Superheating of the steam is effected by returning the steam from the receiver back by the pipe R, Fig. 2, to the upper part of the furnace and passing it through a sufficient number of superheating tubes S, whence it is taken off by the steam pipe T to the engine. The superheating tubes S are arranged and united together in the same manner as the boiler tubes, and are consequently as simple and convenient to get at for erecting and repairing.

ths

The evaporative duty of the boiler with Staffordshire slack has been 5 lbs. of water per lb. of fuel, without covering the receiver and steam pipes to prevent condensation. Steam has been raised from the time the first shovel of fire was placed in the furnace when cold, without wood or forced draught, to 10 lbs. pressure in 25 minutes, when the steam was sufficient to start the circulating pump; in 10 minutes more there was 35 lbs. pressure of steam, when the engine was started; and in 10 minutes more, being 45 minutes from the time the first shovel of fire was put in the furnace, all the machinery driven by the engine was in operation and there was sufficient steam to produce all the power required. This was with onlyths of the boiler or 460 square feet of heating surface, of the boiler being then not at work. The practice at dinner hours and other times when the engine is stopped has been to close the damper, open the firedoors, and cover the fire with ashes and slack, and work the circulating pump as slow as its construction will permit; this entirely prevents generation of steam, and in the meantime saves the tubes from overheating. For starting the engine again, the fire is stirred up and supplied with coals 5 or 10 minutes before steam is wanted, which is ample time to generate a regular and sufficient quantity of steam to commence working all the machinery driven by an engine of 60 horse power. Steam can be regularly maintained in the boiler that has now been in use for ten months, with a variation of from 10 to 15 lbs. pressure when all the work is on the engine with 40 to 55 lbs. steam in the boiler. The pressure cannot be maintained with quite the same regularity in this boiler as in ordinary boilers, on account of the comparatively small amount of steam room; at the same time it is found that a sufficient quantity of steam is made with regularity enough for all practical purposes.

For the purpose of ensuring that the pressure of steam supplied to the engine shall never exceed the intended limit, and of preventing any risk of injury to the engine by over-pressure arising from the comparatively small steam room in the boiler, the regulating valve shown in Fig. 11, Plate 9, has been designed by the writer, and is found to fulfil this object with complete success. It consists of a double-beat valve U, having a piston V below it fixed upon the same spindle and of the same area as the lower valve, and supported by a spiral spring which presses the valves open. The steam from the boiler, passing through both the valve seats, is delivered to the engine by the pipe W; at the same time it acts upon the top of the piston V, compressing the spiral spring below to a greater or less extent according to the pressure of the steam, and thus partially closing the valve and wiredrawing the steam whenever its pressure at entrance approaches the intended limit. The spiral spring is adjusted so as to hold the valve full open until this limit of pressure is nearly reached; but whenever that takes place, the partial closing of the valve checks the supply of steam and prevents the pressure of the steam supplied to the engine from rising above the intended amount. The bottom of the spiral spring is carried by a cylindrical cap X, sliding vertically and supported by the end of the weighted lever Y, which is adjusted to balance the pressure on the piston at the limit of steam pressure. As soon as the intended pressure is exceeded, this lever is depressed immediately, closing the valve entirely and shutting off the supply of steam, thus preventing any increase of pressure in the steam pipe W when the engine is standing, which would otherwise be occasioned by the accumulation of steam gradually passing through the contracted opening of the valve that serves to supply the engine when working. A safety valve Z is added on the top of the casing to make the precaution complete. This regulating valve is in constant work, and maintains the steam supplied to the engine at a uniform pressure. It may also be applied with advantage to low pressure and high pressure engines working in connexion, serving completely to regulate the limit of pressure of the steam supplied to the low pressure engine.

Mr. RUSSELL exhibited specimens of the joints and bends of the boiler tubes, and some of the burst and incrusted tubes that had been taken out of the boiler, as described in the paper. The new boiler had fully answered his expectations since it had been got to work, both in supply of steam and in safety and facility of repairs under any accident that could occur. It occupied only one sixth the space of the two Cornish boilers previously used, and burnt only 3 tons of slack per day against the previous consumption of 5 tons per day for doing the same work; one engine only was working now, instead of two, and was working up to 60 indicated horse power. The safety of the boiler was a great advantage, and they had had two or three instances of tubes bursting; but no injury was done, and the only effect was that the fire was put out by the steam and water, and the burst tube was replaced by a new one with only two hours' delay.

Mr. J. FENTON observed that the evaporative duty shown by the boiler was low, amounting to only 5 lbs. of water per lb. of slack.

