process. Injectors are therefore typically over 98% energy-efficient overall; they are also simple compared to the many moving parts in a feed pump. The motive fluid may be a liquid, steam or any other gas. The entrained suction fluid may be a gas, a liquid, a slurry, or a dust-laden gas stream. Key design parameters Fluid feed rate and operating pressure range are the key parameters of an injector, and vacuum pressure and evacuation rate are the key parameters for an ejector. Compression ratio and the entrainment ratio may also be defined: The compression ratio of the injector, , is defined as ratio of the injector's outlet pressure to the inlet pressure of the suction fluid . The entrainment ratio of the injector, , is defined as the amount (in kg/h) of suction fluid that can be entrained and compressed by a given amount (in kg/h) of motive fluid. Lifting properties Other key properties of an injector include the fluid inlet pressure requirements i.e. whether it is lifting or non-lifting. In a non-lifting injector, positive inlet fluid pressure is needed e.g. the cold water input is fed by gravity. The steam-cone minimal orifice diameter is kept larger than the combining cone minimal diameter. The non-lifting Nathan 4000 injector used on the Southern Pacific 4294 could push 12,000 US gallons (45,000 L) per hour at 250 psi (17 bar). The lifting injector can operate with negative inlet fluid pressure i.e. fluid lying below the level of the injector. It differs from the non-lifting type mainly in the relative dimensions of the nozzles. Overflow An overflow is required for excess steam or water to discharge, especially during starting. If the injector cannot initially overcome boiler pressure, the overflow allows the injector to continue to draw water and steam. Check valve There is at least one check valve (called a "clack valve" in locomotives because of the distinctive noise it makes) between the exit of the injector and the boiler to prevent back flow, and usually a valve to prevent air being sucked in at the overflow. Exhaust steam injector Efficiency was further improved by the development of a multi-stage injector which is powered not by live steam from the boiler but by exhaust steam from the cylinders, thereby making use of the residual energy in the exhaust steam which would otherwise go to waste. However, an exhaust injector also cannot work when