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AUT 236 Automotive HVAC

AUT 236 Automotive HVAC

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AUT 236 Automotive HVAC

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  1. AUT 236Automotive HVAC CPT 7 Air Conditioning Systems

  2. OBJECTIVES After studying Chapter 7, the reader should be able to: • Understand the relationship between the A/C cycle and the components on the low side and high side of a system. • Identify a thermal expansion valve and orifice tube system. • Understand the functions for the low side and high side of a system. • Explain the role of each A/C component.

  3. INTRODUCTION • The automotive A/C system uses the physical principles described in earlier chapters to move heat from the passenger compartment to the condenser and then to the ambient air moving through the condenser. • The heating system uses some of the same principles to move heat from the engine’s cooling system to the passenger compartment.

  4. INTRODUCTION • Air is circulated through the A/C and heating system and the car to either add or remove heat. (Courtesy of Toyota Motor Sales USA, Inc.)

  5. INTRODUCTION • Automotive A/C systems are either a TXV system with a receiver–drier (a) or an OT system with an accumulator (b). Various compressors are used with both systems.

  6. FIGURE 7-3 The high and low sides of an A/C system are divided by the compressor (where the pressure is increased) and either a TXV or an OT (where the pressure drops). INTRODUCTION

  7. FIGURE 7-4 Refrigerant changes state to a vapor as it absorbs heat in the low side and into a liquid as it loses heat in the high side. INTRODUCTION

  8. INTRODUCTION • Refrigerant boils or evaporates in the low side and it condenses in the high side. • In an operating system, you can identify the low and high sides by: • Pressure. • Sight. • Temperature. • Tubing size.

  9. LOW-SIDE OPERATION • When the A/C system is in full operation, the goal of most systems is to maintain an evaporator temperature just above the freezing point of water, 32°F (0°C). • This temperature produces the greatest heat exchange without ice formation on the evaporator fins (evaporator icing significantly reduces the heat transfer).

  10. LOW-SIDE OPERATION • The cold temperature in the evaporator is produced by boiling the refrigerant. • Remember that R-12 and R-134a have very low boiling points, well below 0°F, and that when a liquid boils, it absorbs a large amount of heat, the latent heat of vaporization. • To produce cooling, liquid refrigerant must enter the evaporator, and it must boil inside the evaporator. • The amount of heat an evaporator absorbs is directly related to the amount of liquid refrigerant that boils inside it

  11. FIGURE 7-5 As liquid refrigerant enters the evaporator, the boiling point will try to drop as low as 32°F because of the drop in pressure. The cold temperature causes the refrigerant to absorb heat from the air circulated through the evaporator. LOW-SIDE OPERATION

  12. FIGURE 7-6 If the proper amount of refrigerant enters the evaporator, it has a slight superheat as it leaves (a). A starved condition, in which not enough refrigerant enters the evaporator, does not produce as much cooling (b). If too much refrigerant enters, the evaporator floods because the refrigerant will not all boil (c). LOW-SIDE OPERATION

  13. FIGURE 7-7 The low side begins at the TXV or OT and includes the evaporator and suction line to the compressor (a). The OT system includes an accumulator (b). LOW-SIDE OPERATION

  14. LOW-SIDE OPERATION • Expansion Devices • Thermal Expansion Valves • Orifice Tubes • Evaporators • Accumulator • Refrigerant Charge Level • Evaporator Icing Controls • Cycling Clutch Systems • Evaporator Pressure Controls • Variable Displacement Compressors

  15. LOW-SIDE OPERATION • A TXV is controlled by the pressure on the diaphragm from the heat-sensing tube, the pressure spring, and evaporator pressure through the equalizer pipe (a). • An H-type valve is essentially the same except evaporator pressure goes through an internal passage to the bottom of the diaphragm (b)

  16. Some systems use a suction throttling valve to keep evaporator pressure from dropping to the point at which icing can occur. LOW-SIDE OPERATION

  17. An OT is a simple restriction that limits the flow of refrigerant into the evaporator. The locating dimple keeps the OT from moving downstream. LOW-SIDE OPERATION

  18. What Is an Ejector Cycle? • A TXV or orifice tube is merely a restricted opening that causes a refrigerant pressure drop, and in a sense, the energy of the high pressure liquid is wasted as the pressure drops. • An ejector cycle replaces the expansion device with an ejector that has a very small opening; refrigerant passes through this opening “as fast as a jet airplane.” This action plus the design of the ejector create two flows: • 1. Liquid is directed to the evaporator, where it boils and picks up heat and returns to the ejector. • 2. Vapor is directed/pushed to the compressor inlet. The vapor flow to the compressor is said to reduce compressor motion by 33% and improve efficiency by 50%.

