Document gyxwZELbxLk0xbyrDykoZ5bV

American Society of Heating and Ventilating Engineers Guide, 1925-26 of transmission of the radiating surface is assumed to be 2, and as pre viously stated, only the glass and other exposed surfaces reduced to the equivalent of glass are considered in the calculation. The factors, or divisors, for glass surfaces are derived from the following formula: where p _ (T-DXG (150 - G) X 2 T = temperature desired, fahr.: t = temperature out of doors (0 deg. fahr.); 150 = temperature of water in radiating surface, fahr.: G = glass and glass equivalent surface; 2 = coefficient of transmission; . . R ~ radiating surface. From the above formula the divisors in the table following are derived: Table 52. Factors for Glass Surfaces For 70 to 75 deg. divide sq. ft. of glass and equivalent by 2.0 For 65 to 70 deg. divide sq. ft. of glass and equivalent by 2.28 For 60 to 65 deg. divide sq. ft.of glass and equivalent by 2.62 For 55 to 60 deg. divide sq. ft. of glass and equivalent by 3. For 50 to 55 deg. divide sq. ft.of glass and equivalent by 3.46 For 45 to 50 deg. divide sq. ft. of glass and equivalent by 4.For, 40 to 45 deg. divide sq. ft.of glass and equivalent by 4.67 For 35 to 40 deg. divide sq. ft. of glass and equivalent by 5.5 It will be noted that the temperatures given in the table are not 75 deg., 70 deg., 65 deg., etc., but 70-75 deg., 65-70 deg., 60-65 deg., etc. Greenhouses are so very sensitive to wind and so very unamenable to exact calculation that the initiated greenhouse man allows himself o deg. as leeway or as a factor of safety, and when he intends to heat to 60 deg. he specifies 55-60 deg. Greenhouses do not respond exactly to figures in various ways: The same amount of glass may in two different houses enclose vastly different volumes; the air loss between the laps, though probably never calculated, may be quite different per square foot of glass in two houses of the same size, design and construction, on account of the difference in workman ship or of glass quality, or of both. The humid atmosphere of greenhouses --and for some purposes the atmosphere is much more humid than for others, as for instance, for rose growing--at some temperatures causes the laps to seal with condensation, checking, or stopping the air loss through the laps. At other temperatures these laps are sealed with ice and the inside surface of the glass is entirely frosted over so that its conductivity is changed. It may be much more difficult to heat a greenhouse at 15-20 deg. above zero, or even at 25 deg. above zero, with the wind blowing, than at zero or below, because the low temperature house may be sealed with ice, as stated. And so, the same formula will not work out exactly .90 American Society of Heating and Ventilating Engineers Guide, 1925-26 for outside temperatures below zero; and in some parts where the mini mum outside temperature is above zero, or where the minimum of zero is rarely reached and then for short periods only, as in some Southern states, while it is true that the same amount of heat is required to offset the low outside temperature, the duration of the cold spell is so very short that it is unnecessary to pipe the houses or to provide boilers as large as the same house or houses would require further North. RATIO OF AIR CONTENT TO GLASS SURFACE The ratio of air content of the greenhouses to the glass surfaces increases with the width, so the number of changes of air through laps of glass, however many they may be, are less per hour with a wide house than with a narrow one, and the experienced heating man knows this and judiciously omits a line or two of pipe in very wide houses, after having divided the glass surface by the proper divisor. Fig. 39 shows in section two conventional, even span, adjacent green houses of the ridge and furrow, or saw-tooth type, converted by extending the roof lines until they bisect each other, into one even span house; and the sketch also shows at once that above the eaves line the cubic contents contained by the same amount of glass in the large house is just twice the quantity contained above the eaves lines in the two small ones, and, obviously, it requires less heat units to heat the one large house than it does the two small houses, although the glass surfaces are the same. But how much less heat? Who can say? Glass laid by eye measurement, and glass of varying quality are inconstants. A taljle compiled for purposes of comparing the cubic contents and the glass surfaces in the roofs of the houses in widths of-20 to 80 ft. shows that in the 20 ft. houses: . Glass : Contents : : 1 : 2.34 whereas in 80 ft. houses, with the same roof pitch: -Glass : Contents : : 1 : 9.8 Greenhouse radiation is almost always made up of pipe surface-- horizontal pipe placed on the side walls, on the sides of solid beds and under raised benches, sometimes on the columns supporting the roof. KINDS OF PIPE Steam mains are usually carried overhead on columns, sometimes with the return mains in trenches. The greenhouse heating engineer and con tractor uses 3J/J-in. cast iron pipe mainly for private greenhouse heating with hot water. This pipe holds about 2 qt. per lineal foot, and the super ficial surface is about 1.05 sq. ft. to the lineal foot. This pipe is very popular for heating private greenhouses with water for two reasons: (1) it holds so much water that it cools slowly and does not require close 91