TECHNICAL PRESENTATION

AIR CONDITIONING FUNDAMENTALS

© 2001 Caterpillar Inc.
Property of Caterpillar Inc.
Revised October 1, 2001
 -3-

TABLE OF CONTENTS

INTRODUCTION ..................................................................................................................5

AIR CONDITIONING PRINCIPLES....................................................................................7

Heat Transfer.....................................................................................................................7
Measurement of Heat........................................................................................................9
Sensible Heat...................................................................................................................11
Latent Heat......................................................................................................................12
Latent Heat of Fusion and Latent Heat of Vaporization .................................................14
Effects of Pressure ..........................................................................................................16

REFRIGERANT HFC-134A ................................................................................................20

BASIC AIR CONDITIONING SYSTEM............................................................................21

AIR CONDITIONING SYSTEMS AND COMPONENTS ................................................24

Orifice Tube System .......................................................................................................24
Compressor .....................................................................................................................26
Condenser .......................................................................................................................28
In-line Dryer and Orifice Tube .......................................................................................30
Evaporator Unit...............................................................................................................32
Accumulator....................................................................................................................33
Thermostatic Expansion Valve System...........................................................................34
Thermostatic Expansion Valve .......................................................................................36
Receiver-dryer.................................................................................................................38
"H" Block Expansion Valve System ...............................................................................39
"H" Block Expansion Valve............................................................................................41
Thermostatic Switch .......................................................................................................42
Compressor Clutch..........................................................................................................44
Low Pressure Switch.......................................................................................................45
High Pressure Relief Valve .............................................................................................46
Moisture Indicator...........................................................................................................47

SAFETY PRECAUTIONS...................................................................................................48

AIR CONDITIONING PERFORMANCE TESTS ..............................................................50

Visual Inspection, Engine Off.........................................................................................50

Operation Inspection, Engine On....................................................................................55

 -4-

TABLE OF CONTENTS (continued)

AIR CONDITIONING SERVICE TOOLS ..........................................................................64

Electronic Leak Detector ................................................................................................65
Refrigerant Tanks............................................................................................................66
Recover, Evacuate and Charge Unit ...............................................................................67
Vacuum Pump .................................................................................................................68
Refrigerant Charging Scale.............................................................................................69
Refrigerant Analyzer.......................................................................................................70
Air Conditioning Component Flusher ............................................................................71

CONCLUSION.....................................................................................................................72

SLIDE LIST..........................................................................................................................73

SERVICEMAN’S HANDOUTS ..........................................................................................74

 -5- Air Conditioning Fundamentals

CONDENSER
COIL
COMPRESSOR

INLINE CONDENSER FAN

DRYER

EVAPORATOR
COIL
ACCUMULATOR

ORIFICE TUBE SYSTEM

EVAPORATOR BLOWER FAN

INTRODUCTION:

This module will discuss the natural principles for removing heat as
applied to the operation of vehicle air conditioning systems.
Basic vehicle air conditioning system components and component
functions are explained as they relate to the operation of the air
conditioning system and the procedures for inspecting and servicing the
air conditioning system.
Basic safety practices will also be covered.
The contents of this module should be treated as general information for
air conditioning systems in most earthmoving machines.

➥
 -6 Air Conditioning Fundamentals

The color codes for refrigerant used throughout this presentation are as
follows:
Red - High pressure liquid
Red and White Stripes - Low pressure liquid
Purple - High pressure gas
Purple and White Stripes - Low pressure gas
Green - Refrigerant oil
 -7- Air Conditioning Fundamentals

EVAPORATOR COIL

AIR CONDITIONING PRINCIPLES

Heat transfer

Many know what air conditioning does, but very few understand how it
works. An air conditioner evaporator, surprisingly enough, works
similarly to a pot of boiling water on a stove. In fact, the reason why an
air conditioner can continue to cool the air is because a liquid called the
refrigerant is boiling within the evaporator coil. Or course, everyone
knows a boiling pot is "hot" and an air conditioner is "cold." A cold
substance that boils is usually quite confusing.
Cold is thought to be a definite condition. Actually, the condition
regarded as "cold" does not exist. Cold can only be defined in a negative
way by saying "cold" is the absence of "heat." When heat is removed
from a substance, it becomes cold as a result. Both the pot of boiling
water and the air conditioner are simply devices for removing heat.
 -8- Air Conditioning Fundamentals

The basis of all air conditioning systems is that heat flows from a warmer
object to a cooler object. All substances contain some heat. Theoretically,
the lowest temperature obtainable is 459° below 0°F (no one has reached
that temperature). Anything warmer than 459° below 0°F contains heat.
When making an object cold, the heat in the object being made cold is
transferred to another object. Like water, which always flows downhill,
heat always flows from a warm object to a colder object.
Three ways in which heat is transferred are:
- Conduction, heat travel through a solid object.

- Convection, heat travel through a substance such as water, steam or

air.

- Radiation, when the increase in the temperature of a substance

allows a measurable amount of heat to escape.
 -9- Air Conditioning Fundamentals

Measurement of Heat

Heat is measured by intensity and by quantity. Place a pot of water over

a flame on a stove. The water gets hotter and hotter until the water boils.
A thermometer in the water shows the temperature. The thermometer
tells the intensity of heat, not the quantity of heat present.
The unit for measuring quantity of heat is called a British Thermal Unit,
sometimes abbreviated to BTU. One BTU is specified as that amount of
heat necessary to raise 1 pound of water 1°F (473.6 ml of water 0.55°C).
 - 10 - Air Conditioning Fundamentals

The quantity of heat can best be explained by thinking of heat as drops of

red coloring dye. Each drop of dye corresponds to 1 BTU. If one drop of
red dye is added to a cup of water, the water will turn slightly pink. Two
drops will turn the water reddish in color. Adding more drops will turn
the water succeedingly deeper shades of red.
Correspondingly, adding more BTU's to the water increases the
temperature.
 - 11 - Air Conditioning Fundamentals

