SUPERHEATERS FOR STEAM VEHICLES.
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Second Article :—Superheater Tubes, Their Disposition, Joints, etc.
IN TEE FIRST ARTICLE, of this brief series I showed the value of superheated steam, and explained the two systems (therhigh degree and me low degreeLpf superheating, and gave my reasons for adopting the latter system tor road vehicles.
Dealing.now with the design of the superheater and having settled on the stuokebox as being the most suitable position in which to place it, one of the first points to consider is how to arrange that it does not interfere with the ordinary, functions of the boiler, such as, for instance, sweeping, expanding, or replacing the tubes. It' is therefore evident that the top part rof the smokebox above the tubes is the best position.
Another reason in favour of this position is that it is above the water level of the boiler, and, therefore, is not liable to be flooded with water. The ordinary smokebox des not give sufficient r0Q111 to fit a superheater large enough and with the requisite amount of heating surface necessary to obtain the best result. This can be easily overcome by:extending the smokebox upwards making it box-shaped from the centre line of the boiler barrel, the top being covered in by a removable cover carrying the chimney:nth must not be taken so high, however, that it prevents the pistons being withdrawn from the cylinders. The superheater can be fitted up complete and easily placed in position from the top, arid:also it can easily be withdrawn for attention or repair. . To obtain maximum efficiency in the transmission of heat from the hot gases in the smokebox to the steam in the superheater, certain conditioneraust beaulfilled. The steam must be split up into as many small streams as it is possible, by using small diameter tubes for the coils. The coils should also be arranged in a twisted or zigzag fashion so that the steam is thoroughly mixed and brought into contact with the walls of the tubes and 80 take up the maximum amount of heat. It should also passl through the superheater at a high velocity for two reasons ; first because the faster ,,a substance passes over a hot surface the quicker it will extract the heat, and, secondly, because it helps to prevent overheating of the coils. Care must be taken, however, not to make the pas
sage through the superheater too restricted or the steam will be throttled and the pressure will drop, the objectIbeing to obtain steam in,the cylinder steamchest at the same pressure as the steam in the boiler.
Another important factor is that the steam should flow through the superheater parallel to the flow of the hot gases and in the opposite direction, so that the "wet" steam comes first into contact with the coolest part of the superheater coils and leaves them at the hottest part. -It should, therefore, enter the coils at the top of the smokebox extension and leave them at the bottom just above the boiler tubes.
By this arrangement the greatest superheat temperature is obtained. The hot gases should also be guided by suitably placed baffle-plates, so that they impinge on the superheater coils and do not flow round them.
As previously stated, the heating surface of the superheater should be of sufficient amount, and in steam motor loco.-type boilers it should be at least equal to one-third of the heating surface of the boiler proper, i.e., the firebox and the tubes. Needless to say, after the steam leaves the superheater it should not be brought into contact with the "wet" steam from the boiler, and, for this reason, there should be no connection between the superheater outlet pipe and the boiler, where the "wet" steam could leak through. When the water has finished its day's work and the engine is stopped, the superheater will still be full of steam and, as the boiler cools down, this steam will condense. More gem will also pass into it from the boiler and condense there, if the driver forgets to shut off the steam from the boiler to the superheater. The coils should therefore be so arranged that this water drains to the lowest nart of the superheater, or the bottom header, and a drain cock should be fitted to this part so that the water can be discharged before starting up the engine for its next run. If this water were not drained out of the coils, it would cause them to corrode and also the water would be liable to pass into the cylinder so soon as starting was attempted.
The coils of a superheater are subject to a certain amount of expansion owing to their length, and this must be allowed for without any fear of springing the ioints. For this reason it is best to arrange the coils in a series of loops or bends, so that, although the
,ends are held tight in the leaders, they can expand and contract like a concertina.
This is illustrated in Fig. 3. " Wet " steam enters the top header (A) and leaves at the bottom header (B). Although the ends of thecoil are rigidly held at A and B they are quite free to expand in the middle part. Again; with this shape it is quite evident that no water can lodge in any part of the coils. It would travel down each leg and bend and, filially, settle in the bottom header (B). The steam would also be well mixed in passing round the bends. • Another arrangement is illustrated in Fig. 4, the loops in this case being vertical. Although the coils would be just as free to expand as those shown in Fig. 3, and the steam would be well mixed, the arrangement has very serious defects. For instance, the steam would enter at the top, and leave at the top, thus not fulfilling the condition of the steam leaving the superheater at the hottest part. Also all condensed steam would lodge in the bottom bends, and could not he drained away. If the coils were arranged in the form of a spiral, as shown in Fig. 5, they would still be quite free to expand, and also the steam would be: well mixed owing to the whirling action, and it would leave at the hottest part. But these spiral coils have the same defect as the coils shownlin Fig. 4, thatiis, the condensed steam would lodge in the bottom of the coils. As a steam motor is subject to a great dealtof vibration, it is importantl that the coils should be well stayed, or it will be impossible to keep, them tight in the headers. But at the same time they must also be left free to expand. Referring to Fig?■)3 (this being the best type of coils to use) the correct places to stay them are at C and D. The top and bottom legs are.firmly held and, therefore, theretis no fear of the joints in the headerObeing loosened.1 They could easily expand also, for the tubes would simply slide through the stay, and the middle part is quite free to expand and contract.
With regard to the top and bottom headers, which ad as steam distributors and collectors respectively, it was originally considered necessary to make these of cast steel, but practice has shown that cast iron is quite as good if the headers are properly' designed, and it is much easier to get a sound steam-tight casting in cast iron than in cast steel.
The coils wero also at first connected to the headers by a flange, as is shown in Fig. 6, one flange being fitted to tho ends of a pair of coils and held by a. central bolt or stud, with a packing washer round each coil. It is much cheaper, however, simply to expand the tubes into the headers, as shown in Fig. 3, a screwed plug being fitted opposite each tube to allow the expander to be inserted. This type of joint is also much easier to make steam-tight.
The joints between the inlet and outlet pipes and the superheater headers should be arranged outside the smokebox to preserve them from corrosion and destruction through the action of the heat and soot, and also so that they can easily be connected or disconnected. If they were inside, the soot would get baked on the bolts or studs and it would be very difficult to disconnect them. The packing for the joints must also be of special type, corrugated brass or nickel and asbestos joint rings being found most suitable for this purpose. It is important that these pipes should be well lagged to prevent the loss of superheat by radiation. As the coils are apt to get dirty and coated with soot., provision must be made for easily cleaning them. This is best done with a steam jet, usinga flexible pipe connected to a valve fixed on one of the superheater headers by a short piece of tube extending through the smokebox plate. It is advisable to arrange thettheaders so that the coils can be reexpanded or removed and!:replaced, without taking the superheater out of the smokebox.
To facilitate the working of the superheater and to be able to shut it off' from the boiler,should%any attention to it be required. a valve Should be fitted on the inlet pipe close to the boiler. As it is usual in these engines to take the steam from the top f.the cylinder dome, in order to get it as dry as possible, the valves should be fitted' on the cylinder dome to prevent any accident which might occur, if, say, the driver forgot to open this valve when raising steam, and the condensed water 'in the superheater. was evaporated and raised pressure in it, a small safety valve should be fitted on the top header. It is also advisable to lag the smokebox, like the boiler barrel, to conserve the heat as much as possible.
HE Pli2ESTUS.