Komatsu 830E-AC Shop Manual page 937

Expansion Block Valve
The expansion block valve controls the amount of
refrigerant entering the evaporator coil. Both inter-
nally 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 evapo-
rator. This produces the desired cooling effect.
The amount of refrigerant metered into the evapora-
tor varies with different heat loads. The valve modu-
lates 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 open-
ing wider to allow more refrigerant to pass into the
evaporator. As the load decreases, the valve reacts
and allows less refrigerant into the evaporator. It is
this controlling action that provides the proper pres-
sure 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 refriger-
ant 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 pressure 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.
Accumulator
As the accumulator receives vaporized refrigerant
from the evaporator, moisture and/or any residual liq-
uid refrigerant is collected at the bottom of the com-
ponent. The moisture is absorbed by the desiccant
where it is safely isolated from the rest of the system.
N04031 10/06
The storage of the liquid refrigerant is temporary.
When the liquid vaporizes into a gas it will be pulled
from the bottom of the accumulator into the compres-
sor. This process not only allows the accumulator to
act as a storage device, but also protects the com-
pressor from liquid slugging.
The low side service port is also located on the accu-
mulator.
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 evapora-
tor 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 oper-
ation, depends upon a temperature differential of the
air and the refrigerant. The greater the temperature
differential, 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
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 out-
let of the evaporator.
Operator Comfort N4-11