ASE 7 - HVAC Module 2
Acknowledgements General Motors, the IAGMASEP Association Board of Directors, and Raytheon Professional Services, GM's training partner for GM's Service Technical College wish to thank all of the people who contributed to the GM ASEP/BSEP curriculum development project 2002-3. This project would not have been possible without the tireless efforts of many people. We acknowledge: The IAGMASEP Association members for agreeing to tackle this large project to create the curriculum for the GM ASEP/BSEP schools. The IAGMASEP Curriculum team for leading the members to a single vision and implementation. Direct contributors within Raytheon Professional Services for their support of translating a good idea into reality. Specifically, we thank: Chris Mason and Vince Williams, for their leadership, guidance, and support. Media and Graphics department under Mary McClain and in particular, Cheryl Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & Nancy DeSantis. For his help on the Heating, Ventilation, and Air Conditioning curriculum volume, Subject Matter Expert, Brad Fuhrman, for his wealth of knowledge. Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEP Colleges for their contribution for reformatting existing General Motors training material, adding critical technical content and the sharing of their expertise in the GM product. Separate committees worked on each of the eight curriculum areas. For the work on this volume, we thank the members of the Heating, Ventilation, and Air Conditioning committee: Steve Ash, Sinclair Community College Warren Farnell, Northhampton Community College Rick Frazier, Owens Community College Marvin Johnson, Brookhaven College Chris Peace, J. Sargeant Reynolds Community College Vince Williams, Raytheon
Contents Introduction... 4 Objectives:... 4 General Motors V-5 Compressor... 9 General Motors HR-6 Compressor... 10 Scroll Compressors...11 CCOT Refrigerant System... 12 VDOT Refrigerant System... 13 CCTXV Refrigerant System... 14 VDTXV Refrigerant System... 15 System Lubrication... 16
Introduction The operation of the automobile air conditioning compressor is similar to compressors used in nonautomotive applications. Most are of the reciprocating piston type while the scroll and vane types are less used. A complete understanding of compressor operation is necessary for diagnosis of the system. Servicing of the compressor is now limited to the replacement of the shaft seal or unit replacement, overhaul procedures will not be discussed in this module. Objectives: Describe the function of ththe compressor performs one main function to compress the low-pressure refrigerant vapor from the evaporator into a high-pressure, high temperature vapor. General Motors is currently using compressors made by Harrison and other manufacturers. As a gas is compressed, the temperature also rises because you are concentrating more heat in a smaller area. All materials have a certain amount of heat regardless of temperature, measured in BTU's that can be increase or decreased without sometimes causing a change in temperature. To cause a change in state of one pound of water at 212ºF; you would need to add an additional 970 BTU's. The water will begin to boil at the same 212ºF temperature at sea level. e compressor List the different types of compressors Discuss the operation of each type Describe the four refrigeration systems Discuss the role of the lubricant Service Compressor front shaft seal Diagnose compressor complaints 2-4
The compressor performs one main function to compress the lowpressure refrigerant vapor from the evaporator into a high-pressure, high temperature vapor. General Motors is currently using compressors made by Harrison and other manufacturers. As a gas is compressed, the temperature also rises because you are concentrating more heat in a smaller area. All materials have a certain amount of heat regardless of temperature, measured in BTU's that can be increase or decreased without sometimes causing a change in temperature. Figure 2-1 To cause a change in state of one pound of water at 212ºF; you would need to add an additional 970 BTU's. The water will begin to boil at the same 212ºF temperature at sea level. Figure 2-2 2-5
If we increased the pressure on a contained liquid, such as a radiator, we could effectively raise the boiling point of that liquid proportional to the pressure increase. A typical cooling system with a 15 lb. pressure cap installed will raise the boiling point of the coolant by approximately 45ºF and there by controlling the temperature at which the liquid will change into a vapor or gas. This can be related to the function of the air conditioning compressor by raising the pressure in the system. Figure 2-3 2-6
The atmospheric pressure varies with the altitude and weather conditions as shown below. Look at the temperature at which water boils at different levels in our atmosphere. What effect does this have on an air conditioning system and how does it effect servicing? Figure 2-4 In a basic refrigeration system, the compressor is used to compress the low-pressure vapor and into a high-pressure vapor along with an increase in temperature. The compressor is used to control the temperature at which a change of state will take place in ambient temperatures and to provide the flow of refrigerant through the system. Figure 2-5 2-7
