Heating
This, in connection with horticultural structures, is an absolute necessity for securing and regulating temperatures artificially, to suit the requirements of exotic plants; and for the production of flowers, fruits, and vegetables out of their natural season. Its effects may be derived from fermenting material placed inside the structure, or from causes which arise as the product of combustion by fire in the immediate vicinity, transmitted, by means of water or air, to wherever it is desired. These sources of heat, either used separately or in combination, afford the requisite temperatures for different plants, according as their admission is regulated to the various houses in which the latter are grown. Fermenting material evolves a considerable amount of heat, but by a slower process than combustion, as usually understood. A more genial and moist temperature may be secured from the former than from fire heat, but it cannot be so readily regulated. A fermenting mixture of litter and leaves greatly encourages the growth of young plants in spring, and is also preferable for starting early Vines and fruit-trees. It is advisable, in case of severe weather, to make provision for adding fire heat as well. Gentle hotbeds are also very useful for forcing vegetables, and for the raising of seeds generally. Heating by hot air is not adapted for horticultural purposes, on account of the consequent drying of the atmosphere being very injurious to plant life. Flues are but little better; still, means may be adopted for moistening the heated air transmitted by them, where it is impracticable with a continued influx of dry air. Both of these systems may, therefore, be dismissed in reference to all glass houses of modern construction, and one of the various methods of Heating by hot water should be, in all cases, adopted. Before proceeding to notice some of the most approved boilers for requirements on a large or small scale, it may be well to refer to the principles applied to Heating, as on these being properly understood and carried out in the construction of any hot-water apparatus, success or failure in its action materially depends. Heat always has a tendency to equalise itself, by communicating part of its properties to surrounding substances until they are raised to an equal temperature, so far as the original intensity admits. If generated by the combustion of fuel inside a boiler, heat may be conveyed, by water or air, to a considerable distance; the more remote it is, the less will be the amount that reaches the further extremity. Heated air or water becomes lighter than when cold, and naturally ascends in consequence. Either may be conducted in an upward incline, or in a perpendicular or horizontal direction, but not readily downwards, on account of the disposition of all heated substances to ascend. This transmission of heat in pipes containing water is usually termed circulation, and the arrangement of the pipes throughout, to allow an unimpeded circulation, is one of the main principles of Heating, but is not sufficiently recognised in many instances. The boiler must be placed below the level of any point the heat from it is intended to reach, the upper, or flow pipe, being connected on the top. The return pipe, by which the cold water enters, should be rendered free from the action of the fire by connecting it near the base-at the front preferably-and on both sides, if this is convenient. Dips in the pipes at any point should be specially avoided, as they frequently impede free circulation-generally more so when extra heat is applied. Houses erected for various purposes may have their quantity of pipes in proportion to the heat required, and still be in connection with the same mains conducting heat to others having much higher temperatures. There are no special rules applicable, in all cases, as to how many pipes a certain house will require, so much depending on stoking, and upon the amount of heat that may be available. It is best to provide for emergencies, in the first place, by insuring a sufficiency of piping, and inserting valves in the flow and return pipes, for regulating the admission of heat. In the arrangement of a Heating apparatus, an important part should be taken by the gardener in charge, as, although the workmen employed may understand the principles on which the success of their work depends, they do not similarly understand the requirements of plants. A proper system must be adopted where there are several houses to be heated and kept at different temperatures, by one or more boilers set and connected together. Main flow and return pipes should be fixed, with a gradual rise, at a point below all others in connection, and near the central part of the distance the heat is intended to reach, so that branches may be taken on either side. All houses or pits intended for Heating separately, and irrespective of the one adjoining, should be provided with check valves near the junction with the main pipes. As heat always rises most rapidly to the highest points, it should be arranged that these are in the houses required at the highest temperatures. Pipes 4in. in diameter are those most largely used for top heat; others, only 3in., are well adapted for beds or for small houses. In houses specially devoted to plants requiring a somewhat dry atmosphere in winter-Pelargoniums, for instance-an extra 2in. pipe is sometimes fixed along the lower part of the rafters for drying the air, this being generally attended with excellent results in the production of large, clean flowers, free from damp.
