Case 580SR Service Manual page 783

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EXPANSION VALVE
The expansion valve is located underneath the
evaporator in the pressure line leading from the re-
ceiver/dryer and performs the following functions:
1. METERING ACTION
A metered orifice changes the liquid refrigerant
from a high pressure low temperature liquid to a
low pressure, lower temperature atomized liq-
uid.
2. MODULATING ACTION
A thermostatically controlled valve within the ex-
pansion valve body controls the volume of liquid
refrigerant passing through the orifice and
makes sure the refrigerant is fully vaporized
within the evaporator. Liquid refrigerant at would
damage the compressor reed valves or freeze
the pistons.
3. CONTROLLING ACTION
The valve responds to changes in the cooling re-
quirements. When increased cooling is required,
the valve opens to increase the refrigerant flow
and when less cooling is required the valve clos-
es and decreases the refrigerant flow.
Expansion valve - operation
All of the needed temperature sensing and pressure
sensing functions are consolidated into this basic unit
and no external tubes are required for these purposes.
The refrigerant from the condenser and receiver dry-
er enters the thermostatic expansion valve as a high
pressure warm liquid. Upon passing through the ball
and spring controlled metering orifice, the pressure
and temperature of the refrigerant is reduced and
the refrigerant leaves the thermostatic expansion
valve as a low pressure, lower temperature atom-
ized liquid.
The atomized liquid now passes through the evapo-
rator where it absorbs heat before returning via the
expansion valve to the compressor as a warm low
pressure vapour. There are two refrigerant passag-
es in the valve. One passage is in the refrigerant line
from the condenser to the evaporator and contains
the ball and spring type orifice valve.
The other passage is in the refrigerant line from the
evaporator to the compressor and contains the
valve's temperature sensing element.
Liquid refrigerant flow from the condenser and re-
ceiver dryer is controlled by a push-rod forcing the
orifice valve ball off its seat and the spring exerting
pressure on the ball to keep it on its seat.
During stabilized (vehicle shutdown) conditions, the
pressure on the bottom of the expansion valve dia-
phragm rises above the pressure on the top of the di-
aphragm allowing the valve spring to close the
orifice.
SECTION 50 - CAB HEATING AND AIR CONDITIONING
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