0°C 100°C
(32°F)
WATER
+ 180 BTU'S = (212°F)
WATER
(189.9 kJ)

Sensible heat

Two types of heat also exist: sensible heat and latent heat.
Heat that is measured with a thermometer is called "sensible heat."
Sensible heat can also be felt. Another explanation for sensible heat is the
amount of heat needed to raise 1 pound of water from 0°C (32°F) to
100°C (212°F).
 - 12 - Air Conditioning Fundamentals

0°C (32°F) 0°C (32°F)

Latent heat

The second type of heat is called "latent heat." Latent heat is hidden heat.
("Latent" is the Latin word for hidden.) Latent heat cannot be felt nor can
latent heat be measured with a thermometer.
Latent heat can best be explained by inserting a thermometer into a block
of ice. The thermometer reads 0°C (32°F). Allow the block of ice to melt
and collect the melting water in a container. When the block of ice is
checked a few hours later, the block of ice is smaller because some has
melted away. However, the thermometer reads 0°C (32°F). Where did
the heat go that caused the ice to melt? Some thought the added heat was
in the water that melted from the ice. However, checking the water
temperature as the water melts from the ice shows the water temperature
to be only slightly higher than the temperature of the ice.

➥
 - 13 Air Conditioning Fundamentals

The slight increase in the water temperature does not account for all the
heat the ice has absorbed. The only answer left is that the latent heat has
been used up to change the ice from a solid to a liquid.
All solids soak up huge amounts of heat when changing from a solid to a
liquid.
 - 14 - Air Conditioning Fundamentals

100°C (212°F)

0°C (32°F)

Latent Heat of Fusion and Latent Heat of Vaporization

Water changes into ice or ice changes into water at 0°C (32°F) sensible
heat. The process of changing ice into water or water into ice is called
"latent heat of fusion." 144 BTU's of latent heat is added to change 1
pound of ice into 1 pound of water. Therefore, the ice must absorb 144
BTU's of latent heat. To change 1 pound of water into 1 pound of ice, 144
BTU's of latent heat is removed from the water.
Water changes into steam or steam changes into water at 100°C (212°F).
The process of changing water into steam or steam into water is called
"latent heat of vaporization." 970 BTU's of latent heat is added to change
1 pound of water into steam. Therefore, 970 BTU's of latent heat is
absorbed into 1 pound of water before all of the water is turned into
steam.

➥
 - 15 Air Conditioning Fundamentals

Just as all solids soak up huge amounts of heat when changing to a liquid,
liquids soak up huge amounts of heat when changing to a gas.

Put some water in a pot, place a mercury thermometer in the water, and
place the pot over a flame. As the water heats, the thermometer reading
will rise. At atmospheric pressure, the water boils when the thermometer
reaches 100°C (212°F) sensible heat. Increase the flame and the water
will boil faster.
However, the thermometer reading will not increase above 100°C
(212°F). What happens to the additional heat from the increased flame?
The additional heat is used to change the water from a liquid to a gas.
Since the temperature of the boiling water does not increase above 100°C
(212°F), the boiling must be a natural means for the water to cool itself.
 - 16 - Air Conditioning Fundamentals

E
PH ER
OS
M
AT

EARTH

OCEAN

Effects of Pressure

As previously stated, at atmospheric pressure, water boils at 100°C

(212°F). What is atmospheric pressure?

Atmospheric pressure can be defined as "the weight of the atmosphere

upon an object." Pressure, regardless of how it is produced, is measured
in pounds per square inch (psi). At sea level, atmospheric pressure is 14.7
psi. Any pressure less than sea level (14.7 psi) is known as a "partial
vacuum" or commonly called a "vacuum." Vacuum is measured in inches
of mercury (in. Hg). A perfect vacuum (0 psi) has never been produced.
No one has been able to mechanically obtain ZERO pressure.
 - 17 - Air Conditioning Fundamentals

128 kPa 102.8°C

(18.5 PSI) (217°F)

101 kPa 100°C

(14.7 PSI) (212°F) 80 kPa 60.5°C
(11.7 PSI) (141°F)

10

There is a direct relationship between a liquid’s boiling point and the

pressure on the liquid’s surface.

Shown are three pots of boiling water. The pot on the left has a pressure
of 14.7 psi and the water boils at 100°C (212°F). Increasing the pressure
inside the pot causes the water to boil at a higher temperature. Decreasing
the pressure inside the pot (creating a vacuum) causes the water to boil at
a lower temperature. The pressure can be decreased (a vacuum created) to
a point where the water boils without the flame.
 - 18 - Air Conditioning Fundamentals

VAPOR COMPRESSION

132°F
80°F 134 PSI
84 PSI

32°F
30 PSI

11

There is a direct relationship between the temperature of a vapor and the

amount of pressure on the vapor.
When the pressure on the vapor is increased, the temperature of the vapor
also increases.
 - 19 - Air Conditioning Fundamentals

GAUGE MANIFOLD
0

WATER
VACUUM PUMP

12

There is a direct relationship between a vacuum, the ambient temperature

and the boiling point of a liquid.
Shown is a manifold gauge set connected to a vacuum pump and a flask
with water. The vacuum pump lowers the pressure in the flask thus
creating a vacuum. At a room temperature of 21.1°C (71°F), water boils
with a vacuum of 28.2 in. Hg. (0.7 psi).
Boiling water is a natural cooling process. The boiling water removes the
same amount of latent heat when boiling at 21.1°C (70°F) as when boiling
at 100°C (212°F).
Substances other than water react in the same manner but at different
temperatures.
 - 20 - Air Conditioning Fundamentals

DO NOT HEAT KEEP UPRIGHT

KEEP AWAY FROM FLAME

WEAR SAFETY
GLASSES

DO NOT DROP
DO NOT FREEZE R-134a
DANGER
WEAR GLOVES WHEN HANDLING

13

REFRIGERANT HFC-134A

The substance used in air conditioning systems is called "refrigerant."

