How an Automotive Air Conditioner Works

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First Things First: Before you can understand how an automotive air conditioners works you must understand some things about heat and refrigerant. Below is an explanation of some thermodynamic principles and refrigerant characteristics as I understand them. Sorry if I get any thing wrong.

Heat: Heat is form of energy. It is the internal vibration of the atomic particles of the matter. All matter contains heat or it would disintegrate.

Cold: Cold is just the absence of heat.

Temperature: Temperature is an measurement of the amount of heat. In the United States we use the Fahrenheit system to measure temperature.

Movement of Heat

Heat always moves from hot to cold.
Heat can move by conduction, convection and by radiation.

Conduction is the movement of heat from particle to particle. Conduction takes place most readliy in solids, but also takes place in liquids and gases. An example would be the heat that moves up an iron rod that is held in a fire.
Convection is the movement of heat by a moving medium. Convection takes place only in liquids and gases since solids do not flow. The movement of the forced hot air of a furnace is an example of convection.
Radiation is the movement of heat by an invisible ray. It requires no medium in order to travel! The heat that comes from the rays of sun is an example or radiation.

Amount of Heat: The amount of heat is measured using the British Thermal Unit (BTU).
The BTU is the amount of heat that will raise the temperature of 1 pound of water 1 degree Fahrenheit.

Heat of Compression: If you compress a gas, it creates heat. A gas is made up of small particles, which have a vibrational energy. When you move these particles closer together (compress them), there is heat built up by the collisions of the particles. This heat is called the heat of compression. In an air conditioning system you will see that this is done by the compressor.

Evaporation: When a liquid evaporates (goes from liquid to a vapor), there is a intake of heat from the surroundings. The particles of the liquid are changed into a gas, which has its particle much father apart. The particles are allowed to expand and given more room to vibrate and they take in energy in the form of heat to do this. If you sprayed liquid refrigerant out of a can it would rapidly evaporate. It would take the heat out of what ever it came in contact with. Of course we all know by now we are not allowed to expel refrigerant into the atmosphere. In an air conditioning system the evaporation of refrigerant takes place in the evaporator.

Change of State: There are three states matter can be in, solid, liquid and gas. A change of state happens when a substance changes from one of these states of matter to another. For instance H2O can be solid (ice), liquid (water) or vapor (water vapor). In the air conditioning system there two changes of state. In the condenser high-pressure refrigerant gas changes into a liquid (condenses). In the evaporator liquid refrigerant changes into a gas (evaporates). These changes of state take in or get rid of large quantities of heat. In order to show just how important the change of state is to moving large quantities, consider water about to boil and change state. To change 1 pound of water from 211 °F to 212 °F takes 1 BTU, but to change 1 pound of 212 °F water to 212 °F steam takes 970 BTU's. Another term used to describe the condenser and the evaporator is a heat exchanger, because this is where large quantities of heat are moved.

Refrigerant: Refrigerant is the chemical used inside the closed air conditioning system that moves heat. The only two refrigerants that the automotive manufacturers have used are R-12 and R-134a. They both have a boiling point that is well below freezing (R-12 has a boiling point of -21.7 degrees Fahrenheit). Below that temperature, we could hold R-12 in an open container as a liquid. Above that temperature R-12 has to be kept in a container to keep it from evaporatoring into a vapor. In a closed container, R-12 and R-134a both have certain pressure for each degree of temperature above their boiling point. We are familiar to the fact that water in a pressure cooker creates pressure when it rises above its boiling point. As the temperature of the water in the pressure cooker rises above its boiling point, the pressure inside the cooker also rises. Refrigerant is just like this; we always use it well above its boiling point. If any of you use a refrigerant below its boiling point please let me know why!
Please See and Print out a copy of the Pressure Temperature Table.
If you have two 30 pound bottles of refrigerant, one with 1 pound of refrigerant in it and the other with 20 pounds of refrigerant in it, they will both have the same pressure if they are at the same temperature! Just like two pressure cookers with different amounts of water in them will have the same pressure at 230 degrees Fahrenheit. The above 30 pound containers would have liquid refrigerant sitting on the bottom and vaporus refrigerant filling the rest of the container. The amount of refrigerant is measured by pounds of weight. The small cans of refrigerant in use today have 12 ounces (.75 pound) in them and the large containers that are common have 30 pounds of refrigerant in them. It is very important to not put too much refrigerant into a system. If there is any dought on how much refrigerant that is in a system, remove it all and put in the correct amount by weight. Be careful not mix up pounds of refrigerant (weight) and the pressure of refrigerant measured in PSI (pounds per square inch). From time to time you will here people referring to supper heated and sub cooled refrigerant. This has to do with what temperature the refergerant is at, compared to the pressure temperature relationship table. If the refrigerant temperature is warmer than the PT table then the refrigerant it is said to be super heated. Also remember that the refrigerant can be in the liquid or vapor state. Pressure Temperature Table.

