Air Conditioning Theory
Heat Transfer Diagram

~6 Times more heat
transferred than
sensible heat.
Sensible Heat which is
not very efficient method
of heat transfer

48

Air Conditioning Theory
Temperature/Pressure Relationship
How does the AC system create this phenomenon?
Pressure / temperature relationship: As the
pressure on a liquid is increased, the boiling point
rises. As the pressure on a liquid is decreased, the
boiling point drops.
In an air conditioning system, the refrigerant is
contained in a closed loop plumbing system that can
be pressurized.
The pressure in the evaporator is low, so that all the
refrigerant vaporizes. The pressure in the
condenser is high, so the refrigerant readily
changes state to a liquid.
In an air conditioning system, a compressor is used
to increase the pressure of the refrigerant; this
raises its temperature. The refrigerant vapor
entering the condenser is hot.
In this air conditioning system, an expansion valve
is used to lower the pressure of the refrigerant; the
refrigerant in the evaporator is cold.
Automotive A/C Systems are designed to operate at
pressures that keep the refrigerant at the optimum
temperature for taking heat out of the passenger
compartment.

49

Air Conditioning Theory
Typical A/C Operating Conditions
A/C system typically operates in a 50°F
to 110°F environmental range.
In those given conditions the A/C
operating pressures range from 5 to 30
psig on the evaporator side and 75 to
325 psig on the condenser side.
This correlates in refrigerant
temperature to -2°F to 35°F on the
evaporator side and 73°F to 166°F on
the condenser side.
In 110°F environment, the air flowing
through the evaporator is ~75°F hotter
than the refrigerant and it boils and
becomes a vapor.
In 110°F environment, the air flowing
through the condenser is ~56°F cooler
than the refrigerant and it condenses
and becomes a liquid.

This picture shows the direction of refrigerant and engine coolant flow in the system. The air conditioner
evaporator coil and condenser, and the heater core, are the main points of heat transfer.

50

Air Conditioning Theory
Air Conditioning System
1. The compressor sucks in &
compresses the cool R134a refrigerant
gas, causing it to become hot, high
pressure gas.
2. This hot gas runs through the
condenser & dissipates its heat into its
cooling air flow and condenses into a
liquid.
3. The high pressure liquid enters the
receiver/drier for storage and moisture
removal.
4. The liquid is drawn off the bottom of the
receiver/drier and runs through a
pressure dividing, fixed size orifice hole
in the thermostatic expansion valve.
5. A bulb containing R134a liquid
refrigerant controls the flow of
refrigerant by using a diaphragm to
push down or retract a pin. The pin
pushes downward onto small, metal
ball plugging the orifice allowing liquid
refrigerant to enter the evaporator. The
ball is cradled by an upward spring
force to counter the pin’s downward
force. Together, these forces will
modulate the refrigerant flow through
the TXV.
6. The low pressure liquid refrigerant
travels through the coil and evaporates
thus becoming cold, low pressure gas
which absorbs heat from the hot air
flowing through the coil.
7. A small amount of lightweight oil is
mixed in with the refrigerant to lubricate
the compressor.

51

Air Conditioning Theory
Super Heat
At a certain point in the evaporator the R134a refrigerant is completely vaporized, after that point
any additional heat absorbed by the R134a vapor is described as SUPER HEAT.
The value of this SUPER HEAT is the temperature difference above the point at which
R134a liquid changes to a vapor. A proper Super Heat value is the insurance that vaporized
refrigerant will enter the vapor compressor instead of liquid (i.e. Slugging).
The thermal expansion valve (TXV) super heat setting is established at the factory for particular
applications. Ensure when a valve is replaced that it is of the type suited to the R134a A/C
system.
Saturation temperature = The temperature at which refrigerant in liquid form changes to a
vapor at a given pressure. Saturation temperatures values can be derived from a R134a
Temperature/Pressure Chart.
Actual temperature = The temperature of refrigerant at the evaporator outlet.

