Reversing valve
This type of valve is in common use for hot gas defrost systems and room air conditioners. It effectively reverses the flow of refrigerant to the evaporator and condenser when a defrost period is initiated. In room air conditioners, it reverses the flow when heating is required rather than cooling.
The valve is an electromagnetic type and operates by pressure when the solenoid is energized. Figure 86a shows how the valve is installed. When installation of the valve requires brazing, always remove the solenoid coil (which is detachable) and wrap a wet cloth around the valve body to avoid damaging the valve mechanism when intense heat is applied.
Fusible plugs
Note: Before charging a system with a new or replacement refrigerant check the fusible plug rating.
Condensers and receivers are fitted with fusible plugs. These are safety devices against excessive high side pressures developing in the event of a fire. (Excessive operating pressures are controlled by the high pressure cut-out switch: see Chapter 6.) The plugs are normally brass studs with a 3 mm hole drilled through the centre, which is filled with a low melting point solder.
It is important that, when a replacement is fitted, the correct plug is selected. The following are the required melting points for various systems:
