Checking Compressor Oil Evacuating the System Charging the System 16-58 Figure 16-95.  Checking the compressor oil when the system is open.  catastrophically failed, it can be flushed clean. Special fluid  flush formulated for vapor cycle air conditioning systems  should be used. The receiver dryer is removed from the  system for flushing and a new unit is installed, as it contains  fresh filters. Follow the aircraft manufacturer’s instructions. connected and pressure inside the system is reduced to  vaporize any water that may exist. Continued operation of  the vacuum pump draws the water vapor from the system.  A typical pump used for evacuating an air conditioning  system can reduce system pressure to about 29.62 "Hg (gauge  pressure). At this pressure, water boils at 45 °F. Operate the  vacuum pump to achieve the recommended gauge pressure.  Hold this vacuum for as long as the manufacturer specifies. The compressor is a sealed unit in the vapor cycle system  that is lubricated with oil. Any time the system is purged, it  is an opportunity to check the oil quantity in the compressor  crankcase. This is often done by removing a filler plug and  using a dip stick. Oil quantity should be maintained within  the proper range using oil recommended by the manufacturer.  Be certain to replace the filler plug after checking or adding  oil.  [Figure 16-95] As long as a vapor cycle air conditioning system retains a  charge higher than atmospheric pressure, any leak forces  refrigerant out of the system. The system pressure prevents air  (and water vapor) from entering. Therefore, it is permissible  to recharge or add refrigerant to a system that has not dropped  below atmospheric pressure without evacuating the system. Only a few drops of moisture can contaminate a vapor  cycle air conditioning system. If this moisture freezes in the  expansion valve, it could completely block the refrigerant  flow. Water is removed from the system by evacuation.  Anytime the system refrigerant charge falls below  atmospheric pressure, the refrigerant is lost, or the system is  opened, it must be evacuated before recharging.  Evacuating a vapor cycle air conditioning system is also  known as pumping down the system. A vacuum pump is  Charging capacity of a vapor cycle air conditioning system is  measured by weight. The aircraft manufacturer’s maintenance  manual specifies this amount and the amount and type of oil  to be put into the system when filling. Pre-weighing the  refrigerant or setting the refrigerant weight into the servicing  cart input ensures the system is filled to capacity. Charging a vapor cycle air conditioning system should be  undertaken immediately after evacuation of the system is  completed. With the hoses still connected to the high and  low side service valves, selecting charge on the service  cart panel positions solenoid operated valves so that the  refrigerant supply is available. First, refrigerant is released  into the high side of the system. Observe the low side gauge.  When the low side gauge begins to indicate pressure, it is  known that refrigerant is passing through the tiny orifice in  the expansion valve. As pressure builds in the high side, the  flow of refrigerant into the system stops.  To complete the charge of the system, refrigerant needs to  be drawn in by the compressor. A major concern is to avoid  damage to the compressor by having liquid refrigerant enter  the compressor inlet. After the initial release of refrigerant  into the high side, the high side service valve is closed, and  the remaining charge is made through the low side service  valve. The engine is started and run at a specified rpm, usually  a high idle speed. Full cool is selected on the air conditioning  control panel in the cockpit. As the compressor operates,  it draws vapor into the low side until the correct weighed  amount of refrigerant is in the system. Charging is completed  with a full performance test. Charging with a manifold set is accomplished in the same  way. The manifold center hose is connected to the refrigerant  Dip stick Compressor