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Automotive Air Conditioning Pressure & temperature relationship

Automotive
Air Conditioning
Pressure & temperature relationship




Evaporation:

Is the term used when enough heat is added to a liquid substance to change it into a vapor (gas). For example, when water is boiled.This condition occurs within the A/Csystem.
 
Evaporation

Condensation:

Is the term used to describe the opposite of the evaporation process. If you take a vapor and remove enough heat from it, a change of state occurs.The vapor becomes a liquid.  The change of vapor to a liquid is called condensation. This condition occurs within the A/C system.

Condensation



Freezing:
Is another change of state. Freezing results when heat is removed from a liquid substance until it becomes a solid. Remember that anything above -273 C still contains some heat. In an air conditioning system freezing must be avoided. Otherwise component damage will occur.

Freezing


Pressure & temperature relationship:

point of a substance

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.

The pressure




A good example is the automotive cooling system. The pressure cap keeps the radiator from
boiling over by increasing the pressure on the coolant.

Example:
110 kPa radiator cap allows the coolant  temperature to reach 126 C before boiling.
boiling point of water


This chart opposite shows that the boiling point of water can be altered by changing the pressure upon it. As a comparison with the radiator example above. The substance used in the air conditioning system, called refrigerant, also boils at different temperatures depending on the pressure that it is under.


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Automotive Air Conditioning heat get inside a vehicle

Automotive
Air Conditioning
heat get inside a vehicle


Air Conditioning car

What is heat?

To understand just how an air conditioning system works, we must first understand the
nature of heat.


For a simple definition we may say that heat is energy. The meshing of gears, the turning of
wheels cause friction which results in heat. Combustion (fire) gives off heat. The burning of
sun radiates heat to the earth's surface.
Heat in the correct amount will provide life and comfort. Heat in either extreme - extreme to
much or to little - will be uncomfortable.
The control of temperature means the control of comfort.
Air conditioning is a method of controlling heat.
When is heat hot?
When is heat cold?
All substances contain heat. Something "feels" hot when it is warmer than our body
temperature. When something contains less heat than our bodies, we say it feels cold!
Cold is merely the removal of some heat.
Science tells us that a measurement called "Absolute Zero" is the point at which all heat is
removed from an object (approximately -273 C). Any substance above this absolute zero
temperature retains some heat.
temperature
All substances contain heat:
The average person requires a comfort zone of approximately 21 C to 26 C, with a relative humidity of 45 to 50%. In this temperature and humidity range, we feel most comfortable. All objects within this  same range are comfortable to touch. As the temperature of anything goes
above or below this range, we think of it as HOT or COLD.
feel most comfortable

Heat measurement:

A temperature reading gives us the heat intensity of a substance and not the actual quantity of heat. Heat quantity is measured in "KILOCLORIES" (KCAL's). One KCAL is the amount of heat required to raise the temperature of one kilogram of water one degree Celsius (at sea level). This quantity measurement is used in air conditioning to describe heat transferduring changes of state.



Heat
What causes heat to move?

Heat always moves from the hotter objects to the colder one. Whenever there is a transfer
difference between two objects, the heat energy will be transferred from the warmer
object to the cooler one until both objects stabilize at the same temperature. This is known as the law of heat transfer, and is the basis of air conditioning operation. When a hot cup of coffee is set aside for some time, it becomes cold. Heat moves out of the hot (90 C) coffee into the cooler (25 C) surrounding air. In time the coffee will reach the temperature of the surrounding air.

heat transfer


How does heat get inside a vehicle?
 
When a car is driven or parked in the sun, heat enters the vehicle from many sources.
These sources include:
- Ambient air
- Sunlight
- Engine heat
- Road heat
- Transmission
- Exhaust heat
All of these and other miscellaneous heat sources, increase the air temperature within the
vehicle. In a high ambient temperature situation, (e.g. on a 37 C day), the interior of a
vehicle left standing in the sun with windows closed could reach 65 - 70 C!



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