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TECHNICAL SECTION – Vehicle air-conditioning:
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WHAT IS A CAR AIR CONDITIONER?
An air conditioner is used for the following purposes.
a. Temperature Control
b. Air Circulation Control
c. Humidity Control
d. Air Purification
An air conditioner maintains the air in the room or vehicle at comfortable temperature and humidity as
a. When the room temperature is high, the heat is taken away to lower the temperature
by the air conditioner and conversely, when the room temperature is low, heat is
supplied to raise the temperature.
b. In addition, moisture in the room is removed to maintain a comfortable humidity
To perform these operations, the air conditioner for a vehicle generally consists of a heater,
cooler (or evaporator), ventilation system such as a blower and ducts, and air purifier.
The Four Factors of Air Conditioners
TECHNICAL TERMS AND CHANGE OF STATE:
Heat is one form of energy. 1 Kcal heat quantity changes the temperature of 1 kg of liquid water by 1°C.
Specific heat is heat quantity required to CHANGE THE TEMPERATURE of an object by 1°C.
Temperature is the degree of heat or coldness of an object. The unit generally
used to express it is ’Centigrade degree (°C)” or “Fahrenheit degree (°F)”. In the Centigrade scale, the freezing point (solid point) of pure water is taken as 0°C, and
the distance between the freezing point and boiling point divided into 100 parts and each part designated as 1°C. In the Fahrenheit scale, the freezing point of pure
water is taken as 32°F, and the distance between the freezing point and boiling point divided into 180 parts, and each part designated as 1°F. [°C] = 5/9 ([°F] – 32) [°F] = 9/5 [°C] + 32.
WET Bulb and Dry Bulb Thermometers
The bulb (heat sensitising part) of a glass tube thermometer is wrapped with a gauze or other rough mesh cloth. One end of the cloth is immersed in a water
container to allow the water to be drawn up by a capillary action and to moisten the heat sensitising part.
The water in the cloth surface near the heat sensitising
part evaporates and robs the latent heat of evaporation from surrounding air, causing the air temperature around the heat sensitising parts to drop. The temperature registered by the thermometer at this time is called the
wet bulb temperature.
This is used to find out humidity in combination with the dry bulb temperature.
Dew Point Temperature:
When the air surrounding us is cooled, the air temperature drops, and when the humidity becomes
100%, that is, when the dry bulb and wet bulb temperature become the same, the water vapour
contained in the air will be in a saturated state.
On further cooling, the water vapour reaches a condition where it cannot remain in a vapour state so
that a part condenses and becomes dew. The temperature at which the humidity becomes 100% and
dew is formed is called dew point temperature.
When you pour water and ice into a glass, you notice that drops of water are generated on the glass. Do you sometimes wonder where these drops of water come
from? The drops of water come from the surrounding air. This means humidity is water vapour contained in the air.
Relative Humidity -
The relative humidity is used to measure the humidity. The relative humidity is the amount of water vapour the air contains in comparison with the amount the air could contain at a given temperature. Thus, if the relative
humidity is 50%, the air contains 50% of the water vapour amount the air could contain at a given temperature.
Absolute Humidity -
Absolute humidity is the amount of water the air contains, compared with the dry air.
Pressure is defined as the vertical force applied on a unit area of a solid, liquid or gas. The unit of measurement generally used to indicate pressure is ‘kg/cm2’.
However, instead of ‘kg/cm2’, Pascal is used as an international unit of measurement.
1 kPA (Pascall) = 1.01972 x 10-2 kg/cm2
1 kg/cm2 = 98.06 kPA.
For blower performance, mmAq (Water Column) is generally used.
For pressure below atmospheric (Vacuum) mmHG (Mercury Column) is used.
1. Atmospheric Pressure.
This is the pressure acting on all objects on earth. This pressure is the weight of air surrounding everyone and is equal to 1.03kg/cm2 (1 atmosphere). At this pressure,
the mercury column will be 760 mmHg. 1 atm = 1.03 kg/cm2 = 760 mmHg Practically all pressure gauges are made to indicate atmospheric pressure as 0.
2. Absolute Pressure
Absolute pressure is when perfect vacuum is taken as 0 kg/cm2. Therefore, the atmospheric pressure is 1.03 kg/cm2 in terms of absolute pressure.
