<ul><li>Prepare for ASE Heating and Air-conditioning (A7) certification test content area “A” (Air conditioning System Diagnosis and Repair) and content area “C” (Heating and Engine Cooling Systems Diagnosis and Repair). </li></ul><ul><li>Describe how the heater functions. </li></ul><ul><li>Describe how the refrigeration cycle functions. </li></ul>OBJECTIVES: After studying Chapter 49, the reader should be able to: Continued
<ul><li>List the parts of a typical air-conditioning system. </li></ul><ul><li>Explain how the air-conditioning system removes heat from the passenger compartment. </li></ul>OBJECTIVES: After studying Chapter 49, the reader should be able to:
<ul><li>section 609 (clean air act) • solid • superheat • swash plate </li></ul><ul><li>thermostatic expansion valve (TEV or TXV) system • thermo switch (icing switch or defrost switch) </li></ul><ul><li>vapor </li></ul>KEY TERMS:
<ul><li>Driver and passenger comfort is the primary purpose of the heating, ventilation, and air-conditioning system, abbreviated HVAC . </li></ul>PRINCIPLES OF HEATING AND REFRIGERATION Continued Figure 48–1 Water is a substance found naturally in solid, liquid, and vapor states. Matter is found in three different states: solid, liquid, or vapor (gas). The state depends upon the nature of the substance, the temperature, and the pressure or force exerted on it. Water occurs naturally in all three states: solid ice, liquid water, and water vapor, depending upon temperature and pressure of the location.
<ul><li>Changes of State A solid cannot be compressed and has strong resistance to flow. Molecules of a solid attract each other strongly, and resist changes in volume and shape. A substance is solid at any temperature below its melting point. The melting point is a characteristic of a substance, and the temperature at which a solid turns liquid. For water, melting point is 32°F (0°C), A liquid is a substance that cannot be compressed. A substance in a liquid state has a fixed volume, but no definite shape. The boiling point is the temperature at which a solid substance turns to a vapor. For water at normal seal level conditions, the boiling point is 212°F (100°C). </li></ul>Continued
<ul><li>Vapor can be easily compressed, has no resistance to flow, and no fixed volume. It is considered a fluid just like liquids are. A substance changes to a vapor if the temperature rises above its boiling point. A vapor condenses to liquid if the temperature falls below it. Like melting and freezing, boiling point and condensation point are the same temperature, and vary with pressure. </li></ul>Continued
<ul><li>Heat and Temperature Molecules vibrate rapidly in all directions; disorganized energy is called heat . Intensity of vibration depends kinetic energy , or energy of motion, the atom or molecule contains. We measure the level of this energy as temperature. Heat and temperature are not the same. Heat is measured in calories ( c ). The calorie is a metric unit for the amount of heat needed to raise the temperature of one gram of water one degree Celsius. Heat is also measured in British Thermal Units (BTU). One BTU is the heat required to raise the temperature of one pound of water 1°F at sea level. One BTU equals 252 calories. </li></ul>Continued
If you spray a can of liquid continuously, the can becomes cold, and the liquid being sprayed becomes cold. The can becomes cold because the pressure in the can is reduced while spraying, allowing the liquid propellant inside the can to boil and absorb heat. The liquid being sprayed has also been cooled by the liquid propellant. The propellant vapor is further cooled as it decompresses when it hits the open air. Rapid decompression results in a rapid temperature drop. Why is Liquid Sprayed From a Can Cold?
<ul><li>Latent Heat The “extra” heat that needed to transform a substance from one state to another. When a solid reaches melting point, or liquid reaches boiling point, their temperatures stop rising. The solid begins to melt, and the liquid begins to boil. This occurs without any change in temperature, even though heat is still being added. </li></ul><ul><li>This extra, hidden amount of energy necessary to change the state of a substance is called latent heat. See Figures 48–2 and 48–3. Latent heat is important in air-conditioning system operation because the cooling effect is derived from changing the state of liquid refrigerant to a vapor. The refrigerant absorbs latent heat of vaporization, cooling the air blown into the passenger compartment. You take away the heat to cool the air. </li></ul>Continued
Figure 48–2 The extra heat required to change a standard amount of water at its boiling point to a vapor is called latent heat of vaporization. Continued
Figure 48–3 The latent heat of vaporization that water vapor stores is given off when the vapor condenses to a liquid. The temperature stays the same. Continued
<ul><li>Increasing pressure by compressing a vapor increases temperature. Decreasing pressure by permitting expansion of the vapor decreases temperature. </li></ul>Continued Temperature, Volume, and Pressure of a Vapor Unlike a solid, a vapor has no fixed volume. Increasing temperature of a vapor, while keeping volume confined in the same space, increases pressure. This relationship between temperature and pressure in vapor is why a can of nonflammable refrigerant can explode when heated by a flame—pressure buildup inside the can will eventually exceed the can’s ability to contain the pressure.
