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.
As at this time of the year our heating apparatus, for artificial culture, is inactive operation, a few practical remarks relating thereto may be of service to some of your readers.
We all know that fuel of any kind requires to be heated to a certain degree before combustion commences, and when it does take place, heat is given out, and so continues to be, more or less, according to the bulk of material and supply of oxygen derived from the atmosphere, as draught to the furnace. Whatever principle or kind of mechanism we may adopt, it becomes a matter of consideration how to secure and economize this subtle and quickly radiating medium, so as to apply it to the purposes intended; consequently it is evident the machinery that will collect and freely distribute the greatest amount must be the best, and, also, if the whole which is evolved can be secured, we shall have arrived at the maximum of our wishes. Heat from combustion may influence the surrounding atmosphere by direct radiation, or contact with some conducting material, which afterwards distributes it to the desired spot, but as we cannot accept the former, on account of the noxious gases that are liberated therewith, we have to inclose the fire inside a furnace of some kind, and so pass off the deleterious parts by a flue or chimney.
The Brick Flue is the most primitive, direct, and simple method of warming horticultural structures. The heat in this case is conveyed along the interior of what may be called an elongated chimney, generally placed nearly horizontal on the length of the house, and, as the bricks are composed of partially conducting material, it becomes absorbed, and finally liberated on the opposite side, when it is radiated over the entire cubic bulk of the house. When heat has a free exit from its source, and is allowed to pass quickly away into the external atmosphere, a great portion is wasted, which renders necessary the precaution of preventing such loss. If there is a large surface exposed to the action of radiation from the fire, the heat is collected in the chamber and is more effectually compressed against the brickwork and through it, and if the flue is comparatively capacious, the same thing takes place throughout the whole length; but when the furnace is small, and the flue contracted, there is very little chance of gaining this compression, and the arrangement becomes little better that a channel by which to convey the gases, and heat also, away.
For a small greenhouse, the fire-chamber ought not to be smaller than two feet long, two feet high, and eighteen inches wide; and the flue twelve inches high by eight inches wide, inside measure, and for a larger house in proportion. Three feet long, and correspondingly roomy other ways, will, however, be as large as is advisable to construct any furnace. Owing to the tendency of heat to pass upwards, the flue ought to be placed as near the front of the house as may be convenient, continued across it at the farthest end, and out to the chimney, or back again near the middle, and so on to the place of beginning; but in such case care must be used, so as to secure a gentle rise on the whole length, or the heavy gases and occasional damp will impede the draught, and cause danger from bursting, or a deficiency of circulation. The chimney, also, should be raised above the highest part of the house, and not be exposed to the whirl of cross currents. For the same reasons it is better to keep the furnace somewhat below the level of the flue, thereby enabling the heat to radiate more freely from the continual supply, and all bends in the flue should be rounded.
As common bricks soon burn out, and are liable to crack, fire-bricks ought to be used for constructing all parts around and adjacent to the fire; and fire-clay tiles, which are now manufactured of all sizes, are the best for covers to the flue. It is scarcely necessary to add, all materials should be of the best quality, and the joints closely and smoothly pointed.
With regard to the relative amount of cubic bulk to be heated, if we take for example a house forty feet long, twelve feet wide, five feet high in front, and fourteen feet high at the back, it will contain a little over 4,500 feet to be acted on. Allowing the temperature outside to be 10p or 20° below zero, and the house a lean-to.roof, only ordinarily situated as to protection from cold winds, a flue of the above dimensions, and furnace of the smaller capacity, would raise the internal air to 55° or 60° at midnight; but if the house were to be extended ten feet longer, it would be reduced to 45° or 50°. With a further extension, another fire would be required, and so on according to length. If the cubic volume be greater, without increase of length, the size of both furnace and flue ought to correspond.
Heating by Hot Water is of much more recent introduction than brick flues, and is more worthy of the age we live in. However well the work on flues may be executed, or good the materia], they are subject to crack and crumble away, and need constant repairs, to say nothing of the injurious effects caused by the escape of noxious gases, and the destruction of oxygen by the often nearly red-hot bricks, all of which are objectionable considerations. Fortunately, hot water obviates all these evils, and gives us an atmosphere as pure as can be required, and a warmer temperature without any chemical change worthy of notice, with the further advantage of being easily and securely worked by the most ignorant operator, if he only knows how to light and keep a fire going. When any improved principle of practical utility is introduced to the world, there are frequently many and various ingenious devices brought out to accomplish the same end. Such has been the case with our present subject.
It should be understood here, that the boiler is the prime motor by which the circulation is secured, and the pipes are simply conductors of the water, and radiators of the heat. Of boilers there are many kinds, and if we were to believe the inventors, there is no worst among them. There has not been any reliable set of experiments to test the relative merits of each, neither is it likely there will be at present; but we have practical experience and scientific principles to assist in determining somewhat accurately the amount of work which the different constructions can accomplish with a certain amount of fuel. Taking these two guides as a test point, and we demonstrate that a large surface of metal ought to be exposed to the direct action of the fire, and not only so, but the heat evolved should so come into contact with all parts, that the lines-of radiation may strike, as they emerge from the combustion, upon the inner surfaces, so as to cause a refraction, and, consequently, a compression of the heat, which of itself and alone assists in producing more, and therefore a greater amount with a limited supply of fuel.
A large radiating surface, then, is the principal advantage, and all others over which the draft may be conducted have only an auxiliary effect, and are of secondary consideration. To make our subject more clear, it may be well to explain the construction of a few of the best boilers that have been submitted to public approval, and which the writer has fully tested by practical operation.
The Saddle Boiler is well known, and fifteen or twenty years ago was considered the best in use. When of the right model, it is a good boiler if properly set and constructed, as in Fig. 1. The end opposite to the front is closed; there are two open spaces, one on each side, for the. escape of smoke, and the whole is inclosed in brickwork, with a space left all around, and a flue leading to the chimney. There are many modifications of this principle, but none that we have seen, of such construction, are more powerful than this.
The Cylinder Boiler, as its name indicates, is an upright chamber, having double casings, between which the water is contained; there is a flow pipe at the top, and a return one on the side near to the bottom, while the smoke is liberated by another pipe on the opposite side, immediately under the upper cap, which is also double, and connected with the upright cylinder. Fig. 2 is a longitudinal cross section. The fire in this model is inside the boiler, and there is the advantage of it occupying little space, besides which, there is no brickwork required. This apparatus will do its work very well; but, owing to the perpendicular form, the heat strikes intensely upon the top or horizontal part, and the sides are only slightly acted on, , which produces an unsteady flow of the water in the pipes, and danger of its flowing over at the feed tank when not even at the boiling point. It is best adapted for very small greenhouses, where a more powerful apparatus is not needed. The deviations from this are the many varieties of corrugated cylinders, and upright tubular boilers, all of which are very little superior. We may also include the Horizontal Cylinder, or Cannon Boiler Fig. 3, which is only the above, with a slight alteration, laid down on its side.
Here there is the great objection of much space being required; it is cumbersome, and has to be inclosed in a vault of brickwork. In the example given, the fire first plays on the under side to the back, returns again through the middle, and again returns back, more elevated, to the chimney. This is converting nearly all the passages for heat into flues, and, consequently, although it travels a long distance while in connection with the boiler, there is very little radiating power. This construction will work, we admit, but it is far from being the best.