2System Components All systems contain six major components: Compressor to circulate the refrigerantCondenser to transfer heat to ambient airReceiver dryer or accumulator to store reserve refrigerant, contain the desiccant, and filter refrigerantExpansion device to control amount of refrigerant entering evaporatorEvaporator to transfer heat from in-vehicle air to the refrigerantLines and hoses to connect these parts togetherAnd the refrigerant
3Automotive A/C Systems 2 primary systems used to control evaporator pressure and temperature:TXV systems (thermal or thermostatic expansion valve)OT systems (orifice tube)FixedVariable
4Low Side OperationGoal is to provide constant evaporator temperature of 32 degreesRefrigerants have low boiling pointsWhen liquid boils, it absorbs large amounts of heatAmount of heat absorbed in evaporator is proportional to amount of refrigerant boiled
5Low Side Components Expansion device Evaporator Accumulator (if equipped)
6Expansion DevicesThe 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 pressureMost OTs have a fixed diameter orifice
7TXV SystemA 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
8OT SystemAn 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
9Thermal 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 BalancedExternally BalancedBlock ValveAnimation: TXV Operation
10Thermal Expansion Valves, TXVs Variable valve that can change size of opening in response to system loadOpens or closes depending on evaporator pressure and temperature
11Orifice Tubes, OTsThe 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.
12Orifice Tubes, OTs Fixed diameter orifice Simple and cheap to produce Cannot respond or change according to evaporator temperatureSystem requires accumulator to prevent liquid refrigerant from reaching compressor
13Evaporator OperationHot, 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.
14AccumulatorsAccumulators 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.
15Refrigerant Charge Levels Low Refrigerant Levels:Allow refrigerant to vaporize before evaporatorPrevent proper heat transfer in evaporatorCauses low system pressures
16Refrigerant Charge Levels High Refrigerant Levels:Prevent refrigerant expansion in evaporatorCan cause extremely high system pressuresPrevents heat transfer because of high pressure in evaporator
18Cycling Clutch Systems Pressure controlSwitch mounted on low sideAccumulatorSuction lineCycles compressor on at psiCycles compressor off at psiTemperature controlThermistor senses evaporator temperature
19Evaporator Pressure Controls Devices used to control evaporator pressureSTV’s (suction throttling valves)POA’s (pilot-operated absolute)EPR’s (evaporator pressure regulator)Usually mounted in evaporator outlet or compressor inletUsed to restrict refrigerant flow to compressor
20Variable Displacement Compressors Provide smooth compressor operationMaintain constant evaporator temperatureReduces compressor load on engine when system cooling load is low
21High Side OperationTakes low pressure vapor from evaporator and returns high pressure liquid to expansion deviceMust increase vapor temperature above ambient temperature for heat transfer to occur resulting in change of state from vapor to liquid
22High Side ComponentsHigh begins at compressor and ends at expansion deviceCompressorCondenserReceiver-drier (if equipped)
23CompressorsThere is a large variety of compressors. Some of variations are:The compressor manufacturerPiston, vane, or scroll typeThe piston and cylinder arrangementHow the compressor is mountedStyle and position of portsType and number of drive beltsCompressor displacementFixed or variable displacement
24Compressor OperationOut/Discharge: High Pressure, about 200 psi & High Temperature, above ambientIn/Suction: Low Pressure, about 30 psi & Low Temperature, close to freezingThe compressor increases the refrigerant pressure about five to ten times. This increases the temperature so heat can leave the refrigerant in the condenser.
25Piston CompressorsReed Valve PlatePistonConnecting RodCrankshaftThis 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 SealAnimation: Piston Compressor
26Scotch Yoke Compressors Discharge ReedSuction ReedA 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.PistonsYokeAnimation: Scotch Yoke Compressor
27Scroll CompressorsOrbiting ScrollFixed ScrollThe orbiting scroll is driven by the crankshaft and moves in a small circular orbit. The fixed scroll remains stationaryShaft SealClutch AssemblyAnimation: Scroll Compressor
28Swash Plate Compressors PistonsReed PlateThe 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 PlateShaft SealClutch AssemblyAnimation: Swash Plate Compressor
29Vane CompressorsRotor VaneThe 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 SealDischarge ReedAnimation: Vane Compressor
31Wobble 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.
32Variable Displacement Wobble Plate Compressors Low Angle Minimum StrokeThe evaporator pressure has dropped, and the control valve has increased crankcase pressureHigh Angle, Maximum StrokeNormal operation when cooling is required. Crankcase pressure is low.Control Valve
33Condenser OperationHot, 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.
34Condenser TypesCondensers 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.
35Serpentine CondenserRefrigerant flows from the upper inlet to the bottom outlet through two tubes. These tubes wind back and forth though the condenser.
36Parallel 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.
37Heat ExchangersCondensers 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.
38Receiver Dryers Barb Connections, Note Sight Glass Male Flare ConnectionsA 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
39High Pressure Controls High pressure cutoffSwitches designed to open compressor clutch circuit at high pressuresHigh pressure releaseDesigned to release refrigerant at high pressures
40Pressure ReleasePressure Relief ValveA/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