The Continental engineers, however, soon recognised the reasons of failure, and progress was afterwards rapid. Thus this English invention has made much greater progress on the Continent than it has done at home, and the great majority of large B.F. gas engines have had their rise in Belgium or Germany.

The first really large engine was one of about 650 HP. at the Paris Exhibition of 1900. It was of horizontal type, with a blowing cylinder

in line with the working cylinder. This was made by the Cockerill Co. It was single acting, the piston was 4 ft. 33% in. diameter, with a stroke of 4 ft. in. It was designed to develop its power at 80 revolutions per minute, which, with an initial explosive pressure of from 310 to 325 lb. per square inch, produced a pressure of 300 tons on the piston at each explosion. In Germany, England, and America big engines were being simultaneously built to utilise blast furnace gas, and thus several distinct types have been developed. The single acting engine has given place generally to the double acting, and the design resembles less that of the old gas engines, and much more that of the steam engine in respect of general arrangement, but particularly of the valve gears. One of the big engines of this

Fig. 222.—500 BHP. Gas Engine. End View.

double acting type is of 1,200 HP. at 80 revolutions per minute, built by the Cockerill Co. for their own blast furnaces. The engine piston is 4 ft. 3 in. diameter, the blower piston is 7 ft. 4 in. diameter, and their stroke is 4 ft. 7 in. It supplies air at a pressure of 23.6 in. of mercury.

Several of the Continental designs are being manufactured by British firms. A few may be described.

The 500 BHP. gas engine of Richardsons, Westgarth & Co., Ltd., Figs. 220-222, is of Cockerill type, and has two tandem horizontal 235 x 31 in. cylinders, each of which

is double acting, with piston rods, glands, crossheads, and guides, as in a steam engine. The gas and air valves are placed on the top of the cylinders, the exhaust valves below. The whole of each set of valves are thus worked by one cam on a long side shaft. The cylinder heads are removable. The cylinder body and ends, the stuffing boxes and the valve chambers are all thoroughly water jacketed.

The engine framing consists of two continuous girders extending from the crank pedestal to the extreme end of the engine, and the cylinders are attached by one end only to the frame and are thus free to expand or contract. The Otto cycle being worked to, there are therefore two impulses per revolution.

By means of a telescopic pipe and a pump water is circulated through the pistons and rods at a pressure of 40 lb. Compression is constant, and gas admission is controlled by the

governor.

Magneto-ignition is used, and the engine is started from a compressed-air reservoir. When using B.F. gas, compression is kept high, and the consumption is about 95 cubic feet per BHP. hour. The same firm's 1,200 IHP. engine has four cylinders and two cranks. The two-cylinder engine has ordinarily a cyclic variation not over, and can be used to drive alternators in parallel. An installation of large engines, by Richardsons, Westgarth & Co., Ltd., is also shown in Fig. 223, Plate XII.

Crossley Bros., Ltd., build a double-acting twocylinder engine on steam-engine lines, the main cylinder casing being cast with the front end valve chambers, while the rear valve chambers are carried by a back end casting which is attached to the main casting by long bolts. The working liner of the cylinder is held between the fore end of the main casting and the back casting by these bolts. Full water cooling of cylinder, heads, stuffing boxes, pistons, and rods is arranged. Governing is by moderated charges except at very light loads when some explosions may be omitted.

The Körting gas engine is made by Mather & Platt. As much as 1,000 HP. is obtained from one cylinder in the large sizes. The engine is supplied with its explosive charge

by separate pumps. It is double acting, with explosions every stroke at each end of the cylinder, the exhaust taking place at a ring of ports which are uncovered by a piston of great length. The new charge thus follows the old one. The first part of the new charge is pure air which scavenges out the last burnt charge, when the gas enters a little later and mixes with the air. The mixed charge is compressed on the return of the piston, and fired electrically at two points.

The air and gas pumps are placed on the same rod and parallel with the main cylinder, and are driven by a crank from the main shaft. The admission valves are above the cylinder, the air entering by an annular space round the gas entry. Full water cooling is provided to all parts, the water being led through the crosshead and piston rod to the piston, and thence by a pipe inside the piston rod and away by the opposite side of the crosshead. Governing is by moderated charges, and starting is effected by compressed air at 120 lb. The claims for this type of engine include the absence of an exhaust valve, the smaller cylinder, due to the two cycle arrangements, the scavenging and steady running.

To B. H. Thwaite is due the coupling of horizontal gas cylinders with vertical air-blowing tubes for blast furnace blowing. A descrip

tion of this will be found under the head of Blast Engines.