Example - Calculation for Super Heat
Actual Refrigerant Temperature = 40⁰F
minus
Saturated Refrigerant Temperature = 29⁰F
Super Heat = 11⁰F

25 Psi
40⁰F
Outlet Vapor
Temperature

Air Flow

25 Psi
29⁰F
Saturated Liquid
Temperature

52

Air Conditioning Theory
R134a Temperature / Pressure Chart

The numbers above represent the boiling points for R134a

53

Air Conditioning Theory
Sub Cooling
At a certain point in the condenser the R134a refrigerant is completely condensed, after that point
any additional heat released by the R134a liquid is described as SUB COOLING.
The value of this SUB COOLING is the temperature difference below the point at which
R134a vapor changes to a liquid. A proper Sub Cooling value is the insurance that liquid
refrigerant will enter the thermostatic expansion valve instead of vapor.
Saturation temperature = The temperature at which refrigerant in vapor form changes to a liquid
at a given pressure. Saturation temperatures values can be derived from a R134a
Temperature/Pressure Chart.
Actual temperature = The temperature of refrigerant at the condenser outlet.

Example - Calculation for Sub Cooling
Saturated Refrigerant Temperature = 138⁰F
minus
Outlet Refrigerant Temperature = 125⁰F
Sub Cooling = 13⁰F

225 Psi
125⁰F
Outlet Liquid
Temperature

225 Psi
138⁰F
Saturated Vapor
Temperature

54

Air Conditioning System Components
Refrigerant System Scheme
The system’s refrigerant is
divided into a high pressure
side and a low pressure side –
the dividing points are the
compressor and the expansion
valve.
In the expansion valve system, the
high side begins at the compressor,
continues through the condenser, on
through the receiver/drier and ends
at the expansion valve. The low side
begins where the high side left off
at the expansion valve. From
there it continues through the
evaporator and ends at the
compressor.

The air conditioning system is divided into
two sides, the high side and the low side.
This illustrates the flow of refrigerant
through the system.

.

Let’s take a look at how each part in the system affects the refrigerant, to allow it to
remove heat from the passenger compartment.

Compressor starts refrigerant flow
The compressor is nothing more than a pump (a gas pump, not a
liquid pump); its job is to move the refrigerant and oil through the
system. When restricted, that flow creates the pressure and
temperature differential in the system. The compressor’s mounted on
the engine, and a drive belt from the engine turns the compressor
driveshaft, working the compressor pistons back and forth. The backand-forth movement of the pistons is what draws the refrigerant in on
the low side and pumps it out on the high side.
When the compressor runs, it pushes all of the refrigerant, in the
system, toward the high side of the expansion valve. At the same
time, it pulls all of the refrigerant it can from the evaporator side of the
expansion valve.

55

The compressor
pumps the
refrigerant through
the system to
provide flow.

Air Conditioning System Components
Compressors and Mount & Drive
Mount & Drive
Consists of a bracket,to mount the compressor to the engine, a belt idler pulley, compressor
drive belt and possibly an extra drive pulley for the crankshaft.
Compressor Mount
Manufactured of either plate, cast iron, steel or aluminum, this bracket should exhibit excellent
noise absorption qualities especially if using a piston type compressor.
Idler Pulley
A small pulley normally used in conjunction with a belt adjusting mechanism, also used when a
belt has a long distance between pulleys to absorb belt vibrations.
Drive Pulley
Some vehicles do not have an extra pulley to accommodate an A/C drive belt, in these cases
an extra pulley is bolted onto the existing crankshaft pulley.
Multiple Belt Drive

Serpentine Belt Drive

Idler
Pulley

Poly “V”
Groove

Power
Steering
Pump

“V” Groove

Power
Steering
Pump

Idler
Pulley
Alternato
r

Alternato
r

Compresso
r

Compresso
r

Water Pump
Pulley
Air Pump

Water Pump
Pulley

Crankshaft
Pulley

Air Pump

56

Crankshaft
Pulley

Air Conditioning System Components
Compressor Clutches
The clutch is designed to connect the rotor pulley to the compressor input shaft when the field
coil is energized. The clutch is used to transmit the power from the engine crankshaft to the
compressor by means of a drive belt.
When the clutch is not engaged the compressor shaft does not rotate and refrigerant does not
circulate the rotor pulley free wheels. The field coil is actually an electromagnet, once
energized it draws the pressure plate towards it, locking the rotor pulley and the pressure plate
together causing the compressor internals to turn, creating pressure and circulating
refrigerant.

