DRYING BRICK WITH EXHAUST STEAM FROM THE ENGINE.
In whatever system of steam drying is employed, it is essen tial that the pipes should be of ample size, and that the bends and turns in the pipes be as few as possible, in order to obviate as much as can be done the resistance to the steam, and avoid back pressure upon the engine.
A right-angled turn has the same effect in reducing the velocity as increasing its length about forty times its diameter would have, and consequently these should be avoided when possible.
The entire economy of drying with exhaust steam is neutral ized if the back pressure reaches even as low as four pounds, as the extra fuel required to run the engine will, with loss of power and all things considered, be more costly than to em ploy some separate system of drying.
Of course, with whatever system of steam drying is used, it will be necessary to make ample provision for the use of live steam with which to heat the drier at times when the engine is not in use.
By collecting the water condensed in the pipes in a closed tank and pumping it while hot back into the boiler, saving in fuel should offset any loss from reasonable back pressure. From a paper by Prof. R. C. Carpenter* we glean the following points of practical information in relation to the construction of steam work for driers : the first is the effect of air-traps, the second is the effect of the condensed water, the third is the expansion due to the heating of the pipes.
It is found in steam work that if we have a convex bend in a pipe, as in Fig. 45, air will gather in the upper portion and no amount of pressure will drive it out.
Sometimes it is necessary to have such bends, and in that case a small air-valve will be essential, which can be opened and the air be allowed to escape.
If the bend is the reverse of this, as shown in Fig. 46, con densed water will gather in the low portion ; this may in time be blown out or evaporated by the steam, but its action will be to condense the steam and cause pounding and hammering in the pipes. If such a construction is necessary, a drip-valve must be put in such a place and opened from time to time to allow the water to escape.
To avoid both of these evils as much as possible the pipes should ascend to the highest point at once, or in one continuous run from the engine. A vessel with a water-gauge and a dis charge-valve should be placed so that this water can drain into it, and as soon as it is full, the engineer can empty it. As soon as the room to be heated is reached the radiating surface is in creased to the desired amount, as will be explained later on, and the pipes are then made to descend at least one inch in 12 feet. If the engine is higher than the dry kiln, so that the
pipes can descend continuously, it will be much better. The best results in steam heating are always obtained when the con densed water flows with the steam.
To avoid the ill effects of expansion when the pipes cannot be allowed to expand or contract freely we can use especial ex pansion joints, but better still an arrangement consisting of three elbows.
The method of using the elbows is shown in Fig. 47 ; thus, if the ends E and A are fixed, the effect of expansion in the pipe E D will be to slightly unscrew the elbow C, and the lengthening of the pipe A B will also tend to unscrew the elbow C, but at the other screw thread.
The radiating surfaces are best made by connecting two large pieces of pipe called headers or manifolds together. It will not answer to connect these manifolds by straight pieces of pipe as shown in Fig. 48, for the reason that unequal expansion of these different lengths would certainly take place, in which case they would be broken, but an elbow must be inserted in each length of pipe so as to permit of independent expansion. The arrangement would be somewhat as indicated in Fig. 49.
The pipes that are used in steam-heating are then of two kinds, or, rather, perform two offices : One kind of pipe con veys the steam to the place where the steam is wanted ; this we will call a steam main. -The other pipes expose as much sur face as possible to the cooling influence of the air, and their office is to give off heat ; we will term these heating-pipes, and their outside surface the heating surface. We usually reckon heating surface in square feet, as it is much more convenient. Now we will get one square foot of heating surface with the following lengths of different sized pipes : 3 inch pipe, 3.6 feet; r inch pipe, 2.9 feet ; t inch pipe, 2.3 feet; t inch pipe, 1.8 feet ; 2 inch pipe, 1.6 feet.
It is evident at once that any escaping heat from main pipes is a loss, and should be prevented as far as possible by wrapping with material to prevent the escape of heat. The size of the main pipes, as we have seen, will have to be ltrger as they are increased in length.
With a drying kiln arranged in this way and with the pipes proportioned as explained there is no danger of failure, and the expense can be quite accurately estimated in advance.
The method of forcing an air-current over steam pipes at one end and putting no pipes under the dryer, is one that is even more economical in its results than the method given above. It will require much less heating surface and is certain to be successful.