Many refrigerants are available. In fact, any liquid that will boil at
temperatures near the freezing point of water can be used as a refrigerant.
However, a good refrigerant should be non-poisonous and non-explosive
to be safe. Also, a good refrigerant should be non-corrosive, odorless and
mix well with oil.
The refrigerant that is used in current Mobile Air Conditioning Systems is
known as "Refrigerant HFC-134a." HFC-134a is made from
Hydrogenated Fluorocarbons. HFC-134a has the same advantages of R-
12 plus HFC-134a will not harm the atmosphere.
 - 21 - Air Conditioning Fundamentals

HFC-134a

14

BASIC AIR CONDITIONING SYSTEM

Shown is an open flask of Refrigerant-HFC-134a at room temperature.

The open flask represents the evaporator in an air conditioning system.
When at atmospheric pressure (14.7 psi), HFC-134a boils at -
27°C (-16°F). The heat in the room causes the refrigerant to boil. As the
refrigerant boils, heat is drawn away from the surrounding area. The
absence of heat makes the surrounding area cooler. However, such a
system is not economical nor is it good for the atmosphere.
 - 22 - Air Conditioning Fundamentals

COMPRESSOR

HIGH PRESSURE LOW PRESSURE

15

Continue to build the basic air conditioning system by adding a

compressor and a high pressure flask. The high pressure flask serves the
same function as the condenser in a basic air conditioning system. Cork
both flasks to prevent the refrigerant from escaping.

As the liquid refrigerant boils in the low pressure flask, the vapor is drawn
through a hose into the compressor. The compressor increases the
pressure of the vapor and the intensity of the heat. Since temperature is a
measurement of the heat intensity, the temperature of the vapor increases.
The high pressure, high temperature vapor flows into the high pressure
flask. The temperature of the high pressure vapor is higher than the
temperature of the surrounding area. Therefore, heat flows from the high
pressure vapor to the surrounding area. The high pressure vapor cools and
changes into a high pressure liquid.
 - 23 - Air Conditioning Fundamentals

COMPRESSOR

ORIFICE

HIGH PRESSURE LOW PRESSURE

16

Complete the system by adding a hose to connect the flask of high

pressure liquid to the flask of low pressure liquid. An orifice is inserted in
the hose to maintain a pressure difference between the high pressure
liquid and the low pressure liquid.
When the flask of low pressure liquid refrigerant boils, the boiling process
collects heat from the surrounding area. The low pressure refrigerant
vapor is drawn through a hose into the compressor. The compressor raises
the pressure and temperature of the vapor and stores it in the high pressure
flask. The high pressure, high temperature vapor gives up heat to the
cooler surrounding area, causing the high pressure vapor to cool and
condense into a high pressure liquid. The high pressure liquid refrigerant
flows through a hose and orifice to the flask for low pressure liquid
refrigerant. The low pressure liquid refrigerant boils and repeats the
cycle.
 - 24 - Air Conditioning Fundamentals

CONDENSER
COIL
COMPRESSOR

INLINE CONDENSER FAN

DRYER

EVAPORATOR
COIL
ACCUMULATOR

ORIFICE TUBE SYSTEM

EVAPORATOR BLOWER FAN

17

AIR CONDITIONING SYSTEMS AND COMPONENTS

Orifice Tube System

The standard air conditioning system contains five basic components:

Compressor - Increases pressure and temperature of refrigerant

vapor
Condenser - Removes the heat from the high pressure high
temperature refrigerant vapor causing the vapor to
change into high pressure liquid refrigerant
In-line dryer - Contains the desiccant and the orifice tube. Quick
disconnects allow the in-line dryer to be easily
changed when needed
Evaporator - Low pressure liquid refrigerant boils, collecting
heat from the surrounding area
Accumulator - Acts as a liquid/vapor separator and ensures that
only vapor will reach the compressor
➥
 - 25 Air Conditioning Fundamentals

On the orifice tube system, the liquid refrigerant leaving the evaporator
can damage the compressor. Therefore, an accumulator is located in the
suction line after the evaporator. The accumulator acts as a liquid/vapor
separator and ensures that only vapor will reach the compressor.

On some orifice tube systems, the orifice tube is located in the low
pressure liquid line to the evaporator and the desiccant is in the
accumulator.
On systems with an in-line dryer, the desiccant is in the dryer.
 - 26 - Air Conditioning Fundamentals

COMPRESSOR
INTAKE EXHAUST INTAKE EXHAUST
PASSAGE PASSAGE PASSAGE PASSAGE

EXHAUST
VALVE EXHAUST
INTAKE VALVE
VALVE INTAKE
VALVE

INTAKE STROKE COMPRESSION STROKE

18

Compressor

The dual purpose of the compressor is:

- Increase the temperature and pressure of refrigerant gas from the
evaporator
- Circulate the refrigerant throughout the system.

The compressor has reed valves to control the entrance and exit of
refrigerant gas during the pumping operation.
As the piston moves downward in the bore, the suction reed or intake
valve opens and the discharge reed or exhaust valve closes. The low
pressure, heat laden refrigerant gas is drawn from the evaporator into the
compressor. As the piston moves upward in the bore, the compressor
pressurizes the gas, thus increasing the intensity of the heat.

➥
 - 27 Air Conditioning Fundamentals

Since temperature is a measurement of heat intensity, the temperature of

the gas increases. The high pressure, high temperature gas closes the
suction reed valve or intake valve and opens the discharge reed valve or
exhaust valve. The gas is forced through a hose to the condenser.
The pressure increase is accomplished by adding a restriction in the high
pressure side of the system. The restriction is caused by the orifice tube.
The orifice tube is explained later in this presentation.
 - 28 - Air Conditioning Fundamentals

CONDENSER
FROM
COMPRESSOR

TO INLINE
DRYER

19

Condenser

The purpose of the condenser is to transfer the heat in the refrigerant gas
to the atmosphere and convert the refrigerant gas into a liquid. High
pressure, high temperature refrigerant gas flows from the compressor into
the condenser. As the hot, high pressure gas flows through the condenser,
heat flows from the hot gas to the cooler air flowing through the
condenser coils. The high pressure refrigerant gas cools and condenses
into high pressure liquid. The high pressure liquid flows from the
condenser to the in-line dryer.
Two basic types of condensers are commonly used:
Ram Air - Used in automotive applications
Forced Air - Used on construction equipment.
The ram air condenser depends on machine movement to force large
volumes of air through the condenser coils.
➥
 - 29 Air Conditioning Fundamentals