  19. Two views of a typical OT system; (a) is somewhat realistic and (b) is schematic. Both show the arrangement of the components and the refrigerant flow. (Courtesy of ACDelco) LOW-SIDE OPERATION

  20. FIGURE 7-12 A tube-and-fin (a) and a plate (b) evaporator. Each type has a large contact area for heat to leave the air and enter the refrigerant. LOW-SIDE OPERATION

  21. FIGURE 7-13 Accumulators are designed so that vapor from the top leaves to the compressor. They contain desiccant to absorb water from the refrigerant and many include a fitting for low-side pressure and the clutch cycling switch. LOW-SIDE OPERATION

  22. Are All Desiccants the Same? • A desiccant variety referred to as XH-5 has commonly been used with R-12 in automotive systems. XH-5 has the ability to absorb about 1% of its weight in water, but this chemical suffers damage when it absorbs fluorine from R-134a, and it begins to decompose when it comes into contact with R-134a and PAG oil. Other desiccant types (XH-7 and XH-9) are compatible with R-134a and PAG. The receiver–drier or accumulator used in an R-134a system should use XH-7 or XH-9 desiccant. All new accumulators and receiver–driers contain either XH-7 or XH-9 desiccant. An accumulator or receiver–drier should contain the desiccant type and amount to suit the A/C system that it is designed for.

  23. FIGURE 7-14 Water in an A/C system can combine with refrigerant to form acids. These acids can etch and dissolve components, cause rusting of metal parts, and cause ice blockage at the expansion device. LOW-SIDE OPERATION

  24. What Is a Suction Throttling Valve (STV)? • Many domestic A/C systems in the 1960s and 1970s used suction pressure valves. Suction throttling valves (STVs), pilot-operated absolutes (POAs), and evaporator pressure regulators (EPRs) were the most common. Some import vehicles used an EPR valve in the late 1990s. These valves are mounted at the evaporator outlet, the compressor inlet, or somewhere between. Most of them sense evaporator pressure; when the pressure starts to drop below a certain point, the valve closes down to restrict refrigerant flow to the compressor. When this occurs, the system has three basic pressures: low-side evaporator pressure controlled to about 30 psi (R-12), low-side compressor inlet pressure below 30 psi, and high-side pressure. A system that uses a suction throttling valve maintains an almost constant evaporator temperature of 32°F without cycling the clutch. It should be noted that the compressor drive load drops when the valve restricts the flow.

  25. What Is Hot Gas Bypass? • A few very early automotive systems (before 1960) used a valve that would sense evaporator pressure. When the pressure began to drop below 30 psi, the valve would allow hot gas from the high side to enter the evaporator. This valve was called a hot gas bypass valve. It worked, but now this system is considered crude.

  26. FIGURE 7-15 An automotive A/C system has the potential to lose refrigerant through hoses, the compressor shaft seal, and line fittings. (Courtesy of Everco Industries) LOW-SIDE OPERATION

  27. FIGURE 7-16 A system with the proper charge has the receiver–drier (a) or the accumulator (b) about half full of liquid. (Courtesy of Everco Industries) LOW-SIDE OPERATION

  28. TECH TIP • Cycling clutch systems are less efficient during cool to warm days. The evaporator can be excessively cooled with colder-than-desired outlet temperatures. The temperature control is usually moved to a warmer setting, and the blend door moves to add heat to bring the outlet temperature back up to the desired setting.

  29. FIGURE 7-17 A properly charged system has the condenser filled with condensing vapor and some liquid, a liquid line filled with liquid, a receiver–drier about half full of liquid, and an evaporator with vaporizing liquid (a).An overcharge with too much liquid causes liquid to partially fill the condenser (b). An undercharge has vapor in the liquid line and a starved evaporator (c). LOW-SIDE OPERATION

  30. FIGURE 7-18 The compressor clutch allows us to cycle the compressor off and on to control evaporator temperature and to shut the system off. (Courtesy of Everco Industries) LOW-SIDE OPERATION

  31. FIGURE 7-19 Most TXV systems use a thermal switch to cycle the compressor out when the evaporator gets too cold (a). Most OT systems use a pressure switch to cycle the compressor out when the low-side pressure drops too low (b). LOW-SIDE OPERATION

  32. FIGURE 7-20 A suction throttling valve (STV) stops evaporator pressure from dropping below 30 psi, and this keeps ice from forming on the evaporator. LOW-SIDE OPERATION

  33. FIGURE 7-21 A hot gas bypass system diverts high-side pressure into the evaporator to keep the pressure from dropping to the point at which icing can occur. LOW-SIDE OPERATION

  34. FIGURE 7-22 When the evaporator cools and low-side pressure drops, the piston stroke of a variable displacement compressor is reduced so that compressor output matches the cooling load. (Reprinted with permission of General Motors Corporation) LOW-SIDE OPERATION

  35. HIGH-SIDE OPERATION • The high side of an A/C system takes the low-pressure vapor from the evaporator and returns high-pressure liquid to the expansion device. • To do this, the compressor must raise the pressure and concentrate the heat so that the vapor temperature is above ambient. • This causes heat to flow from the refrigerant to the air passing through the condenser. • Removing the latent heat from the saturated vapor causes it to change state, to a liquid.

