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Automotive Heating and Air conditioning Lecture Notes
CHAPTER THREE
INRODUCTION TO AUTOMOTIVE AIR CONDITIONING
This chapter describes the role of refrigerants in automotive air conditioning
...
The various
components that make up a typical automotive air conditioning system are also described in this
chapter
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1
REFRIGERANTS
A refrigerant is a substance used in a heat cycle usually including a reversible phase change
from a gas to a liquid
...
The desired thermodynamic properties are a boiling point somewhat below
the target temperature, a high heat of vaporization, a moderate density in liquid form, a relatively
high density in gaseous form, and a high critical temperature
...
The physical principle at the heart of the refrigerating or cooling function of an automotive
air conditioning system is concerned with the enforced changes of state of the refrigerant and the
latent heat involved
...
(NB: Latent heat is the amount of
heat required to cause a substance to change its state without changing its temperature)
...
If the
vaporized refrigerant is next compressed by a pump in order to turn it to liquid, then it can be
arranged to give out its latent heat to the outside air, and the heat exchange cycle between the car
interior and exterior can begin again
...
By the same token the refrigerant
must have a low boiling point and for automotive applications, the temperature is about -26 when it
changes from a liquid to a gas
...
In the mid 1980s there was mounting concern that
when CFC’s are discharged into the atmosphere and then enter the stratosphere, the planet is subject
not only to depletion of the ozone layer that allows an increase in ultraviolet radiation to reach the
earth, but also to a larger amount of the heat radiated from the sun to be absorbed by the earth
...
It
was to reduce these factors that from the early 1990s a replacement hydrofluorocarbon (HFC) based
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Automotive Heating and Air conditioning Lecture Notes
refrigerant, designated as R-134a, has been used to charge automotive air conditioning systems
...
The service recommendations of the vehicle manufacturer should therefore be sought in this
matter
...
Installing a component designed for an R-12 system in a system
built to use R-134a or vice versa may cause component failure and could damage the system
...
2
DISPOSAL OF REFRIGERANTS
Due to the hazardous nature of some refrigerants, different countries have different
legislations or guidelines that govern the disposal of refrigerants
...
Refrigerants should never be mixed together outside of facilities licensed to do so for the purpose of
producing blends
...
3
...
Generally, in refrigeration, work is done to extract heat from
a lower temperature heat source and transfer it to a higher temperature heat sink in order to maintain
the temperature of the heat source below that of the surroundings
...
Automotive air conditioning systems run on the
refrigeration cycle
...
Refrigeration systems that provide cooling for air conditioning are
classified into three main categories
...
3
...
1 Vapour Compression Refrigeration Systems
In these systems, a compressor compresses the refrigerant to a higher pressure and
temperature from an evaporated vapour at low pressure and temperature
...
The liquid refrigerant is then throttled to a low-pressure, low-temperature
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Automotive Heating and Air conditioning Lecture Notes
vapour producing the refrigeration effect during evaporation in a component called the evaporator
...
1 shows a schematic of a typical vapour compression refrigeration system
...
1: Schematic of a typical vapour compression refrigeration system
3
...
2 Absorption Refrigeration Systems
In an absorption refrigeration system, the refrigeration effect is produced by means of
thermal or heat energy input
...
The solution is heated by a
direct fired gas furnace or waste heat, and the refrigerant is again vapourized and then condensed
into liquid form
...
3
...
3 Gas Expansion System
In an air or other gas expansion system, air or gas is compressed to a high pressure by
compressors
...
Because the temperature of air or gas decreases during expansion, a refrigeration effect is produced
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E
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OSEI, Department of Mechanical Engineering, Kumasi Technical University, Kumasi, Ghana
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4
AUTOMOTIVE AIR CONDITIONING
Automotive air conditioning is the process by which air is cooled, cleaned and dehumidified
before entering, or re-entering the passenger compartment of an automobile
...
Automotive air conditioning systems are
designed to allow the driver and or passengers to feel more comfortable during uncomfortable
warm, humid or hot trips in a vehicle
...
The cooling effect of an air conditioner is possible by
the use of a chemical, called refrigerant, that flows through the closed circuit of the air conditioning
system
...
