1. Introduction to Air Conditioning

2. Heat Heat is a form of energy
Objects that are hot have more heat energy than objects that are cold
Heat flows from objects that are hotter to those that are cooler.
The metric unit of heat energy is the calorie
One calorie is the amount of heat energy needed to raise one gram of water one degree Celsius.

3. BTU The English unit of heat energy is the BTU – British Thermal Unit
One BTU is the amount of heat needed to raise one pound of water [1 pint] one degree Fahrenheit.
Home air conditioners are rated by the number of BTU per hour the A/C unit can remove from a room.

4. Moving heat
Evaporator
Condenser
An air conditioning system transfers heat from inside the cabin to the condenser where it is transferred to the outside air.
Heat from the cabin air is absorbed in the evaporator and pumped to the condenser where it is transferred to the air flowing through the condenser.

5. Heat and pressure
Air temperature 80 degrees
Air temperature 250 degrees
Whenever a gas is compressed the temperature of that gas is increased.
The amount of heat energy is the same but the intensity [temperature] increases because the heat in the air is now concentrated in a much smaller volume.

6. Heat and pressure
Air temperature 80 degrees
Air temperature 0 degrees
The pressure applied to a gas is released the temperature of that gas decreases.
The same amount of heat energy now fills a larger volume which lowers the temperature.

7. Boiling point
20 psi
212 deg. F
272 deg. F
Water at normal atmospheric pressure boils at 212 degrees F.
Water in a pressure cooker at 20 psi will boil at 272 degrees F.

8. 15 psi outside the chamber
Boiling point
Vacuum chamber
15 psi outside the chamber
0.5 psi inside the chamber
80 deg. F
Vacuum pump
If the pressure applied to a liquid is lowered the boiling temperature decreases.
Water will boil at room temperature when a vacuum is applied.

9. Change of states All mater has 3 states:
Solid
Liquid
Gas [sometimes called vapor
When a solid melts or a liquid boils the process is called a ‘change of state’.
A solid must absorb heat energy to become a liquid and a liquid must absorb heat to become a gas.
A gas must shed heat energy to condense back into a liquid and more energy needs to be shed to freeze into a solid.

10. Latent heat Temp Latent Heat Latent Heat Time
250 deg.
This simple experiment explains how latent heat energy is absorbed as ice changes into water and then to steam.
Fill a pot with ice at -50 deg F and heat it on a stove.
As the ice warms the temperature rapidly rises until the ice begins to melt. As the ice melts the temperature stays at 32 deg.
After all the the ice has melted the temperature rises rapidly until the water boils.
Latent Heat
212 deg.
32 deg.
Latent Heat
The temperature remains at 212 deg. until all the water has boiled away. After all the water has boiled the temperature of the steam will rapidly rise.
-50 deg.
Time

11. Latent heat
Latent heat is the heat energy absorbed when a solid changes state as it changes into a liquid or a liquid is it changes to a vapor.
Latent heat is the heat energy that is needed to change the state of the material but does not increase the temperature of the material.
As ice melts into water the temperature remains at 32 degrees until all the ice is melted.
As water boils the temperature of the water and steam remains 212 degrees until all the water has turned to steam.

12. Boiling and melting points
Each chemical ahs its own melting and boiling temperatures.
Melting and boiling temperatures also depend on pressure. Higher pressures will raise both the melting and boiling temperatures.
The chemicals used as refrigerants have very low boiling temperatures when they are under low pressure and moderately high boiling temperatures when they are at medium pressures.

13. Refrigerants
The chemicals used as refrigerants in modern cars normally boil at about -26 deg. F if the pressure in the refrigeration system was released.
In an enclosed system filled with refrigerant the pressure the pressure [in psi] will be roughly the same as the temperature.
At 70 deg F the pressure in the A/C system is about 70 PSI [when the A/C compressor is not running]
The ratio of temp to pressure is not exactly 1 to 1
At 100 deg F the pressure will rise to 124 psi.

