UNIT I

AUTOMOTIVE AIRCONDITIONING FUNDAMENTALS

1.1 HVAC System:

The acronym HVAC stands for Heating, Ventilation and Air conditioning. The automobile
HVAC system can be thought of as a climate control system having three subsystems:
H Heating
V Ventilation
AC Air conditioning

Heating
The purpose of the heating system is to add heat in the winters. Heating the passenger
compartment is a comparatively easy task, since there is such an abundant supply of waste
heat produced in the engine. This waste heat is expelled into the exhaust system and
absorbed into the engine parts and oil. The heat that is absorbed by the engine parts must
be removed, or the engine would fail in minutes. This is the job of the engine cooling system.
We can tap into this heat source to provide heat to the passenger compartment

1.1.2. Ventilation
The purpose of ventilation air is to keep the car interior fresh, replace stale air, prevent carbon
monoxide from the exhaust, and create positive cabin pressure. The air ducts allow outside air into
the interior via cabin filter to clean the air by trapping dust and pollen particles before they enter the
passenger compartment.

1.1.3. Air Conditioning and Dehumidification

Air conditioning cooling is provided by a vapour compression refrigeration system. The automotive
air conditioner combines the refrigeration system with an air-distribution system and a temperature-
control system to cool, clean, and dehumidify air.

The automobile compartment is heated due to several factors such as:

a. Higher temperature of outside air

b. Solar radiation

c. Engine/exhaust heat

The amount of heat absorbed is dependent upon:

a. Automobile insulation

b. Position of sun and intensity of solar radiation

c. Variation of light and shadow

d. Vehicle color
e. Tinted glass

f. Vehicle speed

g. Wind direction and velocity

The cabin passengers also contribute to heat.

The automobile air conditioner must be capable of removing all the heat inputs. Heat load on the air
conditioner for customary units is expected to be as high as 18000 Btu/hr, which is equivalent to 1.5
tons of air conditioning. About half of this heat is conducted through the body metal and glass and
the remaining comes from air leaks and warm parts within the compartment. The air conditioner can
transfer about one-third of this heat at engine idle and hence full air conditioning is attained only at
raised engine speeds.

Besides temperature, humidity is also one of the comfort factors and in many cases, it is even a
more important factor. Therefore, removal of excess humidity is also one of the requirements of air
conditioning. Water vapour condenses on the cold evaporator fins just as it would on a glass holding
a cold drink. This condensed water then drops off the evaporator and runs out the drain at the
bottom of the evaporator case. This feature is especially useful if it rains outside.

The two main sources of humidity are:

a. Outside air

b. Breathing of passengers

Humidity control is important for safety and defroster operation. In-vehicle humidity is reduced to
about 40% to 45% even on the most humid days if the air conditioning is operated long enough. A
good example of this dehumidification process occurs when a vehicle's A/C is operated on cold day
when the window are fogged up. It usually takes only a short time to dry the air and remove the fog
form the windows.

1.2 Principles of Air Conditioning

The basic principle behind the operation of an HVAC unit is conduction and convection. Heat is
Transferred from a low-temperature region to a high-temperature region in the vehicle, due to the
Pressure difference. This process of heat transfer is called Refrigeration. In order to understand how
air conditioning works, you must first understand the principles of Thermodynamics. Air conditioners
use a fluid, called refrigerant, that absorbs heat when in a liquid State and in the process becomes a
gas (evaporate). For example, when alcohol is rubbed on the Skin, it feels cool. This is because the
alcohol absorbs heat from the area of contact and Evaporates. It is well known that heat is necessary
to change a liquid to a gas. Heat is absorbed From the area in contact with the liquid, thereby cooling
it. The fluids boil at different temperatures depending on the pressure that it is under. To increase Or
decrease the boiling point of a substance, we must alter the pressure on the substance. Increasing
the pressure increases the boiling point. To decrease the boiling point, decrease the Pressure. This
extremely simple principle is the basis of all air conditioning and refrigeration Systems, from home
refrigerators and window A/C units to the largest industrial applications. Thermodynamics deals with
heat and heat transfer, and the refrigeration systems work using the Principles of evaporation,
condensation, and heat transfer. To understand just how an air Conditioning system works, we must
first understand the nature of heat.