The forced air condenser uses fans to move large volumes of air through
the condenser coils. The air is cooler than the refrigerant gas inside the
condenser. Heat flows from the hot refrigerant gas to the cooler air.
 - 30 - Air Conditioning Fundamentals

QUICK DESICCANT
DISCONNECT
OUTLET

QUICK
DISCONNECT
ORIFICE TUBE MOISTURE INLET
ASSEMBLY INDICATOR

O-RINGS
TUBE IN-LINE
DRYER

SCREEN BODY SCREEN TABS

20

In-line Dryer and Orifice Tube

The in-line dryer contains a desiccant bag and two quick disconnects. The
disconnects allows the in-line dryer to be changed without reclaiming the
refrigerant. Some in-line dryers may have a moisture indicator.
On most orifice tube systems, the orifice tube is installed in the in-line
dryer. The orifice tube consists of a small tube through the center of a
plastic body, two o-rings, two screens and two tabs.
The two screens (one on each end) filter the refrigerant that flows through
the small tube. The two o-rings are positioned to seal against leakage past
the outside of the orifice tube. The two tabs engage the tooling when
installing and removing the orifice tube.
The orifice tube separates the A/C System high side from the low side.
High pressure liquid refrigerant enters the orifice tube and low pressure
liquid refrigerant exits the orifice tube.

➥
 - 31 Air Conditioning Fundamentals

The orifice tube has a fixed diameter and does not have the regulating
capability of the expansion valve. The refrigerant flows from the orifice
tube to the evaporator. The amount of liquid refrigerant entering the
evaporator is usually more than the evaporator can boil off, therefore,
some refrigerant will leave the evaporator in the liquid form.
On some orifice tube systems, the orifice tube is installed in the
evaporator inlet line.
 - 32 - Air Conditioning Fundamentals

EVAPORATOR

FROM ORIFICE
TUBE

TO COMPRESSOR

BLOWER FAN

21

Evaporator and Blower Fan

The purpose of the evaporator and blower fan is to transfer the heat in the
operator's compartment to the refrigerant in the air conditioner.
The blower fan draws heat laden air from the operator's compartment over
the evaporator fins and coils where the air surrenders heat to the
refrigerant.
When the low pressure liquid refrigerant enters the evaporator, the
refrigerant is cooler than the air from the blower fan. The heat in the air
flows into the cooler low pressure liquid refrigerant. Some of the
refrigerant boils and changes into refrigerant gas. The heat laden low
pressure refrigerant gas/liquid combination flows to the accumulator. The
cooler air flows back into the operator's compartment.
 - 33 - Air Conditioning Fundamentals

ACCUMULATOR
WITH DESICCANT WITHOUT DESICCANT
DIVERTER
CAP

INLET INLET

VAPOR
LINE

OIL
BLEED HOLE
DESICCANT

OUTLET OUTLET

22

Accumulator

The accumulator stores the refrigerant gas/liquid mixture and allow only
gas refrigerant to flow to the compressor. The refrigerant gas flows
through the opening (inlet) at the top of the vapor line.
Earlier accumulators contain a diverter cap to keep the liquid away from
the opening in the vapor line. The oil bleed hole allows oil to flow back
to the compressor.
Some accumulators contain a desiccant bag to remove moisture from the
refrigerant. On systems with an in-line dryer, the desiccant is removed
from the accumulator an placed in the in-line dryer.
 - 34 - Air Conditioning Fundamentals

THERMOSTATIC EXPANSION VALVE SYSTEM

CONDENSER
COIL

RECEIVER-DRYER COMPRESSOR

CONDENSER
FAN

CAPILLARY TUBE

EXPANSION
VALVE

EVAPORATOR TO
COIL COMPRESSOR

EVAPORATOR
FAN

23

Thermostatic Expansion Valve System

Many earlier model machines are equipped with the thermostatic

expansion valve system. The purpose of the thermostatic expansion valve
is to:
- Restrict refrigerant flow and allow the compressor to increase the
pressure on the high side of the air conditioning system

- Control the amount of refrigerant entering the evaporator

The part of the air conditioning system from the compressor outlet to the
expansion valve inlet is called the "high side." The thermostatic
expansion valve causes a restriction to refrigerant flow that increases the
pressure between the expansion valve (restriction) and the compressor.
The increase in pressure allows the refrigerant to change from a gas to a
liquid.
➥
 - 35 Air Conditioning Fundamentals

Just as the compressor increases the temperature of the refrigerant by

concentrating the refrigerant into a smaller space, the expansion valve
decreases the temperature by allowing the refrigerant to spread out as it
leaves the orifice in the expansion valve. Because the pressure is greatly
decreased, the refrigerant is coldest as the refrigerant leaves the expansion
valve and enters the evaporator. The part of the air conditioning system
from the expansion valve outlet to the compressor inlet is called the "low
side."
The thermostatic expansion valve system is equipped with a receiver-
dryer.
 - 36 - Air Conditioning Fundamentals