<ul><li>Pressure-Temperature Relationships Two aspects are important to understanding the operating of an HVAC system: </li></ul>Continued <ul><li>The temperature at which a liquid boils (and vapor condenses) rises and falls with the pressure. </li></ul><ul><li>Pressure in a sealed system that contains both liquid and vapor rises and falls with the temperature. </li></ul>
<ul><li>Humidity refers to water vapor present in air. Level of humidity depends upon water vapor present and temperature of the air. Absolute humidity is the measurement of the weight of the water vapor in a given volume of air. Relative humidity is the percentage of how much moisture is present in the air compared to how much moisture the air is capable of holding at that temperature. Relative humidity is commonly measured with a hygrometer or a psychrometer . A hygrometer uses sensitive element that expands and contracts, based on the humidity. Hygrometers resemble a clock, with a scale reading from 0% to 100% relative humidity. </li></ul>Continued
Figure 48–4 A sling psychrometer is used to measure relative humidity. <ul><li>A psychrometer uses two thermometers, one with the bulb covered in a cotton wick soaked in distilled water from a built-in reservoir. </li></ul>The wick keeps the bulb of the “wet thermometer” wet so that it can be cooled by evaporation. Continued As the evaporator blows air, the wet bulb’s temperature drops, and the dry bulb reads the temperature of the airflow. Sling psychrometers are spun round in the air a certain number of times.
<ul><li>Water evaporates from the wick at a rate inversely proportional to the relative humidity of the air; faster if the humidity is low, and slower if the humidity is high. The “dry thermometer” registers ordinary air temperature. The higher the relative humidity, the closer the readings of the two thermometers; the lower the humidity, the greater the difference. The different temperatures are compared to a chart, which gives relative humidity. </li></ul>
HEATING SYSTEM <ul><li>All automotive and light-truck heater systems use the hot coolant from the engine to produce heat. Coolant (antifreeze and water) flows through heater hoses and a heater core . The water pump supplies force necessary to circulate the coolant through the heater core, a small radiator with tubes and fins that transfer heat from the coolant to air flowing through core. See Figure 48–5. A blower motor with a squirrel cage-type fan is used to force air through the heater core and into the passenger compartment. Before the heater can function correctly, the cooling system has to be functioning correctly. See Figure 48–6. </li></ul>Continued
Figure 48–5 Typical flow of air through an automotive heat, ventilation, and air-conditioning system when placed in the heat position. Continued
Figure 48–6 A typical heater core as installed in an HVAC housing.
Some vehicles are equipped with an auxiliary electric water pump. The purpose and function of this pump is to help warm the interior of the vehicle by circulating coolant from the engine through the heater core when the engine is at idle speed. At idle speed, the water pump does not pump a sufficient quantity of coolant through the heater core to warm the interior in freezing weather. What is an Auxiliary Electric Water Pump?
AIR-CONDITIONING REFRIGERATION CYCLE <ul><li>All automotive air-conditioning systems are closed and sealed. A refrigerant is circulated through the system by a compressor that is powered by the engine through an accessory drive belt. Older systems used refrigerant CFC-12, commonly referred to by its Dupont trade name of Freon ® or R-12. Manufacturers now use HFC-134a, less harmful to the atmosphere. The basic principle of the refrigeration cycle is that as a liquid changes into a gas, heat is absorbed. The heat absorbed by an automotive air-conditioning system is heat from inside the vehicle. </li></ul>Continued
<ul><li>Automotive air-conditioning function in this manner: </li></ul>Figure 48–7 The evaporator removes heat from the air that enters a vehicle by transferring it to the vaporizing refrigerant. <ul><li>Liquid refrigerant evaporates in a small radiator-type unit called the evaporator . As the refrigerant evaporates, it absorbs heat as it changes from a liquid to a gas. As heat is absorbed by the refrigerant, the evaporator becomes cold. </li></ul>Continued
<ul><li>After the refrigerant has evaporated into a low-pressure gas in the evaporator, it flows into the engine-driven compressor. </li></ul>Figure 48–8 The compressor provides the mechanical force needed to pressurize the refrigerant. The low-pressure refrigerant gas is compressed into a high-pressure gas and forces the refrigerant through the system. Continued
<ul><li>This high-pressure gas flows into the condenser located in front of the cooling system radiator. The condenser looks like another radiator, with the same purpose and function—heat removal. </li></ul>Figure 48–9 The condenser changes the refrigerant vapor into a liquid by transferring heat from the refrigerant to the air stream that flows between the condenser fins. Continued The high-pressure gas changes (condenses) to form a high-pressure liquid as the heat from the refrigerant is released to the air.
