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Air Conditioning, Page 1 of 3

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Theory of air conditioning
See Figure 1

In order to understand how air conditioning works, it is necessary to understand several basic laws about the flow of heat. While it may seem puzzling to talk about heat in the same breath as air conditioning, heat is your only concern. An air conditioner does not cool the air, but rather, removes the heat from a confined space.

The law of entropy states that all things must eventually come to the same temperature; there will always be a flow of heat between adjacent objects that are at different temperatures. When two objects at different temperatures are placed next to each other, heat will flow from the warmer of the two objects to the cooler one. The rate at which heat is transferred depends on how large the difference is between their temperatures. If the temperature difference is great, the transfer of heat will be great, and if the temperature difference lessens, the transfer of heat will be reduced until both objects reach the same temperature. At that point, heat transfer stops.

Because of entropy, the interior of an automobile tends to remain at approximately the same temperature as the outside air. To cool an automobile interior, you have to reverse the natural flow of heat, no matter how thoroughly insulated the compartment might be. The heat which the body metal and glass absorb from the outside must constantly be removed.

The refrigeration cycle of the air conditioning system removes the heat from a vehicle's interior by making use of another law of heat flow, the theory of latent heat. This theory says that during a change of state, a material can absorb or reject heat without changing its temperature. A material is changing its state when it is freezing, thawing, boiling or condensing. Changes of state differ from ordinary heating and cooling in that they occur without the temperature of the substance changing, although they cause a visible change in the form of the substance. While many materials can exist in solid, liquid, or gaseous form, the best example is plain water.

Water is a common material that can exist in all three states. Below 32°F (0°C), it exists as ice. Above 212°F (100°C), at sea level air pressure, it exists as steam, which is a gas. Between these two temperatures, it exists in its liquid form.

Since a change in state occurs at a constant temperature, it follows that a material can exist as both a liquid and a gas at the same temperature without any exchange of heat between the two states. As an example, when water boils, it absorbs heat without changing the temperature of the resulting gas (steam).

The change from a solid to a liquid and vice versa is always practically the same for a given substance-32°F (0°C) for water-but the temperature at which a liquid will boil or condense depends upon the pressure. For example, water will boil at 212°F (100°C), but only at sea level. The boiling point drops slightly at higher altitudes, where the atmospheric pressure is lower. We also know that raising the pressure 15 lbs. above normal air pressure in an automobile cooling system will keep the water from boiling until the temperature reaches about 260°F (127°C)

One additional aspect of the behavior of a liquid at its boiling point must be clarified to understand how a refrigeration cycle works. Since liquid and gas can exist at the same temperature, either the evaporation of liquid or the condensation of gas can occur at the same temperature and pressure conditions. It's just a matter of whether the material is being heated or cooled.

As an example, when a pan of water is placed on a hot stove, the heat travels from the hot burner to the relatively cool pan and water. When the water reaches it's boiling point, its temperature will stop rising, and all the additional heat forced into it by the hot burner will be used to turn the liquid material into a gas (steam). The gas thus contains slightly more heat than the liquid material.

If the top of the pan were now to be held a couple of inches above the boiling water, two things would happen. First, droplets of liquid would form on the lower surface of the lid. Second, the top would get hot very quickly. The top becomes hot because the heat originally used to turn the water into steam is being recovered. As the vapor encounters the cooler surface of the metal, heat is removed from it and transferred to the metal. This heat is the same heat that was originally required to change the water into a vapor, and so it again becomes a liquid.

Since water will boil only at 212°F (100°C) and above, it follows that the steam must have been 212°F (100°C) when it reached the top and must have remained that hot until it became a liquid. The cooling effect of the top (which started out at room temperature) caused the steam to condense, but both the boiling and the condensation took place at the same temperature.

To sum up, refrigeration is the removal of heat from a confined space and is based on three assumptions:

  1. Heat will only flow from a warm substance to a colder substance.
  2. A refrigerant can exist as both a liquid and a gas at the same temperature if it is at its "boiling point." A refrigerant at its boiling point will boil and absorb heat from its surroundings if the surroundings are warmer than the refrigerant. A refrigerant at its boiling point will condense and become liquid, losing heat to its surroundings, if they are cooler than the refrigerant.
  3. The boiling point of the refrigerant depends upon the pressure of the refrigerant, rising as the pressure rises and falling as the pressure falls. The operation of the refrigeration cycle illustrates how these three laws are put to use.

