Komatsu 930E-2, Shop Manual

Page: 537 / 739

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.
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
controlling action that provides the proper pressure
and temperature control in the evaporator.
The externally equalized expansion valve is controlled
by both the temperature of the power element bulb and
the pressure of the liquid in the evaporator.
Some systems may use an internally equalized, block
type expansion valve. With this type valve, the refriger-
ant 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.
NOTE: It is important that the sensing bulb, if present,
is tight against the output line and protected from
ambient temperatures with insulation tape.
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 (ex-
pands) and vaporizes immediately in the evaporator,
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
collecting 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.
M09010 6/97 Air Conditioning System M9-5
for HFC 134a Refrigerant