<ul><li>The high-pressure liquid then flows through a device that meters the flow into the evaporator. When the high pressure of the liquid drops, it causes the refrigerant to vaporize. </li></ul><ul><li>Air is blown through the evaporator by the blower motor and is cooled as heat is removed from the air and transferred to the refrigerant in the evaporator. </li></ul>The underlying principle involved in air-conditioning or refrigeration is that “cold attracts heat.” Therefore, a cool evaporator attracts the hot air inside the vehicle. Heat always travels toward cold and when the hot air passes through the cold evaporator, the heat is absorbed by the cold evaporator, which lowers the temperature of the air. Cooled air is then forced into the passenger compartment by the blower through the air-conditioning vents. How Does the Inside of the Vehicle Get Cooled?
EXPANSION VALVE SYSTEMS <ul><li>An expansion valve is attached to the inlet to the evaporator and controls the amount of refrigerant flow into the evaporator. The valve controls refrigerant flow based on evaporator outlet temperature, measured by a temperature-sensing bulb and tube. When the outlet is warm, the opening of the expansion valve is increased, allowing refrigerant to flow into the evaporator. As the outlet temperature of the evaporator decreases, the sensing bulb causes the expansion valve to restrict flow of refrigerant into the evaporator. This type of system is called the thermostatic expansion valve system—usually abbreviated TEV or TXV . See Figure 48–10. </li></ul>Continued
Figure 48–10 A typical air-conditioning system that uses an expansion valve. A sensor bulb is attached to the outlet of the evaporator to control refrigerant allowed to flow into the evaporator.
ORIFICE TUBE SYSTEMS <ul><li>Many air-conditioning systems today use a fixed-orifice tube at the evaporator inlet. As refrigerant flows through this orifice (small hole), it expands inside the evaporator, where it absorbs heat and expands into a low-pressure gas. A pressure switch located in the low-pressure line at the outlet of the evaporator senses when pressure is too low, which can cause evaporator temperature to drop to below freezing, creating an ice blockage to air flow through the evaporator. When pressure drops below a certain pressure (typically about 3 to 33 psi [214 to 228 kPa]), a pressure switch breaks the circuit to the air-conditioning compressor clutch, which stops the flow of refrigerant through the evaporator. </li></ul>Continued
Figure 48–11 A typical automotive air-conditioning system that uses a cycling clutch and an orifice tube. Continued
<ul><li>When temperature (and pressure) increases in the evaporator, the pressure switch closes, causing the compressor to start forcing refrigerant through the evaporator again. This type of system is commonly called a cycling clutch orifice tube (or CCOT ) system. </li></ul>Figure 48–12 Typical orifice tube.
THERMOSTATIC CONTROL <ul><li>The lower the pressure of refrigerant, the lower the temperature. If evaporator pressure is above 30 psi (220 kPa) for R-12 or 28 psi (193 kPa) for an R-134a system, the temperature of the evaporator will remain about freezing, 32°F (0°C). Temperature control is used to prevent this, as moisture in the air would freezes and clog the airflow through the evaporator. The air-conditioning system would stop functioning. If the A/C is turned off when this happens, heat from surrounding air will melt the ice and the air-conditioning will again function. </li></ul>Continued
<ul><li>A commonly used method to control evaporator temperature is to use a thermostat to control the compressor. Air-conditioning compressors use an electromagnetic clutch . </li></ul>Figure 48–13 A cutaway of an air-conditioning compressor electromagnetic clutch. Continued When the thermostat senses that the temperature is near freezing, 32°F (0°C), a switch stops the compressor from circulating refrigerant. This thermostat switch is also called a thermo switch , icing switch , or defrost switch .
NOTE: Older vehicles used a system to control evaporator pressure when used with a continuously-operating compressor, including: POA valve —(meaning pilot operated absolute ) valve. EPR valve —An evaporator pressure regulator valve maintains at least 30 PSI in the evaporator to prevent evaporator freeze-up.
REFRIGERANTS <ul><li>Air-conditioning refrigerant transfers heat from the inside of a vehicle to the condenser located in the front of the vehicle. A refrigerant absorbs heat when it changes state from liquid to gas. One of the first was CFC - 12 , commonly referred to as R - 12 , or its brand name, Freon ® , a trade name of DuPont Corporation. CFC-12 consists of one carbon atom surrounded by two chlorine (CL) and two fluorine (F) atoms, and is called a chlorofluorocarbon (CFC) compound. Its chemical name is dichlorodifluoromethane . Chlorine atoms that are believed to contribute to the destroying of the ozone layer in our upper atmosphere. </li></ul>Continued
<ul><li>Refrigerant HFC - 134a , called R - 134a , has replaced CFC-12. </li></ul>See the chart on Page 553 of your textbook. HFC-134a contains two carbon atoms and four fluorine atoms, plus two hydrogen atoms. Called a hydrofluorocarbon, the chemical name of R-134a is tetrafluorolthene . Boiling points and operation characteristics of CFC-12 and HFC-134a are similar. Dupont calls HFC-134a Suva ® . Continued
<ul><li>HFC-134a is a small molecule, and can easily leak out through small holes or openings in the system. </li></ul>NOTE: Look at the size of a blue HFC-134a 30-pound container compared to a white CFC-12 30-pound container. The blue R-134a container is larger because it takes more HFC-134a to achieve 30 pounds. Figure 48–14 R-134a is available in 12 oz cans as well as larger 30lb containers. NOTE: Many vehicle manufacturers started using barrier-type refrigerant hoses in the late 1980s, anticipating conversion from CFC-12 to HFC-134a in future models. Suva ® requires refrigerant hoses to contain a barrier to stop penetration through microscopic holes.
