Coach Air Conditioning CHAPTER Coach Air Conditioning 12 Instructor Name: (Your Name)

Learning Objectives Determine the flow of refrigerant through a large bus A/C system. List the components used in a large bus A/C system. Explain the procedures for a triple evacuation procedure. Explain the procedures for a one time evacuation procedure. Determine of suction and discharge pressures are acceptable. Perform service checks to see if refrigerant charge is correct.

Learning Objectives (continued) Perform refrigerant charging procedures. Recover refrigerant from a bus A/C system. Check compressor oil level and adjust to correct level. Perform a superheat test on system. Perform low side pump down procedure.

Coach Refrigeration Schematic Typically use R-134a Utilize the four main components found in any A/C system. Compressor discharge uses a check valve. Receive has inlet service valve Receive has outlet service valve or King Valve. Two stage condenser with sub-cooler. Liquid line passes through a filter dryer.

Condenser and Receiver With Service Valves

Service Valves

Coach Refrigeration Schematic (continued) Main liquid line solenoid valve (LLSV), normally closed. Driver liquid line solenoid valve LLSV, normally open. Parcel rack liquid line solenoid valve LLSV, normally open.

Four Paths of Refrigerant Flow

Coach Refrigeration Schematic (continued) Main evaporator TXV. Main evaporator TXVs maximum operating pressure (MOP) setting. Drivers evaporator module with TXV. Parcel rack evaporator module with TXV. Common suction line returning to compressor.

Thermostatic Expansion Valve

Schematic View Carrier Bus A/C System

Service Procedure Two stage 3 to 5 CFM vacuum pump. Replace vacuum pump oil ever 3 uses maximum or if system is know to be wet or contaminated. Use only the manufactures recommended vacuum pump oil. Use evacuation hoses only. Larger ID. Designed to minimize leaks under vacuum. Use evacuation manifold gauge set.

Service Procedure (continued) Use micron gauge (electronic vacuum gauge). Measure down to 500 microns (1/50” HG) Evacuate from three points in system, compressor suction, compressor discharge, receiver outlet or king valve. A manifold can be used for connection to system. Normally closed solenoids must be energized open. Two basic evacuation procedures, triple and one time evacuation.

Vacuum Micron Gauge and Evacuation Manifold

Triple Evacuation Recover refrigerant from system. Connect vacuum pump and vacuum gauge to system. Back seat all service valves. Energize all normally closed refrigerant valves. Turn on pump and draw all hoses and manifold to deep vacuum. This will ensure vacuum setup is leak free. Mid seat the three service valves.

Triple Evacuation (continued) Evacuate system below 2000 microns. Shut down pump ensure vacuum holds. Close vacuum pump and gauge valve. This protects the gauge. Charge system with roughly 2 PSIG refrigerant or nitrogen. Recover refrigerant or remove nitrogen. Repeat steps 5,6,7,and 8.

Triple Evacuation (continued) Replace filter dryer. Evacuate to 500 microns, monitor system for leaks. Charge system with refrigerant to OEM specifications.

Diagram of an Evacuation Setup

One Time Evacuation Procedure Recover refrigerant from system. Replace filter dryer. Connect vacuum pump and vacuum gauge to system. Back seat all service valves. Energize all normally closed refrigerant valves. Turn on pump and draw all hoses and manifold to deep vacuum. This will ensure vacuum setup is leak free. Mid seat the three service valves.

One Time Evacuation Procedure (continued) Evacuate system to below 1000 microns. Close vacuum pump valve. Check for system leaks. Charge system with refrigerant to OEM specifications.

Compressor Discharge Pressure Record condenser inlet air temperature.5 Add 40 degrees. Take temperature from #2 and convert to pressure. Compare with system discharge pressure. The discharge pressure should be +/- 10%.

Compressor Suction Pressure Record evaporator return air temperature. Subtract 30 Take temperature from #2 and convert to pressure. Compare with system suction pressure. The suction pressure should be +/- 5%.

Check Refrigerant Charge, Large Bus Bus running in cooling mode. Compressor fully loaded. Discharge pressure 150 psig for R-134a and 250 psig for R-22. Refrigerant pressures stabilized. Check Receiver sight glass: Two glass receiver, bottom glass not empty and top not full. One glass system, glass should be half full.

Check Refrigerant Charge, Small Bus Bus running in cooling mode. Discharge pressure at least 150 psig for R-134a. Refrigerant pressure must be stabilized. Check sight glass, it should be clear with an occasional bubble.

Refrigerant Charging for Large Bus Evacuate system, triple or one time method. Connect charging cylinder to receiver outlet valve. Place charging cylinder on scale, note weight. Open liquid valve on charging cylinder and mid-seat receiver outlet valve. Allow refrigerant to flow until correct amount has been dispensed.

Refrigerant Charging for Large Bus (continued) Close the liquid valve on the charging cylinder, back seat the outlet valve on the receiver. Run system and check for proper charge.

Partial Charging Run system and allow to stabilize. Connect hose from charging cylinder to compressor suction valve. Place charging cylinder on scale, note weight. Open vapor valve on charging cylinder and mid-seat the compressor suction valve. Charge until correct level has been reached. Close vapor valve and back seat suction valve. Run system and confirm proper charge.

