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INTRODUCTION

INTRODUCTION. TWO BASIC TYPES. T HERMAL E X PANSION V ALVE (old school) Uses a Receiver/Drier. ORIFICE TUBE. AKA: C ycling C lutch O rifice T ube Uses an accumulator.

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INTRODUCTION

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  1. INTRODUCTION

  2. TWO BASIC TYPES THERMAL EXPANSION VALVE (old school) Uses a Receiver/Drier

  3. ORIFICE TUBE AKA: Cycling Clutch Orifice Tube Uses an accumulator

  4. The high and low pressure 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).

  5. Refrigerant changes state from a liquid to a vapor as it absorbs heat in the low side and into a liquid as it loses heat in the high side. HIGH PRESSURE LOW PRESSURE

  6. Refrigerant boils or evaporates (Gas) in the low side and it condenses (Liquid) in the high side. • In an operating system, you can identify the low and high sides by: • Pressure (depending on type of system & ambient temp) • High (175 psi) • Low (30 psi ) • Location • High (out) • Low (in) • Temperature • High (hot) • Low (cold) • Tubing size • High (small) • Low (large)

  7. 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).

  8. The cold temperature in the evaporator is produced by boiling the refrigerant (lowering pressure). • 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 and boil inside the evaporator. • The amount of heat an evaporator absorbs is directly related to the amount of liquid refrigerant that boils inside it

  9. 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.

  10. If the proper amount of refrigerant enters the evaporator, it has a slight superheat as it leaves.

  11. A starved condition, in which not enough refrigerant enters the evaporator, does not produce as much cooling.

  12. If too much refrigerant enters, the evaporator floods because the refrigerant will not all boil.

  13. The low side begins at the TXV and includes the evaporator, receiver/drier and suction line to the compressor.

  14. The OT system is the same but has an accumulator

  15. EXPANSION DEVICES 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.

  16. An H-type valve is essentially the same except evaporator pressure goes through an internal passage to the bottom of the diaphragm.

  17. Some systems use a suction throttling valve to keep evaporator pressure from dropping to the point at which icing can occur. SUCTION THROTTLING VALVE ON TXV

  18. An OT is a simple restriction that limits the flow of refrigerant into the evaporator. The locating dimple keeps the OT from moving downstream. ORIFICE TUBE

  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.

  20. EVAPORATOR TUBE & FIN PLATE Each type has a large contact area for heat to leave the air and enter the refrigerant.

  21. 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.

  22. 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.

  23. An automotive A/C system has the potential to lose refrigerant through hoses, the compressor shaft seal, and line fittings

  24. A system with the proper charge has the receiver–drier (a) or the accumulator (b) about half full of liquid

  25. 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

  26. An overcharge with too much liquid causes liquid to partially fill the condenser

  27. An undercharge has vapor in the liquid line and a starved evaporator

  28. The compressor clutch allows us to cycle the compressor off and on to control evaporator temperature and to shut the system off

  29. Most TXV systems use a thermal switch to cycle the compressor out when the evaporator gets too cold. Most OT systems use a pressure switch to cycle the compressor out when the low-side pressure drops too low.

  30. A suction throttling valve (STV) stops evaporator pressure from dropping below 30 psi, and this keeps ice from forming on the evaporator. SUCTION THROTTLING VALVE

  31. 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. HOT GAS BY-PASS

  32. 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 (exchange) 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.

  33. HIGH-SIDE OPERATION • Compressor • Crank Piston • Rotary Piston • Vane • Scroll • Electric or Belt Driven • Condensers • Receiver–Drier • High-Pressure Control

  34. CRANKSHAFT PISTON COMPRESSOR (OLD SCHOOL)

  35. REED VALVE IN OUT

  36. ROTARY CRANK

  37. WOBBLE PLATE PISTON

  38. 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. VARIABLE DISPLACEMENT

  39. PISTONLESS WOBBLE PLATE

  40. VANE

  41. VANE MOVEMENT

  42. 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. SCROLL

  43. SCROLL OPERATION Like and Auger or drill bit

  44. ELECTRIC DRIVEN CAN BE SAME TYPES AS BELT DRIVEN COMPRESSORS

  45. A condenser is a heat exchanger that transfers heat from the refrigerant to the air flowing through it. Typically located in front of radiator CONDENSER

  46. REFRIGERANT FLOW Refrigerant follows a winding path through a serpentine condenser. Refrigerant follows a back-and-forth path through a parallelflow condenser.

  47. The volume of gas that enters a condenser is about 1,000 times the volume of liquid leaving it. LIQUID EXPANSION

  48. refrigerant flows from the condenser portion through the modulator/receiver–drier portion and then through the subcooling portion. DUAL CONDENSER

  49. The outlet of a receiver–drier is close to the bottom so liquid flows on to the TXV. Many units include a sight glass so we can observe this flow. RECEIVER DRIER

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