This booklet provides a quick and easy reference to the information presented in the discussional slide film, Chevrolet Air Conditioning Theory and Operation.

Since air conditioning is new to Chevrolet this year and new to mechanics, a copy of this booklet should be kept in the Service Department File of Technical Information. Additional copies are available on request, so that each mechanic will be able to retain one in his own possession for quick on-the-job reference.

Contents

Contents

All-Weather Air Conditioning is available for all Chevrolet passenger cars. This air conditioning system is designed to provide the controlled temperature and humidity requirements for year-round driving comfort.

Controls located on instrument panel provide a means of regulating the system. Conditioned air can be directed through adjustable nozzles at each end of the instrument panel as well as through the floor distributor and defroster openings. (See fig. 1)

When the air conditioning system is in operation, air is directed through the conditioning unit to the passenger compartment. This unit contains both a heater core and cooling coils within an insulated housing to heat, cool, or dehumidify the incoming air. (See fig. 2)

For cold weather driving, the air flowing through the conditioning unit is passed over the surfaces of the heater core. The desired heating capacity is obtained by regulating the flow of engine coolant through the heater core. (See fig. 3)

For warm weather driving, air flowing through the conditioning unit is passed over the surfaces of the cooling coils. These coils are kept at a very low temperature by a complete refrigeration system. (See fig. 4)

Dehumidifying

For driving in warm, humid weather, air flowing through the conditioning unit is directed over the surfaces of the cooling coils where it is rapidly chilled causing excess moisture in the air to condense. (See fig. 5)

Dirt and dust in the incoming air clings to the moist surfaces of the cooling coils and is drained from the system through a drain tube along with the condensate. (See fig. 6)

Finally, the chilled, dry air is passed over the surfaces of the heater core where, if necessary, the temperature may be raised to the desired level before entering the passenger compartment. See fig. 7)

Incoming air contaminated with exhaust fumes from slow-moving traffic may be shut off from entering the passenger compartment by means of the outside air door controlled from within the vehicle. A flexible valve permits inside air to be recirculated. (See fig. 8)

Because the construction, operation and control of the heater is basically the same as the Chevrolet heater used without air conditioning, this film will discuss only the refrigeration system of the air conditioner under the following headings:

• PRINCIPLES OF REFRIGERATION
• CONSTRUCTION AND OPERATION
• PROPERTIES OF FREON-12

A later release, Part II of Air Conditioning, will cover detailed service procedures and maintenance.

Principles of Refrigeration

Refrigeration is the process of removing heat. To understand how this accomplished in a refrigeration system, it is necessary to understand the following laws of liquids and vapors:

1. When a liquid changes into vapor, heat is absorbed.
2. When vapor changes into liquid, heat is given off.

By making use of these laws, it is possible to transfer unwanted heat to an area where additional heat is not important.

LIQUID TO VAPOR

The action of a liquid changing rapidly to a vapor is called boiling. The boiling liquid is, in a sense, absorbing heat from an outside source and carrying it away in the form of a vapor. (See fig. 9)

VAPOR TO LIQUID

The action of vapor changing to liquid is called condensation. Condensation is the reverse of boiling and results in the release of the same amount of heat as absorbed during the boiling process. (See fig. 10)

Construction and Operation

In a refrigeration system, liquid is allowed to boil in an evaporator. The heat in the air surrounding the evaporator is absorbed by the liquid changing to vapor. (See fig. 11)

The vapor is carried off by tubing and allowed to condense in a condenser. Heat is given off to the air surrounding the condenser, causing the vapor to change back into a liquid. (See fig. 12)

Excess heat in the air surrounding the evaporator is absorbed during the boiling process. Heat is then given off to the atmosphere as vapor changes back into a liquid in the condenser. This, then, is the cycle of refrigeration. (See fig. 13)

A receiver-dehydrator is incorporated with the condenser. The receiver serves as a liquid storage tank. The dehydrator traps out water and foreign material which may have entered the system. (See fig. 14)

Pressures and temperatures are used to control the refrigeration cycle. Pressures affect heat transfer in that:

1. Increasing pressure on a liquid raises its boiling point.
2. Increasing pressure on a vapor in a closed container raises its temperature.

Temperature affects heat transfer in that:

1. Heat always travels from a warm object to a cooler object.

Vapor leaving the evaporator is at a temperature lower than the temperature of the air surrounding the condenser. Because heat will only flow from a warm object to a cooler object, the temperature of the vapor must be raised before it can give off heat and condense. (See fig. 15)

Therefore, vapor leaving the evaporator is directed through a compressor. The compressor raises the pressure of the vapor, and as a result, the temperature of the vapor is raised above the temperature of the air surrounding the condenser and condensation can easily take place. (See fig. 16)

Because pressure of the vapor was raised before condensing, the liquid formed by condensing is a high-pressure liquid. This high pressure raises the boiling point of the liquid and prevents it from boiling in the receiver. (See fig. 17)

Liquid leaving the condenser and receiver-dehydrator is directed through a thermostatic expansion valve. This valve lowers the pressure of the liquid, which results in a much lower boiling point for the liquid. And, again, the liquid enters the evaporator and boils freely. (See fig. 18)

Low-pressure liquid enters the evaporator, absorbs heat from the surrounding air and changes into low-pressure vapor. (See fig. 19)

A sight glass is placed in the high-pressure liquid line. This provides a quick and easy method for checking the amount of liquid in the system, but it serves no purpose in the over-all operation. (See fig. 20

THIS, THEN, IS THE CYCLE OF OPERATION, COMPLETE WITH PRESSURE AND TEMPERATURE CONTROLS.

In summing up this portion of the film, let's review the basic cycle of refrigeration.