The compressor is located between the high-pressure and high-pressure side of the systems. The high-pressure side starts with the compressor and ends at the orifice or TXV. The low-pressure side starts with the orifice or TXV and ends at the compressor. Depending on the location and component in the system; the refrigerant is either a vapor or liquid. Figure 2-6 General Motors currently uses one of three types of compressors. They are the fixed displacement and variable displacement compressors. These types can further be broken down into piston, vane, and scroll type compressors. They can also be broken down even further by piston and crankshaft arrangement such as radial and axial. Although all compressors provide the same function, their individual operating characteristics are such that some system components are specific to the system it was designed for. Such things as compression ratios, pumping capacities, pressure ratings, and vibration considerations are just some of the criteria used by engineers to design and select a compressor type. Figure 2-7 2-8
General Motors V-5 Compressor The V-5 compressor is a five-cylinder variable displacement pump which is capable of meeting A/C system demands without cycling. It is available in two different maximum displacement sizes: 8.3 cu. in. and 9.2 cu. in. The minimum displacement of both of these compressors is 0.6 cu. in. The outside size and appearance of the two compressors are identical. They can only be distinguished by checking part numbers. The V-5 compressor mechanism incorporates five axially oriented pistons and a variable angle wobble plate. Compressor displacement (piston stroke length) is controlled by the position of a bellows-actuated control valve in the rear head of the compressor. The control valve senses suction (low side) pressure. The difference between suction and crankcase pressure determines wobble plate angle, which in turn determines piston stroke and displacement. Figure 2-8 2-9
General Motors HR-6 Compressor The HR-6 compressor is a six-cylinder fixed displacement compressor of axial design. It is similar to the DA-6 and A-6 axial compressors used for many years. However, while the HR-6 and DA-6 compressors look the same, they are different. The HR-6 incorporates an oil pump, while the DA-6 uses gravity feed lubrication system. Because the DA-6 has no oil pump, correct positioning in the mounting bracket is essential for proper lubrication. The HR-6 contains three horizontal double-acting pistons. An axial plate, or swash plate, is pressed onto the main compressor shaft. The swash plate drives the pistons. An oil pump mounted at the rear of the compressor provides lubrication to the compressor bearings and internal parts. The oil pump draws oil from a sump at the bottom of the compressor. Reed valves at each end of the HR-6 compressor control the flow of incoming and outgoing refrigerant. Two gastight passages connect chambers at the front and rear of the compressor heads; these passages provide one common suction port and one common discharge port. Figure 2-9 2-10
Scroll Compressors Scroll compressors uses two major components: fixed and moveable scrolls. Each of these forms one side of the pumping chamber and has a spiral-shaped scroll. The fixed scroll is attached to the compressor housing; the moveable scroll is mounted over an eccentric bushing and counterweight on the crankshaft. It does not rotate, but it moves in a orbit relative to the stationary scroll. As the scroll orbits, it forms a pumping chamber that is open at the outer end. This chamber is moved to the center by the scroll's action. Two or three chambers are present at the same time. The outer ends of the scrolls are open to the suction port, and the inner ends connect to the discharge port. A scroll compressor has the advantage of having very smooth operation, and low engagement torque that allows the use of a small clutch. A scroll compressor can also be driven at higher rpms than other designs, so that a smaller drive pulley is used. Figure 2-10 An important part of every automotive compressor is the clutch which allows us to easily turn it on and off. Magnetic clutches are used to perform this function. They have a clutch coil where a magnetic field is generated when electricity is sent to it. This magnetic field pulls the clutch driven plate against the rotating pulley to drive the compressor. Figure 2-11 2-11