EXPANSION VALVES

TUBE TUBE

DIAPHRAGM
DIAPHRAGM

PIN INLET INLET

INTERNAL
EQUALIZER EXTERNAL
ORIFICE ORIFICE
PASSAGE EQUALIZER
SEAT TUBE
PIN SEAT
SUPERHEATER
SPRING
SUPERHEATER
SPRING

THERMAL OUTLET THERMAL OUTLET

BULB
BULB

INTERNALLY EQUALIZED EXTERNALLY EQUALIZED

24

Thermostatic Expansion Valve

Two types of expansion valves are used on machines: internally equalized

and externally equalized. Both the internally equalized and the externally
equalized expansion valves have a thermal bulb connected to a diaphragm
by a small tube. The thermal bulb contains a refrigerant. A clamp holds
the thermal bulb securely to the evaporator exhaust line. The thermal bulb
is sensitive to exhaust temperature. If the exhaust temperature increases,
the refrigerant inside the bulb expands. The expanding refrigerant exerts
pressure against the diaphragm in the top of the valve. The diaphragm is
connected through a pin to the valve seat. Pressure exerted against the
diaphragm causes the diaphragm pin and valve seat to move. As the valve
seat moves away from the orifice, more refrigerant flows into the
evaporator. An increase in the flow of refrigerant causes the evaporator
exhaust to become cooler. The cooler exhaust temperature causes the
refrigerant to condense in the thermal bulb, reducing the pressure against
the diaphragm, pin and valve seat. The valve seat moves to reduce flow
through the orifice.
➥
 - 37 Air Conditioning Fundamentals

In the internally equalized valve, the pressure of the refrigerant entering

the evaporator acts on the bottom of the diaphragm through the internal
equalizing passage. Gas expansion in the thermal bulb must overcome
the internal balancing pressure and the spring before the valve will open
to increase refrigerant flow.
On the external equalizer valve, the pressure acting on the bottom of the
diaphragm comes from the evaporator exhaust line throough an equalizer
tube. The equalizer tube balances the evaporator exhaust pressure against
the pressure caused by the expansion of the gas in the thermal bulb.
The superheat spring prevents surges of excessive liquid from entering the
evaporator. "Superheat" is an increase in temperature of the refrigerant
gas above the temperature at which the refrigerant evaporated. The
superheat spring is installed against the valve and adjusted to a
predetermined setting at the time of manufacture. The expansion valve is
designed so that the temperature of the refrigerant at the evaporator
exhaust line must have 3°C (5°F) of superheat before more refrigerant is
allowed to enter the evaporator. The spring tension is the determining
factor in the opening and closing of the expansion valve. During opening
and closing, the spring tension retards or assists valve operation as
required.
 - 38 - Air Conditioning Fundamentals

RECEIVER-DRYER
FROM
CONDENSER

TO EXPANSION
VALVE

SCREEN

25

Receiver-dryer

The receiver-dryer has three functions: dry, store and filter liquid
refrigerant. As the high pressure liquid refrigerant flows into the
receiver-dryer, the refrigerant is filtered through a desiccant that removes
any moisture that may have entered the refrigerant. The refrigerant is
stored until needed by the system. When the system calls for refrigerant,
high pressure liquid flows through a fine mesh screen fitted on the pickup
tube. (The screen prevents any debris from circulating through the air
conditioning system.) High pressure liquid flows from the receiver-dryer
to the thermostatic expansion valve.
 - 39 - Air Conditioning Fundamentals

CONDENSER
COIL

RECEIVER-DRYER

CONDENSER
FAN COMPRESSOR

"H" BLOCK
EXPANSION "H" BLOCK EXPANSION
VALVE
VALVE SYSTEM
EVAPORATOR BLOWER FAN

26

"H" Block Expansion Valve System

In the "H" Block expansion valve system the thermostatic expansion

valve is replaced with the "H" Block expansion valve.
When the "H" Block expansion valve opens, liquid refrigerant is metered
into the bottom of the evaporator. The low pressure refrigerant begins to
boil as it flows through the evaporator coil. The refrigerant vapor attracts
the heat from the warmer air circulated by the evaporator fan. The
compressor draws the refrigerant vapor out of the top of the evaporator
and past the temperature sensor. The cooler vapor cools the temperature
sensor. As the temperature sensor cools, the gas in the sensor condenses
and decreases the pressure on the top of the temperature sensor
diaphragm. The diaphragm expands upward moving the rod away from
the ball and spring. The ball and spring starts to close restricting flow
through the expansion valve.

➥
 - 40 Air Conditioning Fundamentals

The temperature sensor controls the operation of the air conditioning

system by allowing the exact amount of liquid refrigerant to be metered
past the ball and spring.
 - 41 - Air Conditioning Fundamentals

"H" BLOCK EXPANSION VALVE

DIAPHRAGM
TEMPERATURE SENSOR

FROM TO
EVAPORATOR COMPRESSOR

ROD

TO EVAPORATOR
FROM CONDENSER

BALL AND SPRING

27

"H" Block Expansion Valve

Some air conditioning systems use the "H" Block expansion valve to
control the amount of refrigerant into the evaporator.
During the compressor cut-out mode, the pressure on the bottom of the
temperature sensor diaphragm increases above the pressure on top of the
diaphragm. The diaphragm expands upward retracting the rod and
allowing the ball and spring to close the valve.
During the compressor cut-in mode, the pressure on the bottom of the
temperature sensor diaphragm decreases rapidly. The higher pressure on
the top of the diaphragm causes the diaphragm to contract downward
moving the rod against the ball and spring, thus opening the valve.
 - 42 - Air Conditioning Fundamentals

R-134a
COMPRESSOR CAPILLARY
TUBE
ELECTRICAL
CAPILLARY
CIRCUIT PIVOTING BELLOWS
FRAME ASSEMBLY

CLUTCH

BATTERY
POINT TEMPERATURE
OPENING ADJUSTING SCREW

28

Thermostatic Switch

The thermostatic switch in the compressor electrical circuit cycles the

compressor, allowing the operator to adjust the amount of coolness
desired and prevent the evaporator from freezing.
The thermostatic switch consists of a stationary contact and a pivoting
frame attached to a capillary bellows assembly. The capillary tube is
filled with R-134a or a similar refrigerant. The capillary tube is inserted
between the evaporator core fins. The refrigerant in the capillary tube
expands or contracts, depending on the temperature of the evaporator.
The expanding and contracting refrigerant in the capillary tube causes the
bellows to expand and contract. The expanding and contracting bellows
cause the pivoting frame to pivot.

➥
 - 43 Air Conditioning Fundamentals

Part of the wire to the evaporator clutch coil is connected to the stationary
contact, and the other part is connected to the pivoting frame. The contact
and pivoting frame must come together for the switch to close and operate
the compressor clutch.

