(Pl. V., figs. 1 and 2.)
[Read before the Auckland Institute, 6th October, 1873.]
During the period when low-pressure condensing steam engines were in general use, various plans were adopted to prevent waste of steam and heat in boilers by condensation or radiation. With boilers at 10lbs. to 15lbs. pressure, a simple covering of felt, protected by wood or canvas, answered sufficiently well. But when steam boilers are run, as at present, at 50lbs., 75lbs., or 100lbs. per inch, for working steam expansively, or for working compound engines, it has become much more important that the surface of a
boiler working at these high pressures should be so protected that heat and steam may not be wasted by radiation or condensation.
It is true that common felt cased with bricks will prevent much condensation and radiation, but with the serious disadvantage that a leakage from a rivet, or otherwise, causes rapid corrosion when running unobserved under the brick casing, and with the further disadvantage that the felt is destroyed in a very short time.
Various materials, such as asbestos, cloth, or fabrics saturated with chemical preparations, have been tried, but, so far as my investigations have gone, much the best material yet discovered is common felt.
The proper application of felt has been, and I believe still is, the real difficulty. Applied in contact with the surface of a steam boiler at even 50lbs. pressure, felt will need replacing about once in six months. About two years ago, to lessen this destruction of felt, I made a species of hurdles or gridirons of common hoop-iron, with wooden battens of 1 in. thick rivetted to the hoop-iron. These were placed upon the boiler, and the sheets of felt laid upon them, the upper surface of the felt being protected by canvas. This plan secured a small space between the boiler and the felt, but, though a great improvement upon the old plan, I found that in the course of about fifteen months the wooden battens had become charred and the felt a stratum of dust, slightly adhering, indeed, to the canvas back if undisturbed, but practically useless. Both substances had simply been destroyed, as before, by-too close a contact with the boiler.
A very simple contrivance now presented itself to my mind, which I immediately put in operation. I constructed an iron grid as before, but with one important difference. I placed pieces of hoop-iron AA (Figs. 1 and 2) at 10 in. distance, to lay on the circumference of the exposed portion of the boiler. I then prepared transverse pieces of hoop-iron BBB (Figs. 1 and 2), putting two double cranks in each CC (Fig. 1) 2 ¼in. high x 2 ½in. wide. I placed these transverse pieces at 10 in. distance, and rivetted each of the cranks at D (Fig. 1) to the pieces of hoop-iron intended to lay on the circumference of the boiler. When cranked, the transverse pieces were 21 in. long, about the width of an ordinary sheet of felt. Upon these cranked pieces I placed wooden battens 2 in. broad by 1 ½in. thick EE (Figs. 1 and 2), screwing them together at FFF (Fig. 1). This completed the hurdle, or grid, 21 in. wide, and of sufficient length to lay across the boiler from side to side. I next provided sheets of felt long enough to cover each grid, sewing each sheet to strong canvas 24 in. wide, thus leaving at one side a margin of canvas to lay over the sheet of felt on the adjoining grid. The 4 in. air space (which may be increased at pleasure by increasing size of cranks) between the boiler and felt, besides preventing all charring of wood or felt, is an excellent
non-conductor. To prevent a circulation of cold air from end to end of the boiler, I attached a piece of sheet-iron, cut to the circumference of the boiler, to the ends of the battens on the grids at each end of the boiler G (Fig. 1). A coat of paint on the canvas completed the apparatus.
I exhibit a full-size section of one of the grids, with felt and canvas attached, to be placed for reference in the Museum of the Society at Auckland. I have attached the felt to the grid in this section to show the apparatus complete; but, in practice, the felt and canvas only are attached to each other, but not to the grid, so that each can be stripped from the boiler without difficulty.
I come now to results. I find that loss by radiation and condensation is reduced to a minimum; the canvas covering of the boiler being always quite cool, with steam at 50lbs. As an instance, I may mention that when the engines stop at 6 o'clock p.m., with steam at 50lbs., with the felt on, at 6 o'clock next morning steam is about 25lbs.; without the felt, steam goes down to nil before 6 o'clock next morning, the dampers being in both cases the same. The saving of coal has, of course, been considerable. Nine months have elapsed since I applied this mode of felting at my own works, and I find that both wood and felt are practically uninjured.
Where compound engines are in use, this mode of felting the steam pipes leading from the boilers to the high-pressure engine, and from the high-pressure to the condensing cylinder, will be found most effectual in preventing loss by condensation or radiation, besides being comparatively indestructible. For covering steam domes and engine cylinders it is equally effective. For these latter the grids and felt may be covered with wooden battens, hooped and varnished as usual.
It will be necessary to observe, when covering pipes or other steam chambers of small diameter, that the cranked pieces of hoop-iron must be placed sufficiently near to each other to secure a space of 3 in. or 4 in. between the steam-pipe and felt. In all cases it will be found more practicable to run the pieces of plane hoop-iron round the pipes or cylinders to be covered, running the cranked pieces longitudinally, as already described in the case of steam boilers.