Types of Automotive Air Conditioners

There are several different types of automotive air conditioning systems. It is important to understand each and each has its strengths and weaknesses. I will explain the expansion valve system in detail and then the differences of the other two systems.

Receiver Drier (Filter Drier) - Expansion Valve System
This is the earliest and I think the best system. This system uses an expansion valve to control refrigerant flow and and cycles the compressor clutch to control evaporator temperature.
Accumulator - Orifice Tube System
This system uses a fixed orifice and an accumulator to control refrigerant flow and cycles the compressor clutch to control evaporator temperature.
Suction Throttling Valve System
This system is the most complicated. It uses an expansion valve to control refrigerant flow into the evaporator and a suction throttling valve to control refrigerant flow out of the evaporator. This system does not cycle the compressor clutch, rather it cycles the compressors suction to the evaporator.

I - Receiver Drier (Filter Drier) - Expansion Valve System: This is the earliest and I think the best system. This system uses an expansion valve to control refrigerant flow and and cycles the compressor clutch to control evaporator temperature. The main components are the Compressor, Condenser, Receiver/Filter/Drier, Expansion Valve, Evaporator and Thermoatstic Switch. An air conditioning system has two parts each with a different pressure. The high pressure side starts at the compressor discharge, goes through the condenser and the drier and to the Expansion valve. The low pressure side starts at the output of the expansion valve, goes through the evaporator and back to the compressor. A well designed air conditioning system should not have a drop in pressure due to too small of lines.

Lets take a look at the refrigerant path and the components.

From the Compressor to the Expansion Valve is the High-Pressure (Hot) (Discharge) Side of System
Compressor: The compressor is the heart of the air conditioning system. The compressor is mounted on the engine and is belt driven. At the front of the compressor is a magnetic clutch which when given power engages the compressor. The compressor is the heart of the air conditioning system, it pumps refrigerant in a closed loop through the system. The moving refrigerant moves heat from inside the vehicle to outside the vehicle. The compressor takes in low-pressure gas coming from the evaporator and compresses it into a high-pressure hot gas. Takes in slightly supper heated gas and discharges highly supper heated gas. The compressor discharges this high-pressure gas through its discharge port to the discharge hose. Compressors vary in design, physical size, shape, weight, rotational direction, RPM and displacement. The sizes I have seen, vary from about 6 cubic inches up to about 13 cubic inches. The high side pressure (discharge pressure) should be 150PSI to 285PSI. The high side pressure will depend on load and how well the condenser is being cooled!! With R-12 any pressure over 250 PSI lowers cooling of the system and any pressure over 300 PSI should be considered too high.
The discharge of the compressor is very Hot!! This heat is an indicator of load; it is called the heat of compression.

Discharge Hose: The discharge hose is usually 13/32" or 1/2" in inside diameter and always goes from the compressor to the top of the condenser.