  36. HIGH-SIDE OPERATION • Compressor • Rotary Compressors • Vane Compressors • Scroll Compressors • Electric Compressors • Condensers • Receiver–Drier • High-Pressure Control

  37. Piston compressors can drive the piston through a crankshaft (a), Scotch yoke (b), swash plate (c), or wobble plate (d). A rotary compressor can use vanes (e) or a pair of scrolls (f). (a and e are courtesy of Toyota Motor Sales USA, Inc.; c and d are courtesy of Zexel USA Corporation) HIGH-SIDE OPERATION

  38. FIGURE 7-23 (CONTINUED) Piston compressors can drive the piston through a crankshaft (a), Scotch yoke (b), swash plate (c), or wobble plate (d). A rotary compressor can use vanes (e) or a pair of scrolls (f). (a and e are courtesy of Toyota Motor Sales USA, Inc.; c and d are courtesy of Zexel USA Corporation) HIGH-SIDE OPERATION

  39. FIGURE 7-23 (CONTINUED) Piston compressors can drive the piston through a crankshaft (a), Scotch yoke (b), swash plate (c), or wobble plate (d). A rotary compressor can use vanes (e) or a pair of scrolls (f). (a and e are courtesy of Toyota Motor Sales USA, Inc.; c and d are courtesy of Zexel USA Corporation) HIGH-SIDE OPERATION

  40. FIGURE 7-24 As the piston moves downward in the cylinder, evaporator pressure opens the suction reed and fills the cylinder with refrigerant (a). As the piston moves upward, piston pressure forces the discharge reed open and the refrigerant into the high side (b). HIGH-SIDE OPERATION

  41. FIGURE 7-25 As the rotor turns in a clockwise direction, the vanes move in and out to follow the contour of the housing. This action forms chambers that get larger at the suction ports and smaller at the discharge ports. Evaporator pressure fills the chambers as they get bigger, and the reducing size forces the refrigerant into the high side. (Courtesy of Zexel USA Corporation) HIGH-SIDE OPERATION

  42. FIGURE 7-26 This through-vane compressor has vanes that contact the rotor housing at each end, and they slide to make a seal at each end as the rotor turns. The vanes form a pumping chamber that gets larger at the suction port and smaller at the discharge port. (Courtesy of Toyota Motor Sales USA, Inc.) HIGH-SIDE OPERATION

  43. FIGURE 7-27 A cutaway view of a scroll compressor. Note that one scroll is secured to the housing and the other can be moved through its orbit by the drive shaft. (Courtesy of Sanden International) HIGH-SIDE OPERATION

  44. FIGURE 7-28 As the orbital scroll moves, it forms pumping chambers/ gas pockets that start at the suction ports and force the refrigerant to the discharge port at the center. (Courtesy of Chrysler LLC) HIGH-SIDE OPERATION

  45. FIGURE 7-29 This electric scroll compressor (a) is operated by a DC electric motor operating off batteries. A similar compressor can be used in a heat pump (b). Note that the heat pump is very similar to an A/C system that includes a reversing valve that can swap the high and low sides. (a is courtesy of Sanden International) HIGH-SIDE OPERATION

  46. FIGURE 7-30 A condenser is a heat exchanger that transfers heat from the refrigerant to the air flowing through it. HIGH-SIDE OPERATION

  47. FIGURE 7-31 A tube-and-fin condenser is made up of a series of fins with the tubes passing through them. An extruded tube condenser uses flat tubes with the fins attached between them. Flat tube condensers can use either parallel or serpentine flow. (Courtesy of Four Seasons) HIGH-SIDE OPERATION

  48. FIGURE 7-32 The refrigerant follows a winding path through a serpentine condenser (top); it follows a back-and-forth path through a parallelflow condenser (bottom). HIGH-SIDE OPERATION

  49. FIGURE 7-33 The volume of gas that enters a condenser is about 1,000 times the volume of liquid leaving it. HIGH-SIDE OPERATION

  50. FIGURE 7-34 A dual condenser: the refrigerant flows from the condenser portion through the modulator/receiver–drier portion and then through the subcooling portion. HIGH-SIDE OPERATION