Blowing Engines

BLOWING ENGINES. Appliances for the production of a stream of air under pressure may be divided into three groups, namely compressors, blowing engines and blowers, the latter being again sub-divided into positive blowers and fans. The distinctions between these groups are drawn in several ways by different makers, but the simplest and perhaps the most generally used is based upon the pressures at which they are intended to work; air compressors deliver a stream of air at a pressure almost in variably above 4o1b. per sq.in.; blowing engines work between slb. and 3o1b. per sq.in., occasionally going up to 4o1b.; and blowers below slb. per sq.in., though often considerably lower. It may be noted that appliances of the last group are used for exhausting as well as for forcing air, whereas those of the first two groups are used only for forcing. The boundaries between these two groups are by no means sharply drawn, and there is a certain amount of slackness in the terminology employed ; thus some makers will call an appliance a turbo-compressor, which others would describe as a turbo-blower. To some extent the objects for which the stream of air is required is also taken into account ; thus an appliance pro ducing a stream of air at a pressure of about 4olb. would probably be described as an air compressor if the air were employed to work a compressed air engine or a drill, but would be called a blowing engine if the air were utilized for a metallurgical opera tion. The term blowing engine is practically restricted to appli rapidly. Modern blowing engines consist as a rule of a pair of blowing cylinders direct-coupled to a corresponding pair of steam cylinders, the latter being often low and high pressure respectively, and thus forming a cross-compound engine. A good example of this type is shown in fig. 2. (See also Journal of the Iron and Steel Institute, 1889, No. z, fig. 2, plate 2.) The boiler pressure is r oolb. per sq.in., the two steam cylinders being 36in. and 64in. in diameter respectively. The blowing cylinders work up to a maximum pressure of i olb. per sq.in. ; they are 88in. in diameter and 6oin. stroke, and running at 23 revs. per min. they are capable of blowing 19,00o cu.ft. of free air per minute.

A number of reciprocating blowing engines are now quite often gas-driven instead of steam-driven, the waste gas of the blast ances used in the metallurgy of iron, large blowing engines at a relatively low pressure being used for supplying blast to blast furnaces, whilst those giving a smaller air supply, but at a higher pressure, are employed for blowing Bessemer converters, which are, however, gradually becoming obsolete.

As regards their mode of operation, blowing engines may be divided into two main classes, reciprocating and rotary.

Reciprocating Blowing Engines.

The blowing end of a reciprocating blowing engine consists of a cylinder (or cylinders) fitted with admission and discharge valves, the piston being caused to move to and fro by being direct-coupled to a steam engine or gas engine. As a rule the pressures generated are so low that it is not worth while to make use of any special arrangement for get ting rid of the heat evolved by the compression of the air.

The simplest type of construction of a blowing cylinder is shown diagrammatically in longitudinal section in fig. z. A and A' are the air admission valves and D and D' the air discharge valves which open into a chamber or receiver connected with the air delivery pipe. When the piston is moving in the direction in dicated by the arrow, A' and D are open and A and D' are closed, these relations being reversed when the piston is travelling in the opposite direction. Sometimes the air admission valves are open to the atmosphere, as indicated in fig. z, but in many modern instances the air is drawn from a casing to which an air pipe is connected opening outside the engine house, so that cool air can be used. Sometimes the incoming air is artificially refrigerated, and also dried, by passing it through a "silica gel" tower. More over, a filter can be disposed at the mouth of the suction pipe, so that only dust-free air is aspirated. In the simple form of cylinder here illustrated, the valves are opened and closed by the pressure of the air itself, and hence do not open or close instantaneously the moment the direction of the piston is reversed, because it necessarily takes some little pressure, positive or negative, to overcome the inertia of the valve. This drawback is got over in modern blowing engines by the employment of mechanically actuated valves. There is also considerable difference in the con struction of the valves themselves ; the old form of simple flap valve is now hardly ever seen, annular or disk valves of thin sheet steel, poppet valves, slide valves and cylindrical rotating valves being all used by different makers.

