Thermostatic Expansion Valve; Central System - PREVOST H3-41 Maintenance Manual

TORQUE: 20-40 lb-ft (27-54 Nm)
6. Put back manual lift stem. Tighten lift stem
assembly and seal cap.
TORQUE: 11 lb-ft (15 Nm)
7. Place the coil assembly.
FIGURE 55: TYPICAL REFRIGERANT SOLENOID VALVE
CAUTION
The filter-dryer must be replaced after a
severe system failure or if a line in the
system has been opened over a prolonged
period of time. The line will then have to be
properly evacuated.
Best practice would be to replace the filter-
dryer each time a line is opened.
CAUTION
Be careful not to damage the machined faces
while the valve is apart.
4.11

THERMOSTATIC EXPANSION VALVE

4.11.1

Central System

The expansion valve for the main system is a
thermo-sensitive valve with a remote control
bulb head attached to the evaporator outlet line
and is accessible by the evaporator filter access
door. The valve regulates the flow of refrigerant
liquid into the evaporator coils and is controlled
by the suction gas temperature leaving the
evaporator. The bulb
refrigerant gas temperature as it leaves the
evaporator. High temperature
head senses the
will cause
PA1621 Maintenance Manual All Series | Revised February 2019
SECTION 22: HEATING AND AIR CONDITIONING
expansion and pressure on the power head and
spring. Such action causes the assembly valve
to open, allowing a flow of refrigerant liquid into
the evaporator.
FIGURE 56: EXPANSION VALVE
The remote bulb and power assembly is a
closed system. The pressure within the remote
bulb and power assembly corresponds to the
saturation pressure
temperature leaving the evaporator and moves
the valve pin in the opening direction. Opposed
to this force, on the under side of the diaphragm
and acting in the closing direction, is the force
exerted by the superheat spring. As the
temperature of the refrigerant gas at the
evaporator outlet increases above the saturation
temperature corresponding to the evaporator
pressure, it becomes superheated. The pressure
thus generated in the remote bulb and power
assembly surpasses the combined pressures of
the evaporator pressure and the superheat
spring, causing the valve pin to move in the
opening direction.
Conversely, as
refrigerant
gas
decreases, the pressure in the remote bulb and
power assembly also decreases and the
combined evaporator and spring pressures
cause the valve pin to move in the closing
position.
As the operating superheat is raised, the
evaporator capacity decreases, since more of
the evaporator surface is required to produce
of the refrigerant
the temperature of
leaving the evaporator
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the