Heat engineering; a text book of applied thermodynamics for engineers and students in technical schools . 124 HEAT ENGINEERING Now p2Y 2 = Pl^l Hence work = ^-j Pl7i [l - (^) V] (3) If desired paVf may be substituted for piVi giving workto compress Vf cu. ft. of free air from pi to p2 as work = ^F,[l-g)^] (4) EFFECT OF CLEARANCE Now if there is clearance the indicator card takes the formshown in Fig. 43. The air remaining in the cylinder, d2, at the end of discharge expandsfrom 2 to V on the return strokepreventing air from entering un-til V is reached. The amountof air then taken in is V\ —V\
Heat engineering; a text book of applied thermodynamics for engineers and students in technical schools . 124 HEAT ENGINEERING Now p2Y 2 = Pl^l Hence work = ^-j Pl7i [l - (^) V] (3) If desired paVf may be substituted for piVi giving workto compress Vf cu. ft. of free air from pi to p2 as work = ^F,[l-g)^] (4) EFFECT OF CLEARANCE Now if there is clearance the indicator card takes the formshown in Fig. 43. The air remaining in the cylinder, d2, at the end of discharge expandsfrom 2 to V on the return strokepreventing air from entering un-til V is reached. The amountof air then taken in is V\ —V\, or V\ - Vi = Vi of theprevious discussion. The net work required todrive the compressor is area 1Fig. of clearance. 221 or area l2dc - 12dc. This, assuming the same formof expansion and compression lines, givesNet work with clearance = -^i^-^][-©vl. This is the same expression as (3) for the compression of Vicu. ft. of air from pressure pi to p2, or in other words clearancehas no effect on the work required to compress a definite volumeof air. The effect of clearance is to decrease the amount of airtaken in for a given displacement or to increase the displacement AIR COMPRESSORS 125 for a given amount of air taken in. Thus in Fig. 42 the air takenin is Vi and the displacement D = Vi, but in Fig. 43 the airtaken in is Vi but the displacement D = V\ + lc — 2d, aquantity greater than Vi. In other words the displacement hasbeen increased. This increase of displacement means a largercylinder in stroke or area of piston and hence there will be morefriction and consequently more work will be required to drivethe compressor, but the work indicated on the air for the com-pression of a certain amount is the same with or withoutclearance. The volume 2d represents the volume of the clearance spaceand since this is usually expressed as a pe
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