A typical example of refrigerant used in automotive air
conditioning is R-134a
...
2: Heat entering the cabin of an automobile
3
...
The type depends on whether the air conditioning
system uses an expansion valve or an orifice tube
...
5
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Compressor
2
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OSEI, Department of Mechanical Engineering, Kumasi Technical University, Kumasi, Ghana
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Receiver/drier
4
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Evaporator
The compressor is driven by the engine; the condenser is located at the front of the vehicle, in front
of the radiator; the receiver/drier is located in the engine compartment; the expansion valve is
usually attached to the evaporator; and the evaporator is located inside a housing in the dash, with
the heater core
...
3
...
Figure 3
...
The high-pressure vapour is pumped into the condenser, where it is condensed by the
cooling air flow to a high-pressure liquid
...
The clean,
dry high-pressure liquid is pumped to the expansion valve, where it is changed to a low-pressure
mixture of liquid and vapour
...
From the evaporator, the low-pressure vapour passes back
to the compressor, which pumps it through the system again
...
5
...
Compressor
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Automotive Heating and Air conditioning Lecture Notes
2
...
Orifice tube
4
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Accumulator
The compressor is driven by the engine; the condenser is located at the front of the vehicle, in front
of the radiator; the orifice tube is located in the refrigerant line, or in the evaporator; the evaporator
is located inside a housing in the dash, with the heater core; and the accumulator is located in the
engine compartment
...
4
...
Figure 3
...
The high-pressure vapour is pumped into the condenser, where it is condensed by the
cooling air flow to a high-pressure liquid
...
The low-pressure
liquid is then pumped to the evaporator, where the liquid is partly converted to a vapour, cooling
the air passing over the evaporator as it does so
...
(Note: liquid lock of the compressor means liquid refrigerant
entering the compressor)
...
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Automotive Heating and Air conditioning Lecture Notes
3
...
A number of
additional components, whose function is to control and maximize the efficiency of specific
systems are also employed in all systems
...
7
COMPRESSOR
The two main functions of the compressor are to pump refrigerant through the system and to
increase the refrigerant's pressure and temperature so it will condense and release heat
...
The
refrigerant is then pumped through the compressor to the discharge or outlet side where it is
compressed, raising the temperature
...
All automotive air conditioning compressors are belt-driven from the engine's crankshaft
...
5) disengages the compressor from the belt when
compressor operation is unnecessary, or unwanted
...
6 shows a typical compressor
...
5: An exploded view of a typical electro-magnetic compressor clutch assembly
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Automotive Heating and Air conditioning Lecture Notes
Figure 3
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In some systems, the clutch constantly cycles compressor operation on and
off
...
Basically, all compressors fall into one of the following two categories namely: piston type
compressors and rotary vane type compressors
...
Rotary vane
compressors have no pistons
...
Figure 3
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7
...
On the intake stroke, the low-pressure refrigerant vapour from the evaporator is drawn into
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Automotive Heating and Air conditioning Lecture Notes
the compressor (see Figure 3
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The intake of refrigerant occurs through reed valves
...
During the compression stroke,
the gaseous refrigerant is compressed
...
The
discharge side reed valves then open to allow the refrigerant to move into the condenser
...
8: Schematic of rotary vane type compressor operation
3
...
2 Rotary Vane Type Compressor
Rotary vane type compressors consist of a rotor with several vanes and a precisely shaped
housing
...
8)
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The discharge port is located at the point where the gas is completely compressed
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9: Schematic of scroll type compressor operation
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Automotive Heating and Air conditioning Lecture Notes
3
...
3 Scroll Type Compressor
Scroll type compressors have two metal scrolls, one fixed and one movable, which provide
an eccentric motion (see Figure 3
...
As the compressor shaft rotates, an eccentric bushing on the
shaft drives the movable scroll, and refrigerant is forced against the fixed scroll, and towards its
centre
...
The refrigerant
vapour moves in a circular pattern, and its pressure is increased as it moves toward the centre of the
scroll
...
Scroll type compressors provide a longer effective compression stroke, and a smoother startup than other compressor designs, and they produce less vibration
...
8
CONDENSER
The condenser (see Figure 3
...
