Komatsu 830E Shop Manual page 561

EXPANSION BLOCK VALVE
The expansion block valve controls the amount of
refrigerant entering the evaporator coil. Both internally
and externally equalized valves are used.
The expansion valve is located near the inlet of the
evaporator and provides the functions of throttling,
modulating, and controlling the liquid refrigerant to the
evaporator coil.
The refrigerant flows through a restriction creating a
pressure drop across the valve. Since the expansion
valve also separates the high side of the system from
the low side, the state of the refrigerant entering the
valve is warm to hot high pressure liquid; exiting it is
low pressure liquid and gas. The change to low pres-
sure allows the flowing refrigerant to immediately begin
changing to gas as it moves toward the evaporator.
This produces the desired cooling effect.
The amount of refrigerant metered into the evaporator
varies with different heat loads. The valve modulates
from wide open to the nearly closed position, seeking a
point between for proper metering of the refrigerant.
As the load increases, the valve responds by opening
wider to allow more refrigerant to pass into the evapo-
rator. As the load decreases, the valve reacts and
allows less refrigerant into the evaporator. It is this con-
trolling action that provides the proper pressure and
temperature control in the evaporator.
This system uses an internally equalized, block type
expansion valve. With this type valve, the refrigerant
leaving the evaporator coil is also directed back
through the valve so the temperature of the refrigerant
is monitored internally rather than by a remote sensing
bulb. The expansion valve is controlled by both the
temperature of the power element bulb and the pres-
sure of the liquid in the evaporator.
NOTE: It is important that the sensing bulb, if present,
is tight against the output line and protected from
ambient temperatures with insulation tape.
M09010 05/02
EVAPORATOR
The evaporator cools and dehumidifies the air before it
enters the cab. Cooling a large area requires that large
volumes of air be passed through the evaporator coil
for heat exchange. Therefore, a blower becomes a vital
part of the evaporator assembly. It not only draws heat
laden air into the evaporator, but also forces this air
over the evaporator fins and coils where the heat is
surrendered to the refrigerant. The blower forces the
cooled air out of the evaporator into the cab.
Heat exchange, as explained under condenser opera-
tion, depends upon a temperature differential of the air
and the refrigerant. The greater the temperature differ-
ential, the greater will be the amount of heat
exchanged between the air and the refrigerant. A high
heat load condition, as is generally encountered when
the air conditioning system is turned on, will allow rapid
heat transfer between the air and the cooler refrigerant.
The change of state of the refrigerant in and going
through the evaporator coil is as important as that of
the air flow over the coil.
All or most of the liquid that did not change to vapor in
the expansion valve or connecting tubes boils
(expands) and vaporizes immediately in the evapora-
tor, becoming very cold. As the process of heat loss
from the air to the evaporator coil surface is taking
place, any moisture (humidity) in the air condenses on
the cool outside surface of the evaporator coil and is
drained off as water.
At atmospheric pressure, refrigerant boils at a point
lower than water freezes. Therefore, the temperature in
the evaporator must be controlled so that the water col-
lecting on the coil surface does not freeze on and
between the fins and restrict air flow. The evaporator
temperature is controlled through pressure inside the
evaporator, and temperature and pressure at the outlet
of the evaporator.
Air Conditioning System
for HFC 134a Refrigerant
M9-5