This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
About twenty-five years ago it became recognised that the fire grate consumed fuel too wastefully, and the "slow-combustion" grate became famous. The best known of these had for its chief feature the lowering of the fire-box down to the hearth, while the bottom of the fire was formed with a solid firebrick slab. This was a distinct improvement, and no maker has since reverted to the high fire. The solid firebrick bottom gave the required degree of slow combustion, but it had two drawbacks. Firstly, it made the fire always slow, whereas it was only required to be so when burning brightly. Secondly, the ashes and the debris of the fire remained in the fire-box and collected until, with poor coal, the fire appeared to be more ashes than anything else. To rake the fire out was not to be thought of, consequently by evening time, when the fire should look its best, it was very unsatisfactory.
About this time Dr. Pridgin Teale, in a lecture which he delivered relating to economy of coal in house fires, laid down some excellent rules, a summary of which is as follows:- That except for the front bars and bottom bars there should be no metal in contact with the fire. The back and sides of the fire-box (composed of firebrick slabs) should, above the level of the fire, lean or slope over the fire and not slope away from it. The lean over of the back should be at an angle of 70 degrees; the sides much less. The depth of the fire, front to back, should not be less than 9 inches. The sides of the fire, all the way up, should be an angle from front to back of 45 degrees. The bottom of the fire should be a grating, to allow the ashes to fall through, but there must be a plate to close in the front opening of the ash pit. This plate was designated an "Economiser," as by its use air was prevented from passing through the bottom bars and a slow-combustion fire resulted.
Fig. 87 shows a fireplace made on Dr. Teale's design, and although the sketch suggests little that is attractive, the effectiveness of the grate is beyond question. Realising that the heating efficiency of a grate must lie in the extent to which it radiates heat (whether to warm occupants or the inanimate objects and parts of a room), it was natural to seek for this in the form or arrangement of the firebricks. To avoid the use of iron in every possible way is essential, as iron is not only a rapid conductor of heat, but its conductivity (in grates) keeps the fire, in contact with it, in a semi-dead state. The form of the back brick, the section of which has been likened to a "dog's hind-leg," has two distinct effects in heat distribution, as it both radiates and deflects heat rays. Fig. 88 will serve to describe this, the solid lines representing heat radiated in direct lines downwards from the intensely hot brick; while the broken lines show the deflection or rebounding of heat rays that are projected from the top of the glowing fuel. Very brief experiment will show that heat rays will strike and rebound from a surface (at an angle according to that of the surface) in the same way as rays of light can be deflected by a polished surface. Thus the overhanging brick affords heat in two ways, while, in addition, its leaning over the fire causes it to become intensely hot, so much so as to burn quite clean and be nearly as hot and effective as the glowing fuel beneath it. Compared with the old Register grate, it may be fairly computed that this brick alone has increased effectiveness quite four times or more. The heat rays from the brick (the solid lines) being projected towards the floor is a most effective feature in itself.
The design of the brick suggested by Dr. Teale has been departed from in Fig. 88, so far as that part hidden by the fire is concerned. The brick is shown coming forward at the bottom, and having quite a narrow grating between it and the front bars. This is an improvement that has been introduced with the idea of lessening the bulk of fuel at the back bottom corner, where it serves no specially good purpose. The original Teale brick had a quite vertical back surface in the fire-box.
In arranging that the side firebricks of the fire-box, and particularly above the fire, be at a 45 degree angle, the intention was to project radiant rays into the room, to the right and left at a 45 degree angle, as Fig. 89 will show. This result is very effectually obtained.
In Fig. 88 will be seen, in section, the "Economiser." This is an ornamental casting that is made to rest on the hearth and come closely against the bottom bar of the front grating. It is simplicity itself, but its effect is all important. As already stated, slow combustion is obtained by preventing air from passing through the bottom of the fire. This not only reduces the rate of combustion, but it also prevents the bottom of the fire being a bright glowing part for no good purpose. In obtaining the slowness, however, it is important that the fire shall not always be slow, and to hasten combustion, when needed, the economiser can be drawn out. It is a most effective detail, and figures, either directly or indirectly, in all modern grates.
The grate having the largest number of novel practical features in it is that made by the Eagle Range Company. This grate has the Teale improvements (as practically all grates now have), but in addition there are three pairs of hinged doors in the front that can be operated to produce certain desired effects. Fig. 90 shows this grate, and the illustration A represents it with all doors closed. This is to afford a good appearance in summer when the fire is not in use. B shows the lower pair of doors open, which makes the grate a stove for the time being, with a fierce draught to hasten the fire, when first lighting or whenever it may need reviving. C shows the grate as ordinarily used when the fire is in a normally bright condition. D shows the lower doors only closed, and this is the most novel if not the most useful feature of all.