1.2.1. Heat and Heat Transfer

Heat is the amount of energy transferred due to the temperature difference between two bodies
(flowing from the high-temperature system to the low-temperature system). It is a boundary
Phenomenon and depends on the path followed in the process. Heat is typically measured in Btu,
Calories, or joules. Heat Transfer is the term used for the movement of heat across the border of the
system due to The difference in temperature between the system and its surroundings at any instant
(which is Given as the amount of heat transferred per unit time). This transfer of heat can happen in
three Ways: conduction, convection, and radiation.

a. Conduction is the mode by which heat is transferred in solid materials from one molecule To
another. The rate at which each specific material can transfer heat depends on thethermal
conductivity of each specific material. A heat exchanger in an HVAC system (evaporator and
condenser) or home furnace uses conduction to transfer heat. In general, the heat exchangers are
constructed from materials, which are good thermal conductors.

b. Convection is the second mode of heat transfer and is defined as the transfer of heat through
the movement of fluids. An HVAC system uses convection in the form of air, water, steam, and
refrigerants in ducts and piping to convey heat energy to various parts of the system. In an
automobile engine, the engine coolant flows around the cylinders, carrying away the heat of
combustion.

c. Radiation carries heat through waves. Heat transferred by radiation travels through space
without heating the space. Radiation or radiant heat does not transfer the actual temperature value.
The first solid object that the heat rays encounter absorbs the radiant heat.

In an automobile air conditioning system, heat is removed from the air entering the passenger
compartment and released from the condenser in front of the radiator, into the atmosphere. Heat
transfer occurs in two main places in an air conditioning system, in the evaporator in the passenger
compartment and in the condenser.

1.2.2. Heat versus Temperature Heat and temperature are not the same.

a. Temperature is the measure of heat intensity (levels of energy). Hot objects have relatively high
heat intensity, while cold objects have relatively low heat intensity. Temperature is measured in
degrees Celsius (°C) or in degrees Fahrenheit (°F).

b. Heat is measured in the metric unit called calorie and expresses the amount of heat needed to
raise the temperature of one gram of water one degree Celsius. Heat is also measured in British
Thermal Units (BTU). One BTU is the heat required to raise the temperature of one pound of water
1°F at sea level. One BTU equals 252 calories.

A temperature reading gives us the heat intensity of a substance and not the actual quantity of heat.
To understand the difference, think of two containers of water—one containing 10 gallons and one
containing 1 gallon. The water in both containers is 50°F. Although they are the same temperature,
the larger container holds 10 times more heat than the smaller one. The larger container has more
thermal mass and therefore has more heat capacity.

Important Fact: Air conditioning is a method of controlling heat. The control of temperature means
the control of comfort.

1.3 Classification of Heat

There are two kinds of heat:

1.3.1. Sensible Heat

Sensible heat is the energy that causes a change in the temperature of an object with no phase

change. When an object is heated, its temperature rises, and this increase in heat is called

sensible heat. Similarly, when the heat is removed from an object and its temperature falls, the

heat removed is also called sensible heat.

1.3.2. Latent Heat

All pure substances in nature are able to change their state. Solids can become liquids (ice to

water) and liquids can become gases (water to vapour), but changes such as these require the

addition or removal of heat. Latent heat is the “extra” heat that is needed to transform a substance

from one state to another. It does not affect the temperature of a substance - for example, water

remains at 100°C while boiling. The heat added to keep the water boiling is latent heat. The heat

that causes a change of state with no change in temperature is called latent heat.

Latent heat is important in the operation of an air-conditioning system because the cooling effect

is derived from changing the state of liquid refrigerant to vapor. The liquid refrigerant absorbs the

latent heat of vaporization, making the air cooler. The cooler air is then blown into the passenger

compartment. It also explains why the terms 'total capacity' (sensible & latent heat) and 'sensible

capacity' are used to define a unit's cooling capacity. During the cooling cycling, condensation
1.3 Automobile Air-conditioning System
Now that we know the heat transfer and refrigeration principles, let’s take a more detailed look at
the automobile air conditioning system.