CCOT Refrigerant System All Cycling Clutch Orifice Tube (CCOT) refrigerant systems use fixed displacement compressors. In addition to a compressor, these systems typically include a condenser, accumulator, orifice tube, evaporator, muffler and various control components. CCOT systems cycle the compressor on and off to maintain the refrigeration cycle within predetermined limits. Control of the refrigeration cycle and compressor is achieved through the use of a switch which senses low side system pressure as an indicator of evaporator temperature. This switch, the pressure cycling switch, is usually located on a Schrader valve low side fitting. A Schrader valve is a spring-loaded pressure valve similar to the valves used in tire stems. At air temperatures above 10 degrees Celsius (50 degrees Fahrenheit), the equalized pressures in a fully charged A/C system close the contacts of the pressure cycling switch. When the switch is closed and an A/C mode is selected, voltage is supplied through the pressure cycling switch to the compressor clutch coil. This action engages the compressor. When the compressor reduces evaporator pressure to approximately 175 kpa (25 psi), the pressure cycling switch opens and de-energizes the compressor clutch coil. This action disengages the compressor. Pressures between the high and low side of the refrigerant system gradually equalize until approximately 315 kpa (46 psi) is reached. At this time, the pressure cycling switch contacts close, and the cycle is repeated. These pressures and temperatures vary slightly from vehicle-to-vehicle. Figure 2-12 2-12
VDOT Refrigerant System The main difference between CCOT and VDOT refrigerant systems consists of the manner in which the compressor achieves control of the refrigerant cycle: CCOT systems cycle the compressor on and off, while VDOT systems use constantly-running compressors. Variable Displacement Orifice Tube (VDOT) refrigerant systems use the V- 5 variable displacement compressor. The V-5 compressor clutch remains engaged whenever an A/C mode is selected, except during high load conditions such as wide open throttle or high power steering pressure. A constantly running compressor allows smooth, steady engine operation by avoiding the intermittent load changes that accompany compressor cycling. In addition to a compressor, VDOT refrigerant systems typically include a condenser, accumulator, orifice tube, evaporator, muffler and various control components. Refrigerant system components operate in the same manner in CCOT and VDOT systems. Figure 2-13 2-13
CCTXV Refrigerant System The Cycling Clutch Thermostatic Expansion Valve (CCTXV) refrigeration systems have a fixed displacement compressor with a PCM-controlled clutch. The PCM or pressure switch cycles the compressor ON and OFF to maintain the refrigeration pressure within predetermined limits. The CCTXV system uses a Thermostatic Expansion Valve to control refrigerant flow and separate the high-side pressure from the low-side pressure (see Figure 2-14). The TXV meters the amount of refrigerant entering the evaporator to more accurately control heat absorption. The CCTXV refrigeration system also uses the following components: Condenser Receiver-Dryer Evaporator Muffler (application dependent) Thermostatic Switch Pressure Cut-Off Switches (compressor dependent) Pressure Sensor (application dependent) Figure 2-14 2-14
VDTXV Refrigerant System The Variable Displacement Thermostatic Expansion Valve (VDTXV) refrigeration systems have a variable displacement compressor with a PCM-controlled clutch. Internal pressure controls within the compressor maintain the low-side pressure within predetermined limits. Internal pressure controls eliminate the need to cycle variable displacement compressors. Pressure switches on the high side of the compressor protect the compressor from damage if refrigerant pressure is not within operating specifications. The VDTXV system uses a Thermostatic Expansion Valve to control refrigerant flow and separate the high-side pressure from the low-side pressure (see Figure 2-15System Lubrication The VDTXV refrigeration system also uses the following components: Condenser Receiver-Dryer Evaporator Muffler (application dependent) Pressure Cut-Off Switches (application dependent) Pressure Sensor (application dependent) Figure 2-15 2-15
System Lubrication The A/C system refrigerant carries a charge of lubricating oil. Since the refrigerant and oil mix, the refrigerant carries globules of oil throughout the system to lubricate moving parts. The most common compressors used today rely totally upon the oil-saturated refrigerant to lubricate their internal moving parts. The A-6 and V-5 compressors also have an internal oil reservoir. However, since the majority of A/C compressors rely completely on oil-saturated refrigerant, it is imperative that the proper level, viscosity, and purity be maintained for proper operation and longevity. As the refrigerant enters the compressor, some of the oil droplets that separate from the refrigerant fall into the crankcase and lubricate the moving parts. The oil is then picked up by the exiting refrigerant and continues through the system. Since some oil remains in parts of the system and the accumulator dryer acts as a reservoir, an oil bleed hole at the bottom of the vapor inlet tube ensures that enough oil is supplied to the compressor through the suction side. This bleed hole is critical to the lubrication of the compressor and is protected by a filter assembly. General Motors currently uses only two types of lubricating oil. 525- viscosity mineral-based oil is used with R-12 systems, and PAG (Polyalkaline Glycol) oil is used with R-134a systems. Both lubricating oils will absorb water vapor and therefore should not be exposed to air for prolonged periods. Figure 2-16 2-16