Several pipelines agreements are used for hotgas defrosting systems, one of which is shown in Fig. 6-25. Part of the gas at compressor discharge passes through the electromagnetic valve evaporator. Solenoid valve can be operated manually or automatically for this debt. Hot-gas defrost valve requires a slightly different approach from simple pressure drop against tonnage. Be sure to consider the evaporator temperature correction factors to make sure that the selected valve has sufficient capacity.
Normally open solenoid valves have many uses. Perhaps the most popular is their adaptation to the heat recovery system. Use one normally closed valves and one valve is normally open to shunt the discharge gas or outdoor condenser or indoor heat-recovery coil provides positive opening and closing action.
This eliminates the problems found in some three-way valves, which have a tendency to leak hot gas in a heat-drying coil when not needed. When this leakage occurs during the cooling season, it puts more load on the cooling system, which wastes energy than take care of her.
If leakage occurs during the heating season, when the discharge gas should go in the heating coils, a good portion of the liquid fee may be entered in the inactive capacitor.
For a simple schematic heat-recovery cycle, see Fig. 6-26. Many original equipment manufacturers (OEMs) have developed their own heat cycle, which may be completely different from the one illustrated. In addition, some may include head pressure control, so it is always advisable to consult with manufacturerР‚'s Bulletin on its special design.
Expansion valves 