In an air-conditioning system, the refrigerant picks up heat from the passenger compartment and is
pumped to the condenser where it gives up its heat to the outside air and then flows back through
the system to pick up more heat. The system is shown schematically in the figure below. Refrigerant
is recycled in a closed system. Refrigerant R-134a is currently the most commonly used heat transfer
medium in an automatic air-conditioner refrigeration system.
This refrigerant has a good heat transfer property in the required temperature range, has zero ozone
depletion potential, and does not corrode system components. The heat from the passenger
compartment evaporates the refrigerant in the evaporator thereby changes its phase from liquid to
vapour and cools the air. The vapour is then pumped from the evaporator by the compressor, which
compresses the vapour to increase its pressure. This high pressure, high-temperature vapour passes
through the condenser where it is cooled with outside air and condensed into high-pressure liquid.
This liquid is throttled down to lower pressure in the expansion valve before passing to the
evaporator. This constitutes a continuous cycle in which heat is carried from the evaporator placed in
the passenger compartment to the condenser where it is given up to outside air. During operation,
the refrigerant continuously changes its phase from liquid to gas and then back to liquid again

1.8.1. The Automotive Air Conditioning v/s Room Air Conditioning:

The automotive application places very special demands on the air conditioning system. A typical
Vehicle system has a similar cooling capacity to that required for the air conditioning of a small
House despite the vast difference in volumes to be cooled. The reasons for this are twofold.

a. Firstly, cooling duty per unit volume is much higher for the vehicle because heat transfer
Coefficients between hot ambient air and the outside surfaces are much higher due to the
Movement of the vehicle through the air.

b. Secondly, the proportion of the enclosure consisting of glass is very high for the vehicle – A factor
that makes the effect of direct solar radiation heating very high. On top of this, a Particularly
demanding requirement is to cool the cabin very rapidly after the vehicle has Been soaked in an
ambient temperature of 40°C or higher. At the start of the cool down, Temperatures in the cabin can
be as high as 60 or 70°C.

Another significant way in which automotive air conditioning differs from domestic or air
Conditioners is the compressor drive. The engine-driven compressor has numerous constraints. The
major implications are:

a. In the vehicle, the compressor is belt-driven by the engine so that independent control Over the
compressor speed is not possible. This obviously has significant implications for System control, and
means that there can be calls for high system performance at times When the compressor speed is
very low. Also, the hermetic compressors are not possible, And very effective shaft seals must be
used.

b. The second implication of the external drive is that the compressor must be engine Mounted so
that lengths of the flexible hose must be introduced to accommodate relative Movement between
engine and chassis mounted components.

c. And finally, there are stringent constraints on size, weight, and cost. There are lots of Constraints
to arrange air conditioning components to fit in a compact space and allow for The serviceability.

Because of these constraints, an electrically driven system could be thought of to provide a better
Solution. But it is also not feasible on current vehicles due to insufficient electrical power. The
Applications in which an electrical compressor would make real sense, however, would be the
Electric, hybrid, or fuel cell vehicles where sufficient electrical power is readily available
Environmental concerns.

Few motorists know that the air conditioner in their vehicle has been a focus of worldwide
environmental concerns.

The use of AC systems in automobiles has increased energy consumption, resulting in climate change
and adverse environmental effects.

Scientists have found that using the AC reduces pollutants inside a car by 20-34%.

The choice of refrigerants used in automotive air conditioning systems can have environmental
impacts. The United States Environmental Protection Agency (EPA) provides information on
acceptable refrigerants and their impacts.

Recent advances and sustainable solutions are being explored to mitigate the environmental impact
of automotive air conditioning systems.

Ozone Layer
The ozone layer acts as a blanket in the stratosphere that protects us from harmful ultraviolet

(UV) radiation. Scientists worldwide believe that man-made chemicals such as CFC-12 (also

known by the trade name Freon) are rapidly destroying this layer of gas 10 to 30 miles above the

earth's surface. Strong UV radiation breaks the CFC-12 molecules apart, releasing chlorine. A

single chlorine atom can destroy over one hundred thousand ozone molecules. Ozone loss in the

atmosphere is likely to lead to an increase in cataracts and skin cancer, which is now one of the

fastest-growing forms of cancer and could weaken the human immune system. In the U.S., one

person dies of skin cancer every hour. Agriculture, as well as plant and animal life, may also be

dramatically affected. Remember that ozone is "good up high, bad nearby": even though it protects
us when it is in the

stratosphere, ozone at ground level can be harmful to breathe and is a prime ingredient in smog.

Many man-made sources such as tailpipe emissions from cars contribute to ground-level ozone
Global Action to Protect the Ozone Layer
The United States has joined over 160 countries as a Party to the international treaty known as

the Montreal Protocol. All developed countries agreed to phase out the production of most
ozonedepleting substances, including CFCs, by the end of 1995. The 1990 Clean Air Act Amendments

(the Act) incorporated this production ban date and directed EPA to develop regulations to

maximize recycling, ban nonessential uses, develop labelling requirements and examine safe

alternatives for ozone-depleting substances.

[7:29 pm, 07/02/2024] jenish: MAJOR CAR AC PARTS AND THEIR FUNCTIONS

car ac system diagram

Car AC system diagram shows the essential components for proper functioning of air-conditioner

When you see signs of car AC failure such as reduced cabin cooling or noise from AC, there may be a
problem with the components. Hence, understanding the major car AC parts can prove helpful in
identifying the problem. The car AC parts include:
Major components of a car Air conditioning system
COMPRESSOR

The compressor is the power unit of an air-conditioning system mounted in front of the engine. It is a
pump which distributes the refrigerant or Freon to all the parts of a car AC system. Besides, the
compressor has a clutch that compresses the low-pressure refrigerant gas into high-pressure and
temperature gas.

If your AC compressor is not working, it may not carry out the following functions:

 Compressing the refrigerant

 Controlling the temperature change and output
 Smoothly moving air to the next component, the condenser

Condenser
A condenser is mounted behind the grille in front of your car. The function of a condenser is to cool
the high-temperature refrigerant coming from the compressor. It radiates the heat from the
pressurised Freon or refrigerant changing its state from vapour to liquid. A condenser comprises the
following components:

 Condenser coil
 Compressor
 Condenser fan
 Motor
 Circuit board

A fan is an essential component in the condenser. It cools the refrigerant which flows to the
evaporator. A malfunctioned condenser can be a reason behind the hot air coming from the car’s AC.
Hence, cleaning the clogged pipes of the condenser will help maintain its efficiency

Dryer
Another essential part of a car AC system is the dryer located between the condenser and expansion
valve (orifice tube). The high pressure and cooled-down refrigerant from the condenser flows
through the dryer’s inlet point. After passing through filters and drying agents (desiccants), the liquid
moves to the thermal expansion valve.

The dryer serves as a decent filter removing dirt and debris from the liquid. That said, when the
liquid moves into the expansion valve, it doesn’t clog the orifice tube or thermal expansion valve
Thermal Expansion valve
The thermal expansion valve, also known as the metering device or orifice tube expands the high-
pressure liquid. The expansion process results in lowering the pressure and temperature of the
refrigerant. The metering device is mounted on the high-pressure side of the AC system between the
dryer and firewall. When the refrigerant leaves the expansion valve, it is still in liquid form

Evaporator
Situated behind the dashboard in the cabin, the evaporator is one of the major parts of the car’s AC
system. It changes the low pressure and temperature liquid to the gaseous state which flows through
the AC vents in your car. If the evaporator is damaged, it can result in weak airflow from your car’s
AC.

Refrigeration system
A refrigeration system is a mechanical process or arrangement that is responsible for lowering the
temperature between two points. For this process to take place, the thermodynamic properties of
matter are involved, which are responsible for transferring thermal energy or heat between two
points.

The main difference between a system and a refrigeration circuit is the complexity of the circuit, as
systems are only a simple arrangement of the circuit, in which other variables such as mass balance,
energy, heat transfer, etc. become more important.

There are two basic configurations of a refrigeration system depending on the method of refrigerant
injection or its construction.

Compression refrigeration
It is based on the operation of the mechanical energy of the circuit through the compression of a
refrigerant fluid. When it condenses, this fluid gives off latent heat at a lower temperature than that
which was absorbed when the refrigerant itself evaporated. The compression of the refrigerant fluid
in the compressor is responsible for maintaining the cycle which, after passing through the
evaporator, the expansion valve and the condenser, is subsequently repeated, passing through
different pressures and therefore different temperature conditions.

Absorption refrigeration
Its operation is the same as in compression refrigeration, since it involves producing cold through the
transfer of heat by means of changes in the state of certain refrigerant fluids. In this case, it is not the
compressor that is the key aspect but the properties of the refrigerant fluid, which is able to absorb
the properties of another substance in the vapour phase…
Examples of such substances are lithium bromide, ammonia or water in vapour phase, although this
method is usually only used when there is a cheap or waste heat source, because it is an economical
method, but not very efficient, since, depending on the fluid used, it cannot cool to temperatures
below the freezing point of water, for example.

