This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
One of the most primitive boilers was in form like a cone, and was in use thirty years since. This apparatus gave satisfaction at the time. It was simply a double-cased cone, with the water between, a flow pipe at the top, a return one at the side near the base, and the whole was inclosed in a hollow chamber of brickwork. The inner surface of this construction was well formed for the radiation of heat, providing the fire had come into direct contact with the inside, and the outlet been from the upper part of this interior portion; but the first examples of this kind were not so, for the fire was in a grate below, and the boiler raised some distance above, notwithstanding which a considerable radiation, with compression, was produced, and hence the good quality of this original model. After a time the error as to the position of the fire was corrected, and we had the cone elongated at the base, a door made at the side, a grate fixed inside below, fire-bricks around where the fire was built, and the smoke-pipe placed near the top. This was a great improvement, but not yet complete. Various adjuncts were afterwards added to the interior space, and water passages connecting with the main casings.
Each of these alterations have been a progressive step towards connecting the two best models we have so far had a knowledge of, viz., the Cone and Saddle forms, and we have, at last, got the two so effectually combined as to make one boiler do nearly the work that two of these would do separately, with the extra advantages of occupying very little room, no brickwork being required; a small grate; large space for fuel, (which enables the operator to work a steady heat;) sufficient volume for radiation, and a large surface furnished in the right position for direct contact Fig. 4 is an upright cross section of this combination. After having practically operated with nearly all kinds, I have not found any to be so efficient as this, either for power, steadiness of action, or the little attention required. What is hitherto mentioned of warming by hot water, is simply a circulation gained by specific gravity without any confined pressure. A given bulk of cold water is heavier than the same cubic contents when heated; and, of course, when there is a difference in temperature between the upper and lower pipes, and a free egress around the boiler, the heavier and colder part will fall to the lowest point, and the heated, or lighter, is drawn onwards along the upper, until an equilibrium of temperature is arrived at.
The action of the fire, when working, is continually heating the water in the boiler, which travels along the flow pipe, cools on its passage onwards, and returns to the boiler much reduced in temperature, and consequently heavier. In this principle there is always a feed-tank placed on the length of the pipes, which is an egress in case of the water boiling, or at the point of forming steam, so that an explosion or any amount of dangerous pressure never can take place; for so long as the water is kept below the boiling point, the stream is only a placid circulation. The water may be boiled out of the pipes by carelessness, but no danger arises therefrom. When such does occur, steam will be evolved into the house; the fire ought to be drawn out immediately, the pipes refilled, and set to work as before.
Various kinds of material have been tried for circulating water, but nothing answers so well as copper or cast-iron pipes; and as the former are more expensive, without any counterbalancing benefit, the latter are almost universally adopted. These are generally made four inches in diameter, and jointed together with iron filings and sal-ammoniac. Taking this size as a guide, it may be of service to speak of the proportion in length, according to the amount of warmth required, and the cubic bulk to be heated. Some allowance ought to be made for exposed situations and double span houses, but taking a medium position, and an ordinary lean-to house, where the thermometer may fall sometimes to 10° or 20° below zero, a safe calculation may be considered to be one lineal foot of four-inch pipe for every fifteen cubic feet in the house, when the inside temperature is required to reach 70°, at midnight, (which is as high as is ever wanted for any purpose.) Or, if only 45° to 50°, which is suitable for most common greenhouse culture, then one foot of pipe to twenty-five cubic feet will be sufficient.
The heat of the water is taken at 200°, and from these reckonings the intervening differences of other requirements may be estimated.
There are other methods of heating, such as radiating from large surfaces of iron cylinders, which parch the air, and, therefore, are more objectionable than the common flue; High Pressure, or confining the water in small pipes without any outlet, when they may be heated to some 400°; but there is great danger of bursting in this method, even with care, and it is always unsafe. The Open Tank has been much spoken of, and is good, when a more than usual ratio of moisture is needed in the atmosphere ; but as the same can be accomplished by open flanges fixed on the pipes, this latter is not worth discussing.
[There arc other Hot Water boilers of recent introduction, some of which seem to possess merit; but they have not yet been tested so as to be spoken of confidently. There is much room for improvement, however, in the best of them. We have seen boilers which we consider, in some important respects, superior to either of those above named. The Cone Boiler is undoubtedly the best of those mentioned by Mr. Chorlton. - Ed].