1244

THEORY AND PRACTICE OF ENGINEERING

Book II.

at F, which prevents its flowing back again; at G isa valve for the air to escape. The engine beam liasattached to it mechanism which opens and closes thevalves; and water is supplied at the top of the piston tomake the packing around it steam-tight. When the beamis in the proper position, steam is admitted below thepiston by the valve V: this valve is then, by the action ofthe beam, shut off, and the injection cock at D opened;water then flows through the pipe at I, and condenses thesteam within the cylinder, after which it passes off throughthe pipe E ; the atmosphere then acting on the piston, P,it is forced down, the air contained in the cylinder, andproduced by the condensation of the water, being sufferedto escape at G : connected with the beam is a bar havinga vertical movement; in this is attached lappets, whichopen the steam and injection cocks or valves: the steam-valve closing, and the injection cock opening when theup-stroke is completed, the steam-valve is made to openwith the rise of the piston.

This kind of engine may be regulated by turningoff the steam, and cutting off the injection: a hot well Fig. 1954. common atmospheric.supplies the water for the boiler, and about 16 pounds ofcoal per hour for each horse-power is the quantityrequired to work it; the boiler, whatever its form,should have the steam limited to 1 pound on thecircular inch.

When this kind of engine has a separate condenser,it is constructed as shown in fig. 1955. ; the steam isadmitted by the pipe S, through the slide B, into thecylinder at D; A is a pump with a solid piston,which expels the condensed steam, air, and water,the injection being made into the pipe E ; I is theinjection cock ; F is a cock to let out any air thatmay accumulate when the engine is not at work,below' the piston P.

When it is required to set the engine to work, theslide B is raised above S, and sufficient steam is admittedto drive out the air at the valve Q, the pistons, bothof the cylinders and pump, being up : the steam is thenshut off by the slide B, and the injection opened,when, in consequence of the condensation which takesplace, both pistons descend, during which the cock Fmust be opened, and then closed ; the injection beingstopped, the slide B closes the opening to the con-denser, and by again admitting the steam, the pistonsrise, and the air and water are driven out at Q; by opening and closing these passages alter-nately, the action is continued: by closing the valve B during the ascent, and the cock Iduring the descent, the engine may be regulated: the power of the atmospheric engine,when applied to raising water, should have its velocity in feet per minute 98 times thesquare root of the length of the stroke, and the diameter of the cylinder should behalf its length. The area of the steam passages should be to the area of the cylinder,as the velocity of feet per minute is to 4800: the stroke of the air-pump should be halfthat of the piston, and its diameter three-eighths that of the cylinder.

To ascertain the quantity of steam, multiply the number of feet contained in the areaof the cylinder by half the velocity in feet, w’ith the addition of one-fifth of loss for cooling ;this result divided by 1480 gives the quantity of water requisite to supply the boiler; andfor injection, twenty-four times that quantity will be required. The aperture by whichthe injection is made should have its diameter one-tliirty-sixth that of the cylinder, andthe pipe one-ninth.

The power of the engine is found by multiplying 6*25 times the square of the diameterof the piston by half the velocity in feet per minute, and the product is the effective pow’erin pounds raised one foot high per minute; this divided by 93,000 gives the horse-power.The cylinder being 32 inches in diameter, and the velocity 220 feet per minute, we have6*25 x 32 8 x 110 = 704,000 pounds, or 21 horse-power; 29’4 feet of water is requiredper hour, and 246 lbs. of caking coal, or 11*7 lbs. for each horse-power.

Steam Pressure Engines. — Boulton and Watt's Single Engine , as applied to the raising ofwater. The boiler is surrounded with brickwork, and the steam passes by the pipe b to

Fig. 1955. ATMOSPHERIC ENGINE WITH

SEPARATE CONDENSER.