Sunday, January 22, 2017 Detailed Auto Topics
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Most people understand that air conditioning cools the inside of a vehicle. The mechanical process is not complicated, and a bit of physics helps to understand how it all works.

Air conditioning works by removing heat from an area. This heat is moved to the outside and given off at the condenser. The absence of heat is cold, which is the affect that is felt. Humidity is also removed in the process, making the area far more comfortable.

Basic physics of air conditioning

When matter changes state, energy is absorbed or released in the process. For instance, water placed in a freezer gives off stored heat energy and becomes solid ice. The change of state releases energy which is absorbed by the freezer. Heating water to 212 degrees Fahrenheit, at sea level, turns it to steam, the gaseous state. Energy, as heat is absorbed by the water in the process.

If water is put under pressure, the boiling point will rise. This is much how a pressurized radiator keeps coolant from boiling. Pressure affects the point at which matter changes state. Water in a vacuum will boil at a lower temperature than water at atmospheric pressure.

The same holds true for the refrigerants used in an air conditioning system. Under pressure the boil point is raised, when pressure is lowered the boil point is lowered.

Basic mechanics of air conditioning

basic automotive air conditioning

An air conditioning compressor raises the pressure of the gaseous refrigerant. The high pressure gas flows from the compressor, into the condenser. In the condenser, air flow removes heat (energy) from the high pressure gas. This is the heat you feel when standing near a condenser. This heat is the energy that was removed from the air conditioned area.

When air flow removes this energy the gaseous refrigerant changes state to a high pressure liquid. Heat is given off in the change of state of the refrigerant. Cooler high pressure liquid refrigerant now flows through the lines and hoses of the system.

parts of an Automotive air conditioner orifice tube

To absorb the heat in the vehicle this high pressure liquid must again change state. Most modern systems use an orifice tube or expansion valve to accomplish this. The orifice tube is a restriction that causes pressure to drop. An expansion valve does the same thing but is variable in operation.

Low pressure liquid now leaves the orifice and enters the evaporator core. A fan moves hot air (energy) through the evaporator core, which causes the low pressure liquid to boil. The change of the refrigerant from liquid to gas absorbs the heat energy. This heated low-pressure gas leaves the evaporator and is stored in the accumulator or receiver.

cutaway view of automotive air conditioner accumulator

The accumulator or receiver traps any liquid that may have come through the evaporator. Liquid is not compressible and would destroy the compressor if allowed to enter. There is also a desiccant and sometimes a filter which helps to remove moisture and debris in the system. This is why an accumulator is sometimes also called a dryer or filter/dryer.

Low-pressure gas, that is now cleaned and dried, leaves the accumulator and travels to the compressor.  At the compressor the process is repeated.

What controls air conditioner temperature

In the simplistic example above, no control of cooling was considered. If controls did not exist, heat would be removed until the evaporator core would freeze. If this were allowed, cooling would cease and liquid refrigerant could overflow the accumulator and damage the compressor.

Several means have been used over the years to prevent this. Older vehicles used expansion valves that closed with temperature to block the flow of refrigerant and regulate cooling. The compressor continued to run which took energy (fuel) to turn.

magnetic air conditioner clutch

Later systems use sensors to determine freeze point and turn off a clutch on the compressor. This is known as cycling or cutting the compressor on and off to regulate temperature. Additional sensors are sometimes added to cut off the compressor when the refrigerant leaks out or if pressure rises too high.

More modern systems use variable displacement compressors. The variable displacement compressors can reduce the amount it pumps based on need. Zero to full pressure is controlled internally by the compressor. This cuts energy consumption when it is not needed and can eliminate the compressor clutch in many instances.

Temperature within the vehicle is further regulated by reheating the cooled air using the heater core. This offers the advantage of dehumidifying the air without over-cooling the passenger compartment. This also allows for heating and cooling zones in the passenger compartment. For instance the driver side can be one temperature and the passenger side another.

Multiple zone and automatic temperature control adds a lot of complexity to the system and can be very expensive to maintain. A bad actuator motor or broken blend door often involves removing the entire dash to repair.

Repairing modern air conditioning

Modern climate control repair must be performed in a near sterile environment. Any debris that enters the system will cause problems. Dirty working conditions will not produce quality repair and can be very expensive.

Accurate diagnosis of problems is also imperative. For instance when a compressor fails. Compressors are operating in an extremely clean and well lubricated environment. Compressors DO NOT just go out, something causes them to fail. Replacing a compressor without finding the cause of failure is worse than just throwing money away.

cross section of a condenser tube showing passage size

When a compressor fails, metal contaminates the entire system. Tiny metal particles collect in the condenser, hoses, lines, evaporator and accumulator. The passages of the condenser cannot be cleaned. They are very small and loop back in forth for many yards. After compressor failure the condenser should be replaced, along with the orifice or expansion valve and accumulator. All hoses and the evaporator must be thoroughly cleaned as well.

If a single particle of metal remains, it can circulate back to the new compressor and cause it to fail. Now the system is again contaminated and worse than before. The same thing can happen with a low quality rebuilt air conditioner compressor. Each failure makes the situation less able to be repaired.

Cost effective air conditioner repair requires the most knowledgeable technician, using the best parts available. Anything less is just too expensive.

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