The exhaust from the cylinders is used in two ways, first to create a draft for the fire, and second to heat the feed water pumped into the boiler. The draft is increased by exhausting a portion of the steam into a nozzle placed directly under the stack. The friction of the steam on the surrounding air, draws the air with it, forming a partial vacuum over the grate at each puff, and in this way it causes additional air to rush through the fuel and increases the temperature of the combustion. As the load increases the “puffs” increase in intensity due to the greater terminal pressure and the fire is accelerated in proportion. This is a simple but rather expensive way of increasing the draft. A considerable proportion of the heat in the exhaust steam is saved by using it to heat the feed water supplied to the boiler. Besides the saving in fuel, affected by heating the water from steam that would otherwise be thrown away, the strains on the boiler due to the injection of cold water are greatly decreased as the difference between the temperatures of the boiling water in the boiler and the hot feed water are much less than in the former case. The feed water heater consists essentially of a series of tubes in a cylindrical shell. The tubes are surrounded on the outside by the feed water, and are filled with the exhaust steam which passes from end to end through the tubes. The hot water is pumped from the heater into the boiler. An efficient feed water heater adds greatly to the steaming capacity of the boiler. (152) Feed Pump. A small steam pump is furnished for pumping the water into the boiler. This device consists of a small steam cylinder connected directly with the pump plunger and is absolutely independent of the main engine so that it can be used whether the engine is running or not. The exhaust of the pump should be turned into the feed water heater when the engine is not running so as to heat the water, but should be directed to atmosphere when the main exhaust is passing through the heater. An injector is usually supplied with the engine for feeding the boiler in emergencies. The injector forces water into the boiler by means of a steam jet which is arranged so that a high velocity is imparted to the water in the injector nozzle by the condensation of the steam furnished by the jet. In this way water is pumped into the boiler against a pressure that is equal to the pressure of the steam acting on the water. Except for a check valve there are no moving parts. No feed water heater connection is made with the injector for this device raises the temperature of the feed to a considerable temperature. The temperature is not as high, however, as the temperature of the water from the feed water heater and pump, and because of the comparatively low temperature coupled with the fact that live steam is used in heating the injector water, it is not an economical method of pumping. (153) The Boiler.