Four-inch cast-iron pipe was formerly used for hot-water heating, but two-inch wrought-iron pipe is now more commonly used for the coils, and the same size will answer for the flow pipes in houses less than 100 feet in length. In deter mining the amount of pipe to be used in a green house, it will be safe under ordinary conditions to use one square foot of pipe for three square feet of glass, when a temperature of 60° is desired, or for four square feet of glass if 50° will suffice. All of the glass in the roof, sides and ends of the house should be computed, and it will be safe also to con sider the exposed woodwork as equivalent to 20 per cent as much glass.
After determining how many feet of radiation will be required in the house, the size and number of flow pipes should be determined. As a rule, two-inch pipes can be used in houses 50 feet in length if they are not more than 20 feet wide, but they should not be used to carry more than 200 square feet of radiation, including that in the main itself. While a larger number might be used in short houses, when the boiler is some distance below the coils, the circulation will be more even when not more than two two-inch returns are supplied by a two-inch flow pipe. A two and one-half - inch flow pipe will ordi narily handle 400 square feet of radiation, in cluding its own surface. Unless the houses are rather long, it will be best not to use flow pipes within the houses larger than two and one-half inches.
It is an easy matter to adjust the radiation in a greenhouse. If a house is 20 feet wide and 100 feet long, and has two feet of glass in each of the side walls, it will require about 1,000 feet of radi ating surface to heat it to a temperature of 60° in zero weather, provided the house is reasonably well built and is not too much exposed to strong winds. From the above, it will be seen that three two-and one-half-inch flow pipes should be used. These will supply 225 square feet of radiation, while twelve two-inch returns will supply the remainder of the radiation required. In addition to the data given above, one merely needs to know that a two and one-half-inch pipe has .75 of a square foot of sur face, while a two-inch pipe has .621 of a square foot for each foot in length.
In arranging the pipes, it will ordinarily be well to place the return pipes on the walls and under the benches, or in the walks when beds are used. The flow pipes may also be under the benches, provided the returns are above the level of the heater; but a better circulation can gen erally be secured if there is one flow pipe placed on each of the plates. When more than two flows are required and are not placed under the benches, one or two may be carried on the center posts two to four feet below the ridge, and, in wide houses, one can be on each row of purlin posts. In all systems of heating, the return pipes should be given a fall of one inch in ten or fifteen feet to allow the air to escape. The flow pipes give the best circulation when they also are given a slight fall, but they can run uphill with but little loss of circulation. If the downhill system is used, it will not be necessary to use air-valves, provided the pipe which connects the system with the expansion tank leads from the highest part of the main flow pipe. It will also be well to place a valve on each of the flow pipes to the different houses so that the circulation of the water can be regulated. If the lower ends of the returns are
higher than the top of the boiler, there will be little difficulty in securing a good circulation, even though the flow pipes are on the same level. By giving the flow pipes considerable elevation, a fairly good circulation can be secured even when the returns are only slightly above the bottom of the boiler.
The above applies to what is known as the open tank system. This will always be most satisfactory for small ranges, but by the use of a closed system, the water, which with an open tank seldom has an average temperature of more than 160', can be raised above the boiling point. This makes it possible to use fewer and smaller heating pipes, thus reducing the cost of installing the plant ; but it is less economical of fuel, requires greater care, and may become somewhat dangerous.
Stearn heating.
In a general way, much that has been said re garding hot-water heating plants applies to steam heating. Both wrought- and cast-iron boilers are used, the latter being rather more expensive and lasting but little longer than tubular boilers that are well cared for. The ordinary return tubular boilers seem well adapted to the heating of green houses containing more than 5,000 square feet of glass. Aside from the steam boiler fittings, there is but little difference in the arrangement of a hot water and a steam-heating plant except that the pipes used for the latter are much smaller and the air-valves are placed at the lower end of each coil. In a general way, it can be said that the number of one-inch steam-pipes required to heat a greenhouse will be about the same as the number of two-inch pipes when hot water is used. In all except very small houses, it will be better to use one and one fourth-inch steam-pipes for the returns. The flow pipes also can be much smaller than with hot water, a two and one-half-inch pipe being amply large for a house 20 by 100 feet.
Two methods are commonly used for arranging the steam heating pipes in greenhouses. In one, the flow-pipe is carried to the farther end of the house where it is joined by means of branch pipes to the coils, which are distributed about the same as with hot water. The other way is to connect the flow pipes with the coils at the end nearest the boiler. Each of the coils may be provided with a return pipe for the drip, or all of the coils may be connected at the farther end of the house with one pipe which serves as a common return pipe for the series.
Literature.
Greenhouse Construction and Greenhouse Man agement (two books), L. R. Taft, Orange Judd Co., New York. For chapters on the building and care of greenhouses, see also Gardening for Pleasure, Peter Henderson ; Success in Market Gardening, W. W. Rawson ; Vegetable Gardening, S. B. Green; Vegetables Under Glass, Henry A. Dreer. The Forc ing-Book, L. H. Bailey. All recent garden books are likely to offer good advice. Recent years have seen great changes in methods of constructing glass houses for both vegetable gardening and floricul ture under glass. The reader will need to consult the current horticultural periodicals to keep in touch with the progress. The tendency is toward very wide houses of simple construction. Some of the newer forms are shown in Figs. 184-188, as well as in the pictures on preceding pages. Fig. 184 is redrawn from a print in The Florist's Ex change.