14. R-134a The refrigerant currently used in the United States is R-134a.
R-134a has been installed on all cars sold in the USA since model year 1993
The chemical name for R-134a is tetrafluoroethane (CF3CH2F).
R-134a does not contain chlorine and does not harm the earth's ozone layer.
There are plans to replace R-134a with a more environmentally friendly alternative.

15. Temperature / pressure relationship
The pressure of refrigerant in a container is roughly the same as the temperature.
If the temperature drops a small amount of vapor in the top of the container will condense into liquid.
The pressure will drop because the condensed liquid takes up less volume than the vapor.
70 psi pressure

16. Temperature / pressure relationship
If the temperature rises a small amount of refrigerant will boil.
This additional refrigerant vapor increases the pressure inside the container.
When the pressure equals the temperature the refrigerant stops boiling.
The boiling and condensation inside the container as temperature changes maintain a pressure / temperature equilibrium.
124 psi pressure

17. Superheated liquids
When pressure applied to a liquid is released so that the liquid is now above its boiling point the liquid is said to be superheated.
The liquid wants to boil but it can’t boil with additional heat.
The superheated liquid is a heat magnet – it will pull heat from anything that it comes into contact with.

18. Superheated liquids experiment
70 degrees F
70 psi pressure
Fill a cup with R-134a from a container at 70 degrees F.
Drop a nut into the liquid in the cup.
Vapor bubbles will form around the nut as the heat from the nut is absorbed by the R-134a.
The nut cools to -26 deg. F. as the refrigerant surrounding it boils.
The heat energy that was in the nut is carried away by the R-134a vapor.
R-134a
R-134a vapor at 70 degrees F

19. Reusing the refrigerant
Heat energy is absorbed by liquid refrigerant when the refrigerant changes from a liquid to a vapor [gas].
A pound of refrigerant can absorb only a small amount of heat.
The A/C system uses the refrigerant over and over again in a continuous cycle of boiling and condensing.
To reuse the refrigerant it must be pressurized and cooled so that it turns back into a liquid.

20. Recovering the refrigerant
Low pressure vapor
High pressure vapor
The process of turning a vapor into a liquid is called condensation.
When the refrigerant is pressurized to 200 psi it will condense back into a liquid when it is cooled to 130 deg. F. or lower.
Condenser
Compressor
Air passing over the condenser coils removes the heat that was absorbed by the refrigerant.
High pressure Liquid

21. Accumulator [reservoir]
Refrigerant cycle
Accumulator [reservoir]
Condenser
Evaporator
Compressor
Restriction
[orifice]

22. Pressure drop
The A/C system runs about 2 lbs. of refrigerant through a continuous cycle of pressurization and pressure release.
When high pressure refrigerant passes through a restriction [orifice] the pressure drops.
There are two ways the cause this drop in pressure:
Fixed orifice [orifice tube]
Expansion valve

23. Fixed orifice
An orifice tube is a small brass tube with an inner diameter of about .060”.
The tube is enclosed in a nylon sleeve that has mesh filters of prevent small particles of dirt clogging the orifice.
The orifice tube is located at the end of the liquid line where it connects to the evaporator inlet.
The orifice tube is inexpensive and is normally replaced anytime an A/C component is replaced.

24. Expansion valve
An expansion valve opens and closes to control the flow of refrigerant into the evaporator.
The valve is operated by a diaphragm that is connected to a thermal bulb via a capillary tube.
The diaphragm responds to changes in the outlet temperature of the evaporator.
The valve is often called a TXV [Thermal Expansion Valve].

25. Expansion valve The expansion valve connected to the evaporator inlet.
The thermal bulb is clipped to the evaporator outlet .
When the temperature of the bulb drops below 32 deg. F the diaphragm closes the expansion valve, stopping the flow of refrigerant into the evaporator.
Outlet
Thermal bulb
Capillary tube
Liquid line
Equalizer line
Inlet

26. Compressor
Inlet and outlet ports
Compressor clutch
Mounting boss
The compressor takes refrigerant vapor at low pressure [ typically 20 – 30 psi] and compresses it to 180 psi or higher.
Compressor clutch electrical connector
Image courtesy of General Motors corp.

