Let us then see what quantity of water at the common temperature would be necessary to produce these effects.
If the latent heat of steam be taken at 1000°, a cubic inch of water in the state of steam may be considered for the purposes of this computation, as equivalent to one cubic inch of water at 1212°. Now the question is, how many cubic inches of water at 60° must be mixed with this, in order that the [Pg141] mixture may have the temperature of 100°? This will be easily computed. As the cubic inch of water at 1212° is to be reduced to 100°, it must be deprived of 1112° of its temperature. On the other hand, as many inches of water at 60° as are to be added, must be raised in the same mixture to the temperature of 100°, and therefore each of these must receive 40° of temperature. The number of cubic inches of water necessary to be added will therefore be determined by finding how often 40° are contained in 1112°. If 1112 be divided by 40, the quotient will be 27·8. Hence it appears, that to reduce the water in the condenser to the temperature of 100°, supposing the temperature of the water injected to be 60°, it will be necessary to supply by the injection cock very nearly twenty-eight times as much water as passes through the cylinder in the state of steam; and therefore if it be supposed that all the water evaporated in the boiler passes through the cylinder, it follows that about twenty-eight times as much water must be thrown into the condenser as is evaporated in the boiler.
From these circumstances it will be evident that the cold cistern in which the condenser and air-pump are submerged, must be supplied with a considerable quantity of water. Independently of the quantity drawn from it by the injection valve, as just explained, the water in the cistern itself must be kept down to a temperature of about 60°. The interior of the condenser and air-pump being maintained by the steam condensed in them at a temperature not less than 100°; the outer surfaces of these vessels consequently impart heat to the water in the cold cistern, and have therefore a tendency to raise the temperature of that water. To prevent this, a pump called the cold pump, represented at L in [fig. 21.], is provided. By this pump water is raised from any convenient reservoir, and driven through proper tubes into the cold cistern. This cold pump is wrought by the engine, the rod being attached to the beam. Water being, bulk for bulk, heavier the lower its temperature, it follows that the water supplied by the cold pump to the cistern will have a tendency to sink to the bottom, pressing upwards the warmer water contained in it. A waste-pipe is provided, by which this [Pg142] water is drained off, and the cistern therefore maintained at the necessary temperature.
From what has been stated, it is also evident that the hot well B, into which the warm water is thrown by the air-pump, will receive considerably more water than is necessary to feed the boiler. A waste-pipe, to carry off this, is also provided; and the quantity necessary to feed the boiler is pumped up by a small pump, O, the rod of which is attached to the beam, as represented in [fig. 21.], and which is worked by the engine. The water raised by this pump is conducted to a reservoir from which the boiler is fed, by means which will be hereafter explained.
We shall now explain the manner in which the machine is made to open and close the valves at the proper times. By referring to the explanation already given, it will be perceived that at the moment the piston reaches the top of the cylinder, the upper steam valve G must be open, to admit the steam to press it down; while the exhausting valve I must be opened, to allow the steam to pass to the condenser; and the condensing valve E must be opened, to let in the water necessary for the condensation of the steam; and at the same time the lower steam valve H must be closed, to prevent the passage of the steam which has been admitted through G. The valves G, I, and E must be kept open, and the valve H kept closed, until the piston arrives at the bottom of the cylinder, when it will be necessary to close all the three valves, G, I, and E, and to open the valve H, and the same effects must be produced each time the piston arrives at the top and bottom of the cylinder. All this is accomplished by a system of levers, which are exhibited in [fig. 21.] The pivots on which these levers play are represented on the framing of the engine, and the arms of the levers G′, H′, and I′, communicating with the corresponding valves G, H, and I, are represented opposite a bar attached to the rod of the air-pump, called the plug frame. This bar carries certain pegs and detents, which act upon the arms of the several levers in such a manner that, on the arrival of the beam at the extremities of its play upwards and downwards, the levers are so struck that the valves are opened and closed at the proper [Pg143] times. It is needless to explain all the details of this arrangement. Let it be sufficient, as an example of all, to explain the method of working the upper steam valve G. When the piston reaches the top of the cylinder, a pin strikes the arm of the lever G′, and throws it upwards: this, by means of the system of levers, pulls the arm of the valve G downwards, by which the upper steam valve is raised out of its seat, and a passage is opened from the steam pipe to the cylinder. The valve is maintained in this state until the piston reaches the bottom of the cylinder, when the arm G′ is pressed downwards, by which the arm G is pressed upwards, and the valve restored to its seat. By similar methods the levers governing the other three valves, H, I, and E, are worked.
Fig. 23.