Condenser: The condenser is usually in front of the radiator. The air flowing through the condenser removes heat from the refrigerant that is flowing through it. The condenser takes in hot high-pressure gas and cools it. The gas goes in at the top as a supper heated vapor and changes to a sub cooled liquid as it cools. The output of the condenser is at the bottom. There should be no pressure drop across the condenser if it is designed properly. The discharge pressure is an indicator of the average temperature of the condenser. Somewhere around the center of the condenser the pressure and the temperature of the refrigerant equal the pressure temperature relationship that is on your gauges and on the PT chart. Look at your gauges, the outer numbers are the reading in PSI. The inner numbers gives you the PT relationship of the refrigerant as designated.
See Pressure Temperature Table.
The high (discharge) side pressure should be 150-285 PSI. I mention this so you can get an idea of how hot it is, and what high discharge pressure means. If you do not cool the condenser properly (not enough air flow) the head pressure will go higher the normal and the system will not cool well. This is a real common problem!! I blow cold water on the condenser if the pressure goes over 250 PSI. With R-12 any pressure over 250 PSI lowers cooling of the system and any pressure over 300 PSI should be considered too high.

First Liquid Line: The First Liquid Line is usually 5/16" in inside diameter and goes from the bottom of the condenser to the drier.

Receiver/Filter/Drier

The vapor liquid combination coming out of the bottom of the condenser comes into the drier.

The Receiver/Filter/Drier provides three functions.

It is a tank that holds excess refrigerant and the refrigerant that leaves it usually goes up a pickup tube and past a sight glass.
It has a filter in it to remove small amounts of debris from the system.
It has a chemical in it that treats the refrigerant by removing moisture and acid.

Second Liquid Line: The Second Liquid Line is usually 5/16" in inside diameter and goes from the drier to the expansion valve.

From the Expansion Valve back to the Compressor is the Low-Pressure (Suction) Side of System
Expansion Valve: The expansion valve controls the flow of refrigerant into the evaporator. The flow varies with need, but the average flow is about the size of an ink pen tube. You have to keep in mind that there will be wide ranges of heat to be removed from the inside of the vehicle (warm days versus hot days). A important thing to note, is that by design the compressor can way out pump the flow of refrigerant through the expansion valve. This is why there is low-pressure from the expansion valve back to the compressor. The expansion valve has a capillary tube with a thermal bulb that controls how far open or closed it is. The thermal bulb and the internal pressure of the refrigerant balance to control just the exact amount of refrigerant needed. The thermal bulb is clamped to the output of the evaporator. If not enough refrigerant is flowing to cool the evaporator this bulb will sense it and open more or vice versa. Some expansion valves have two capillary tubes, they are called externally equalized expansion valves. If you remove the thermal bulb and cool it or warm it you can see the results on your gauges. Warming the thermal bulb will open the expansion valve and your suction pressure will rise. If you cool the thermal bulb well below freezing the expansion valve will close an your suction pressure will drop. We used to spray the thermal bulb with liquid refrigerant to test it. If an expansion valve is working properly there will be no liquid refrigerant leaving the evaporator. There is a small safety margin (super heat adjustment) so no liquid refrigerant leaves the evaporator and goes back to the compressor. Super heat of the expansion valve is the adjustment so there is a small amount of heat added to the refrigerant over and above what it needs to be a full vapor leaving the evaporator. It is important you have this and do not adjust this because liquid refrigerant going back to the compressor will damage it.

