SINGLE ACTING STEAM ENGINE.

41

closed by means of handles, marked respectively G', H', I'. From the cylinder, the tube T' opens into the condenser D, which communicates by a valve M with the air-pump, N; the condenser and the air-pump are placed in a cistern of cold water, called the cold well, W; a jet of cold water is allowed to play into the condenser by means of a valve E. The piston of the air-pump is furnished with valves which open upward, to admit of the passage of air and water to the upper part of the pump, where they escape into the cistern by the valve K, which prevents their return; the piston of the air-pump is worked by a rod Q, connected with the lever beam.

31. Operation of the Single Action Steam Engine.—The above description of this engine will enable us readily to understand its mode of operation.

(1.) The first process consists in what is called blowing through; that is, clearing the cylinder, condenser, and pipes, from atmospheric air, and filling them with pure steam, preparatory to working the engine. For this purpose, let the three valves G, H, and I, be opened, and steam admitted from the boiler through the pipe S; the steam will presently displace all the air contained in the cylinder, the tubes, and the condenser; the pressure exerted by the steam and heated air in the condenser will open the valve M, the valves in the piston of the air-pump will be opened upward, the steam and air will escape through the valve K into the cold cistern, and the several vessels will be filled with pure steam. The engine is now in a fit state for effective operation.

(2.) The piston P being at the top of the cylinder, and the injection valve E being opened, a jet of water plays into the condenser, forming a vacuum, into which the steam from the cylinder instantly rushes through the tube T' and is condensed, leaving a vacuum in the cylinder below the piston. The valves H and I being now closed, and steam admitted by the valve G to the upper surface of the piston, this is immediately pressed down with a force corresponding to the pressure of the steam and the perfection of the vacuum in

42

SINGLE ACTING STEAM ENGINE.

the cylinder. This portion of the first stroke* of the engine, elevates the pump-rod R on the other side of the building, and raises the water to the height required.

(3.) The upward motion of the piston is effected as follows. The upper steam valve G being closed, and the lower valve H opened, the steam is admitted below as well as above the piston, which, so far as the steam is concerned, is in a state of equilibrium, there being no force to resist its motion upward or downward, except its friction against the surface of the cylinder. In this passive state of the piston, the weight of the water on the piston of the pump, together with the weights attached to the piston-rod of the pump, draw down the corresponding side of the lever beam, and thus raise the piston in the cylinder on the other side.

(4.) To effect the second stroke of the piston, the valve H must be closed, the valves G and I opened, and the condensing valve V opened, in order to admit a jet of water into the condenser. The steam in the cylinder below the piston will immediately rush through the valve I into the vacuous condenser, and be converted into water, leaving a vacuum in the cylinder, into which the steam, admitted by the valve G, will immediately force down the piston; and thus by the alternate opening and closing of the valves, the engine performs its work.

(5.) It is well known that water contains a considerable quantity of air, which, being uncondensible, remains in the gaseous state after the steam has been condensed; unless this air were removed from the condenser, it would collect and impede the motion of the piston. The mode by which this air, together with the injected water, and the water produced by condensation of the steam, are removed from the condenser, is as follows. The piston of the air-pump

By a stroke of the piston is meant a double motion, viz., from the top of the cylinder to the bottom, and upward again. Its motion in one only of these directions, is called the length of the stroke. Hence, if the length of the stroke be five feet, the stroke itself is ten feet.

APPLICATION OF WATT'S PRINCIPLE.

43

is raised by the lever-beam to the top of its barrel, at the same time that the steam piston is raised to the top of the cylinder; hence, a vacuum is produced in the barrel of the air-pump, and the water and air pass into it from the condenser through the valve M. As the piston of the air-pump descends, it presses first upon the air, which opens the piston valves upwards and passes through to the upper part of the piston, then upon the water, which also passes through the valves; these valves are then closed by the weight of the air and water, and the piston, in its next ascent, carries them both upwards, where they are discharged through the valve K into the cistern B.

32. Application of Watt's Principle.-The application of the principle of separate condensation, as introduced by Watt, depended first, on keeping the cylinder as hot as the steam; and secondly, on keeping the condensing water as cool as will suffice to render the vacuum perfect. 1. It is evident that by the cooling of the cylinder from exposure to the atmosphere, the steam within it would be liable to a diminution of force. To prevent this loss of heat, Watt first proposed to surround the cylinder with a casing of wood, this substance being a slow conductor of heat. He afterwards enclosed the cylinder within another cylinder, called a jacket, and kept the space between the two cylinders constantly filled with steam. 2. It is evident that the condensing water, injected into the condenser, must be sufficiently cold, not only to condense all the steam which issues from the cylinder, but to reduce its own temperature, when mixed with the condensed steam, below the point at which it would produce vapour of sufficient pressure to impede the action of the piston in the cylinder. From experiments made by Watt on the pressure of steam at different temperatures, he found, that for the effective operation of the piston, the temperature of the water in the condenser must be reduced to 100°.

33. Quantity of injection Water.—The quantity of water required to condense a given quantity of steam, so as to pro

44

MECHANISM OF THE VALVES.

duce a constant and effective vacuum in the condenser, may be readily calculated. The latent heat of steam being computed at 1000° (page 5), and its sensible heat being 212o, or that of boiling water, the total amount of the heat of steam is 1212°; and this amount is to be reduced by the injection water to 100°. Supposing the injection water to be at the ordinary temperature of 60°, it is necessary for this to be raised to 100°. Hence it follows, that the steam must be deprived of 1112o, and the water must receive 40° of heat. If the former of these numbers be divided by the latter, the result will be 27'8, or, in a round number, 28. If, therefore, the entire contents of the boiler be supposed to be converted into steam, the amount of injection water must be 28 times greater than that of the water in the boiler. To meet this demand, it is necessary that the cistern, in which the condenser is placed, should be supplied with a sufficient quantity of water at the ordinary temperature of 60°. This is effected by means of a pump, called the cold pump, L, which is worked by the lever-beam of the engine, as represented in the engraving; as the water is pumped in, it sinks to the bottom of the cistern from its greater weight, while the portions heated by contact with the condenser rise to the top and are carried off by a waste pipe; the contents of the cistern are thus maintained at a uniform temperature. The pipe O

forms a communication between the hot cistern and the boiler, and is intended to supply the latter with water. It will be more particularly noticed hereafter.

34. Mechanism of the Valves.-The several kinds of valve employed in steam engines, will be described in a subsequent chapter. The valves used in the Single Acting Engine of Watt, are those termed spindle, or button valves, and sometimes puppet clacks; they are seen in the engraving at G, H, and I. Each of them is provided with a lever, which plays on a pivot attached to the frame-work of the engine; the levers are furnished with arms or handles, G', H', and I'. The valves are opened and shut by means of a rod or bar, called the plug-tree, attached to the rod of the air-pump.