REFRIGERANTS AND THE ENVIRONMENT <ul><li>Air-conditioning refrigerants have been discovered to be harmful to the ozone layer. The ozone (O 3 ) layer is in the upper atmosphere and blocks out ultraviolet rays from the sun. </li></ul>Continued Figure 48–15 A depletion of the ozone layer allows more ultraviolet radiation from the sun to reach Earth’s surface. It has been discovered that certain chemicals, such as chlorofluorocarbon (CFC), are rapidly destroying this layer of ozone, 10 to 30 miles above Earth’s surface.
Figure 48–16 Chlorofluorocarbon molecules break apart in the atmosphere. <ul><li>How ozone is destroyed: </li></ul>Continued
<ul><li>Montreal Protocol A conference was held in Montreal, Canada, in 1987, where the United States and 22 other countries agreed to limit the production of ozone-depleting refrigerants. The Clean Air Act of 1990 specified that the production of R-12 refrigerant would cease at the end of 1995. Section 609 of the Clean Air Act required the following: </li></ul>Continued <ul><li>All techs who service or repair automotive air-conditioning systems shall be properly trained and certified. </li></ul><ul><li>Recovery and recycling equipment must be properly approved. </li></ul><ul><li>Each shop performing automotive air-conditioning service should comply to Environmental Protection Agency (EPA) that it is using approved recycling equipment and that only properly trained and certified technicians are using the equipment. </li></ul>
<ul><li>Not likely. While carbon dioxide (CO2) (R744) is being used on prototype vehicles, such as the Toyota Fuel Cell Hybrid Vehicle (FCHV), it requires extremely high pressures, up to 2000 PSI and is not as efficient as a refrigerant as R-134. </li></ul>Is Carbon Dioxide the Next Refrigerant? Figure 48–17 The label on a Toyota Fuel Cell Hybrid Vehicle (FCHV) showing that CO 2 is being used as the refrigerant.
REFRIGERANT OILS <ul><li>The oil carried by the refrigerant through various components is often the only source of lubrication for the compressor. The oil used must be able to be mixed without separating in the refrigerant. This characteristic of being able to be mixed is called miscible . CFC-12 systems must use mineral oil. Mineral oil is not miscible in HFC-134a; those systems must use synthetic polyalkyline glycol, usually referred to as PAG oil. There are numerous different PAG oils, and each vehicle manufacturer (or air-conditioning compressor manufacturer) recommends which PAG to use. See Figure 48–18. </li></ul>Continued
Figure 48–18 PAG oil used in Chrysler vehicles equipped with HFC-134a refrigerant. Notice that different oils are used for different systems depending primarily on the manufacturer of the compressor. Also notice that both PAG oils are in metal cans. PAG oil absorbs moisture so readily that it can even absorb moisture that is in the air through plastic—that is why metal containers are used. Continued
<ul><li>Another type of refrigerant oil is called ester oil , a classification of hydrocarbons, specified for use in air-conditioning systems that have been retrofitted from CFC-12 to HFC-134a. Ester oil will mix with any remaining mineral oil and will work to lubricate the system even if some CFC-12 is still in the system. </li></ul>Figure 48–19 Ester refrigerant oils are often specified for use when retrofitting an R-12 system to R-134a by companies who supply refit kits. Ester refrigerant oil is not recommended by many vehicle or air-conditioning compressor manufacturers. Always use the recommended refrigerant oil for the vehicle and system being serviced. Continued
<ul><li>All refrigerant oils have a viscosity rating. Viscosity is the measure of the oil’s thickness or resistance to flow. Always use the type and viscosity of oil specified by the manufacturer. </li></ul>CAUTION: Failure to use correct refrigerant oil in an air- conditioning system can cause serious (and expensive) damage to the air-conditioning compressor. Always use the refrigerant oil specified by the manufacturer.
