EX. B 7.-Make working dimensioned drawings, half full size, of a steel crank shaft for horizontal engine of Ex. B, and as in Fig. 196. Crank pin size from Ex. B 5. Crank webs, 41′′ wide, 24" thick; diameter of shaft, 33"; journals, 3" diameter; pressure per square inch = 156 lbs.; weight of flywheel, 8 cwt. Loose counterbalance weights can be fitted, for proportions of which see § 217.

(The webs need not extend in length beyond crank pin and shaft a greater distance than their width extends beyond the diameter. Note increased length of flywheel journal.)

It is unnecessary to show the whole length of the crank shaft, or the exact distance between the journals in either example, hence it will be convenient to draw it broken off in parts, and then the lengths between centres of journals and total length can be left until after designing the engine framing and bed plate.

SECTION XXXI.

JET CONDENSERS AND AIR PUMPS.*

(205) Condensers.-Steam-engine condensers are of two kinds— the first and most common are called jet condensers, owing to condensation being effected by the exhaust steam meeting a jet of cold water and mixing with it; the second are known as surface condensers, in consequence of the steam being condensed by coming in contact with a cold surface, and without mixing with the condensing water, the steam passing inside a large number of small tubes and the water passing outside them. Surface condensers are used for engines of large power, especially when the water supply is too dirty to be used as feed for the boilers, but jet condensers are most common for the class of engines we are considering.

(206) Air Pumps.- Condensers are fitted with a pump worked from the engine which removes the condensed steam, and in a jet condenser, the condensing or injection water also, from the mixing chamber to a part of the condenser called the hot well, from which it passes to the boilers as "feed." But in doing this the pump also produces a partial vacuum in the space between the mixing chamber and the engine cylinder, by exhausting the

* Students working the examples for the vertical engine of Ex. A should pass over this section to Section xxxii.

air and steam from behind the piston, and is known in conse

quence as the " air pump." Air pumps are of different forms,

but we shall only deal with the type as fitted to the jet condenser of Ex. B, the construction of which we shall describe.

(207) Jet Condensers.-In ordinary jet condensers the mixing chamber, air pump, and hot well are contained within the one casting, which is usually of cast iron. In Figs. 197a, b, c, are shown three views of the jet condenser as fitted to the horizontal engine of Ex. B. Fig. 197a is a front section through the centre, Fig. 1976 an end section, and Fig. 197c the plan. The condenser casing consists of an oblong box, containing the mixing chamber, M C, the pump chamber, PC, and the hot well, H W. The air pump, A P, is a simple cast-iron plunger, cast hollow, and attached at the front end by means of a cottered joint to the back end of the piston-rod. The exhaust steam enters from the cylinder at the opening, ES I, in the top of the condenser, and meets a jet of cold water which enters from the side through the opening marked CWI, and is directed upwards immediately under the steam inlet by a pipe not shown. The steam is condensed and mixes with the water in the chamber, M C, falling around the barrel of the air pump, into the lower part next the suction valves, S V. As the plunger moves outwards these valves open, and the water passes into the pump chainber, P C. On the return stroke of the plunger as it moves inwards, it forces a certain quantity of the water through the delivery valve, DV, into the hot well, H W, the same process being repeated every revolution of the engine. The air pump is, therefore, single acting-that is, it only discharges each revolution, not each stroke. The hot well is connected to a discharge or overflow pipe at the opening marked H W O, or the water may pass down the sloping division plate, P, into the triangular-shaped passage P, and along to the other side of the condenser from which a pipe marked F P leads to the engine feed pump. The pump barrel, PB, forms the stuffing box for the plunger, the back cover, BC, being for access to the suction valves, and the hot well cover, H W C, for access to the delivery valve.

(208) Suction and Delivery Valves.-These valves are of indiarubber, covering gratings of the design shown in Fig. 169, and are fitted together so that a turned projection on the seating fits into holes in the division plates between the chambers. The suction valves are then held in position by the set screws, SS, which pass through the crossbar, C B, shown in the end view, fitting in the lugs, LL, cast on the inside of the condenser. The delivery valve is fixed by the vertical stay, D V S, the upper end of which fits into a groove in the valve seating, while the lower

H.W.

Hot well.

H.W.C

Hot wel cover.

Air pump.

Exhaust steam

Figs. 197a, b, c.-INDEX TO PARTS.

Sectional Front and End Elevations, and Plan of
Jet Condenser.

F.P. for Feed pipe to feed

pump for boiler.
Hot well overflow.
Air pump barrel.
Back cover.
Crossbar for fixing
suction valves.
Screws in cros bir
for suction valves.
Delivery valve stay.
Flanges for bolting
to engine bed-plate.
Lugs or bosses for
holding down bolts.

S.S.

D.V.S.

F.F.

L.L.

end is screwed, and rests in the circular boss, B, cast on the condenser bottom, adjustment being effected by the nuts shown. Notice that the indiarubbers of the suction valves are placed inside the pump chamber, while that of the delivery valve is inside the hot well, so that water cannot return from the pump chamber into the mixing chamber, nor from the hot well into the pump chamber. The condenser is provided with flanges along the two sides, lugs or bosses being provided for the holding-down bolts, as shown.

(209) Injection Spray Pipe. The pipe which directs the condensing water upwards to meet the exhaust steam is made of cast iron, is bolted by a flange to the outside of the inlet, CW I, and is turned upwards immediately under the centre of the exhaust steam inlet, and to within 51" of it, the end being closed, enlarged in diameter, and pierced by a number of slits or holes so that the water meets the incoming steam in the form of a spray.

(210) Arrangement of Details.-Other points to be noticed about the design of such a condenser are—that the exhaust steam inlet should be at the highest point, so that no water may possibly find its way along the exhaust pipe to the cylinder; that the suction valves should be at the lowest point, so that full advantage may be taken of any head of water in the mixing chamber, to assist the working of the suction valves; that the delivery valve should not be too high above the pump; and that the valves, when of the indiarubber pattern, must be arranged so that, when the engine is not working, they remain covered with water, otherwise the rubber would dry and crack. This is effected in the condenser shown by having the suction valves below the plunger, so that when the water level in the pump chamber sinks to below the plunger, no more water will be displaced, and by casting the rib, K, across the hot well, a little higher than the delivery valve, thus forming a kind of trough, which always remains full of water. The hot-well overflow, H W O, must also be above the top of the valve. The object of inclining the bottom of the mixing chamber is to better direct the water against the suction valves.

(211) Size of Condenser and Air Pump. A small condenser is liable to become flooded, and a large condenser simply means so much more space to produce a vacuum in. In the condenser illustrated, the mixing and pump chambers have each a volume of approximately three times the cylinder volume, and the hot well a volume of about 1.3 times. In commencing to design a condenser of this pattern, we first of all recognise that the air pump must be large enough to get rid of the condensing