Basic Clutch Information
To assure you get the right part the first time, it is important to identify the compressor the
clutch is to be used on, and the characteristics or specifications of the clutch. If possible, check
for label or stamped numbers on the clutch body or the coil assembly to identify the clutch.
These are general guidelines to check and identify the compressor clutch as part of the AC
system, consult the chassis manufacturer or the compressor clutch supplier before any repair
or replacements to the compressor clutch. Evans does not supply the compressor and clutch
for RV A/C systems.

57

Air Conditioning System Components
Compressor Clutches (Continued)
If the clutch marking or label is illegible, the following
information will be needed:
1. Compressor Make and Model
2. Voltage – 12 or 24 Volt
3. Diameter of Clutch Pulley
4. Number of Grooves; Width of Drive Belt
5. A-B Distance or Gauge Line

A “How to Measure” guide is shown in each clutch
section to help in identification.
IF PART NUMBER ON CLUTCH BEING REPLACED CANNOT BE READ, USE THE
FOLLOWING PROCEDURE(S) TO DETERMINE REQUIRED CLUTCH.
HOW TO MEASURE:
1. DETERMINE IF 12 OR 24 VOLT (BLACK WIRE = 12V, GREEN WIRE = 24V)
2. DETERMINE IF CLUTCH IS SINGLE, DOUBLE OR POLY GROOVE
3. MEASURE OUTSIDE DIAMETER OF PULLEY
4. MEASURE WIDTH OF PULLEY GROOVE
a. Single and Double Groove = measure the width of pulley groove
b. Poly Groove = count the number of grooves
5. MEASURE ACCURATELY THE A-B DISTANCE
a. Single and Double Groove = measure the distance from the compressor

mounting hold on the side of the compressor to the middle of the first
belt groove on the pulley
b. Poly Groove = measure the distance from the compressor mounting hole

on the side of the compressor to the middle of the first groove on the
pulley
6. DETERMINE THE NUMBER OF WIRES

58

Air Conditioning System Components
Compressor Lubrication
R134a is part of the air conditioners lubrication system. Never operate an A/C system
without refrigerant as there will be no lubrication for the compressor and internal damage
will occur.
Refrigerant oil is circulated around the A/C system saturated in the refrigerant.
All Evans R-134a Air Conditioning systems currently supplied to the vehicle industry use
PAG oil as the system lubricant. This oil is completely different than the mineral oil used
in R-12 systems.
PAG oil must never be used in an R-12 system, and mineral oil must never be used in an
R-134a system. Use of an incorrect oil can cause serious A/C system damage, and will
void warranty coverage.
When an air conditioning system is serviced, it is important to maintain the original
quantity and type of oil within the A/C system. Oil type and total system quantity will be
listed on the compressor data label attached to the compressor body.
If additional oil is determined to be necessary, only use the exact oil type and viscosity
shown on the compressor label. Do not mix oil brands or types. Ford, Spartan
Freightliner, Workhorse , and other OEM's supply and warrant their own vehicle A/C
compressors. They also supply their own specified compressor oil, which is available
through the standard dealer network.
There are only three situations that justify adding oil to an A/C system:
1. Replacing oil that was removed in the refrigerant recovery process. Measure the
oil quantity that settled in the oil-catch bottle of the recovery machine. Add an
equal quantity of new oil back into the A/C system.
2. Replacing oil that was removed with a defective system component. Add an equal
quantity of new oil back into the A/C system.
3. Replacing oil lost due to system damage or severe leakage. This lost oil will be
visible on or around a damaged or leaking component. Only add new oil if more
than 1-2 ounces is believed to have been lost.