CONDENSER <ul><li>The condenser looks like a cooling system radiator, because it is designed to radiate heat from the refrigerant to the outside air. When refrigerant leaves the compressor it is over 300°F (150°C) as it enters the condenser. Even on a hot 100°F (38°C) day, there is a difference in temperature between the outside air and the refrigerant inside the condenser. Heat always travels hot to cold. Heat in the hot refrigerant radiates into the outside air. As heat travels into the air, high-pressure gas refrigerant changes state and becomes high-pressure liquid. The condenser condenses refrigerant from a gas (vapor) to a liquid. See Figure 48–20. </li></ul>Continued
Figure 48–20 The condenser serves the same function for both the orifice-tube and the expansion valve–type air-conditioning system, and that is to remove the heat from the refrigerant and cause the hot refrigerant vapors to condense into a hot liquid. Continued
<ul><li>The wise tech will carefully inspect and replace any and all worn engine mounts if a broken aluminum condenser line is discovered to prevent a premature failure of a replacement condenser. </li></ul>Most air-conditioning systems use aluminum and flexible rubber lines between the compressor and the condenser. Because the compressor is mounted on and driven by the engine and the condenser is mounted to the body, these lines can break if the engine mounts are defective. The rubber hoses attached between the aluminum fittings of the compressor and condenser are designed to absorb normal engine movement. Worn engine mounts allow the engine to move too much. Aluminum lines cannot stand to be flexed without crushing and breaking. Broken Condenser Line? Check the Engine Mounts! Figure 48–21 A repaired condenser refrigerant line.
EVAPORATOR <ul><li>The evaporator looks like a small radiator located in a housing on the passenger side of the bulkhead (firewall). </li></ul>NOTE: If the carpet or floor of the vehicle is wet on the passenger side, the cause is often a clogged evaporator drain hose. The opening, called the condensate line , is frequently clogged with mud, debris, or leaves. To check the drain opening, hoist the vehicle and insert a wire or screwdriver into the end of the hose opening at the bottom of the evaporator housing. The evaporator transfers heat from the air to refrigerant flowing through it. Heat from the air causes low-pressure liquid inside the evaporator to evaporate into low-pressure gas. As the refrigerant changes state, it absorbs heat. A blower motor with a squirrel cage fan circulates air through the evaporator and forces the cooler air into the passenger compartment. See Figure 48-22.
Figure 48–22 The evaporator serves the same function for both the orifice-tube and the expansion valve–type air-conditioning system, and that is to allow the liquid refrigerant to evaporate and absorb heat from the passenger compartment.
RECEIVER-DRIER <ul><li>A receiver-drier, used on air-conditioning systems with an expansion valve, is located between condenser and evaporator. This section of the air-conditioning system contains high-pressure liquid refrigerant . The receiver provides temporary storage for liquid refrigerant and includes a debris filter and a desiccant to remove moisture. Many receiver-driers contain a sight glass that provides a view of the liquid refrigerant in the system. A drier is needed to remove moisture from the system. The drier contains a desiccant (silica alumina or silica gel), a drying agent that absorbs any moisture (water) in the air-conditioning refrigerant system. Moisture can combine with refrigerant to form an acid. Water can also freeze and form ice in the system. </li></ul>Continued
<ul><li>The desiccant is classified as XH-5 for CFC-12 systems and XH-7 or XH-9 for HFC-134a systems. The desiccant used on a CFC-12 system is not compatible with HFC-134a systems. The desiccant (accumulator or receiver-drier) should also be replaced on any air-conditioning system that has been left open to the atmosphere for any length of time (over 24 hours) or whenever the system has been left in a discharged condition. See Figure 48–23. </li></ul>Continued
Figure 48–23 Expansion-valve systems store excess refrigerant in a receiver-drier, which is located in the high-side liquid section of the system, whereas orifice-tube systems store excess refrigerant in an accumulator located in the low-side vapor section of the system.
ACCUMULATOR <ul><li>An accumulator, used on systems with an orifice tube, is located between evaporator and compressor. Refrigerant in this section of the cycle is a low-pressure gas . The purposes of the accumulator: </li></ul>Continued <ul><li>Preventing liquid refrigerant from reaching the compressor </li></ul><ul><li>Holding a reserve of refrigerant </li></ul><ul><li>Holding the desiccant (helping remove moisture) </li></ul>NOTE: A liquid cannot be compressed. If liquid refrigerant were to enter the compressor, the compressor would lock up and be damaged.
Figure 48–24 A typical accumulator used on a cycling clutch orifice-tube (CCOT) system.
REFRIGERANT LINES AND HOSES <ul><li>Aluminum tubing is used to connect many stationary items like condenser to receiver-drier and receiver-drier to evaporator. Rubber lines are used to and from the compressor, because it is attached to the engine, which is mounted on flexible rubber mounts. There is movement between the compressor and the other air-conditioning components attached to the body of the vehicle. Flexible refrigerant hoses are constructed from many layers of rubber and fabric. Hoses used on vehicles since the early 90s use a nonpermeable inside layer that prevents the loss of refrigerant. Called barrier hoses , they are required for use with HFC-134a refrigerant. See Figure 48–25. </li></ul>Continued
Figure 48–25 Rigid lines and flexible hoses are used throughout the air-conditioning system. The line to and from the compressor must be flexible because it is attached to the engine, which moves on its mounts during normal vehicle operation.
