How Air Conditioning Works
The History Of Air Conditioning and How It Works
In our cars is a system that lets us enjoy the same comfort that we enjoy at home and at work. It is Air Conditioning. At the flick of a switch or the push of a button, we go from unbearable heat to comfortably cool air. We don't even think about it too much unless something goes wrong.
Air conditioning in cars arrived in the 1940's. The A/C systems in use then were crude by today's standards but did supply some measure of relief from the heat. In the years that followed there were some improvements, such as computerized automatic temperature control (ATC) that allow you to set the desired temperature and have the system adjust automatically and improvements to overall durability. This has also added to the complexity of the A/C system as well.
In addition to the complexity of the new A/C systems, there are tighter environmental regulations that govern even the simple process of adding refrigerant to a system. Scientific studies have established the fact that R-12 does damage the ozone layer and the United Kingdom and many other countries have banned its manufacture. These countries have joined together to sign the Montreal Protocol, a landmark agreement that was introduced in the 1980's to limit the production and use of chemicals known to deplete the ozone layer.
Should you have a problem with your R-12 A/C system, you may have heard of new words and terms. Words like "retrofit" and "alternative refrigerant" are now in your mechanics glossary. You may be given an option of "retrofitting" as opposed to merely repairing and recharging with R12.
In 1992, a new type of refrigerant for automotive air conditioning systems began to appear in new vehicles. R-134a was introduced to replace R-12 because R-12 contains ozone-damaging chlorofluorocarbons (CFCs). R-134a is ozone-friendly because it contains no CFCs. It is also nontoxic and nonflammable, and meets all of the Environmental Protection Agency's criteria for alternative refrigerants. By 1995, all new vehicles were factory-fitted with R-134a A/C systems after the production of R-12 was phased out.
Retrofitting involves making any necessary changes to your system, which will allow it to use the new industry accepted, "environmentally friendly" refrigerant. This new refrigerant has a higher operating pressure, therefore, your system, dependant on age, may require larger or more robust parts to counter its inherent high pressure characteristics. If not performed properly, may reduce cooling efficiency that equates to higher operating costs and reduced comfort.
There are three basic types of air conditioning systems. And as each type is different, the operating principles and designs are similar. The components that each system has are: the compressor, condenser, evaporator, orifice tube, thermal expansion valve, receiver-drier, and accumulator.
The compressor is the heart of the air conditioning system. The compressor is a belt driven pump that is fastened to the engine. At the front of the compressor is a magnetic clutch which when given power engages the compressor. It is responsible for compressing and transferring refrigerant gas. As 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 A/C system is split into two sides, a high-pressure side and a low-pressure side; defined as discharge and suction. The compressor takes in low-pressure vapour coming from the evaporator or in some cases the accumulator and compresses it into a high-pressure liquid and sends it to the condenser, where it can then transfer the heat that is absorbed from the inside of the vehicle. The compressor discharges this high-pressure refrigerant through its discharge port to the discharge hose.
This is the area in which heat dissipation occurs. The condenser is usually in front of the radiator and will have much the same appearance as 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 refrigerant and cools it. The refrigerant goes in at the top as a super heated vapor and changes to a sub cooled liquid as it cools. As hot compressed gasses are introduced into the top of the condenser, they are cooled off. As the gas cools, it condenses and exits the bottom of the condenser as a high-pressure liquid.
In a rear wheel drive vehicle the engines cooling fan supplies the airflow required to draw heat from the condenser. In some cases there will be an auxiliary, electric, cooling fan to provide additional airflow. On front wheel drive cars there will be one possibly two electric cooling fans to provide additional airflow through the radiator and condenser.
The evaporator is very similar in function to the heater core, except for the fact that it absorbs heat from the passenger compartment instead of supply heat. It is usually mounted under the passenger side dash and mounted to the inside bulkhead. The low-pressure liquid refrigerant enters the bottom of the evaporator, or a high-pressure liquid that is sprayed into the evaporator by the expansion valve, goes through a rapid evaporation and changes state into a vapor. Its primary duty is to remove heat from the inside of your vehicle but the secondary benefit is remove humidity.
