GLASS HOUSES FOR VEGETABLE CROPS 



127 



a boiler and then carried through the houses in 

 a series of pipes. The circulation is due to the 

 fact that cold water is heavier than hot water, 

 and as one end of the circuit of pipes is attached 

 to the bottom, while the other is connected with 

 the top of the boiler, the heavy cold water in 

 the greenhouse flows back in the pipes and pushes 

 the light hot water out at the top to flow oflf 

 into the system to take its place. A great variety 

 of hot-water boilers, of both cast- and wrought- 

 iron, are made for greenhouse heating. The cast- 

 iron boilers are to be preferred for small plants, 

 but there are a number of tubular boilers that are 

 made for hot-water heating that answer very well. 

 An ordinary tubular steam boiler will also be 

 found very satisfactory for hot-water heating, 

 although if a tubular boiler is to be constructed 

 for the purpose it would be better to have tubes 

 also in the top of the boiler. 



Pour-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 pjpes 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 

 ene-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 difliculty 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. 



Steam 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 