THERMOSTATIC EXPANSION VALVES <ul><li>Thermostatic expansion valve (TXV) systems use a temperature-sensitive bulb located on the evaporator outlet tube. The sensing bulb is insulated with a special tape, reacting only to temperature changes from the outlet tube. </li></ul>Continued Figure 48–26 A typical expansion valve which uses an inlet and outlet attachment for the evaporator, and a temperature-sensing bulb that is attached to the evaporator outlet tube The sensing bulb works with a pressure-sensitive diaphragm inside the TXV body to regulates the rate of refrigerant flow into the evaporator.
<ul><li>The key to operation of the expansion valve is the variable orifice. The outlet from high- to low-pressure side is a variable-diameter hole. A pintle valve is a ball-and-seat valve used to increase or decrease the size of the opening. See Figure 48–27. The expansion valve uses the pintle valve to control how rapidly refrigerant enters the evaporator. The expansion valve controls the refrigerant flow in response to the temperature of the evaporator outlet, measured by the remotely mounted sensing bulb and capillary tube . See Figure 48–28. The sensing bulb may be clamped to the outlet pipe or mounted inside a passage near the outlet of the evaporator. The bulb and tube contain refrigerant. The rise or fall of the evaporator outlet temperature causes the refrigerant in the bulb to expand or contract, resulting in a rise or fall of pressure inside the capillary. </li></ul>Continued
Figure 48–27 A slot cut in the ball seat inside the expansion valve permits a small amount of refrigerant and oil to pass through at all times, even when the valve is closed. This flow through the system is necessary to ensure the compressor receives the oil it needs for lubrication. Continued
Figure 48–28 The sensing bulb is attached to the evaporator outlet tube. Refrigerant inside the bulb expands or contracts in response to the evaporator temperature.
<ul><li>This outlet-temperature-sensitive pressure is applied to one side of the spring-loaded diaphragm in the expansion valve. As the capillary tube warms, the refrigerant inside expands, forcing the diaphragm downward. The diaphragm magnifies this pressure and uses it to open the valve by pushing the pintle and ball away from its seat. This increases the size of the orifice and allows more refrigerant into the evaporator, increasing the cooling capacity. When the evaporator cools in response to the boiling of the added refrigerant, the refrigerant in the capillary tube contracts. This relieves the pressure on the expansion valve diaphragm, which closes the pintle and ball, and reduces refrigerant flow. See Figure 48–29. </li></ul>Continued
Figure 48–29 Pressure from the capillary tube pushes on the spring-loaded diaphragm to open the expansion valve. As the pressure in the capillary tube contracts, the reduced pressure on the diaphragm allows the valve to close. Continued
<ul><li>Pressure on top of the diaphragm is applied through the capillary tube. Equalizing pressure on the underside of the diaphragm can be internal (from the evaporator inlet) or external (evaporator outlet): </li></ul>Continued <ul><li>An internally equalized expansion valve has a passage that permits evaporator inlet pressure to reach the underside of the diaphragm. </li></ul><ul><li>An externally equalized expansion valve has an extra line mounted to the underside of the diaphragm housing. This line monitors the outlet pressure of the evaporator. The connection can be either at the outlet of the evaporator or at the outlet of the evaporator pressure control device. </li></ul>In an expansion-valve system, refrigerant vapor that leaves the evaporator is warmer than the liquid refrigerant that entered it. The heat that warms the refrigerant is referred to as superheat .