Just as water condenses on an ice filled glass, water condenses on the evaporator. Airborne contaminants entering the system stick to the wet evaporator and are drained to the outside. Temperature and pressure regulating devices must be used to control its temperature. While there are many variations of devices used, their main functions are the same, keeping pressure in the evaporator low and to keep the evaporator from freezing. Pressure Regulating Devices:
The evaporator temperature can be controlled by regulating the refrigerant pressure and flow into the evaporator. There are several ways to do this and the most common are:
The most commonly used pressure-regulating device is the orifice tube. This is used in most Ford and General Motors cars. It is installed either in the inlet tube of the evaporator or in the liquid line between the condenser and evaporator. The down side of the orifice tube is that it can become clogged with debris and can be costly to replace.
Another pressure-regulating device is the expansion valve. This is used by most import and aftermarket systems. The expansion valve can sense the pressure and temperature of the refrigerant, which makes it very efficient in controlling refrigerant to the evaporator. There are several different varieties of expansion valves. The down side is that, like the orifice tube, it can become clogged with debris and because they contain moving parts can stick or fail over time. An 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.
The receiver-drier is placed on the high side of expansion valve systems. Since this type of valve needs liquid refrigerant to function a receiver is used to ensure that the valve gets the liquid it needs.
The Receiver/Filter/Drier provides three functions:
1. 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. Under normal operating conditions, vapor bubbles should not be visible in the sight glass.
2. It has a filter in it to remove small amounts of debris from the system.
3. It has a chemical in it that treats the refrigerant by removing moisture and acid.
There are different types of receiver-driers and different desiccant materials used in them. Some of these desiccants are not compatible with R-134a and receiver-driers will be marked for use with either R-12 or R-134a. Newer type receiver-driers use a special desiccant that is compatible with both refrigerants. R-134a requires a different desiccant (XH7) in the accumulator or receiver/drier because the molecules are smaller than R-12 and quickly fill up the "holes" that absorb moisture in XH5 desiccant. This can lead to acid formation and sludging as well as desiccant breakdown in some applications. It is a good idea to replace the receiver-drier each time the system is opened for repair or anytime moisture and/or debris is of concern.
Orifice type systems use an accumulator that is connected directly to the evaporator outlet and stores excess liquid refrigerant. Since liquid refrigerant going into a compressor can do serious damage to the compressor, the main purpose of the accumulator is to isolate the compressor from any damaging liquid refrigerant. Accumulators, like receiver-driers, also remove debris and moisture from a system. It is also a good idea to replace the accumulator each time the system is opened for repair or anytime moisture and/or debris is of concern. The New Refrigerant - R134a:
The reason vehicle manufacturers choose R-134a to replace R-12 is because R134a comes closer to the cooling properties of R-12 than any other alternate refrigerant. Even R-134a is so close to R-12; it is not a direct replacement. They are chemically different and incompatible because each requires a different type of compressor oil and desiccant. Since they are different the current law prohibits mixing the two to prevent cross contamination. It is also required that A/C technicians doing A/C work are required to recover and recycle R-12 whenever they perform any repairs to an R-12 system.
Factory R-12 systems generally use mineral oil while R-134a systems use various types of PAG (polyalkylene glycol) oil because mineral oil does not mix with R-134a and PAG oil does not mix with R-12. Some conversions also require replacing the high-pressure cutoff switch and/or orifice tube or expansion valve with ones calibrated for R-134a.
The demand for R-134a conversions will grow as the remaining supplies of virgin and recycled R-12 disappear. The reason vehicle manufacturers choose R-134a to replace R-12 is because R-134a comes closer to the cooling properties of R-12 than any other alternate refrigerant. Even R-134a is so close to R-12; it is not a direct replacement. They are chemically different and incompatible because each requires a different type of compressor oil and desiccant. Since they are different, environmental law prohibits mixing the two to prevent cross contamination.