SURFACE-CONDENSERS.] 



APPLIED MECHANICS. 



873 



applied in his engine. We have already (see p. 851) alluded 

 to another mode of condensation, by means of cold surfaces, 

 which now begins to be adopted, and which, from the great 

 advantages attending its use, will probably soon entirely 

 supersede the injection method. Oil considering the 

 mode in which a jet of cold water admitted into a 

 vessel containing steam acts upon it, we readily see that 

 the water, entering with considerable force through an 

 aperture (usually ntted with a rose), presents to the 

 steam a very great and minutely subdivided surface. 

 Thus every minute particle of the water is at once sur- 

 rounded by an atmosphere of steam, from which it 

 abstracts so much heat, that the steam is almost im- 

 mediately reduced to its liquid condition as soon as it 

 comes in contact with the water : as a mode of con- 

 densing, then, rapidly and effectually, probably no better 

 arrangement could be desired. But, after condensation, 

 the injection-water, which, as we have seen, amounts to 

 15 or 20 times the quantity of water produced from the 

 steam, forms the principal part of the water fed from the 

 hot well to the boiler. If, therefore, the injection-water 

 be impure, as it generally is ; and still more, if it be salt, 

 as it always is at sea, the boiler is fed with the impure 

 water, and is thus subjected to all the evils of deposit and 

 incrustation, to which we have before alluded. The 

 quantity of water actually required for feeding the 

 boiler, is precisely the quantity derived from the con- 

 densation of the steam, with a small allowance for 

 MBAvoidable losses by leakages. If, then, the steam could 

 be rapidly and effectually condensed without mixing it 

 with impure water, it would itself supply almost enough 

 water for feed, and that of the purest possible quality, 

 because it would be distilled. There appears to be no 

 mode of obtaining the rapidity of condensation required 

 for the efficient working of an engine, except that of 

 exposing the steam to a very extended cold surface, just 

 aa it is exposed to the minutely subdivided surface of 

 the water of injection, but without mingling with it. 

 The new condensers, called surface-condensers, are there- 

 fore constructed generally like tubular boilers. A box 

 or casing, like the body of a boiler, lias a great number 

 of very small tubes passing from end to end, and tightly 

 jointed, so that no communication can take place between 

 the interiors of the tubes and the space through which 

 they pans. In some condensers, cold water is made to 

 flow through the tubes, while the steam from the cylinder 

 is admitted to the box, and plays round the outside of 

 the tubes in the spaces surrounding them. In other 

 condensers the steam is made to pass through the tubes, 

 which are surrounded by cold water supplied to the box 

 in which they are contained. There are numerous 

 modifications of form and arrangement of parts, and 

 varieties in the modes of constructing the casing and of 

 jointing the tubes to it, of causing the water to circulate, 

 and in other details. But in all surface-condensers the 

 general principle is the same namely, that of exposing 

 a very large cold surface to the steam, and of subdividing 

 the steam so that every minute portion of its bulk is 

 brought rapidly into contact with the cooling surface. 

 In the best condensers of this kind now in use, a vacuum 

 is obtained quite as good as was formerly obtained by 

 the injection method ; and, although an increased quantity 

 of cold water may be demanded for keeping the surface 

 cold, yet this is readily attainable at sea, and in most 

 situations where condensing engines are employed. The 

 very leakages are provided against by supplying for the 

 loss of feed from this cause, by distilled water procured 

 from a small supplementary boiler, so that scarcely a 

 particle of impurity can enter the main boiler, and all 

 the evils of deposit, incrustation and priming over of 

 viscid water, are effectually remedied. As water, before 

 attaining its boiling point, parts with the air which it 

 may have held in solution, the condensed steam contains 

 no air, and thus the vacuum in the surface-condenser 

 is maintained by the use of a much smaller pump. With 

 the ordinary condenser, worked by injection, the pump 

 is properly called an air-pump, because a large portion 

 of ita capacity is at every stroke filled by the attenuated 

 air given off from the steam and from the water of in- 



VOL. I. 



jection. But with the surface-condenser, the function 

 of the pump is almost exclusively confined to the re- 

 moval of condensed water, the quantity of air evolved 

 from the distilled water in the condenser being so small 

 as to be almost inappreciable. The size of the air-pump 

 is thus considerably reduced when surface-condenpcvtion 

 is adopted, and less of the engine power is consequently 

 absorbed in working it. 



The indicator applied to a condensing engine produces 

 a figure similar to those we have already discussed. But 

 as during one stroke of the piston the communication to 

 the condenser is open, the pencil traces a line below that 

 of atmospheric pressure. Thus if A (Fig. 206) be the 



Fig. 206. 

 20- 



15- 



10 

 6 

 0- 



,0 



IS-l. 



line of atmospheric pressure, another line B, drawn at 

 the pressure of 15 Ibs. below A, would be the line of ab- 

 solute vacuum, or of no pressure. The lower limb of 

 the figure would more nearly approach this line, the 

 more perfect the vacuum in the condenser. The area of 

 the figure represents the power during a stroke ; and the 

 mean effective pressure is found by taking the average 

 of the lengths of numerous ordinates drawn within the 

 figure as before.* 



PARALLEL MOTION. Non-condensing engines 

 are generally of the form called beam-engines. The pis- 

 ton-rod does not act directly on the crank in the case of 

 the double-acting engine, nor on the pump ill the case 

 of the single-acting engine, but on one end of a lever or 

 beam, working on central bearings, from the other end 

 of which the crank or pump is worked by a connecting- 

 rod. By jointed rods connected with pins on the beam, 

 the air-pump is worked with half the stroke of the 

 piston ; the feed-pump and the cold-water pump are 

 also worked with their proper strokes. As the piston- 

 rod, and also the air-pump rod, pass through stuffing- 

 boxes in the cylinder and air-pump covers respectively, 

 and must therefore move in straight lines, while the parts 

 of the beam to which they are connected vibrate in cir- 

 cular arcs, some arrangement is required for controlling 

 the motion of the rods so that it shall not partake of 

 the circular vibration. This could be effected by making 

 them slide between rectilineal guides, but not without 

 considerable friction and some difficulty in arranging 

 the guides in a suitable manner. We are indebted to 

 Watt for a simple and elegant manner of effecting this 

 object by the apparatus called the parallel motion. 



If A D! and B Cj (Fig. 207) be two equal levers vi- 

 brating in circular arcs JJ, D D 2 , Cj C C 2 respectively, 

 their extremities being connected" by a rod D 1 C 1( it will 

 be found that E, the middle point of this rod, moves 

 from E, to E 2 in a line which is very nearly straight. 

 Again, if F G, one of these levers, be prolonged to H so 

 as to be doubled in length, and a parallelogram G H M L 

 be formed of jointed rods so that F K and L are always 

 in one straight line, then K being controlled so as to 

 move in a straight line or nearly so, Lwill also move in 

 nearly a straight line. Applying this principle to the 

 beam of an engine, K is the point to which the air-pump 

 rod is connected, and L that to which the piston-rod is 

 connected ; G M and H L are called parallel motion links, 

 and M N, the subsidiary lever which controls the move- 

 ment of K, is called the radius rod. In the case of 

 single-acting engines, where the end of the beam oppo- 

 site to that worked by the piston works a pump, a 



See ante, p. 868. 



61 



