When two pumps are placed side by side, and have a common delivery pipe, the machine is called a duplex pump. It is usual to set the steam valves so that when one piston is at the end, the other is at the middle of its stroke. A duplex pump having a large airchamber and valves set to act in this manner, delivers water with an approximately constant velocity. Injectors.Water may be forced into a boiler by an injector or inspirator. By means of this instrument, the energy of a jet of steam is used to force the water into, the boiler. That there is sufficient energy to do this work is evident from the fact that each pound of steam, in condensing, gives up about 1,000 B.T.U., and a B. T. U. is equivalent to 77S foot-pounds. Not all the energy of the jet of steam is used in forcing water into the boiler; some is wasted, and much is used to heat the feed-water.
The action of the injector is briefly as follows: The steam escapes from the boiler with great velocity, and, as it passes through the cone-shaped passage, draws air along with it, thus creating a partial vacuum in the suction pipe. Atmospheric pressure forces water up into the suction pipe, and the jet of steam which it meets is partly condensed.
The energy of the jet carries the water along with it into the boiler. Experiments show that the injector, if considered as a pump, has a very low efficiency. When used for feeding a boiler, it has a thermal efficiency of nearly 100 per cent, since all the heat of the steam passes to the water except the slight amount lost in radiation. The pump, however, has one great advantage over the injector; it can force hot water from a heater into the boiler, while an injector can be used only with cold or moderately warm water.
Figs. 54 and 55 show the interior section and exterior of a "Hancock" inspirator. To inject water to the boiler, first open overflow valves 1 and 3; close valve 2; and open starting valve in the steam pipe. When the water appears at the overflow, open 2 one quarter-turn, close 1, and then close 3. The inspirator will then lx in operation. When inspirator is not working, open both 1 and 3 to allow water to drain from it.
Both temperature and quantity of delivery water can be varied by increasing or decreasing the water supply. When the water in the suction pipe is hot, either cool oti both pipe and injector with cold water, or pump out the hot water by opening and closing the starting valve suddenly.
Circulating Apparatus. There is always more or less danger in starting a fire under a boiler. If the circulation is poor, the result will be that not only will the water be of an uneven temperature, hot near the top and cold at the bottom, but the boiler shell is likely to be subjected to severe strain, owing to the difference of temperature arising from the stagnation of the cold water near the bottom. The fire must be started slowly, and a considerable time consumed in getting up steam. To overcome the difficulty of poor circulation, several mechanical devices have been applied.
The first device tried was a hydro-kineter—a sort of injector—in which jets of steam driven through a conical nozzle drew in the surrounding water. This was so arranged as to induce the cold water to flow from the bottom toward the top, where it was more intensely heated. This arrangement is efficient, but slow of action. In large marine boilers—in which the fire is cautiously started, as is proper— the temperature at the surface of the water, four hours after lighting up, has been found to be as high as 205°, while at the bottom it was only 73°. Several observations with a hydro-kineter in action have shown the temperatures to be 205° and 144° respectively. It was six hours more before the temperature was equalized throughout.
In naval vessels, where it is frequently necessary to raise steam rapidly, this device is altogether too slow. It has, moreover, two other drawbacks. There must be an auxiliary boiler under steam pressure, and it will cease to act when the temperature and pressure of steam in the main boiler has reached that in the auxiliary boiler. The steam jet, in the American Navy, has been replaced by a jet of feed-water forced through a conical nozzle. This arrangement answers very well so long as steam is being drawn from the boiler; but when the boiler is at rest and steam is being raised, it is inoperative.
The best service can be had by means of small centrifugal pumps fixed beside the boilers, which take water from the bottom of the boilers and discharge it. a little below the water-level. The pumps may be turned by hand while raising pressure, and may be worked by steam when sufficient pressure has been attained. A small engine of perhaps 1-1/2 horse-power is sufficient to give a proper circulation to a large boiler. With such a circulating device, steam can be raised with safety, in a comparatively short time.