59

Air Conditioning System Components
Compressor Lubrication (Continued)
The quantity of oil typically removed with a replaced component is shown below:
1. Compressor:

Drain and measure oil from old compressor. Typically 2-4 ounces will
be found. Drain and refill new compressor with new oil, using the
same quantity found in the old compressor. Use a minimum of 2
ounces in the new compressor. Never add more than 8 ounces to a
new compressor, to avoid compressor start-up damage.

2. Condenser:

2 ounces.

3. Receiver-Drier:

1 ounce.

4. Evaporator:

1 ounce.

5. System Hoses*: Typically none -- drain and measure.
*Systems with hoses 15 feet or longer, contact Evans for assistance.

Precautions when using PAG oil:
- Do not allow PAG oil to contact bare skin or vehicle paintwork.
- Flush skin immediately when using PAG oil.
- Avoid breathing PAG oil/R134a mixture.
- PAG oil is highly hygroscopic. Open containers only when ready to use. Cap
container immediately after use.

60

Air Conditioning System Components
Evaporators
R134a enters the evaporator coil as a cold low-pressure liquid. As this liquid passes
through the evaporator coil, heat moves from the warm air blowing across the
evaporator fins into cooler refrigerant. This air that has now been cooled is then ducted
into the cabin via the blower motor.
When there is enough heat to cause a change of state, a large amount of the heat
moves from the air to the refrigerant. This causes the refrigerant to change from a lowpressure cold liquid into a cold vapor. (Latent heat of vaporization).
As the warmer air blows across the evaporator fins, moisture contained in that air
(humidity) will condense on the cooler evaporator fins. Condensed moisture then runs
off through the drain tubes located at the underside of the evaporator case.

Evans uses 4 different versions of combination heater core and evaporator,
tube and fin heat exchangers in the RV industry:

61

Air Conditioning System Components
Block Type Thermostatic Expansion Valve (TXV)
The Thermostatic Expansion Valve (also referred
to as TXV) is located at the evaporator inlet;
controlling the flow of refrigerant entering the
evaporator, thus the cooling load or the evaporators
temperature.

TXV - Open

10.
F2

Sensing temperature changes via the diaphragm
(11), the metering valve constantly opens or closes
as it precisely meters the amount of refrigerant
needed. The pathway internally, is smaller than the
refrigerant line, causing the pressure to drop,
changing it from a high pressure liquid to a low
pressure liquid mist. The expansion valve, along
with the compressor, are the dividing lines between
the high and low pressure sections of the system.

F1 - Temperature sensing
This is a sealed diaphragm and sensor containing
refrigerant. As refrigerant leaving the evaporator
coil outlet passes over sensing element (12) the
refrigerant (9) above the diaphragm (11) expands
moving pin (8) downwards pushing ball valve (6)
away from the metering orifice (5).
F2 - Pressure compensation
This is a passage (10) in the block valve outlet side
where refrigerant can build up under the diaphragm
(11) to act as an opposing pressure to help
regulate the amount of refrigerant into the
evaporator coil inlet side.
F3 - Pressure spring
This spring (7) is located under the ball valve (6)
and acts as an opposing force trying to move the
ball valve towards the metering orifice (12) and to
reduce refrigerant flow to the evaporator coil inlet.

12.

3.

4.

Low
Pressure
2. Liquid

As the refrigerant from the outlet side of the
evaporator passes over the sensing element (12),
expansion or contraction of the refrigerant takes
place causing the activating pin (8) to move the ball
valve (6) away or closer to the metering orifice. This
allows more or less refrigerant to enter the
evaporator coil inlet.
Pressures in control
As shown in the illustration, the block valve controls
refrigerant flow by using a system of opposing
pressures which we will call:

1.

High
Pressure
Liquid

5.
6.

8.

7. F3

1.
2.
3.
4.
5.
6.
7.

From Filter Drier
To Evaporator Inlet
From Evaporator
To Compressor
Metering Orifice
Ball
Spring

8. Activating Pin
9. Refrigerant