Evaporator

The evaporator is the heat exchanger that removes heat from (cools) the inside of the vehicle. It is located in or adjacent to the passenger compartment, usually mounted on the fire wall. The high pressure liquid refrigerant that is sprayed into the bottom of the evaporator by the expansion valve goes through a rapid evaporation and changes state in to a vapor. This evaporation is posible only if there is heat removed from the air going through the evaporator fins. Do not get confused, but I will mention here that the refrigerant enters the expansion as a sub cooled liquid and comes out the top of the evaporator as a supper heated vapor. Note also that it is the compressor suction that keeps the evaporator pressure down! There are different types of evaporators but they all have tubes or passages to flow the refrigerant and attached fins. Air is either blown through or drawn through the evaporator. As the air goes through the evaporator it is cooled by the fins of the evaporator. The air is cold because the heat is removed from it by the evaporator! In recirculate mode, inside air is moved through the evaporator. In fresh air mode, outside air goes through the evaporator. You can look at the suction pressure on your gauges and see the pressure of the evaporator. The suction pressure is an indicator of the average temperature of the evaporator. Somewhere around the center of the evaporator the pressure and the temperature of the refrigerant equal the pressure temperature relationship that is on your gauges and on the PT chart. Look at your gauges, the outer numbers are the readings in PSI. The inner numbers gives you the PT relationship of the refrigerant as designated.
See Pressure Temperature Table.
The low side pressure (suction pressure) should be down to about 15-35 PSI. After the vehicle has been driven down the road and cooled down, the temperature of the air coming out of the vents should be down to about 45 °F.

A couple of things to keep in mind when thinking about the evaporator.

It has to be clean.
Systems either blend warm air to the cold air or give hot water to the heater core to provide proper temperature of the air outlets. Your Temperature slide control either moves a blend door or a heater control valve. I have seen many vehicles not cool well or at all because the blend door was not adjusted or the heater was heating the air back up!!! In some systems the air goes through the evaporator core and the heater core all the time.
Its cooling can be hampered by the heater core.
Some systems like VW draw (suck) air through the evaporator. I have seen them not cool because they are not drawing any air through the evaporator.

I always touch the output line of the evaporator (suction line) to get an idea of how the system is cooling!

Suction: The suction hose is usually 1/2" or 5/8" in inside diameter and goes from the evaporator to the compressor.

Thermostatic Switch: The thermostatic switch cycles the compressor off when the evaporator gets too cold. The thermostatic switch has a capillary tube that is stuck into the evaporator and picks up a temperature signal that moves an internal electrical switch. This switch makes and breaks electrical power to the compressor clutch. This switch could also be called a defrost switch. With out compressor cycling the evaporator will frost up some times to the point of no air going through the evaporator (no cooling).

II - Accumulator - Orifice Tube System: This system uses a fixed orifice and an accumulator to control refrigerant
flow and cycles the clutch to control evaporator temperature.
The orifice is not as small or as large as an expansion valve could be. There is no control of too much refrigerant. The accumulator replaces the drier and has one or two dessicant bags inside. The accumulator does not work as well as a filter drier at removing moisture or acid, that is why they went to two bags. It is on the suction side of the system located between the output of the evaporator and the input of the compressor. The accumulator has a stand pipe in it that functions by not allowing liquid refrigerant to go back to the compressor. The preset thermostatic switch or a pressure cycling switch helps slugging and controls evaporator temperature. The pressure cycling switch seems to work better than the thermostatic switch. This is the system that GM and Ford now use.

III - Suction Throttling Valve System: This system is the most complicated.
It uses an expansion valve to control refrigerant flow into the evaporator and a suction throttling valve to control refrigerant flow out of the evaporator. This system does not cycle the compressor, rather it cycles the compressors suction to the evaporator. This system goes by many names GM's POA & STV, Fords STV & Combo-valve and Chryslers EPR & ETR. They all control Evaporator pressure and temperature by controlling compressor suction to it. The suction throttling valve is located between the compressor and the evaportor. If the evaporator pressure drops below its' setting, compressor suction is cut off to the evaporator. There is still input to the evaporator from the expansion valve so evaporator pressure rises and suction is re-applied. This happens so quickly that it is a throttling action that just controls evaporator pressure. If we can control the minimum pressure in the evaporator we can control the minimum temperature of the evaporator. I like to think of suction throttling valve like a pressure valve that takes about 25PSI 28PSI to go through it. To test a STV valve I disconnect the blower motor and watch my gauges and where the system frosts up. It is alright if the line from the STV valve to the compressor frosts up. It is not alright for the evaporator or the line from it to the STV valve to frost up.

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