Brochure

Catalog CIC-2003-1/US
Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
185
Service
& Check
Valves
Thermostatic and Constant Pressure (Automatic) Expansion Valves
TXVs & AXVs
Thermal Electric ValvesOperation
Thermal Electric Valve Model 625 Operation
the bimetal heater inside the valve head and the bi-
metal deflects upward. This deflection opens the valve
more.
The valve stays open until enough liquid refrigerant is
fed into the evaporator to reduce superheat. Once
superheat is eliminated, wet refrigerant gas again con-
tacts the thermistor. The wet gas cools the thermistor.
The thermistor’s resistance increases and less voltage
is sent to the valve. The valve moves toward closing.
Ambient Effect
The electric valve is calibrated to close in a 70°F
(21°C) ambient when no voltage is applied. Above
70°F (21°C) the valve opens and bleeds off refrigerant.
The valve has a normal tendency to lose capacity at a
lower ambient temperature. The thermistor, however,
compensates for these changes automatically.
Bleed Connections
In many cases, the electric valve is manufactured with
a third connection that bleeds off small quantities of
liquid refrigerant that may collect in the head of the
valve where the bimetal heater is located, which limits
capacity. With the bleed connection, full capacity of the
valve is available at all conditions. The bleed tube is
always connected to the system suction line.
Easy Servicing
The electric valve makes system analysis trouble
shooting fast and easy. Service personnel need only
attach a voltmeter to the electric valve and thermistor.
The readings obtained from the voltmeter will tell how
the valve is operating at a glance. A simple check of
system conditions will indicate thermistor function and
identify problems elsewhere in the system.
The thermal electric valve is operated by, and responds
to, low voltage electricity. The valve’s operation is
simple and easy to understand.
The operating parts
of the valve are
shown in the figure
at the right. They
are: a wire-bound
bimetal heater and
a spring-loaded
needle.
The amount of volt-
age applied to the
heater controls the
degree of the valve
opening. At zero
voltage, the valve is
closed. As voltage is
applied, the heater
deflects the bimetal
upward. The stain-
less steel needle follows the bimetal deflection and
opens the valve. The more voltage applied to the valve,
the greater the valve opening.
Because system pressure or temperature doesn't affect
it, one valve will work for all applications from low tem-
perature freezers to unitary air conditioners.
A special device, called a liquid
sensing thermistor (shown here),
regulates voltage to the electric
valve. The thermistor is installed in
the suction line at the exact point
where complete change of refriger-
ant from liquid to gas is desired.
Here, the thermistor reacts to the
amount of liquid present in the re-
frigerant as it leaves the evaporator.
The schematic at the top of the right
column illustrates the operation of
the thermistor and the electric valve
when they are wired in series. When voltage is applied,
the thermistor acts like a small heater. An increase in
evaporator load causes superheat to increase. This
means the refrigerant changes to superheated gas at
the thermistor location. Because the gas is super-
heated, there is no liquid present and the thermistor
has nothing to cool it. When exposed to dry refrigerant
gas in this manner, the thermistor is heated to a high
temperature by the voltage applied to it and the resis-
tance drops. This causes an increase in voltage across
Model 625 Thermal Electric Valve
Liquid Sensing
Thermistor
Operation of the Thermistor and the Electric Valve when They
are Wired in Series
Optional
Bleed
Connnection
625
Electric
Valve
Evaporator Coil
Compressor
Condenser Coil
Receiver