<ul><li>Superheat ensures all (or almost all) refrigerant vaporizes before leaving the evaporator. DaimlerChrysler uses a valve called an H - valve , which has two refrigerant passages that form the legs of the “H” as shown here, and in Figure 48-31. </li></ul>Figure 48–30 An H-valve (H-block) combines the temperature-sensing and pressure-regulating functions into a single assembly. Continued
<ul><li>The lower passage is the refrigerant line from the condenser to the evaporator, and contains the ball and spring valve. The upper passage is the refrigerant line from the evaporator to the compressor, and contains the temperature-sensing element. </li></ul>Figure 48–31 An H-valve as used on a Chrysler minivan. A pushrod connects the diaphragm of the sensor located at the top of the block to the valve ball at the bottom. The cycling-clutch switch, which senses the suction line, detects the evaporator outlet temperature and cycles the compressor clutch to control system cooling. Continued
Figure 48–32 In this Chrysler system, a low-pressure cutoff switch and a cycling-clutch switch are mounted on the H-valve. <ul><li>This capillary device does not directly control the metering orifice. </li></ul>
Figure 48–33 The orifice tube is usually located at the inlet tube to the evaporator. <ul><li>Liquid refrigerant flows from the condenser to the orifice tube. Fixed-orifice tubes provide a restriction that separates the high-pressure from the low-pressure side of the system. </li></ul>FIXED ORIFICE TUBES When it reaches the fixed-orifice tube, refrigerant undergoes rapid expansion & changes from a warm, high-pressure liquid to a cold, low-pressure liquid/vapor mix. Continued
<ul><li>As it passes through the restriction to the low side, the refrigerant changes state from a liquid to a vapor because the pressure in the evaporator is so much lower than in the refrigerant line upstream from the orifice tube. The refrigerant begins to vaporize quickly as it absorbs the heat from the evaporator. The orifice tube, located between the condenser and the evaporator inlet, may be inserted in the refrigerant line or may be part of the inlet refrigerant line assembly. </li></ul>
COMPRESSORS <ul><li>The air-conditioning compressor is driven by the engine with an accessory drive belt. A magnetic clutch is usually used to connect and disconnect the drive pulley to the compressor as needed for cooling or defrosting. The compressor: </li></ul>Continued <ul><li>Raises temperature of the gas so there is a temperature difference between outside (ambient) air and refrigerant in the condenser </li></ul><ul><li>Acts as the pump used to circulate refrigerant through the system </li></ul><ul><li>Often switches on and off (cycles) to control evaporator temperatures </li></ul><ul><li>Is the major reason why there is refrigerant oil in the system. Oil in the refrigerant lubricates the moving parts of the compressor. </li></ul>
Figure 48–34 In a positive-displacement compressor, the descending piston creates a drop in pressure inside the cylinder. The resulting pressure differential allows low-side pressure to force the suction valve open. Refrigerant then flows into the cylinder. On the piston’s discharge stroke, the pressure caused by the ascending piston closes the intake valve and forces the refrigerant out the discharge valve. <ul><li>Positive-Displacement Piston Compressors Most automotive air-conditioner compressors are positive - displacement compressor designs. They displace a uniform volume of refrigerant for each revolution or operating cycle, and have from one to 10 cylinders. </li></ul>Continued
<ul><li>The compressor intake stroke is also known as the suction stroke. As the compressor operates, the maximum possible charge of refrigerant vapor has been drawn in at bottom dead center of the compressor piston’s suction stroke. When the piston begins its discharge stroke, the pressure increases, shutting the suction valve and opening the discharge valve at the same time. The charge of refrigerant vapor is pushed out of the compressor into the high-side refrigerant line and travels toward the condenser. The refrigerant vapor is compressed simultaneously throughout the entire high side of the air-conditioning system. </li></ul>Continued
<ul><li>All piston compressors use one suction valve and one discharge valve for each piston. The typical valve used in compressors is the reed valve : a one-way, flap-type check valve that is built into the valve plate which seals one or more cylinders. </li></ul>Figure 48–35 A reed valve is a one-way check valve that flaps away from the valve plate to open, and toward the valve plate to close. Pressure pushes the valve in one direction or the other. The suction reed valve is located on the underside of the valve plate. The refrigerant vapor fills the partial vacuum created by the moving piston. Continued
<ul><li>The discharge reed valve is on the top side of the valve plate. The partial vacuum in the cylinder also pulls the discharge valve tightly against its seat, sealing off the system’s high side from the cylinder during the suction stroke. The reeds behave the opposite way during the piston’s discharge stroke. The increasing cylinder pressure pushes the suction valve tightly against its seat, sealing off the system’s low side. The discharge valve on the opposite side of the valve plate is unseated as the pressure in the cylinder increases as a result of the upward moving piston. </li></ul><ul><li>The piston pushes the refrigerant through the discharge valve, out of the compressor, and into the air-conditioning system high side. </li></ul>Continued
<ul><li>Pistons and Rings The two-stroke cycle depends upon the pistons and sealing rings to provide an adequate seal against high-side refrigerant pressure. </li></ul>Figure 48–36 The swash plate, attached to the crankshaft at an angle, converts the pulley’s rotary motion to axial motion, which drives the pistons in a reciprocating motion. Continued Some compressors use a swash plate , or axial plate, rigidly mounted to the belt-driven shaft at an angle. As the pulley turns, the shaft and angled swash plate assembly rotate. This forces the pistons back and forth in their bores. Compressors that use a swash plate are often called axial compressors .
<ul><li>Variable Displacement Compressor Some systems use variable displacement to control amount of refrigerant flow through the evaporator. The pressure difference between high side and low side causes the swash plate to move inside the compressor. As the swash plate changes its angle, the stroke of the piston is increased for more cooling or decreased to reduce the amount of cooling. </li></ul>Figure 48–37 A V-5 variable displacement compressor. Internal pressures act on the swash plate, which changes the stroke of the piston and then displacement based on the pressures in the system.
