Mobile Air Conditioning

1. Mobile Air Conditioning Basic A/C Operation

2. System Components • All systems contain six major components: • Compressor to circulate the refrigerant • Condenser to transfer heat to ambient air • Receiver dryer or accumulator to store reserve refrigerant, contain the desiccant, and filter refrigerant • Expansion device to control amount of refrigerant entering evaporator • Evaporator to transfer heat from in-vehicle air to the refrigerant • Lines and hoses to connect these parts together • And the refrigerant

3. Automotive A/C Systems • 2 primary systems used to control evaporator pressure and temperature: • TXV systems (thermal or thermostatic expansion valve) • OT systems (orifice tube) • Fixed • Variable

4. Low Side Operation • Goal is to provide constant evaporator temperature of 32 degrees • Refrigerants have low boiling points • When liquid boils, it absorbs large amounts of heat • Amount of heat absorbed in evaporator is proportional to amount of refrigerant boiled

5. Low Side Components • Expansion device • Evaporator • Accumulator (if equipped)

6. Expansion Devices • The expansion device separates the high side from the low side and provides a restriction for the compressor to pump against. • There are two styles of expansion devices: • The TXV can open or close to change flow. It is controlled by the superheat spring, thermal bulb that senses evaporator outlet temperature, and evaporator pressure • Most OTs have a fixed diameter orifice

7. TXV System A TXV controls the refrigerant flow from the high pressure side to the evaporator. A receiver dryer is mounted in the liquid line of all TXV systems. Animation: A/C Systems, TXV

8. OT System An OT controls the refrigerant flow from the high pressure side to the evaporator. An accumulator is mounted in the suction line of all OT systems. Animation: A/C Systems, OT

9. Thermal Expansion Valves, TXVs The three major types of expansion valves: Internally balanced TXVs are the most common. Externally balanced TXVs are used on some larger evaporators. Block valves route the refrigerant leaving the evaporator past the thermal sensing diaphragm so a thermal bulb is not needed. Internally Balanced Externally Balanced Block Valve Animation: TXV Operation

10. Thermal Expansion Valves, TXVs • Variable valve that can change size of opening in response to system load • Opens or closes depending on evaporator pressure and temperature

11. Orifice Tubes, OTs The OT used in a modern vehicle is a tubular, plastic device with a small metal tube inside. The color of the OT is used to determine the diameter of the tube. A plastic filter screen is used to trap debris that might plug the tube. Some older General Motors vehicles used an OT that resembled a brass fuel filter.

12. Orifice Tubes, OTs • Fixed diameter orifice • Simple and cheap to produce • Cannot respond or change according to evaporator temperature • System requires accumulator to prevent liquid refrigerant from reaching compressor

13. Evaporator Operation Hot, liquid refrigerant flows through the expansion device in the low side to become a fine mist. Refrigerant boils or evaporates to become a gas inside the evaporator. The boiling refrigerant absorbs heat from the air during this change of state.

14. Accumulators • Accumulators are used in the suction line of all OT systems. • The accumulator: • separates liquid refrigerant so only gas flows to the compressor. • Allows oil in the bottom of the accumulator to return to the compressor. • provides storage for a refrigerant reserve. • contains the desiccant bag for water removal. • provides a place to mount low pressure switches and sensors.

15. Refrigerant Charge Levels • Low Refrigerant Levels: • Allow refrigerant to vaporize before evaporator • Prevent proper heat transfer in evaporator • Causes low system pressures

16. Refrigerant Charge Levels • High Refrigerant Levels: • Prevent refrigerant expansion in evaporator • Can cause extremely high system pressures • Prevents heat transfer because of high pressure in evaporator

17. Evaporator Icing Controls • Cycling Clutch Systems • Evaporator Pressure Controls • Variable Displacement Compressors

18. Cycling Clutch Systems • Pressure control • Switch mounted on low side • Accumulator • Suction line • Cycles compressor on at 42-49 psi • Cycles compressor off at 22-28 psi • Temperature control • Thermistor senses evaporator temperature

19. Evaporator Pressure Controls • Devices used to control evaporator pressure • STV’s (suction throttling valves) • POA’s (pilot-operated absolute) • EPR’s (evaporator pressure regulator) • Usually mounted in evaporator outlet or compressor inlet • Used to restrict refrigerant flow to compressor

20. Variable Displacement Compressors • Provide smooth compressor operation • Maintain constant evaporator temperature • Reduces compressor load on engine when system cooling load is low

21. High Side Operation • Takes low pressure vapor from evaporator and returns high pressure liquid to expansion device • Must increase vapor temperature above ambient temperature for heat transfer to occur resulting in change of state from vapor to liquid

