Welcome To,Air Conditioning Operation & Diagnosis,SECTION 12,2,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,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.,TXV System,Animation: A/C Systems, TXV,8,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,OT System,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.,Animation: TXV Operation,Internally Balanced,Externally Balanced,Block Valve,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 STVs (suction throttling valves) POAs (pilot-operated absolute) EPRs (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