It is similar in
design to the evaporator, the heater core and the radiator
...
This configuration affords the maximum surface area for heat transfer within a
minimum amount of engine compartment space
...
The
refrigerant vapour enters at the top of the condenser and flows through the coils, conducting heat
through the walls of the tubing and into the cooling fins, then radiating off the fins into the cooler
atmosphere
...
Figure 3
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OSEI, Department of Mechanical Engineering, Kumasi Technical University, Kumasi, Ghana
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9
RECEIVER/DRIER
The receiver/drier is installed in-line on the high side of the system, somewhere between the
condenser and the expansion valve
...
11 and 3
...
The
receiver/drier assembly is always installed on the high side of the system, downstream from the
condenser, but before the expansion valve
...
Figure 3
...
12: Cutaway View of a Typical Receiver/Drier
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Automotive Heating and Air conditioning Lecture Notes
The receiver/drier has two main functions
...
(Note: The evaporator requires
a varying amount of refrigerant, depending on operating conditions)
...
This
agent, which is usually in the form of a silica gel, is known as desiccant
...
If moisture is allowed to accumulate, it can wreak havoc on air
conditioning components and eventually cause system failure
...
10
ACCUMULATOR
Accumulators are used on systems that accommodate an orifice tube to meter refrigerants
into the evaporator (see Figure 3
...
It is connected directly to the evaporator outlet and stores
excess liquid refrigerant
...
The chief role of the
accumulator is to isolate the compressor from any damaging liquid refrigerant
...
It is usually a good idea to replace
the accumulator each time the system is opened up for major repair and anytime moisture and/or
debris is of concern
...
If moisture is
present, it mixes with refrigerant and forms a corrosive acid which causes long term damage to
system components
...
13: Cutaway View of a Typical Accumulator Assembly
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Automotive Heating and Air conditioning Lecture Notes
3
...
The main function of the expansion valve is to
meter, modulate and control the flow of refrigerant
...
14: Cutaway View of a Typical Expansion Valve
A metered orifice within the valve changes the pressure of the incoming liquid refrigerant from high
pressure to low pressure (see Figure 3
...
A thermostatically-controlled valve located inside the
expansion valve body fluctuates toward an open or closed position as required to control the liquid
refrigerant passing through the orifice
...
The low pressure created at the expansion valve makes it possible for the liquid
refrigerant to vapourize as it passes through the evaporator coils, absorbing heat from the vehicle's
interior
...
The pressure in the
diaphragm chamber varies in accordance with the pressure inside a temperature sensing bulb
located close to the evaporator and the capillary tube which links it to the diaphragm chamber
...
A decrease in refrigerant temperature decreases the pressure in the bulb and tube,
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Automotive Heating and Air conditioning Lecture Notes
lessening the pressure on the diaphragm and allowing the valve to close
...
12
ORIFICE TUBE
The orifice tube performs the same function as the expansion valve but have a different
configuration (see Figure 3
...
It is located in the inlet tube of the evaporator, or in the liquid line,
somewhere between the outlet of the condenser and the inlet of the evaporator
...
Figure 3
...
The intermittent operation of the compressor controls the refrigerant flow and pressure
...
Figure 3
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OSEI, Department of Mechanical Engineering, Kumasi Technical University, Kumasi, Ghana
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17: An Exploded View of a Typical Blower Motor
3
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The
evaporator provides several functions
...
A secondary function is dehumidification (dehumidification means the evaporator removes
moisture from the incoming air)
...
The evaporator is usually
installed in a housing under the dash panel
...
The evaporator receives refrigerant from the thermostatic expansion valve or orifice tube as
a low-pressure liquid
...
When the liquid refrigerant receives enough heat, a change
of state – from a low-pressure liquid to a low-pressure vapour takes place
...
The low-pressure refrigerant vapour then continues on to the inlet
(suction) side of the compressor
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OSEI, Department of Mechanical Engineering, Kumasi Technical University, Kumasi, Ghana
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17)
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The blower motor is controlled by a
fan switch with settings from Low to High
...
A reduction in speed decreases the air volume
...
The result is more heat transfer to the cooler refrigerant
...
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29