Fig. 101.—Cornish Pumping-Engine, 1880.
In the figure, A is the cylinder, taking steam from the boiler through the steam-passage, M. The steam is first admitted above the piston, B, driving it rapidly downward and raising the pump-rod, E. At an early period in the stroke the admission of steam is checked by the sudden closing of the induction-valve at M, and the stroke is completed under the action of expanding steam assisted by the inertia of the heavy parts already in motion. The necessary weight and inertia is afforded, in many cases, where the engine is applied to the pumping of deep mines, by the immensely long and heavy pump-rods. Where this weight is too great, it is counterbalanced, and where too small, weights are added. When the stroke is completed, the “equilibrium valve” is opened, and the steam passes from above to the space below the piston, and an equilibrium of pressure being thus produced, the pump-rods descend, forcing the water from the pumps and raising the steam-piston. The absence of the crank, or other device which might determine absolutely the length of stroke, compels a very careful adjustment of steam-admission to the amount of load. Should the stroke be allowed to exceed the proper length, and should danger thus arise of the piston striking the cylinder-head, N, the movement is checked by buffer-beams. The valve-motion is actuated by a plug-rod, J K, as in Watt’s engine. The regulation is effected by a “cataract,” a kind of hydraulic governor, consisting of a plunger-pump, with a reservoir attached. The plunger is raised by the engine, and then automatically detached. It falls with greater or less rapidity, its velocity being determined by the size of the eduction-orifice, which is adjustable by hand. When the plunger reaches the bottom of the pump-barrel, it disengages a catch, a weight is allowed to act upon the steam-valve, opening it, and the engine is caused to make a stroke. When the outlet of the cataract is nearly closed, the engine stands still a considerable time while the plunger is descending, and the strokes succeed each other at long intervals. When the opening is greater, the cataract acts more rapidly, and the engine works faster. This has been regarded until recently as the most economical of pumping-engines, and it is still generally used in freeing mines of water, and in situations where existing heavy pump-rods may be utilized in counterbalancing the steam-pressure, and, by their inertia, in continuing the motion after the steam, by its expansion, has become greatly reduced in pressure.
In this engine a gracefully-shaped and strong beam, D, has taken the place of the ruder beam of the earlier period, and is carried on a well-built wall of masonry, R. F is the exhaust-valve, by which the steam passes to the condenser, G, beside which is the air-pump, H, and the hot-well, I. The cylinder is steam-jacketed, P, and protected against losses of heat by radiation by a brick wall, O, the whole resting on a heavy foundation, Q.
The Bull Cornish engine is also still not infrequently seen in use. The Cornish engine of Great Britain averages a duty of about 45,000,000 pounds raised one foot high per 100 pounds of coal. More than double this economy has sometimes been attained.
Fig. 102.—Steam-Pump.
A vastly simpler form of pumping-engine without fly-wheel is the now common “direct-acting steam-pump.” This engine is generally made use of in feeding steam-boilers, as a forcing and fire pump, and wherever the amount of water to be moved is not large, and where the pressure is comparatively great. The steam-cylinder, A R, and feed-pump, B Q ([Fig. 102]), are in line, and the two pistons have usually one rod, D, in common. The two cylinders are connected by a strong frame, N, and two standards fitted with lugs carry the whole, and serve as a means of bolting the pump to the floor or to its foundation.