Boilers. Of these, there are numerous forms in use. Some are composed of one or two series of cast-iron pipes placed in an upright or horizontal direction, and exposed to the action of the fire. Others are made of welded or wrought iron, and as they can be purchased in such a variety of sizes, and invariably answer well, their use is somewhat extensive. The more simple a boiler is in construction, and the greater surface it exposes to the direct action of fire used, the better. Tubular boilers frequently become choked with fuel amongst the pipes or tubes, and, if this is not prevented, a great loss of heat is sustained. Boilers having complicated arrangements of any sort are seldom so effective as those of a simple form, the divisions between the parts in the furnace soon becoming choked with soot. The requisite size of boiler depends on its approximate Heating power, the length of pipes connected, and the amount of heat required. It is advisable to make provision, in the first place, by fixing a Heating power considerably higher than that absolutely necessary. A great deal depends on the sort of fuel used, the rapidity of draught, and the manner of stoking. The plain Saddle Boiler is well known as being one of the oldest types, but, when properly set, still amongst the most efficient. There are various modifications of it, which claim various advantages, such as economising fuel, heat, &c. One of the most useful and efficient forms is the Flue and Terminal End Saddle It is a wrought welded boiler, made in sizes varying in length from 2ft. to 5ft., heights and transverse inside measurements being in proportion. The approximate Heating power of one of these boilers, 2ft. long, is given as 500ft. of 4in. piping; 3ft. long as 800ft.; and 5ft. long as 2000ft.; the unequal proportion, in results corresponding with length, being accounted for by the enlargement of all parts, and the variation in height and width. The flue extends nearly to the back, and through it the whole of the heat must pass from the fire. Sometimes, another flue is formed with bricks on the outside surface, as with the ordinary Saddle; at others, the whole is covered with an arch without any division. As the full surface of the boiler on both sides is exposed to heat, nearly the fullest possible amount is absorbed before reaching the chimney. The Gold Medal Boiler, so named from that award being conferred on it after a working competition at the Birmingham Exhibition in 1872, is of wrought iron, and virtually a flued Saddle with a terminal end, the flue being in the form of three chambers instead of one, as in that previously noticed. This boiler maintains a high position, and a large number are in use. Various sizes are made, ranging from 2ft. to 6ft. long, and proportionately large in all parts. The approximate Heating power of one 2ft. long, is 500ft.; that of 4ft., 1700ft.; and that of 6ft. in length, 3500ft.-all of 4in. piping. Another modification of a saddle boiler is the Cruciform. It combines great Heating power with economy of fuel; the formation of its flues, in the shape of a cross, being such as conduces to a free circulation of water, without the disadvantage of resting-places being in them for sediment. Approximate Heating power is much the same as in the Gold Medal Boiler.
The Climax is a wrought-iron saddle boiler, somewhat like the Gold Medal, but having only two chambers in the interior, instead of three. It has a waterway both at back and front, and is fed from the top instead of the furnace door. This latter arrangement has now been introduced into other forms of saddle boilers, it being considered advantageous in saving labour in stoking. There are various other modifications of the saddle in use, where more chambers are made in the crown part or on the sides. Those already noticed will be found thoroughly efficient, and, not being complicated in construction, are much to be preferred.
A powerful boiler for Heating great lengths of piping, on account of its form being specially adapted for sustaining heavy pressure, is the Improved Cornish or a, Flow Pipe; b, Return Pipe; c, Furnace Door; d, Upper Flue Door; e, Lower Flue Door and Front Stand; f, Back Stand.
Trentham Boiler, represented in Fig. 175. It consists of two wrought-iron cylinders, strongly riveted together, about 2in. of water space being allowed between them. The door frame is attached to one end, and the fire bars are inside the cylinder near the bottom, which forms an ash-pit, the upper, or larger, portion being the furnace. In fixing, the boiler is stood on two cast-iron stands, the front one forming a frame for the lower flue doors. Walls are built clear of the boiler on either side, and upper and lower flues formed by a course of fire bricks being fixed against the side of the cylinder about half-way up; an arch spanning the top from this. The heat is conducted through the centre, over the top by the upper flue, and then returns by the bottom one to the chimney, thus exposing the fullest possible amount of water space to the action of the fire. The minimum approximate Heating power of this boiler, 5 1/2ft. long by 3ft. diameter, is given as 2000ft. of 4in. piping; and one 8ft. long by 3 1/2ft. diameter is calculated to heat 5500ft. of the same sized pipes.
Tubular boilers are composed of a series of cast-iron tubes placed either in an upright or a horizontal direction, and connected together for the free circulation of water in all parts. Some are cast in one piece-an objectionable system, as any defect in casting, or an accident, may cause a leakage at any time, which, if serious, would render the whole useless. To meet this objection, and effect further improvements, Messrs. Weeks and Co., of Chelsea, have provided, in their notable and widely-used Duplex Upright tubular boilers with diaphragm, a system by which the whole may be worked together, or, in the event of an accident to one part, that half of the boiler may be removed and the other still kept working until repairs are finished. Duplicate parts are kept for replacing those which become defective, without the necessity of substituting a new boiler. Water tubes inclose the furnace, and small horizontal ones are placed as fire bars. The fuel is admitted at a circular hole in the top, which is provided with an iron cover. Rivers' Patent is a rather expensive, but a powerful, tubular boiler, which may be practically termed indestructible. It has a double row of horizontal tubes, forming a semicircle above the fire, which is fed from the furnace door. On any one of the tubes becoming defective, it can be replaced by a duplicate in a very short time, and the Heating conducted as before. These boilers are in use, in some instances, where enormous lengths of pipes are connected; but it should be stated that they have not been sufficiently tried to prove whether they would be equally satisfactory in all cases. They are noted for rapid circulation.
