Document By31M0aE95xZNm7dVo3GQe4Vk

American Society of Heating and Ventilating Engineers Guide, 1928 Long runs of pipe are used and expansion and' contraction require more consideration than in house heating work; piping must be tied up, anchored, installations must be flexible; expansion joints are no more desirable in greenhouse heating than elsewhere; expansion must be com pensated for by "spring" of pipe and by swivel fitted joints, and expansion joints used where expansion may not be compensated otherwise.- Table 1. Temperature Required for Different Purposes House Temp. Required, Dec. Fahr. 55 to 60 45 " 60 60 " 65 65 " 70 60 " 65 60 " 65 65 " 70 50 " 55 65 " 70 50 " 55 55 " 60 45 " 55 40 " 45 55 " 60 45 " 50 Cool and Damp 65 to 70 65 "70 65 " 70 Cool Peach House (Cold Damp Weather), Early Peach House (Start 65 " 70 Second Peach House (Start February and March), Late Peach House 65 " 70 65 " 70 .40 " 45 55 " 60 .60 " 65 The matter of levels also affects the design and proportions of the heat; ing mains. In many greenhouse ranges the walk levels are not more than 2 ft. 6 in., or 3 ft., above the top of th^:.boiler and where all of the radiating surface is made up of pipe coils on a level not higher than 1 ft. 6 in. above the floor, especially if there are short benches requiring short coils under them, special care is necessary to avoid short circuiting, or interference with the flow through low temperature drops in the short coils. The temperatures required in houses are given in Table 1. ESTIMATING HEATING REQUIREMENTS Heating requirements, i.e., the amounts of radiation,for greenhouses, are not obtained by scientific calculation or intricate formulae; at least, they are not so obtained by greenhouse specialists. In this calculation only the exposed glass surfaces and other exposed surfaces reduced to the ,138 Chapter V--Greenhouse Heating Systems equivalent of glass surfaces are considered. The loss of heat by conduc tion through the glass is, of course, by far the greatest loss, but there is another considerable loss which it is very difficult, if not absolutely impossible to calculate, that is, the loss by air leakage between the lights of glass at the laps. These two make up practically all of the losses. The only other losses are those through open doors and through vents. The latter are generally intentional, and used to reduce the greenhouse to the required temperature; but a change of air is necessary for plant growth, so the vents are not opened except for reducing the inside temperature. Modern greenhouses are glazed almost exclusively with double-thick glass in lights 16 in. wide and 24 in. long. The glazing bars, or ribs, are then usually 16J4 in. center to center. The lap of the glass is J-g in. by eye measurement. Sometimes 24 x 24 in. glass is used, but not often, and with this size, the bow, or spring, of the glass when the wind blows is greater, and the heat loss through the laps may accordingly be greater as a consequence. The cubic contents in ratio to the surrounding glass surface, the size and the shape are, of course, more or less factors, but as previously stated, only the glass and glass equivalent enter into the calculation for the quantities of radiating surface. The engineer may modify somewhat the quantities so obtained because of the ratio of the contents to the enclosing glass, or the size or shape of the structure, or because of its geographical position or its elevation, or because the greenhouse is in a particularly exposed position. The calculation is merely , that of dividing the glass and the equivalent surface by the proper divisor; and where proper, allowances are made for special conditions and where boilers are suf ficiently large so that a liberal factor of safety or proper surplus is pro vided to cover these conditions over which the designer has no control, there is no better method known than the application of the table of divisors given in Table 2. The conditions for which the engineer must provide, and which are largely out of his control are as follows: Workmanship in construction and glazing of glass house or houses; ratio of cubic contents enclosed to the glass surface; draft, if chimney is not designed by the engineer; fuel and its quality; firing habits of the operator, time of turning on steam and making up temperatures inside against falling temperature outside; attention to venting greenhouse heating coils--there is no one thing that the heating engineer can do which will so efficiently offset these negative factors, as the providing of a generous boiler power. WATER TEMPERATURE The average temperature of . the water in a gravity hot water heating system is assumed to be about 150 deg. fahr. and the average co-efficient of transmission of the radiating surface (pipe surface) is assumed to be 2. Pipe in different locations and banked pipes have different values, it is true; overhead mains, or pipes overhead free and unobstructed have the highest co-efficient, but because of their proximity to roof glass are not the best possible heaters; flat coils of parallel lines are more efficient than