Refrigerant Management Center

Refrigerant Recovery Connect refrigerant management center (RMC) high side hose to receiver outlet valve. Connect RMC low side hose to compressor suction service valve. Mid seat both valves. Energize any normally closed liquid solenoid valves open. Operate the RMC per manufacturers instructions until all refrigerant has been removed.

Check Compressor Oil Level Run A/C in cool mode. Ensure compressor is fully loaded. Discharge pressure must be 150 psig for R-134a and 250 psig for R-22. Run until pressures are stabilized. After 20 minutes of operation shut unit down and examine the oil level in compressor. The level should look like those in the following slide.

Compressor Oil Levels

Adding Compressor Oil System off, disconnect power to compressor clutch. Front seat compressor suction and discharge valves. Recover refrigerant from compressor. Stop recovery when system reaches 1 to 2 psig (positive pressure). If pressure rises continue recovery process. Once crankcase pressure is stable continue.

Adding Compressor Oil (continued) Remove oil fill plug, add oil. Once proper level has been achieved replace plug. Evacuate compressor. Front seat manifold or management center hand valves and mid seat compressor suction and discharge service valves. Run unit and check refrigerant charge.

Removing Refrigerant Oil System off, disconnect power to compressor clutch. Front seat compressor suction and discharge valves. Recover refrigerant from compressor. Stop recovery when system reaches 1 to 2 psig (positive pressure). If pressure rises continue recovery process. Once crankcase pressure is stable continue.

Removing Refrigerant Oil (continued) Remove oil fill plug. Remove oil drain plug. Drain to correct level by viewing sight glass. If removing oil allow to fully drain. Reinstall oil drain plug. If oil was removed, add oil through fill plug to proper level as viewed through sight glass. Reinstall fill plug.

Removing Refrigerant Oil (continued) Evacuate the compressor. Front seat manifold or management center hand valves, mid seat compressor suction and discharge service valves. Run unit and check refrigerant charge.

Superheat Test Procedures Calibrate manifold gauge set. Hook up manifold gauge set to suction and discharge service valves. Hook up thermometer to evaporator tail pipe and insulate sensing probe. Operate A/C system, run for 20 minutes. Allow system to stabilize. Record evaporator tail pipe temp and pressure at suction valve. Repeat step 5 minimum of 5 times at 2 to 3 minute intervals.

Superheat Test Procedures (continued) After recording five readings calculate superheat for each reading. Using pressure temp chart, find the saturated temp at the suction pressure recorded. To calculate superheat, subtract the saturation temperature determined above from the temperature measured at eh evaporator tail pipe. Calculate the average of the 5 readings and compare to OEM specifications for unit.

Low Side Pump Down Procedure Hook up manifold gauge set to unit. Operate system on cool mode and allow to operate 15 to 20 minutes. Front seat receiver outlet valve (king valve) Run until compressor suction pressure is 1 to 2 psig. WARNING Do not allow Compressor to run in vacuum. Compressor damage may occur. Observe suction pressure gauge, watch for rise.

Low Side Pump Down Procedure (continued) If pressure rises high, run compressor a few more seconds. If pressure is to low, momentarily mid-seat manifold valve to leak high side pressure to suction side until suction side pressure is 1 to 2 psig. When pressures are stabilized, front-seat suction service valve. Al refrigerant is now contained in compressor, condenser, and receiver.

Low Side Pump Down Procedure (continued) Once repairs are made filter dryer should be replaced. Energize all normally closed liquid solenoid valves open. Evacuate low side of system through suction service valve and front seated receiver outlet valve. Check for leaks. Back seat the suction service valve and receiver tank outlet. Performance test the system.

Summary A typical large bus systems using R-134a uses the same 4 main components of any A/C system along with components that enhance system performance, control operation, and protect the system. The condenser coil removes heat from the vaporous refrigerant. The receiver acts as a storage vessel for liquid refrigerant.

Summary (continued) More sub-cooling of the refrigerant will result in less flash gas and improve system capacity and efficiency. The filter dryer removes moisture in the system and filters out any solid particles. The main LLSV is normally closed and will energize and open whenever the system is operating. The main evaporator is always in use when the system is operating. Parcel racks are evaporators that are used to provide A/C from above the passenger's head.

Summary (continued) If suction pressure of the evaporator outlet reaches the MOP setting the valve will close, preventing further refrigerant flow until suction pressure is reduced. Proper evacuation and dehydration procedures are extremely important . They will improve operating performance and compressor life. The use of a micron gauge is the only real means of ensuring the a good evacuation has be achieved. There are two basic evacuation procedures, the triple evacuation and the one-time procedure.

Summary (continued) For maximum performance, and A/C system must have the correct charge of refrigerant. For the compressor to receive proper lubrication, the compressor crankcase oil level must be maintained. When a bus A/C system in not cooling properly, one of the first checks is the system super heat setting. The low side pump down allows work to be performed down stream from receiver without removing the refrigerant from the system.