In low-temperature installations the shutoff valve between the evaporator and the suction line should offer as little flow resistance as possible during refrigeration. A normally-open valve meets this requirement, but in addition the valve must be capable of closing positively, for example, during a hot-gas defrost. A valve designed to meet these requirements, and first shown in Fig. 6.49 among the defrost piping arrangements, is the gas- powered suction valve, as shown in Fig. 11.7. If the external gas pressure is removed, the pressure above the piston vents through the bleed hole and the spring opens the valve. When the valve is to be closed, high-pressure gas from an external source is applied to the piston, which closes the valve, working against the spring compression.