The operator regulates evaporator cooling by varying the space between

the stationary contact and pivoting frame. Moving the contact and
pivoting frame farther apart (decreasing cooling) causes the bellows to
expand farther before closing the switch. Moving the contact and
pivoting frame closer together (increasing cooling) causes the switch to
close with less bellows movement.
Adjustable thermostats have provisions for regulating the range between
the opening and closing of the switch. The adjustment screw is located
under a removable cover. If the adjustable screw is not found in this
location, the thermostat is non-adjustable.
The non-adjustable thermostat system (sometime called a Freeze Control
System) contains one temperature control knob. The knob is connected to
the heater control valve, which controls the flow of coolant through the
heater coil. The evaporator air flow temperature is controlled by the non-
adjustable thermostat. The cab temperature is maintained by monitoring
the air flow across the heater and evaporator coils. When air flow across
the heater and evaporator coils reaches 2.2° C (36° F), the non-adjustable
thermostat turns the compressor ON. When air flow temperature
decreases to -1.1° C (30° F), the non-adjustable thermostat turns the
compressor OFF.
 - 44 - Air Conditioning Fundamentals

PULLEY ASSEMBLY

DRIVE PLATE

HUB

COMPRESSOR
CLUTCH

SHAFT

BEARING

COIL ASSEMBLY

29

Compressor Clutch

The clutch is driven by the engine crankshaft through a belt to the pulley
assembly on the magnetic clutch. The pulley assembly turns on the
bearing and is not connected to the shaft. The drive plate is splined
through the hub to the shaft. The coil assembly is mounted on the frame
of the compressor and does not rotate.
The electrical current from the thermostat creates a magnetic field in the
coil assembly. The magnetic field pulls the drive plate against the pulley
assembly. The pulley assembly then turns the drive plate, hub and shaft to
operate the compressor.
 - 45 Air Conditioning Fundamentals

30

Low Pressure Switch

Shown is the low pressure sensing switch (arrow) threaded into the
receiver-dryer. The low pressure sensing switch is used to protect the
system from damage due to the lack of oil. Located in the electrical
circuit to the magnetic clutch, the switch opens when system pressure
decreases below 175 kPa (25 psi) and shuts off the compressor. The
switch can be located on the dryer, expansion valve, liquid line, or on the
compressor.
A similar high pressure switch (not shown) is used on some machines to
shut off the system before system pressure reaches the high pressure relief
valve setting. The high pressure switch is located in the electrical circuit
to the magnetic clutch. High system pressure opens the switch and shuts
off the compressor.
 - 46 Air Conditioning Fundamentals

31

High Pressure Relief Valve

The high pressure relief valve (arrow) is located on either the compressor
or the receiver-dryer. The high pressure relief valve allows the refrigerant
to be released to the atmosphere if system pressure increases above 3450
kPa (500 psi). On todays systems, the high pressure relief valve opens a
high pressure switch. This prevents refrigerant from being vented into the
atmosphere.
 - 47 Air Conditioning Fundamentals

32

Moisture Indicator

Shown is the moisture indicator. The moisture indicator is located in the

line between the receiver-dryer and the expansion valve. The moisture
indicator measures the relative moisture in the system. A moisture
reference color chart is on the face of the indicator. The color blue
represents a dry system and the color pink represents a wet system.
The moisture indicator should be checked at the end of each shift. To
check the moisture indicator, look at the indicator ring (2) through the
sight glass (1). If indicator ring is blue in color, the system is dry. If
indicator ring is pink in color, the system has moisture. The moisture
must be removed and the receiver-dryer must be changed.
 - 48 - Air Conditioning Fundamentals

33

SAFETY PRECAUTIONS

The following safety precautions should be followed when servicing air

conditioning systems, operating air conditioning equipment or handling
refrigerants.
1. Wear safety goggles. Escaping refrigerant coming in contact with the
eyes can cause serious injury.

2. Do not use excessive heat on refrigerant containers during the

charging process. Never use direct heat. Use a container of water
that does not exceed 52°C (125°F).

3. Do not discharge refrigerant to the atmosphere. In addition to being

harmful to the earth's ozone layer, Refrigerant 12 when subjected to
an open flame results in a very deadly phosgene gas.

➥
 - 49 Air Conditioning Fundamentals

4. Always work in a well ventilated area. Inhaling refrigerant, even in

small amounts, can be cumulative and cause light-headedness.
Refrigerants can also cause irritation to the eyes, nose and throat.

5. Do not weld or steam clean an air conditioning system. Excessive

pressure could build up in the system.

6. Do not mix R-134a with air for the purpose of leak testing. When
under pressure the mixture could explode.

7. When charging a system with the engine running, be sure the high
pressure gauge valve is closed.

8 Be alert when the engine is running and stay clear of rotating

components.

9. Do not recover or transfer refrigerant into a disposable tank. Always

use a DOT approved tank. Look for DOT4BA or DOT4BW on the
tank.

10. Do not fill a storage tank to more than 60% of its gross weight rating.

11. Do not weld or steam clean near vehicle installed air conditioning
lines. The heat can cause excessive refrigerant pressure.