Mr. RUSSELL said the boiler was at present very unfavourably circumstanced as to evaporative duty, owing to the steam pipes and receiver not being protected in any way; and much heat from the fire was also lost by passing away into the chimney. The advantage to be obtained by the new boiler in economy of fuel would be fully shown when steam of very high pressure was used, which could be safely done only with a boiler upon that construction.

Mr. G. A. EVERITT thought that the consumption of 18 tons of slack per week for an engine of 60 indicated horse power was certainly far from economical; for with Cornish boilers he was burning at his works only 16 tons of slack altogether per week for two engines of 60 nominal horse power, working up to 170 indicated horse power.

Mr. W. RICHARDSON mentioned that in Green's economiser, which he had used for several years past for heating the feed water by the waste heat passing to the chimney, consisting of a stack of upright pipes placed in the chimney flue, through which the feed water was passed on its way to the boiler, cast iron pipes were first used, but they had tried substituting wrought iron pipes, to obtain a thinner metal that would conduct the heat better; these however all became riddled through with small holes in 18 months, by the destructive action of

the condensed water, and had all to be taken out again and replaced by cast iron pipes. The same result had been experienced at several other places where wrought iron pipes had been tried in the economiser; and he feared therefore the wrought iron tubes in the boiler would be destroyed in the same way by their direct exposure to the fire.

Mr. RUSSELL said there had been such long experience of the use of wrought iron tubes in boilers that there was no fear for their durability, and they had been found to last for many years' working with regular circulation through them.

Mr. W. RICHARDSON enquired what degree of superheating was obtained by the superheating tubes in the boiler; he thought this could not be great, as they were placed in the coolest part of the furnace, furthest from the fire. He had tried superheating the steam by tubes placed in the flue beyond the boiler, but found that steam of 70 or 80 lbs. pressure could not be superheated by that arrangement, since the temperature in the flue was scarcely higher than that of the steam itself.

Mr. RUSSELL replied that at 60 or 70 lbs. pressure the steam was superheated about 220° or 240° by passing through the superheating tubes; and after taking out three sections of the boiler tubes the steam was superheated more than 500°, having a temperature of more than 900° after passing through the superheating tubes, in consequence of their having in that case a greater extent of surface exposed direct to the fire, while less of the heat was taken up by the boiler tubes.

Mr. C. W. SIEMENS observed that the amount of superheating which had been mentioned would go far to explain the low evaporative duty of the boiler; for if the steam were superheated to upwards of 900° by the superheating tubes in their present position close to the chimney, the heat passing away into the chimney must be more than 1000°, which would produce a great loss of fuel. The tubular construction of boiler, in which the entire heating surface consisted of small tubes having great strength to resist internal pressure, was he thought one that might be advantageously employed, and it had been tried in this country by Dr. Alban many years ago, with steam of 150 to 200 lbs. pressure. In the present boiler the circulating pump was the novel feature, producing an artificial circulation of the water

through the tubes; but he questioned the desirability of introducing such a system, on account of the additional complication involved, and thought the plan might be simplified by some alteration in the arrangement, so as to rely on natural circulation alone.

Mr. A. MASSELIN remarked that the superheating tubes at the top of the boiler next to the chimney were in the least effective position for superheating the steam; and would have been placed with much greater advantage at the bottom of the boiler, immediately over the furnace, if sufficiently durable to stand so close to the fire.

Mr. BENSON said the special feature of the boiler was the forced circulation of the water, to prevent the tubes ever being short of water; he was satisfied that a boiler of this construction would not last morethan five or six months, were it not for the mechanical circulation, for the tubes would soon tear themselves to pieces by unequal expansion and contraction if exposed to the risk of being alternately full and empty of water, which they would be liable to if dependent on natural circulation. In the case of water heaters that had been referred to, the tubes were soon eaten through by corrosion, and became forced out of position and twisted round, owing to the small quantity of water passing through them; but no such results had been experienced in the tubes of the boilers, because the quantity of water passed through them by the forced circulation was so much greater than that evaporated. The bottom tubes were made 14 inch diameter for one third the height of the boiler, then 14 inch for the next third, and 12 inch at the top, which gave an additional security for the bottom tubes being always thoroughly filled with water, while greater freedom of passage was allowed at the top for the mixed water and steam escaping into the receiver.

The chief improvement made since the erection of the first boiler on this construction was the mode of fixing the tubes, in such a manner as to allow of removing and replacing any tube without taking out an entire section of the boiler; a tube could now be taken out and put in in as short a time as 15 to 20 minutes, when the

a new one

boiler

was again ready for work at once.

When the boiler was properly constructed, he had found the evaporative duty was equal to that of any tubular boiler; but in the present

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