To distinguish from absolute pressure, the pressure measured with a gauge is called a gauge pressure. For identification, absolute pressure is indicated by kg/cm2abs and gauge pressure by kg/cm2G. The relationship between absolute pressure and gauge pressure is as follows:
Absolute press. [kg/cm2abs] = Gauge press. [kg/cm2G] + 1.03 kg/cm2 Pressure.
CHANGE OF STATE
Now, we will consider how ice changes its state when we add heat to it. We will use water as an example as it is the easiest matter to understand heat and state change. If we add heat to ice until the temperature of ice reaches 0 degree Centigrade (32.0 F), ice melts into water, and
while the ice is melting, the temperature of ice and water remains at 0 degree Centigrade. After the ice has melted, the temperature of water begins to rise. When the temperature of water reaches 1000C, water begins to become steam. Until all the water becomes steam, the temperature of water remains 100.0C.
Sensible Heat and Latent Heat
There are two kinds of heat called sensible heat and latent heat.
can change the temperature of water but cannot change the state of water. Therefore, the sensible heat raises or lowers the temperature of water.
In the case of water, 1kg of water at 00C must absorb 100 Kcal of sensible heat to change to 1 kg of water at 1000C. The Latent Heat can change the state of water, but cannot change the temperature of water. Ice melts into water by adding latent heat and water evaporates into steam by adding latent heat.
In the case of water, 1 kg of ice at 0 degree must absorb 80Kcal of latent heat to change to 1 kg of water at 0 degree, and 1 kg of water at 100 degrees must absorb 539 Kcal of latent heat to change to 1 kg of steam.
Three States of Matter
As you know, matter exists in thee states; solid, liquid and gas. In the case of water, the solid state is ice, the liquid state is water, and the gas
When solid melts into liquid, heat is absorbed from the surrounding matter.
In the opposite situation, when liquid changes into a solid, heat is released to the surrounding. In the case of water, when 1 kg of ice melts into 1 kg of water, under atmospheric pressure, 80
kcal of heat is absorbed from the surrounding.
When liquid evaporates into gas, heat is absorbed from the surrounding.
In the opposite situation, heat is released to the surrounding.
In the case of water, when 1 kg of water evaporates into 1 kg of steam 539 kcal of heat is
absorbed from the surrounding. In the opposite situation, 539 Kcal of heat is released to the surrounding matter.
In rare case such as dry ice (solid carbon dioxide) and naphthalene, heating causes the solid to directly turn into gas.
The reverse process is called adhesion.
PRINCIPLE OF COOLING
A cooler cools and dehumidifies the air inside the
vehicle or fresh air from outside the car so as to produce
BASIC THEORY OF COOLING
After swimming on a hot day we feel a little cold. This is because the water remaining on our body eventually
vaporises. Whilst vaporising, this is drawing heat from our body.
For the same reason, we feel cool when we apply alcohol to our skin. The alcohol is placed as a liquid on our skin. After a short time the liquid starts to change
into a gas (vaporise). In this process heat is drawn from our skin (our skin feels cold). Using this principle we can apply it to an air conditioning system. ie: Heat being drawn when a liquid vaporises to a gas.
Refrigerant is a substance that services as a moving fluid in a refrigerator and circulates through functional parts to produce the cooling effect by absorbing heat through the expansion valve and vaporising. The refrigerant used in new vehicle today is now HFC 134a,
which has no-ozone-destroying properties (does not contain chlorine).
Characteristics of HFC134a:
Water boils at 100 deg Celsius under atmospheric pressure, but HFC134a boils at –26.90C under atmospheric pressure. Water boils at 1210C under 1 kg/cm2G (98kPa) of pressure, but HFC134a boils at –10.60C under 1kg/cm2G (98 kPa) of pressure.
If HFC134a were released to the air under normal room temperature and atmospheric pressure, it will absorb the heat from the surrounding air and boil immediately, changing into a gas. HFC134a is also easily condensed back into liquid under a pressurised conditions by removing the heat.
Characteristics of Refrigerant (HFC134a).
The gaseous refrigerant can be converted into the liquid refrigerant by increasing the pressure without changing the temperature.
The gaseous refrigerant can also be converted into a liquid by decreasing the temperature without changing the pressure.
The liquid refrigerant can be converted into gas by decreasing the pressure without changing the temperature.
The liquid refrigerant can be converted into a gas by increasing the temperature without changing the pressure.
HFC134a has very good characteristics and properties to be used in automotive air conditioning systems. It is nonflammable, non-explosive, non-poisonous, non-corrosive, odourless and harmless to clothes and food.
Have a look at this link: More on car air-conditioning
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