Komatsu 930E-2, Shop Manual

Page: 535 / 739

REFRIGERATION -
THE ACT OF COOLING
•
There is no process for producing cold; there is
only heat removal.
•
Heat is always drawn toward cold objects. This
principle is the basis for the operation of a cooling
unit. As long as one object has a temperature
lower than another, this heat transfer will occur.
•
Temperature is the measurement of the intensity
of heat in degrees. The most common measuring
device is the thermometer.
•
All objects have a point at which they will turn to
vapor. Water boiling is the most common exam-
ple of heating until vapor is formed. Boiling is a
rapid form of evaporation. Steam is a great deal
hotter than boiling water. The water will not in-
crease in temperature once brought to a boil. The
heat energy is used in the vaporization process.
The boiling point of a liquid is directly affected by
pressure. By changing pressure, we can control
the boiling point and temperature at which a
vapor will condense. When a liquid is heated and
vaporizes, the gas will absorb heat without
changing pressure. This gas is in a superheated
condition.
•
Reversing the process, when heat is removed
from water vapor, it will return to the liquid state.
Heat from air is attracted to a cooler object.
Usually the moisture in the cooled air will con-
dense on the cooler object.
•
Refrigerant - Only R-134a should be used in the
new mobile systems which are designed for this
refrigerant.
THE REFRIGERATION CYCLE
In an air conditioning system, the refrigerant is circu-
lated under pressure through the five major compo-
nents in a closed circuit. At these points in the system,
the refrigerant undergoes predetermined pressure and
temperature changes.
The compressor (refrigerant pump) takes in low pres-
sure heat laden refrigerant gas through the suction
valve (low side), and as its name indicates, pressurizes
the heat laden refrigerant and forces it through the
discharge valve (high side) on to the condenser.
Ambient air, passing through the condenser removes
the heat from the circulating refrigerant resulting in the
conversion of the refrigerant from gas to liquid.
The liquid refrigerant moves on to the filter-receiver
drier where impurities are filtered out, and moisture
removed. This component also serves as the tempo-
rary storage unit for the liquid refrigerant.
The liquid refrigerant, still under high pressure, then
flows to the expansion valve. This valve meters the
amount of refrigerant entering the evaporator. As the
refrigerant passes through the valve, it becomes a low
temperature, low pressure liquid and saturated vapor.
The remaining low pressure liquid immediately starts
to boil and vaporize as it approaches the evaporator.
This causes the refrigerant to become cold. The hot,
humid air of the cab is pulled through the evaporator
by the evaporator blower. Since the refrigerant is
colder than the air, it absorbs the heat from the air
producing cool air which is pushed back into the cab.
The moisture in the air condenses upon movement into
the evaporator and drops into the drain pan from which
it drains out of the cab.
The cycle is completed when the heated low pressure
gas is again drawn into the compressor through the
suction side.
This simplified explanation of the principles of refrigera-
tion does not call attention to the fine points of refrig-
eration technology. Some of these will be covered in
the following discussions of the components, controls,
and techniques involved in preparing the unit for effi-
cient operation.
M09010 6/97 Air Conditioning System M9-3
for HFC 134a Refrigerant