When the oil for screw compressors is cooled by refrigerant circulating by means of a thermosyphon, some of the same principles of condenser draining apply, but there are some additional considerations. A vessel, called the thermosyphon receiver, is needed to separate the liquid from the liquid/vapor mixture that rises from the oil cooler. The vapor thus separated passes to the discharge gas header, as shown in Figure 7.33. The return of liquid/vapor from the oil cooler should flow to the thermosyphon receiver and not to the discharge header of the condenser. Were the liquid from the oil cooler to enter the condenser, it would overload it with liquid and reduce its heat-transfer capacity.
A significant fact is that there is appreciable flow of vapor through the vapor line from the thermosyphon receiver, so there is a pressure drop in this line. Thus the pressure in the thermosyphon receiver is higher than that of the condenser inlet and certainly higher than the condenser outlet. To overcome this pressure difference a liquid column must develop somewhere. Figure 7.33 shows bottom inlet to the thermosyphon receiver with individual liquid columns for each condenser. Entrance of the condensate to the top of the thermosyphon receiver would work satisfactorily if the drains were equipped with the P-traps of Figure 7.32a. Even if the installation consists of only one condenser, there is still the need to build up pressure from the outlet of the condenser to the thermosyphon receiver. A convenient means of providing a liquid seal to form a liquid trip is to immerse the drain line from the condenser below the liquid level maintained by the overflow to the system receiver.