Cool, low-pressure vapor, leaving the evaporator, is directed through the compressor where the pressure is raised and the vapor is heated as a result of compression. (See fig. 21)

Warm, high pressure vapor leaves the compressor and is directed through the condenser. Here it gives up the heat absorbed in boiling and changes back into a high-pressure liquid. As a high-pressure liquid, it has a very high boiling point. (See fig. 22)

The high-pressure liquid passes through the thermostatic expansion valve where the pressure is reduced. This reduction of pressure results in lowering the boiling point of the liquid. (See fig. 23)

Low-pressure liquid enters the evaporator. In the evaporator the low-pressure liquid absorbs heat, boils, and changes into a low-pressure vapor to complete the cycle. (See fig. 24)

Throughout the system, operating pressures vary. Pressures and temperatures are closely related. Refrigerant vapor in a closed system has a definite temperature corresponding to each pressure. By checking the vapor temperature at any point in the system, it can be determined what the pressure should be and vice versa.

This fact of temperature-pressure relationship is often used in diagnosing the operation of the refrigeration system. A temperature-pressure chart, and a gauge set are diagnosing tools which will be discussed in Part II. (See fig. 25)

Because the thermostatic expansion valve plays a major part in the refrigeration cycle, a detailed explanation of its operation in conjunction with the power element and the equalizer line is as follows: (See fig. 26

High-pressure liquid leaving the condenser and receiver-dehydrator is directed through the thermostatic expansion valve. The purpose of the valve is to reduce the pressure of the liquid and to regulate the amount of liquid supplied to the evaporator. (See fig. 27)

Liquid must be admitted to the evaporator at the same rate that the vapor is leaving the evaporator. (See fig. 28)

The thermostatic expansion valve consists of a power element, body, operating pins, screens, stationary seat orifice, needle and needle carriage and the adjusting spring and stem. (See fig. 29)

When the system is not operating, the spring tension is sufficient to hold the needle valve closed so that no liquid passes through the valve. (See fig. 30)

The power element is a closed circuit filled with inert gas. It is clamped to the low-pressure line leaving the evaporator. Any temperature change at the thermobulb results in a pressure change in the gas which provides a force acting against spring pressure within the valve. (See fig. 31)

Power element pressures are applied to a bellows which operates three pins bearing on the needle carriage. Any increase in power element pressure will act through the pins to move the needle away form its seat. (See fig. 32)

An equalizer line is required in all high-capacity refrigeration systems. The equalizer line joins the expansion valve to the evaporator outlet. This allows evaporator outlet pressure to oppose the power element pressure in the expansion valve. (See fig. 33)

High-pressure liquid enters the valve through the screen and maintains a constant pressure at the needle seat orifice. (See fig. 34

Pressures from the power element and the high-pressure liquid are trying to force the needle away form its seat, while pressure exerted by the spring and equalizer line are trying to hold the needle against its seat. (See fig. 35)

Therefore, the operation of the expansion valve consists of an alternate opening and closing of the needle valve as controlled by the forces of the spring tension, power element and equalizer line. This permits the flow of refrigerant into evaporator to be controlled as the needs for cooling vary.

The adjusting stem of the expansion valve provides a means of increasing or decreasing the spring pressure which regulates liquid flow through the valve. Service procedures for adjusting the spring tension will be detailed in Part II. (See fig. 36)

A liquid which is used for refrigeration purposes is called a refrigerant. Several liquids are suitable for use as refrigerants, but most widely used is Freon-12. This is the refrigerant use in Chevrolet's air conditioning system.

Freon-12 is a colorless and transparent refrigerant in both the gaseous and liquid state. It has a boiling point of 21.7F. below zero, and therefore, at all normal temperatures and pressures it will be a vapor. (See fig. 37)

The vapor is heavier than air and is nonexplosive, noninflammable, and nonpoisonous except when in contact with an open flame. It is also noncorrosive except when in contact with water.

THE FOLLOWING PRECAUTIONS SHOULD BE OBSERVED WHEN HANDLING FREON-12:

Freon-12 is stored in vapor state under pressure in drums and disposable containers. Never store Freon-12 in an area where there is open flame or excessively high temperatures, or resulting pressure increase may burst the container. (See fig. 38)

When adding Freon-12 to the system, it may be necessary to raise the pressure within the storage container. To do this, a bucket of warm water, not over 125 F. or warm wet rags wrapped around the container will supply all the heat necessary. (See fig. 39)

Protection of the eyes when working around Freon-12 is of vital importance. Wear goggles at all times if there is danger of escaping gas. If Freon-12 strikes the eyes, flood with cold water, apply cod liver or antiseptic oil and see a doctor immediately. (See fig. 40)

Freon-12 mixes well with oil. This fact is utilized as a means of lubricating the internal parts of the expansion valve and compressor. As the refrigerant travels through the compressor it picks up oil and carries it throughout the system. (See fig. 41)

Extreme care should be taken to prevent water or other foreign matter from entering the Freon system. Cleanliness is important. Wipe all connections before opening the system. (See fig. 42)

Whenever it is necessary to disconnect a refrigerant or gauge line, it should be plugged or capped immediately to prevent moisture, dirt or foreign matter from entering. Tools also should be kept clean and dry. (See fig. 43

Escaping Freon gas will change the color of a propane flame in direct proportion to the size of the leak. Because of this, a propane gas cylinder, burner head, and sampling hose is used to detect Freon leaks. The detailed use of the leak detector will be outlined in Part II. (See fig. 44)

A thorough knowledge of the refrigeration cycle, especially understanding the reasons why pressures and temperatures are different at different points of the circuit, will be extremely helpful in diagnosing refrigeration problems. This knowledge of temperature-pressure, and proper use of the gauge set and leak detector which is outlined in Part II, make the refrigeration system an easily serviced unit.