Page:EB1911 - Volume 03.djvu/727

 are a number of openings controlled by valves opening inwards so that air can flow freely in but cannot return. The piston is shown moving downwards. Air is now being drawn into the space above the piston through the valves v at the top, and the air in the space A below the piston, drawn in during the previous up-stroke, is being expelled through the valve v′ into the discharge chamber B, thence passing to the outlet pipe O. The action is reversed on the up-stroke. Thus it will be seen that air is being delivered both during the up-stroke and the down-stroke, and therefore flows almost continuously to the furnaces. There must, however, be momentary pauses at the ends of the strokes when the direction of movement is changed, and as the piston, though worked from an evenly rotating crank shaft, moves more quickly at the middle and slows down to no speed at the ends of its travel, there must be a considerable variation in the speed of delivery of the air. The air is therefore led from O into a large storage chamber or reservoir, whence it is again taken to the furnace; if this reservoir is made sufficiently large the elasticity of the air in it will serve to compensate for the irregularities, and a nearly uniform stream of air will flow from it. The valves used in this case and in most of the older blowing engines consist of rectangular metal plates hinged at one of the longer edges; these plates are faced with leather or india rubber so as to allow them to come to rest quietly and without clatter and at the same time to make them air-tight. It will be seen that some of these valves hang vertically and others lie flat on the bottom of the cover. The Dowlais cylinder is very large, having a diameter of 12 ft. and a piston stroke of 12 ft., giving a discharge of 44,000 cub. ft. of air per minute, at a pressure of 4 ℔ to the square inch.

A later design of blowing engine, built in 1871 for the Lackenby iron-works, Middlesbrough, is shown in section in fig. 4, and is of a type which is still the most common, especially in the north of England. Here A, the high-pressure steam cylinder, and C, the low-pressure one, are placed in tandem with the air cylinders B, B, whose pistons they actuate. In these blowing cylinders the inlet valves in the bottom are circular disk valves of leather, eighteen in number; the inlet valves T on the top of the cylinder are arranged in ten rectangular boxes, having openings in their vertical sides, inside which are hung leather flap valves. The outlet valves O are ten in number at each end of the cylinders, and are hung against flat gratings which are arranged round the circumference. The blast is delivered into a wrought iron casing M which surrounds the cylinder. The combined area of the inlet valves is 860 sq. in., or one-sixth the area of the piston. The speed is twenty-four revolutions per minute and the air delivered at this speed is 15,072 cubic ft. per minute, the horse-power in the air cylinders being 258. The circulating pump E, air pump F, and feed pumps G, G, are worked off the cross-head on the low-pressure side.

A more modern form of blowing engine erected at the Dowlais works about the end of the 19th century, may be taken as typical of the present design of vertical blowing engine in use in Great Britain. The two air cylinders are placed below and in tandem with the steam cylinders as in the last case. The piston rods also terminate in connecting rods working on to the crank shaft. The air cylinders are each 88 in. in diameter, and the high and low pressure cylinders of the compound steam engine are 30 in. and 64 in. respectively, while the common stroke of all four is 60 in. The pressure of the air delivered varies from 4 to 10 ℔ per sq. in. and the quantity per minute is 25,000 cub. ft. Each engine develops about 1200 horse-power. It is to be noted that flap valves such as those used in the 1851 Dowlais engine have in most cases given place to a larger number of circular steel disk valves, held to their seats by springs. In a large blowing engine built in 1905 by Messrs Davy Bros. of Sheffield for the North-Eastern Steel Company at Middlesbrough (see Engineering, January 6, 1905) the same arrangement was adopted as in that just described. The two air cylinders are each 90 in. diameter and have a stroke of 72 in. The capacity of this engine is 52,000 cub. ft. of air per minute, delivered at a pressure of from 12 to 15 ℔ per sq. in. when running at a speed of thirty-three revolutions per minute. The air valves consist of a large number of steel disks resting on circular seatings and held down by springs, which for the delivery valves are so adjusted in strength that they lift and release the air when the desired working pressure has been reached. It is worthy of note that in this engine no attempt is made to make the air pistons air-tight in the usual way by having packing rings set in grooves round the edge, but the piston is made deeper than usual and turned so as to be a very good fit in the cylinder and one or two small grooves are cut round the edge to hold the lubricant.

To illustrate a blowing engine driven by a gas engine supplied with blast furnace gas, fig. 5 gives a diagrammatic view of the blowing cylinder of an engine built by Messrs Richardsons, Westgarth & Co. of Middlesbrough about 1905. The gas cylinder is not shown. It will be seen that the air cylinder is horizontal, and it is arranged to work in tandem with the gas motor cylinder. The chief point of interest is to be found in the arrangement of the details of the air cylinder. Its diameter is 86 in. and the length of piston stroke 55 in. As to the arrangement of the valves, if the piston be moving in the direction shown, on the left side of the piston at A air is being discharged, and follows the course indicated by the arrows, so as first to pass into the annular chamber which forms a continuation of the