22. High Side Components • High begins at compressor and ends at expansion device • Compressor • Condenser • Receiver-drier (if equipped)

23. Compressors There is a large variety of compressors. Some of variations are: The compressor manufacturer Piston, vane, or scroll type The piston and cylinder arrangement How the compressor is mounted Style and position of ports Type and number of drive belts Compressor displacement Fixed or variable displacement

24. Compressor Operation Out/Discharge: High Pressure, about 200 psi & High Temperature, above ambient In/Suction: Low Pressure, about 30 psi & Low Temperature, close to freezing The compressor increases the refrigerant pressure about five to ten times. This increases the temperature so heat can leave the refrigerant in the condenser.

25. Piston Compressors Reed Valve Plate Piston Connecting Rod Crankshaft This two-cylinder compressor uses a crankshaft to move the pistons up and down. Refrigerant flow is controlled by the suction and discharge reeds in the valve plate. Shaft Seal Animation: Piston Compressor

26. Scotch Yoke Compressors Discharge Reed Suction Reed A Scotch yoke compressor has two pairs of pistons that are driven by a slider block on the crankshaft. The pistons are connected by a yoke. Pistons Yoke Animation: Scotch Yoke Compressor

27. Scroll Compressors Orbiting Scroll Fixed Scroll The orbiting scroll is driven by the crankshaft and moves in a small circular orbit. The fixed scroll remains stationary Shaft Seal Clutch Assembly Animation: Scroll Compressor

28. Swash Plate Compressors Pistons Reed Plate The swash plate is mounted at an angle onto the drive shaft. It drives three double-ended pistons. Two sets of reeds control the refrigerant flow in and out of the cylinders, Swash Plate Shaft Seal Clutch Assembly Animation: Swash Plate Compressor

29. Vane Compressors Rotor Vane The rotor is driven by the clutch and driveshaft. The vanes move in and out of the rotor to follow the outer wall to pump refrigerant. Shaft Seal Discharge Reed Animation: Vane Compressor

30. Wobble Plate Compressors Piston Wobble Plate Bearing Angle/Drive Plate Animation: Wobble Plate Compressor

31. Wobble Plate Compressors The wobble plate does not rotate; it just wobbles, being driven by the angled drive plate that does rotate. Variable displacement, wobble plate compressors can change the angle of the drive plate, and this changes piston stroke and compressor displacement. Most wobble plate compressors have 5 to 7 pistons.

32. Variable Displacement Wobble Plate Compressors Low Angle Minimum Stroke The evaporator pressure has dropped, and the control valve has increased crankcase pressure High Angle, Maximum Stroke Normal operation when cooling is required. Crankcase pressure is low. Control Valve

33. Condenser Operation Hot, high pressure gas is pumped from the compressor to enter the condenser. The gas gives up its heat to the air passing through the condenser. Removing heat from the hot gas causes it to change state and become liquid.

34. Condenser Types Condensers A and C are round tube, serpentine condensers. Condenser B is an oval/flat tube, serpentine condenser. Condenser D is an oval/flat tube, parallel flow condenser. Flat tube condensers are more efficient.

35. Serpentine Condenser Refrigerant flows from the upper inlet to the bottom outlet through two tubes. These tubes wind back and forth though the condenser.

36. Parallel Flow Condenser Refrigerant flows from the upper inlet to the bottom outlet through groups of parallel tubes. Some carry refrigerant from the right to the left, and others move it back to the right side.

37. Heat Exchangers • Condensers have to move heat from the refrigerant to the air. • Evaporators must move heat from air to the refrigerant. • Both require a lot of contact area for both air and refrigerant. • Both require free movement of air and refrigerant.

38. Barb Connections, Note Sight Glass Receiver Dryers Male Flare Connections • A receiver dryer is mounted in the liquid line of a TXV system. It is used to: • to store a reserve of refrigerant. • hold the desiccant bag that removes water from the refrigerant. • filter the refrigerant and remove debris particles. • provide a sight glass so refrigerant flow can be observed. • provide a location for switch mounting. Male O-ring Connections, Note Switch

39. High Pressure Controls • High pressure cutoff • Switches designed to open compressor clutch circuit at high pressures • High pressure release • Designed to release refrigerant at high pressures

40. Pressure Release Pressure Relief Valve A/C systems can include a pressure release valve that is usually mounted at the compressor or a fuse plug mounted on the receiver dryer. The relief valve can open at a preset pressure and then reclose. The center of the fuse plug melts to let pressure escape. Fuse Plug