Saturday, January 12, 2013
Working of and Problems with Car Air-conditioner -- Part 1
In our hot climate. the air-conditioning system is no longer a luxury but a necessity. The air-conditioning system cleans and humidifies the outside air entering the car, making it comfortable to drive. Yet how the air-conditioner works is a baffle to many people - in fact, some people think an air-conditioner adds coldness to hot air! A basic understanding of how the car air-conditioner works can make you appreciate the importance of regular servicing, and trouble-shoot possible causes when problems arise.
We all know that if you blow air over something cold such as ice, the latter absorbs heat and rises in temperature. The result is cold air. The basic principle behind the working of the car air-conditioner is similar. Hot air entering a radiator-like component called the evaporator under the dashboard is cooled and flows out as cold air into the passenger compartment. The evaporator is a heat exchanger as it contains a special cold stuff (not ice!) that absorbs heat from the air that flows past its fins and tubes. This stuff, called the refrigerant, gets hotter as the air gets colder. The refrigerant should not be confused with the coolant found inside the radiator in the engine compartment.
A compressor driven by a belt from the engine pumps the refrigerant to the evaporator. The refrigerant absorbs heat from the air passing through the evaporator to the passenger compartment. As the refrigerant flows back to the compressor, it carries the heat back. From the compressor, the refrigerant flow to a second heat exchanger called the condenser, which is usually mounted in front of the radiator. Air flowing through the condenser cools the refrigerant before it is pumped back to the evaporator again.
The magic in the working of the air-con system is actually the refrigerant. If the air-conditioning system is filled with cold water (or some other liquid), it will cool the system for a while but eventually it will heat up. Though the cold water is also circulated through the condenser, it will not cool down as it is of the same temperature as the outside air. One of the laws of thermodynamics is that there must be a difference in temperature for heat transfer to take place. Since cold water does not work, why does the refrigerant work? The answer lies in a phenomenon called "phase change", otherwise simply termed a "change in the physical state of matter."
In the 1800s, scientists discovered a direct relationship between the temperature and pressure of a gas. If the temperature of gas is increased, its temperature also increases. On the other hand, if its pressure is decreased, its temperature will decrease.
The scientists also discovered that if a gas was compressed by a piston-driven compressor and allowed to cool, it would undergo a "phase change" and turn into a liquid. As long as the gas was kept under pressure inside a sealed container, it would remain in a liquid state. Once the pressure on the liquid was released, it would return to its gaseous state. During the process, it would absorb tremendous heat from its surroundings, producing a chilling effect.
The discovery was soon put to practical use by the invention of the first compression refrigeration system. The gas used in the system was ammonia as it would turn into a liquid easily when compressed. Ammonia-filled refrigeration systems were soon used widely in the food industry, and home refrigerators replaced the ice box. When air-conditioners first appeared in cars, ammonia was substituted with a non-toxic and non-flammable gas called diflurodichloromethane (CF2CL2), commonly referred by its trade name Freon or R-12.
Later, it was discovered that Freon (R-12) and other man-made chlorofluorocarbons contained chlorine. The chlorine was responsible for the thinning of the earth's ozone layer, which shields us from the sun's harmful ultra-violet rays. So the use of Freon and other CFCs as a refrigerant was discontinued in 1996. Modern car air-conditioners are now filled with tetrafluoroethane (CH2FCF3), commonly called R-134a. Containing no chlorine, R-134a is also non-toxic and non-flammable.
The refrigerant is, therefore, responsible for the working of the air-conditioning system. As explained earlier, it cools the air by absorbing heat and carrying it away. The chilling effect is made possible by a phase change that occurs in the refrigerant as it enters the evaporator.
The compressor is actually a pump that drives the refrigerant round the whole system. It compresses the refrigerant and sends the high-pressure vapour to the condenser through a high-pressure discharge hose. Once inside the condenser, the refrigerant is cooled by air, which condenses and changes into a liquid. The high-pressure refrigerant then travels from the condenser through the "liquid line" hose to an "orifice tube" or "expansion valve" just before it reaches the evaporator. The orifice tube keeps the liquid under pressure and controls the flow of refrigerant to the evaporator. The liquid line hose is commonly called the high-pressure line; the orifice tube, the low-pressure line.
As the refrigerant passes through the orifice tube or expansion valve to enter the evaporator, the liquid begins to change back into a gas. This produces the chilling effect that absorbs heat from the air passing through the evaporator. From here, the refrigerant vapours are pulled back through a suction hose to the compressor to begin the circuit all over again.
The compressor contains from two to six pistons, and is belt driven off the crankshaft via a magnetic clutch. In most systems, the on and off cycling of the compressor clutch regulates the flow of refrigerant but in some systems the compressor runs continuously and changes displacement to control cooling. At the condenser outlet, there is a drier to remove moisture from the refrigerant. The drier is actually a small reservoir containing a desiccant. Another similar device called the accumulator is located at the evaporator outlet, which also functions to remove moisture from the refrigerant.
Published by Monsoon Books, Singapore. Sold in Malaysia at Kinokuniya, Popular Book Store, Borders and MPH nationwide. An e-book edition is also available. Please check out monsoonbooks.com.sg.