27. Compressor
The compressor is driven by engine power via a serpentine belt.
An electrically actuated clutch allows the compressor to be turned off when the A/C system is not needed.
In some systems the compressor clutch is cycled on and off to control the temperature of the evaporator.

28. Coldest point in the A/C system
Evaporator
Outlet [vapor]
Liquid refrigerant under low pressure enters the evaporator at the bottom.
The superheated refrigerant liquid pulls heat from the cabin air passing over the evaporator fins.
As the refrigerant absorbs heat from the cabin air vapor bubbles float to the top.
By the time the refrigerant gets to the top all of the refrigerant should have changed from liquid to vapor
Coldest point in the A/C system
Orifice
Inlet [liquid]

29. Evaporator core
Since space ia at a premium under the dash the evaporator core is much thicker than the condenser.
Note: the evaporator shown here is upside down. It would not work if it was installed in the car this way.
Evaporator inlet
Evaporator outlet

30. Evaporator case
Heater core
Evaporator
Blend door
The evaporator is located inside the heater box between the blower fan and the heater core.
All of the air from the blower fan passes through the evaporator.
If the temperature is desired to be warmer than maximum cool the blend door diverts some of the cooled air through the heater core.
This has the added benefit of dehumidifying the air as well as cooling it.

31. Condenser The condenser changes refrigerant vapor back into liquid.
Inlet [vapor]
The condenser changes refrigerant vapor back into liquid.
Refrigerant vapor enter the condenser at close to 200 psi.
At this pressure refrigerant will change into a liquid when cooled below 130 deg. F.
Air passing through the condenser fins absorbs the heat from the refrigerant.
Outlet [liquid]

32. Condenser The condenser is normally mounted ahead of the radiator.
Inlet [vapor]
Outlet [liquid]
The condenser is normally mounted ahead of the radiator.
Normally the same electric fan that draws air through the radiator pulls air through the condenser as well.
In some vehicles the condenser has a dedicated fan

33. Piston type compressor
Vapor inlet [suction]
Outlet [discharge]
A/C compressors use power from the engine crankshaft to compress the vapor leaving the evaporator.
There are three types of compressors used in modern A/C systems:
Piston
Vane
Scroll
Outlet reed valve
Intake reed valve
Piston type compressor

34. Refrigerant reserve - Accumulator
Condenser
Evaporator
Compressor
Orifice tube
In normal operation the system needs a little over one pound of refrigerant for normal operation plus a few additional ounces of reserve for expected leakage over time.
Cars that use a fixed orifice have an accumulator [reserve] located between the evaporator and compressor.

35. Accumulator
Service fitting
This accumulator has been cut open to reveal the desiccant bag.
Desiccant is a chemical that absorbs water.
Beside being a reservoir of liquid refrigerant the accumulator functions to prevents liquid refrigerant from entering the compressor.
Outlet
Inlet
Desiccant

36. Refrigerant reservoir – Receiver Drier
Thermal bulb
Capillary tube
Receiver drier
Desiccant bag
Expansion valve
Systems that use an expansion valve store their refrigerant reserve in a receiver drier between the condenser output and the thermal expansion valve.

37. Refrigerant cycle - summary
5 principle components:
Evaporator
Condenser
Compressor
Orifice tube or expansion valve
Refrigerant reserve tank – accumulator or receiver/drier
Refrigerant absorbs heat when it is liquid and under low pressure.
Refrigerant releases this heat when it is a gas[vapor] at high pressure.

38. Change of state
Refrigerant absorbs heat energy as it goes from liquid to vapor as it passes through the evaporator.
Refrigerant goes from vapor back into a liquid as gives up its heat energy inside the condenser.
The orifice tube or expansion valve causes a drop in pressure as refrigerant passes through it.
The compressor increases the pressure of refrigerant vapor.
A reserve of liquid refrigerant is stored in the accumulator or receiver drier.