12. Do not transport refrigerant in passenger compartment of a vehicle.

13. Do not expose refrigerant to open flames, high temperatures or direct

sunlight.
 - 50 - Air Conditioning Fundamentals

34

AIR CONDITIONING PERFORMANCE TESTS

Visual Inspection, Engine Off

Correct air conditioning system performance is the number one objective

whether conducting preventive maintenance or a major repair. When
doing a performance test, the first step is a visual inspection of the air
conditioning system components. The visual inspection is performed with
the engine OFF.
 - 51 - Air Conditioning Fundamentals

35

The compressor drive belt may be damaged or loose. A damaged drive

belt must be replaced. When installing a new belt or tightening a loose
belt, use a belt tension gauge. See the appropriate machine Service
Manual for belt tightening specifications.
Inspect the condenser for trash, dirt and other debris that can restrict air
flow. Insufficient air flow through the condenser can cause poor cooling
and lead to compressor damage.
 - 52 - Air Conditioning Fundamentals

36

The evaporator blower or fan can only be effective when air passages are
clear. Condensation traps dirt and debris on the blower side of the
evaporator. The dirt and debris form a coating that restricts evaporator air
flow. The coating must be removed.
Inspect the fresh air and recirculating air filters. Clean or replace the
filters as needed.
 - 53 - Air Conditioning Fundamentals

37

Check the blower motor for satisfactory operation. Operate the blower
motor at all speeds. (Turn the key switch ON if needed to provide power
to the blower motor.) Make repairs if the air flow does not increase as the
control is moved from low speed to higher speeds, if the motor is noisy
and/or if the motor fails to operate in some speeds.
 - 54 - Air Conditioning Fundamentals

38

Operate all air ducts and louver controls. The controls should move freely
without sticking or binding.
 - 55 - Air Conditioning Fundamentals

39

Operation Inspection, Engine On

When making the air conditioning operation checks, the engine should be
at normal operating temperature and the air conditioning system must be
stable.
- Install the manifold gauge set.
- Start the engine and increase the engine speed to approximately 1000
rpm.
- Turn on the air conditioning system. Move the temperature control
to the MAXIMUM position. Move the fan switch to the HIGH
position. Operate for 10 - 15 minutes.
- Increase the engine speed to 1300 - 1400 rpm.
- Conduct the air conditioning system operational checks.
 - 56 Air Conditioning Fundamentals

1 3
2

8
4

5
7
6

40

The manifold gauge set is an important tool in checking performance,

diagnosis and servicing of the air conditioning system. The gauge set is
composed of a low side (compound) gauge (1), a manifold (2) to which
the gauges are connected and a high side gauge (3). The high side hand
valve (4) and low side hand valve (8) allow the system to be evacuated
and serviced through the manifold.
The low side hose connector (7) and high side hose connector (5) connect
the gauge manifold to the air conditioning system. The center service
hose (6) connects the manifold gauge to an external source.
Manifold gauge pressures will be affected by the ambient or outside air
temperature. High side pressures are affected more than the low side
pressures.
When the ambient temperature is above 21°C (70°F), the low side
pressure should read from 70 to 210 kPa (10 to 30 psi) depending on the
ambient temperature and the machine being tested. The high side
pressure should read from 820 to 2075 kPa (120 to 300 psi) depending on
the ambient temperature and the machine being tested.
No two systems will have the exact same manifold gauge readings.
Allow for variations in pressures. Refer to the appropriate service manual
for the machine.
 - 57 Air Conditioning Fundamentals

41

Schrader valves are used to attach the manifold gauge set to the air
conditioning system. Schrader valves eliminate the need for service
valves in the system. The Schrader valves effectively seal the refrigerant
inside the system until the Schrader valves are opened.
The Schrader fitting on the high side (1) is smaller than the fitting on the
low side (2). The difference in fitting sizes is to prevent connecting the
manifold gauge set to the wrong pressure side.
 - 58 - Air Conditioning Fundamentals

SCHRADER VALVE AND SERVICE HOSE

SCHRADER VALVE CORE

VALVE PIN DEPRESSOR

SERVICE
GAUGE PORT

COMPRESSOR
FITTING

42

Shown is a sectional view of a Schrader valve and a service hose with a

Schrader core depressor.
As the high and/or low side pressure hose is threaded onto the Schrader
valve service port, the Schrader core depressor in the hose depresses the
pin in the center of the Schrader valve. The valve is opened allowing
refrigerant to flow between the manifold gauge set and the compressor.
When the hose is removed, the valve closes automatically.
 - 59 - Air Conditioning Fundamentals

PERFORMANCE TEST
LOW SIDE (COMPOUND) HIGH SIDE
PRESSURE GAUGE PRESSURE GAUGE

40 2
80
3
20
600 750 1
450 20
900 120
10
300
10 1050 4
150
1200 30
0 160 0
0 25
50 5
100 40

kP SI
PS Pa
10

a
k

P
I
20
30 6

CENTER INTERNAL
PASSAGE

LOW SIDE HIGH SIDE

HAND VALVE HAND VALVE

HIGH SIDE INTERNAL PASSAGE

LOW SIDE INTERNAL PASSAGE

LOW SIDE SERVICE CONNECTOR HIGH SIDE SERVICE CONNECTOR

CENTER SERVICE CONNECTOR

43

Shown is a sectional view of the manifold gauge set used in a

performance test.
The compound gauge is connected through the low side internal passage
to the low side service connector. The low side service connector is
connected through a hose (not shown) to the low pressure side of the air
conditioning system. When the low side hand valve is closed, the
compound gauge shows only the low side pressure reading.
The high pressure gauge is connected through the high side internal
passage to the high side service connector. The high side service
connector is connected through a hose (not shown) to the high pressure
side of the air conditioning system. When the high side hand valve is
closed, the high pressure gauge shows only the high side pressure reading.
The center internal passages in the manifold connect the center service
connector to the low and high side passages. During a performance test,
the closed hand valves isolate the low and high side passages from the
center service connector.
 - 60 - Air Conditioning Fundamentals

ADDING REFRIGERANT
LOW SIDE (COMPOUND)
PRESSURE GAUGE

40 2
80
3
20
600 750 1
450 20
900 120
10
300
10 1050 4
150
1200 30
0 160 0
0 25
50 5
100 40

kP SI
PS Pa
10

a
k

P
I
20
30 6

CENTER INTERNAL
PASSAGE

LOW SIDE
HAND VALVE

LOW SIDE
INTERNAL PASSAGE

CENTER SERVICE CONNECTOR

44

Shown is a sectional view of the manifold gauge set when adding

refrigerant to the system.
Opening the low side hand valve opens the center service connector to the
low side service connector and the low side gauge. Refrigerant flows into
the center service connector, through the manifold gauge and out through
the low side service connector. The compound gauge registers the low
side pressure during the operation.
 - 61 - Air Conditioning Fundamentals