<ul><li>Most air-conditioning compressor clutch circuits contain a diode used to suppress high-voltage spikes generated whenever the compressor clutch coil is disengaged (turned off). If this diode were to fail, a high voltage (up to 400 volts!) could damage sensitive electronic components in the vehicle including the electronic air-conditioning compressor clutch control unit (if so equipped). </li></ul>The Radio “POP” Trick The radio will often turn off, then back on, whenever the electronics inside the radio detect a high-voltage spike. This can create a “pop” in the radio that is very intermittent because it only occurs when the air-conditioning compressor clutch cycles off. To check this diode, simply tune the radio to a weak AM station near 1400 Hz and cycle the air-conditioning compressor on and off. If a “pop” is heard from the radio, the diode is defective and needs replaced. NOTE: While some compressor diodes can be replaced separately, some air-conditioning clutch diodes are part of a wiring harness assembly.
COMPRESSOR CONTROLS <ul><li>Most air-conditioning compressors use an electromagnetic clutch. A coil of wire inside the clutch creates a magnetic field that connects the input shaft of the compressor to the drive pulley. About 3 to 4 amperes of current are required to energize the air-conditioning compressor clutch. Most vehicle manufacturers connect several components in series with the compressor clutch so that all have to be functioning before the compressor clutch can be engaged. </li></ul>Continued
<ul><li>The most commonly used switches include: </li></ul>Figure 48–38 Typical air-conditioning pressure switches. A service manual would be needed to determine the function of each switch. One switch could be the low-pressure switch and the other a high-pressure switch. Continued <ul><li>Low-pressure switch This switch is electrically closed only if there is at least 25 PSI of refrigerant pressure. This amount of pressure means the system is sufficiently charged to provide lubrication for the compressor. This switch also prevents the compressor from being engaged when the temperature is low (low temperature means low refrigerant pressures). </li></ul>
<ul><li>High-pressure switch This pressure switch is located in the high-pressure side of the air-conditioning system. If the pressure exceeds a certain level (typically 375 PSI [2,600 kPa]), the pressure switch opens, thereby preventing possible damage to the air-conditioning system due to excessively high pressure. </li></ul><ul><li>Power steering pressure switch This switch is used on many vehicles, especially those with four-cylinder engines. It opens the circuit to the air-conditioning compressor clutch when the driver turns the steering wheel. This reduces the load on the engine at the same time power is needed by the power steering pump. </li></ul>Because the wheel is seldom held in a turning maneuver for a long period of time, this stoppage of the air-conditioning compressor has little, if any, effect on passenger cooling.
A service technician was tracing the cause of an inoperative air compressor on a Saab. The service manual showed a schematic of the air-conditioning compressor that indicated a number of switches that had to be closed for the compressor clutch to be supplied with battery voltage. Besides the low pressure switch (to assure that the system is charged so as not to damage the compressor), a throttle switch was shown on the schematic. Obviously, someone else had worked on the vehicle because the throttle switch was missing entirely—just two wires remained to indicate that anything had been installed. Connecting the two wires together provided voltage to the air-conditioning compressor clutch. The customer decided not to replace the throttle switch after learning that its purpose was to disconnect (open circuit) the air-conditioning compressor when the throttle was at wide open positive to allow the maximum power for passing. What Throttle Switch?
SUMMARY <ul><li>Engine coolant flows through heater hoses and through a heater core to provide heat to the inside of the vehicle. </li></ul><ul><li>The refrigeration cycle uses a compressor to circulate a refrigerant through a closed system. </li></ul><ul><li>Refrigerant expands in the evaporator. When the refrigerant expands, both its pressure and its temperature drop. Air from inside the vehicle is cooled as it passes through the evaporator. </li></ul><ul><li>The compressor forces the refrigerant through the closed system and raises the temperature of the refrigerant so that the refrigerant will condense back into a liquid in the condenser. </li></ul>Continued
SUMMARY <ul><li>Airflow through the condenser removes heat from the hot refrigerant, which condenses back into a liquid. </li></ul><ul><li>The expansion valve (or orifice tube) causes the refrigerant to expand. When the refrigerant expands, its pressure and temperature both drop, thereby cooling the evaporator. </li></ul><ul><li>Temperature (thermostatic) or pressure controls (EPR, POA, or VIR) prevent the freezing of the evaporator by keeping the temperature of the evaporator above freezing (32°F or 0°C). </li></ul><ul><li>CFC-12 is commonly called Freon and is a chlorofluorocarbon (CFC). </li></ul>Continued ( cont. )
SUMMARY <ul><li>HFC-134a is less harmful to the environment because it does not contain any CFCs. </li></ul><ul><li>A desiccant is used either in the receiver-drier or accumulator to remove any moisture that may get into the system. Moisture and refrigerant combine to form harmful acid. </li></ul><ul><li>The airflow through the evaporator and heater core help condition the air by removing humidity and directing the airflow where needed. </li></ul>( cont. )