Figure 1 The same substances can exist in three states, depending on the temperature.
The same substances can exist in three states, depending on the temperature.

How the air conditioner works

Refrigeration cycle
See Figure 2

Any automotive air conditioning system employs four basic parts-a mechanical compressor, driven by the vehicle's engine; an expansion valve, which is a restriction the compressor pumps against; and two heat exchangers, the evaporator and the condenser. In addition, there is the refrigerant that flows through this system.

The belt-driven compressor uses engine power to compress and circulate the refrigerant gas throughout the system. The refrigerant passes through the condenser on its way from the compressor outlet to the expansion valve. The condenser is located outside the passenger compartment, usually in front of the vehicle's radiator. The refrigerant passes from the expansion valve to the evaporator, and after passing through the evaporator tubing, it is returned to the compressor through its inlet. The evaporator is located inside the vehicle's passenger compartment.

When the compressor starts running, it pulls refrigerant from the evaporator coil and forces it into the condenser coil, thus lowering the evaporator pressure and increasing the condenser pressure. When proper operating pressures have been established, the expansion valve will open and allow refrigerant to return to the evaporator as fast as the compressor is removing it. Under these conditions, the pressure at each point in the system will reach a constant level, but the condenser pressure will be much higher than the evaporator pressure.

The pressure in the evaporator is low enough for the boiling point of the refrigerant to be well below the temperature of the vehicle's interior. Therefore, the liquid will boil, remove heat from the interior, and pass from the evaporator as a gas. The heating effect produced as the refrigerant passes through the compressor keeps the gas from liquefying and causes it to be discharged from the compressor at very high temperatures. This hot gas passes into the condenser. The pressure on this side of the system is high enough so that the boiling point of the refrigerant is well beyond the outside temperature. The gas will cool until it reaches its boiling point, and then condense to a liquid as heat is absorbed by the outside air. The liquid refrigerant is then forced back through the expansion valve by the condenser pressure.

Figure 2 Basic components of an air conditioning system and the flow of refrigerant.
Basic components of an air conditioning system and the flow of refrigerant.

Refrigerant

A liquid with a low boiling point must be used to make practical use of the heat transfer that occurs when a liquid boils. Refrigerant-12 (R-12) is the refrigerant that was universally used in automotive air conditioning systems. At normal temperatures, it is a colorless, odorless gas that is slightly heavier than air. Its boiling point at atmospheric pressure is -21.7°F (minus 6°C). If liquid R-12 is spilled into the open air, it would be seen for a brief period as a rapidly boiling, clear liquid.

R-12 was nearly an ideal refrigerant. It operated at low pressure and condenses easily at the temperature ranges found in automotive air conditioning systems. It is also non-corrosive, non-toxic (except when exposed to an open flame), and nonflammable. However, due to its low boiling point and the fact that it is stored under pressure, certain safety measures must be observed when working around the air conditioning system. Unfortunately it was discovered the carboflurocarbons (CFC's) which were chemicals in the same group as dichlorodifluoromethane, which you know as R-12 or Freon were depleting the ozone layer of the atmosphere.

On December 31, 1995, CFC-12 production essentially ended in the U.S. However to avoid release into the atmosphere it is still legal to use the existing stockpiles of CFC-12.

The replacement for CFC-12 has been a non-CFC refrigerant R-134a. This has been used since the 1994 model year. Some of the older R-12 systems are being changed over to R-134a but this can be a costly and complicated process on some vehicles.

There is a third category for refrigerants that substitute CFC-12, these also contain ozone depleting HCFC's such as R-22, R142b, and R-124.

There are strict governmental regulations enforced by the clean air act of 1990 section 609. These include specific regulations for the use and handling of each of the three types of refrigerants.

Continue to page 2 of Air conditioning

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©1998 W. G. Nichols - Chilton's Easy Car Care