Encyclopaedia Britannica, 11th Edition, "Bedlam" to "Benson, George" / Volume 3, Slice 5

by Various

Fig. 4.—Vertical Section of Lackenby Blowing Engines (1871).

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 4½ 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.

Fig. 5.—Richardsons, Westgarth & Co.’s Blowing Engine.

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 12½ 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 86½ 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 space A, and thence, through the spring-controlled steel disk valves v′, into the discharge chamber C, which ultimately leads to the blast pipe. It will be seen that the valves v on the other side of the annular chamber are closed. At the same time a partial vacuum is being formed in the space B, to be filled by the inflow of air through the valves v which are now open, the corresponding discharge valves v′ being closed. These valves on the inside and outside of the annular spaces referred to are arranged so as to form a circle round the ends of the barrel of the cylinder. The free air, instead of being drawn into the valves v direct from the air of the engine house, is taken from an enclosed annular chamber E, which may be in communication with the clean, cool air outside. It will be seen that the piston is made deep so as to allow for a long bearing surface in the cylinder. Two metal packing rings are provided to render the piston air-tight. The horse-power of this engine, which is designed on the Cockerell system, is 750.

Air valves of other types than those which have been mentioned have been tried, such as sliding grid valves, rotatory slide valves and piston valves, but it has been found that either flap or disk lift valves are more satisfactory for air on account of the grit which is liable to get between slide valves and their seatings. In some of the blowing engines made by Messrs Fraser & Chalmers (see Engineer, June 15, 1906), sheets of flexible bronze act as flap valves both for admission and delivery, the part which actually closes the opening being thickened for strength.

The pressure of the air supplied by blowing engines depends upon the purposes for which it is to be used. In charcoal furnaces the pressure is very low, being less than 1 ℔ per sq. in.; for blast furnaces using coal an average value of 4 ℔ is common; for American blast furnaces using coke or anthracite coal the pressure is as high as 10 ℔; while for the air required in the Bessemer process of steel-making pressures up to 25 or 30 ℔ per sq. in. are not uncommon. According to British practice one large blowing engine is used to supply several blast furnaces, while in America a number of smaller ones is used, one for each furnace.

Fig. 6.—Thwaites’ Improved Roots’ Blower.

Rotary blowers occupy a position midway between blowing engines and fan blowers, being used for purposes requiring the delivery of large volumes of air at pressures lower than those of blowing engines, but higher than those of fan blowers. The blowing engine draws in, compresses and delivers its air by the direct action of air-tight pistons; the same effect is aimed at in a rotary blower with the difference that the piston revolves instead of moving up and down a cylinder.