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SUPERHEATERS FOR STEAM VEHICLES.

2nd January 1919, Page 14
2nd January 1919
Page 14
Page 15
Page 14, 2nd January 1919 — SUPERHEATERS FOR STEAM VEHICLES.
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Fourth Article : Further .Patent Superheater Designs. Points When Employing a Superheater

THE PATENT TYPE of superheater fitted by Messrs. J. and H. McLaren to their steam tractor is shown in Fig. 11. "Wet" steam enters at A and circulates round the two top spiral coils, after which it .passes to the two bottom spiral coils, and, having completed its course through the latter, it finally passes to the cylinders through the pipe (B).

In order to ensure that the whole of the hot gases pass through the coils, instead of part passing round them, asbestos rope pa,ckings (0) are laid between the . coils. The defects in this type of superheater are as follow :—The coils are not drained ; the steam is not sufficiently sub-divided ; the joints are inside the smokebox and are therefore liable to be burnt out ; the coils are not easily accessible for cleaning or repairing, and they are riot sufficiently elastic to withstand the vibration.

Another type of superheater, in which zigzag coils are used, is Marshall's patent superheater illustrated in Fig. 12. steam enters one compartment of the top header, passes down the front series of coils to the bottom 'header, and then back again to the top header by the back series of coils, as shown by the arruirs. From the top header it passes to the cylinders. Long bolts extending across the smokebox stay the coils to resist, vibration. The coils are drained from the bottom header, and are easily cleaned from the smokebox door.

This arrangement does not call for any alteration to the shape of the smokebox, but, although the coils are arranged so that all the boiler tubes can be swept by passing the tube brush between the coils, it inter feres with expanding or renewing the boner tubes. Again, the coils are below the water level of the boiler, and there are too many joints inside the. smokebox.

In Fowler's patent superheater, illustrated in Fig. 13, looped coils are employed similar to those used on the locomotive high-degree superheaters, except that they are placed wholly in the smokebox instead of projecting inte,the boiler tubes. No alteration is made to the shape of the smokebox. The header is so sub-divided that the steam passes thiough a series of thercoils, which gives it a lengthy passage -through the superheater. The coils are supported by the bolts (A) passing across the smokebox, and at B steam pipes with jets are carried through between the coils so that they can be readily cleaned. The coils are not properly drained, and some of them are below the water level of the boiler, and also interfere with the tubes. With the small size of smokebox on steam wagon boilers, it is not possible to fit a superheater of this type with sufficient heating surface to obtain a degree of superheat high enough to give the best results.

• A very good design of smokebox superheater is illustrated in Fig. 14, which shows Davey-Paxmaafs patent arrangement. It consists of a series of zigzag coils

connected to top and bottom headers, arranged above the boiler tubes in an extension of the snaokebox. Steam enters the top header at A, and passes down to the bottom header, then up to the top header and down again to the bottom header, finally leaving the superheater at B. The baffle plates (U and l)) cause the hot gases to flow parallel to, and in the opposite direction to, the flow of the steam through the superheater.

The smokebox is also provided with two dampers (E and F) worked simultaneously. When E is in the vertical position, F is horizontal, and the hot gases pass over the superheater coils. When E is placed horizontal and. F vertical, the gases are by-passed straight to the chimney, which is an advantage when raising steam. As the headers are fixed on the side of the smokebox, the coils are readily accessible for expanding or renewing them. The only drawback is that it entails rather awkward inlet and outlet

pipes. Practically all the conditions for successful superheating are fulfilled by this arrangement. Having dealt with the design of superheaters, and the special features in the construction of the other parts of the engine, it is now necessary to refer to certain points in the working of the engines which must be regarded.

In the first place, special attention must be paid to the lubrication of the cylinders. Good mineral oil must be used. Vegetable oil would decompose at the high tempera, ture and carbonize in the cylinders. The oil must be delivered into the steam after it has passed through the super. heater, and an efficient type of oil pump is necessary. There are plenty of good brands of oil on the market and also suitable types of oil pumps. The amount of cylinder oil required by an. engine using superheated steam is slightly more than that required by one using wet steam. In locomotive practice it has been found that the increased consumption is about 5 per cent. for superheater engines.

It is also necessary to use good smokeless steam coal to obtain the best results, otherwise the coils would become quickly coated with soot, which reduces their efficiency, and they would require constant cleaning. Coke is not suitable for a loco-type boiler with superheater, because it does not give off hot gases like coal, and, consequently, there is not sufficient heat in the smokebox to be of practical value. In fact, coke is not suitable for a loco-type boiler without a superheater. Provision must be made for readily cleaning the coils, and this is easily done by fitting: a. short length of flexible metallic pipe on one of the superheater headers, the free end being provided with a nozzle which is so arranged that the steam is spread out like a fan and so reaches all the surface of the coils. The nozzle should be long enough to reach to the badk oi the coils, and be provided with suitable grips for handling it. When the boiler is held for repairs, it is advisable to scrape the coils, especially if it has been necessary to use inferior coal. The soot sometimes gets baked on and is not readily removed with the steam jet. . I Occasionally the coils will require re-expanding in the headers, and, if the engine is properlymanipulated, no other attention should be required. Experience with locomotives has proved that, even with high-degree superheaters, they are ordinarily not the things most in need of attention when a loco is held for repairs.

There are two special points which must be observed daily when driving a. motor with a superheater. Before starting up the engine, when about to commence a run, the drain cock on the bottom superheater header must be opened to blow out any water which might be there. So soon as the steam whieh, issues from this valve has become invisible, it is a proof that it is quite dry, and the engine can then be started. When the motor has finished its day's work, the valve between the boiler and the superheater should be closed and not opened until steam is going to be raised

for the next trip. BIEPHESTITS.

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