In the engine of the Premier Gas Engine Co., Ltd., the cylinders, two in tandem, are single acting, and there is a scavenging cylinder or pump placed in an inclined position above. one cylinder and driven off the connecting rod. Air at 3 lb. pressure is discharged into the two cylinders alternately, the ordinary valves of the working cylinders providing all that is necessary to distribute this scavenging charge. In a blowing engine with a 72-inch blowing tub the gas cylinders are 38 inches diameter, and the power is about 1,000 HP. The stroke is 4 feet, and the speed 75 to 85 revolutions per minute. The scavenging cylinder is omitted, the scavenging air coming from the blowing tub. Types of "Premier" engines are illustrated in Figs. 224 and 225, Plate XIII.

The analyses of gases given in the table below are culled from various sources.

Furnaces making ferro-manganese, spiegel, and basic pig usually produce more dusty gas than furnaces making foundry and Bessemer pig.

In certain tests by M. Greiner there were produced from two furnaces which turned out 300 tons of pig per day a weight of 13 tons of heavy dust, or 10 grammes per cubic meter, and 3 tons of light dust, or 2-2 grammes per cubic meter.

From 2 to 3 grammes of dust are often found in washed gas. Mr Keith of Middlesborough gives 2 to 5 tons of dust from a make of 600 to 700 tons of Cleveland iron, but another furnace gave scarcely 1 ton from a make of 600 tons. The blast weighs about 4.47 times the weight of coke used, and the effluent gases (neglecting steam) about 5.83 times the coke.

All processes of cleaning blast furnace gas are based on Thwaite's methods. The necessary operations are cooling to eliminate the steam, washing to take out the dust, and a final passage through a filter to eliminate the last few particles of finest dust. Among other things Thwaite has employed a fan to propel the gas, and the centrifugal action of this to throw the dust against the outer casing, a stream of water being admitted to flow through the fan and ensure washing away of the dust now thoroughly wetted. Thwaite lays particular stress on his box washer. He passes the gas first through a series of two box washers so arranged that the gas enters one of them and is partially purified and cooled. It then passes through the second washer which further purifies and cools it, and then it is suitable for introduction to the centrifugal fan on the blades of which jets or sprays of water are maintained. The fan is capable of drawing the gas if necessary through 20 inches head of water. The top of the washer box may be a water tank in which hard water is heated until it deposits its lime salts, and the water can then be used in the jackets of the gas engines, or soft water may be heated for steam purposes.

Partitions are fixed to distribute the flow of gas evenly, and hung from the partitions are perforated frames which can be agitated from outside the box. The draught of the fan sucks the gas through these perforated frames in fine streams or small bubbles, and an elongated seal permits the deposited dirt to be drawn from the box bottom. The water acts better as a cleanser when it becomes thickened by deposit, and the addition of some liquid tar is generally useful. Some tar oil may also be added with the water spray in the fan to prevent adhesion of the dust to the fan blades. From the fan the gas is forced through a solid resistance filter to a gasholder, and any excess

of gas is allowed to escape back again to the suction side of the fan, the escape valve being operated by the action of the fully-filled holder.

The filter is of circular form, with filling of sawdust or shavings. The layers are circular in plan enclosing a space into which the gas enters after passing the 6-inch wall of filling. The sawdust is held by a wire netting, and a means is provided for agitating the sawdust to help it to shake off its collected dust. This agitation refers to the looser material in the central space of the filter, not to the more tightly packed annular 6-inch wall.

Only by thorough cleaning can B.F. gas be used in gas engines with satisfaction, but the cleaned gas is an ideal fuel. It contains little hydrogen, and will therefore admit of high compression without danger of pre-ignition, and it burns when ignited with sufficient slowness as to provide a time for the piston to travel away nearly or quite as fast as the volume of gas in the cylinder is increased by the combustion and added temperature. Thus the heat of combustion is directly converted to work, and efficiency attained.

Blast Furnace Hoist.-Early blast furnaces were charged by hand, by men who tipped the ore from wheelbarrows after it had been brought up an inclined way by a loaded truck, ascending while an empty one came down. Later, vertical lifts have been used freely, for which various motive agencies have been employed, hydraulic and pneumatic, and balanced lifts, besides direct acting steam hoists.

The

With the intense driving of furnaces now prevalent, the vertical lift is disappearing. limitations are largely those of human endurance, and it is doubtful if more than about 1,500 tons of material can possibly be dealt with by this method in twenty-four hours, which is not adequate for present requirements. Hence the return to the inclined plane now so apparent, but in greatly improved forms. Skips or cars are now hoisted to the top of the furnace, and are discharged automatically. To these, when the locality is favourable, the long-armed Brown cranes become valuable adjuncts in the work of unloading vessels or cars, and conveying the ore to the place of storage. The rope