For Heating a small or moderate-sized house, such as those frequently possessed by amateurs, a portable Upright Cylinder Boiler and Furnace, similar to that shown in Figs. 176 and 177, is well suited. It may be placed near one end, or even inside a house, as no bricks are required for setting, and the smoke may be conducted from the flue to the outside by a circular pipe or chimney. The exterior view of these independent boilers presents a neat appearance; but it is not advisable to place them inside the plant house if it can be avoided, on account of their drying effect on the air. Independent cylinder boilers are made both with and without an extended top for adding fresh fuel.
Small greenhouses are occasionally heated with boilers warmed by gas instead of ordinary fuel. This method is rather expensive to keep sufficient water in circulation for raising or maintaining a medium temperature in a large glass house. It is, however, a convenient mode of excluding frost from small structures in places where a plentiful supply of gas can be obtained. It has an advantage for those who do not require much heat, and who are unable to attend to fires. When once started at the proper rate, the water will continue to warm and circulate so long as the gas keeps burning. A little additional water is necessary in the supply cistern occasionally. Gas boilers, of which Messrs. Wright and Co.'s is a good arrangement consist of a heating coil of pipes arranged above one or more Bunsen burners inside an inclosed case, and having a flow pipe attached, which branches into another, as shown in the illustration, and returns to the lower part of the boiler. With a small flue attached, the whole apparatus can stand in the house it has to warm, and thus the full amount of heat will be utilised. The product of combustion from a Bunsen burner is merely a slight vapour, sufficient oxygen being incorporated with the gas, so soon as it leaves the pipe, to cause its whole consumption by the fire without any soot being left. Two stoves heated by gas, and answering without flues, are Ritchie's Lux Calor and Clark's Syphon Condensing Stove, represented in Fig. 180; both having Bunsen burners attached. In the Lux Calor, the products of combustion, with the exception of carbonic acid, are condensed in tubes on either side of the burner. There is little fear of the small amount of carbonic acid gas doing injury, as, being heavier than atmospheric air, it falls to the lowest point, and is removed by any feeble current. This stove is calculated to warm any fairly good structure, not too much exposed, with an interior capacity not exceeding 1000 cubic feet. The Syphon Condensing Stove is constructed on somewhat the same lines as the Lux Calor; but, unlike it, the warm air is-after parting with the products of combustion--conveyed through a tube over the flame and into the space to be warmed; an additional amount of purity in the air being claimed by the inventor in consequence of this process. The Syphon Condensing Stove is considered useful and available for small lean-to or other houses, containing a cubic capacity of from 600ft. to 800ft., in places inaccessible to a hot-water apparatus.
Mineral oil stoves are perforce used by amateurs for excluding frost from small houses in winter. They are objectionable on account of the strong smell caused by the oil when burning, but are useful where no other means of Heating can be procured, or as a subatitute at times when a permanent apparatus gets out of order. Large oil stoves with double burners emit a considerable amount of heat, and materially raise the temperature in a small house. All boilers should be provided with a tap near the bottom, for emptying, in case of repairs, or for removing sediment that collects inside. Air taps must be fixed in the highest points of the flow pipes, or, better still, a small lead tube may be connected and carried up the inside of the house, higher than the level of any part of the apparatus. Hot-water pipes are usually made of cast iron, and the joints may be connected with various compositions, such as cement, red and white lead mixed, steel filings, &c. Each substance is largely used by different Heating engineers, the last-named being perhaps the oldest, and--when mixed with the proper proportion of sal-ammnoniac, and a little sulphur, to cause rusting--the most substantial method; it is, however, more difficult to disconnect joints made with this preparation than when red lead is employed. Strong hemp packing, in addition, will be requisite in either case. Some persons prefer joints made with flanges, and screwed together, with vulcanised indiarubber washers between. These occupy more space than ordinary sockets, but have the advantage of being easily replaced. Indiarubber rings also make good joints, and are quickly renewed or removed in cases of necessity. They are made of the proper size, and placed on the smaller end of one pipe, which is then pushed into the socket end of the other. All pipes in use should rest on firm walls or stands prepared for them; sufficient room for expansion being allowed on each side, and at the ends. To insure a free circulation, the interior must be kept free from air, and all valves should be capable of opening a waterway as nearly as possible the full size of the pipe. A water cistern must be provided, and fixed at a higher level than any part of the apparatus it has to keep supplied.