrangement provides zero de-
gree superheat between high and low pressure lim-
its. Note that the compressor suction line is still al-
lowed to superheat.
Evaporator
Condenser
Receiver
Compressor
625
Electric
Valve
24 Volts
Thermistor
Low
Pressure
Switch
High
Pressure
Switch
Accumulator
Compressor
Condenser
Chiller
625
Electric
Valve
Thermistor
24 Volts
Compressor
Evaporator
Condenser
Receiver
24 Volts
625
Electric
Valve
Pressure
Switch