Blowing cylinders may be arranged either horizontally, if space is not important ; or vertically, in which case they are more difficult to repair. Large blowing engines, such as are used for blast-furnaces, are mostly of the vertical type in Great Britain and of the horizontal type in the United States. It is a matter of in difference whether in the vertical engine the blowing cylinder is above or below the steam cylinder, but the former is the more usual place. Blowing engines, whether vertical or horizontal, must be arranged with a heavy fly-wheel, for it must be remembered that the air pressure in the blowing cylinder reaches its maximum at the same time that the steam pressure in the steam end is falling furnace being as a general rule employed for this purpose. Since such gas is of low calorific value (say about ioo B.T.U. per cu.ft.), these engines must be of large capacity. The first person to utilize blast-furnace gas in explosion engines was B. H. Thwaite about 1894 ; and the first gas-driven blowing engine (of 60o h.p.) was built at Seraing, Belgium, in 1899. One of the most serious difficulties connected with this use of the blast-furnace gas is the necessity of thoroughly cleaning the waste gas from the dust with which it is always charged ; after leaving the ordinary fur nace dust-catchers, the gas contains about 5 grams of dust per en.

metre, and for use in gas engines this must be reduced to below o•oi gram per cu. metre (see IRON AND STEEL). Various methods of cleaning have been employed successfully. A gas blowing en gine suitable for this purpose is shown in vertical section and plan (fig. 3) . This is a double acting tandem, two-cylinder gas engine, developing 1,25o b.h.p., the gas cylinders being 392in. in diameter by '472in. stroke, direct-coupled to a blowing cylinder of 93in.

diameter. This engine made 90 strokes per minute, giving a blast of free air equal to 29,40o cu.ft. per minute, which it delivered at a pressure of ffolb. per sq.in., the actual volume of air delivered at this pressure being 17,60o cu.ft. per minute. The consumption of blast-furnace gas per b.h.p. per hour in these engines was ap proximately ioo cu.ft. at full load, therefore about io,000 B.T.U. per b.h.p. per hour.

Where coke-oven plants are combined with blast-furnaces, it is possible to mix a proportion of rich coke-oven gas (with about B.T.U. per cu.ft.) with the blast-furnace gas, such a mixture giving excellent results.

Turbo-blowers.

A form of blowing engine that has obtained much popularity in recent years since its introduction in 1902 is the turbo-blowing-engine, or, as it is often called, the turbo-blower —which may be described as a reversed steam turbine. In the turbo-blower the discs are mechanically rotated and become im pellers which force the air through the guides and thus gradu ally raise it to the required pressure ; the greater the number of stages the greater is the pressure obtained. Turbo-blowing engines are at their best when delivering large quantities of air at relatively low pressures, and are, therefore, particularly well adapted for blast furnace work. In practice, since their speed of revolution has to be tolerably high, they are generally direct coupled to steam turbines, though occasionally they are driven in the same way by electric motors. The general arrangements of a turbo-blowing engine may be seen from fig. 4, which is a vertical section through the blowing end of one of a pair of such engines ; as will be seen, it is a five-stage blowing engine, the five impellers being shown attached to the central shaft, and it is in tended to compress 5o,000 cu.ft. of free air per minute up to a pressure of i 51b. per sq. inch. The usual American practice is to build such turbo-blowing engines with capacities ranging from 30,00o to 8o,000 cu.ft. of free per min. and for pressures ranging between 12 and 25 or exceptionally up to 3olb. per sq. inch. For the higher pressures the turbine casing is jacketed to receive a flow of cold water and thus absorb the heat generated by the compression of the air. These blowing engines run at between 2,75o and 4,00o revs. per min., and the horse-power de veloped by the steam turbine is from 3,00o to 9,00o b.h.p.

Fig. 6 shows a vertical section through steam turbine and turbo blower complete, built for British blast-furnaces. As will be seen from the section, the turbo blower is a single flow three-stage blower and is intended to com press 20,00o cu.ft. of free air per minute against a maximum pres sure of iolb. per sq. inch. Gen erally speaking, British blast f urnace practice demands both smaller quantities of air and lower pressures than are usual in the United States. As compared with gas-driven blowing engines, turbo-blowing-engines are some what less efficient, but they gen erally cost less in up-keep and are, generally speaking, more re liable, this latter quality being of the utmost importance for ap pliances that are required to work continuously for long periods, as is the case with blast-furnace blowing engines.

Positive Blowers.—Finally, mention may be made of the positive blower, such as is generally used for foundry cupolas and for water-jacket furnaces for smelting copper and lead. The most widely employed blower for these purposes is shown in cross tion (fig. 5). It will be seen that the blower consists of a couple of impellers, dumb-bell shaped in cross section, which are rotated within a casing in such a way as to force a continuous stream of air through it. These blowers are in the majority of cases belt driven, but may also be driven by gearing or at times direct coupled to a dynamo. They are positive, inasmuch as the quantity of air delivered by their agency depends entirely upon the speed of rotation, and is not affected by the pressure against which they are called upon to work. What are known as low-pressure blowers are usually built for pressures up to i alb. or 21b. per sq.in., whilst high-pressure blowers will work up to pressures of 51b. per sq. inch. For ordinary cupola practice about 'lb. per sq.in. is required. (H. Lo.)