It is neither wise nor necessary to go into a detailed description of an injector.  The young reader will be likely to become convinced if an injector works for five minutes, it will continue to work, if the conditions remain the same.  If the water in the tank does not become heated, and no foreign substance is permitted to enter the injector, there is nothing to prevent its working properly as long as the conditions are within the range of a good injector.  It is a fact that with all injectors as the vertical distance the injector lifts is increased, it requires a greater steam pressure to start the injector, and the highest steam pressure at which the injector will work is greatly decreased.  If the feed water is heated, a greater steam pressure is required to start the injector and it will not work with as high steam pressure.  The capacity of an injector is always decreased as the lift is increased, or the feed water heated.  To obtain the most economical results the proper sized injector must be used.  When the exact quantity of water consumed per hour is known it can be easily determined from the capacities given in the price lists which sized injector must be selected.

An injector must always be selected having a maximum capacity in excess of the water consumed.  If the exact amount of water consumed per hour is not known, and cannot be easily determined, the proper size can be approximately determined from the nominal H. P. of the boiler.  The usual custom has been to allow 7 I/2 gallons of water per hour, which is a safe rule for the ordinary type of boiler.

What A good injector ought to do.

With cold feed water, a good injector with a two foot lift ought to start with 25 pounds pressure and work up to I50 pounds.  With 8 foot lift, ought to start at 30 pounds and work up to I30.  With feed water heated to I00 degrees Fahrenheit it should start with the same lift, that is, will say 2 foot, at 26 and work Up to I20, and at 8, from 33 up to I00.  You will see by this that conditions, consisting of variation of temperature in the feed water and different lifts, change the efficiency of your injector very materially, and the water can soon get beyond the ability of your injector to work at all.  The above refers more particularly to the single tube injector.  The double tube injector under the same conditions as above should work from I4 pounds to 250, and from I5 to 2I0, but as this injector is not generally used on farm engines you will most likely not meet with it very often.

The injector should not be placed too near the boiler, as the heat from it will make it difficult to start the injector each time after it has been standing idle.