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The diameter of a single-acting air-pump, having a length of stroke equal to one-half that of the engine, may be diameter of the cylinder multiplied by '6.

Width of air-pump piston or plunger = diameter of air-pump multiplied by 3 to 7 according to the description of packing-rings used. Packingrings are frequently dispensed with, in which case the width of the plunger is generally from one-third to one-half the diameter of the air-pump. Diameter of air-pump rod diameter of air-pump divided by 8. The pump-rod should be of iron, cased with brass.

Metallic-packing is the best packing for the air-pump-rod, as it considerably reduces friction, requires little attention, and is least liable to admit air.

The area of delivery and suction-valves should each be diameter of airpump multiplied by 7. For a high speed of plunger, the valves should not be less in area than that of the pump.

In applying the rules for air-pumps and condensers to compoundengines, the diameter of the low-pressure cylinder is to be employed as the unit of measurement. For the discharge of pumps, see rule on page 9o. A disc-valve for an air-pump is shown in Fig. 68. The disc is either of vulcanised fibre or india-rubber. The guard and grid are of gun-metal.

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Fig. 69.-Valve for Air-Pump.

Air-pumps work very well without footvalves when there is an ample supply of sufficiently cool injection-water.

Efficiency of Air-Pumps. -A vertical single-acting air-pump is capable of maintaining the highest vacuum in a condenser: a horizontal double-acting air-pump generally maintains the lowest vacuum.

The efficiency of single-acting vertical air-pumps averages 52 per cent., and that of double-acting vertical air-pumps 40 per cent. of the theoretical efficiency. The efficiency of horizontal single-acting air-pumps averages 40 per cent., and that of horizontal double-acting air-pumps averages 33

per cent.

Speed of Air-Pumps.-The low efficiency of air-pumps is frequently due to the speed of the plunger being so high that the water cannot follow it in a continuous volume, and a space is formed between the water and the plunger, in which air accumulates and impairs the action of the pump. A low speed of the bucket or plunger of an air-pump conduces to the maintenance of a good vacuum in the condenser. To obtain a high efficiency, the speed of the plunger should not exceed 100 feet per minute and to obtain a moderate efficiency, the speed should not be greater than 200 feet per minute. At speeds of from 300 to 400 feet per minute, the efficiency must necessarily be low.

Horizontal High-Pressure Condensing Steam-Engine.-A hori

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Fig. 69.-Horizontal Single-Cylinder Condensing Steam-Engine.

zontal single-cylinder condensing steam-engine of simple construction, having all working parts easy of access, and provided with a high-speed governor and an equilibrium throttle-valve, is shown in Fig. 69. It is fitted with a jet-condenser with air-pump and hot-well combined in one casting, fixed on the bed behind the cylinder, the piston of which is fitted with a tail-rod carried through a stuffing-box in the back cylinder-cover and coupled to the rod of the air-pump-plunger. This type of engine is usually made of the following sizes:

Table 5.-SIZES OF HORIZONTAL SINGLE-CYLINDER CONDENSING STEAM ENGINES, AS SHOWN IN FIG. 69.

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The Weight of Horizontal Single-Cylinder Condensing SteamEngines having a length of stroke equal to twice the diameter of cylinder, with fly-wheel complete, as shown in Fig. 69, may be estimated approximately by the following Rule

Weight in tons=

(diameter of cylinder in inches) 2
26

For the weight in cwts. use 13 as a divisor instead of 26.

For instance the weight of a horizontal single-cylinder condensing steamengine, as shown in Fig. 69, with cylinder 20 inches diameter, and 40 inches stroke, may be estimated at (20 × 20 inches) ÷ 26 = 15°3 tons.

COMPOUND STEAM-ENGINES.

The Cylinder of a Steam-Engine is alternately exposed to a high temperature from the entering steam, and a low temperature from the exhaust-steam. If, for instance, steam enters a cylinder at a pressure of 70 lbs. per square inch, and leaves it at a pressure of 15 lbs. per square inch, the metal is exposed to a range of temperature of 303° - 212° = 91° Fahr. resulting in considerable loss from condensation. By dividing the range of the temperature of the steam between two or more cylinders, greater uniformity of temperature is obtained in the cylinder and condensation is diminished.

The Steam in a Compound-Engine after performing part of its work by driving the piston in one cylinder, is exhausted into, and performs work

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in, another cylinder, or in several cylinders in succession, before being discharged into the condenser to be condensed in a condensing-engine, or before being finally exhausted in a non-condensing-engine. By expanding the steam through a succession of cylinders the range of temperature in each cylinder is diminished, and the mean temperature in each cylinder is much nearer that of the initial temperature of the steam used in it, than is obtained in the cylinder of a simple engine, and the loss due to cooling is minimised.

The compound-engine permits the principle of expansion to be carried out to the fullest extent, that is, it enables steam to be used in the most economical manner, by employing its full expansive force; and it effects a considerable saving of fuel in comparison with a simple engine. The power developed by a compound-engine is theoretically the same as would be developed if the quantity of steam used by it were expanded in a single cylinder of the same capacity as the low-pressure cylinder. Hence, the area of the low-pressure cylinder of a compound steam-engine is calculated as if all the power were to be developed in that cylinder, which therefore requires to be of the same area as the cylinder of a simple engine of the same power. When separate pistons act upon separate cranks, the latter are placed at an angle of 90° apart on the same shaft in the case of doubleexpansion engines, and at equal angles of 120° apart in the case of trebleexpansion engines, in order to obtain uniformity of rotative-pressure upon the cranks. The working-pressure of double-expansion engines is usually 90 lbs. per square inch, of triple-expansion engines 160 lbs. per square inch, and of quadruple-expansion engines 180 lbs. per square inch.

A Horizontal Compound, or Double-Expansion, Condensing Tandem-Steam-Engine of compact and strong construction is shown. in Fig. 70. The pistons of both cylinders are attached to one piston-rod. The condenser and air-pump are placed for compactness below the floor, in the space occupied by the projection of the fly-wheel beyond the end of the engine-bed. The air-pump is single-acting, and is worked by a crank attached to the crank-pin of the engine.

To find the Area of the Low-pressure Cylinder of a compound, or double-expansion, steam-engine Rule: Multiply the number of horse-power the engine is required to indicate by 33,000, which will give the number of footpounds required per minute, divide this by the speed of the piston in feet per minute, and the result will be the total effective pressure on the piston at that speed to develop the given number of indicated horse-power; divide the quotient by the mean effective pressure per square inch on the piston, and the final quotient is the area in square inches of the low-pressure cylinder. The speed of the piston in compound, or double-expansion, engines is usually not less than 420 feet per minute.

The ratio of expansion is found by dividing the initial absolute pressure of the steam in the high-pressure cylinder by the final pressure in the lowpressure cylinder.

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