Types of refrigeration systems

As mentioned above, refrigeration systems can be classified according to the method of refrigerant
injection or construction.

Refrigeration system for cold rooms

Different types of refrigeration systems can be used in cold rooms due to the many factors that
directly affect the refrigeration needs for the preservation and maintenance of the stored product.
For example, the dimensions of the cold room, the number of door openings, the load of the cold
room, the requirements of the stored product, the type of construction material of the cold room
itself…
The main cooling systems are:

 Direct refrigeration systems: compressed and condensed refrigerant gas leaves the
equipment and is distributed to the remote units (evaporators). This type of system is mainly
used in smaller, commercial applications due to refrigerant charge limitations in the industry.
 Indirect refrigeration systems: the refrigerant gas is confined in the cooling generation zone,
where the cooling power is transferred to a secondary fluid by means of an exchanger. The
fluid (water, glycol or brine), driven by a pumping system, conveys it to the final elements
such as air coolers, exchangers, tank coils, etc. This type of system is mainly used in large and
industrial applications because the use of a secondary fluid such as glycol, brine or water
lowers the cost of the refrigerant charge in this case, showing a high performance.

Basic refrigeration system

It is made up of:

 Compressor: responsible for compressing the refrigerant fluid and increasing its pressure in
order to transport heat throughout the circuit.
 Evaporator: this is responsible for reducing the pressure and temperature of the refrigerant
fluid after leaving the compressor until it becomes a liquid at a lower temperature and
pressure, transferring the heat to the outside.
 Expansion valve: responsible for increasing the pressure of the refrigerant until it becomes a
liquid after passing through the condenser.
 Condenser: this is responsible for sending the thermal energy of the refrigerant liquid to the
outside which, until it reaches the compressor again, will be converted back into gas due to
the increase in pressure caused by the change in temperature of the refrigerant exchange.

Vapour compression refrigeration system

This is a compression system in which compressors are responsible for activating the refrigerant by
compressing it to a high pressure and temperature after having produced the refrigerant effect. In
this way, the compressed refrigerant transfers its heat to the outside and is condensed, returning to
its liquid form, thus achieving the refrigerant effect during evaporation, which is the most common
and basic process in refrigeration.

Ammonia refrigeration system

It should be noted that ammonia or NH3 refrigeration is currently an economical alternative and is
more efficient. Its thermodynamic properties make it highly efficient, and it reaches temperatures
down to -70ºC in direct expansion systems.
Ammonia refrigeration has certain advantages for its use:

 Superior thermodynamic performance compared to other refrigerants.

 It is an environmentally safe refrigerant with zero GWP.
 It has a low cost, and a smaller quantity is required for the same application than other
refrigerants.
 Its strong smell can be a sign of quick identification in case of leakage.
 Ammonia refrigeration equipment is safer for preventive purposes

Disadvantages:

 Ammonia is not compatible with copper, so we cannot use it for its construction.
 Although a safe refrigerant, ammonia is a toxic refrigerant.

CO2 refrigeration system.

The main difference between CO2 and other refrigerants is the operating pressure at which it works.
However, this makes it a high-density gas, achieving a higher cooling effect with a low circulating
mass. Advantages of using CO2 as a refrigerant:

 GWP = 1.
 It is not flammable.
 Low toxicity (only dangerous in high concentrations).
 Contains a high heat transfer coefficient.
 High efficiency, low energy consumption.
 It has no long-term side effects.
 It is cost-effective and has no risk of obsolescence.
 High availability, as it is obtained as a by-product of different processes.
 Can be blended with POE, PGA and PVE lubricants.

CO2 also has a number of drawbacks and restrictions for its use in refrigeration:

 It works at higher temperatures and pressures than HFCs and other refrigerants.
 In case of leakage, CO2 accumulates on the ground, displacing the air; and being odourless, it
cannot be detected olfactorily.
 CO2 is only suitable for new systems. As a high pressure, low critical temperature refrigerant,
it is not suitable for conversion of systems with existing fluorinated refrigerants.
 The price of the system is high.
 It is governed by the Regulation F-GAS.