45

With the air conditioner running, carefully check the relative temperatures
at the HIGH and LOW SIDE of the system.
HIGH SIDE temperature should vary from "hot" at the compressor
discharge to "warm" at the expansion valve. Any sudden drop in
temperature indicates a partial blockage at that point.
LOW SIDE temperature should be "cool." There may be large sweating
or frosting of the suction line from the evaporator to the accumulator
depending on the ambient temperature.
 - 62 - Air Conditioning Fundamentals

46

With the engine speed set at 1300 to 1400 rpm, set the temperature control
to the MAXIMUM cool position and the fan switch in the HIGH position.
Run the air conditioning system for 15 to 20 minutes. Place a
thermometer in the blower air outlet duct and record the reading. Then
use the thermometer to read the ambient (outside) air temperature. The
temperature difference between the air from the air duct and the ambient
air should be as follows:
Ambient Air Temperature Difference
(minimum)
Below 24°C (75°F) 11°C (20°F)
Between 24 - 32°C (75 - 90°F) 14°C (25°F)
Above 32°C (90°F) 17°C (30°F)
 - 63 Air Conditioning Fundamentals

47

The moisture indicator may be added to some air conditioning systems to

aid in determining the amount of moisture in the system. The moisture
indicator is located on the "HIGH SIDE" between the receiver-dryer and
the expansion valve. The moisture indicator consists of the sight glass (1)
and the indicator ring (2). The indicator ring is viewed through the sight
glass.
When checking for moisture, the air conditioning system must operate for
approximately 3 hours. If the indicator ring is blue, the system is dry. If
the indicator ring is pink or white, the system has moisture. The system
refrigerant must be recovered and recycled, a new dryer installed, the
system evacuated and properly charged. This procedure will remove all
moisture from the system.
NOTE: The moisture indicator reading is most efficient after the air
conditioning system has been in operation 3 hours or more. For best
results, have the operator inspect the moisture indicator at lunch time
and at the end of each shift.
 - 64 - Air Conditioning Fundamentals

48

AIR CONDITIONING SERVICE TOOLS

When servicing an air conditioning system, many special tools are needed
in addition to the basic mechanic's tool box. Several special tools are
covered in the following materials.
 - 65 Air Conditioning Fundamentals

49

Electronic Leak Detector

The electronic leak detector is consider the most safe and the most
accurate means of finding a leak in the system. Many electronic detectors
can detect small leaks equivalent to 1/2 oz. per year. The detector will
"beep," activate a light or both when a leak is found.
To obtain accurate results, leak detection must be performed with the
system under pressure. A 50% refrigerant charge in the system is enough
to locate most leaks. However, very small leaks may require that the
system pressure be increased above normal before the leaks can be
located.
 - 66 Air Conditioning Fundamentals

2
1

50

Refrigerant Tanks

The standard tank (1) in which refrigerant is sold should never be used to
reclaim refrigerant. Refrigerant tanks (2) used on recovery/recycling
equipment must be approved by the Department Of Transportation
(DOT). DOT approval is indicated by "DOT 4BW" or "DOT 4BA"
stamped into the tank.
Safety codes recommend that closed tanks not be filled with liquid over
80% of the tank volume. The remaining 20% (called "head pressure
room") is left for liquid expansion.
 - 67 Air Conditioning Fundamentals

51

Recover, Evacuate and Charge Unit

The refrigerant recovery unit should be used to recover refrigerant from

the air conditioning system when making repairs. The refrigerant can
then be recycled and reused in the system after the repairs are completed.
An automatic air conditioning recover, evacuate and charge unit may be
used to perform a one step recovery, evacuating and charging operation.
The evacuation time and the amount of refrigerant charge are both
programmed into the unit. After the refrigerant has been recovered, the
unit will evacuate and charge the air conditioning system automatically.
A large variety of units are available. Some units (such as the unit
shown) are used to recover, recycle, evacuate and charge refrigerant.
Other units may only recover the refrigerant. The refrigerant is then
transferred to a recycling unit to be recycled.
 - 68 Air Conditioning Fundamentals

52

Vacuum Pump

The vacuum pump completely removes all air and moisture from the air
conditioning system by lowering the pressure within the system to a point
where moisture turns to a vapor. The vapor is them pumped out of the
system with the air.
To remove all moisture from the system, the vacuum pump should operate
with the low pressure gauge at 981 mbar (29 in. Hg) for a minimum of 30
minutes.
NOTE: All refrigerant should be recovered from the system before
connecting the vacuum pump.
 - 69 Air Conditioning Fundamentals

1 2

53

Refrigerant Charging Scale

The two types of refrigerant charging scales are the manual operated type
(1) and the automatic type (2). Each type allows the specified amount of
refrigerant to be added to the system regardless of the ambient
temperature.

Currently, the charging scale is the recommended method when charging

the air conditioning system on earthmoving machines.
 - 70 Air Conditioning Fundamentals

54

Refrigerant Analyzer

The refrigerant analyzer has become a vital tool in air conditioning

service.
The refrigerant analyzer identifies the refrigerant, measures the
percentage of purity, indicates the percent of air in the system and
indicates blends and contaminated refrigerants.
Using the refrigerant analyzer will prevent possible contamination of the
recovery equipment with refrigerants other than the one specified for use.
 - 71 Air Conditioning Fundamentals

55

Air Conditioning Component Flusher

The A/C component flusher uses shop air to atomize the flushing solution.
The solution is used to remove residue and other contaminants from the
hoses, evaporator and condenser.
 - 72 Air Conditioning Fundamentals

56

CONCLUSION

This presentation has discussed the basic air conditioning principles, the
basic vehicle air conditioning system components and the component
functions as they relate to the operation of the air conditioning system.

Basic safety practices and the procedures for inspecting and servicing the
air conditioning system have also been covered.

Always check the appropriate Service Manual for the latest service
information and specifications when servicing, testing and adjusting,
and/or making repairs.