by R. C. Carpenter)
Methods of proportioning radiating surface for heating of greenhouses.
Radiating surface, whether from steam or hot-water pipes, is estimated in square feet of exterior surface. All projections, ornaments, etc., on the exterior of pipes or radiators are counted as efficient surface. Formerly, cast-iron pipe of about 4 inches in diameter was used almost altogether for greenhouse work; it is still used to some extent for hot-water heating, but the great majority of houses are now piped with wrought iron or steel pipe, which is made of standard size and thickness, and is a regular article of trade.
The heating surface in a boiler or hot water heater is that portion of the boiler, or heater, which is exposed to the direct heat of the fire or of the heated gases.
Grate surface is the number of square feet of grate in the boiler or heater.
In estimating the heat required for greenhouses, the area expressed in square feet of glass in the roof and walls is taken as the basis from which computations are made. Certain rules of practice have been adopted, and appear to give fairly good results in proportioning radiating surface, grate surface, and heating surface. The ratio of heating surface to grate surface in heaters will depend upon the kind of coal to be burned and the economy desired. The more heating surface provided per unit of grate surface, the higher the economy, but the greater the first cost of the heater. The usual practice in large boilers is to employ 40 square feet of heating surface to 1 of grate surface for hard coal, and 80 feet of heating surface to 1 of grate surface for soft coal.
In small cast-iron heaters the proportion of heating surface to grate is frequently one-third to one-fourth that given above.
If the greenhouse is maintained at 70° when the outside temperature is zero, one square foot of radiation will supply 5 square feet of glass surface, if steam is used at 5 pounds pressure, or 4 square feet of glass surface if water at a temperature of 180° F. is used. The following table gives the ratio of radiation to glass surface for various temperatures: —
(A) Table showing relation of glass surface, radiating surface, and heating surface l
Temperature of radiating surface . . .
(5 lbs. Pressure)
(10 lbs. Pressure)
Square feet of glass for 1 square foot radiating surface.
Temp. 100° F. above surrounding air . .
Temp. 90° F. above surrounding air .
Temp. 80° F. above surrounding air . .
Temp. 70° F. above surrounding air .
Temp. 60° F. above surrounding air .
Temp. 50° F. above surrounding air . .
Temp. 40° F. above surrounding air .
Temp. 30° F. above surrounding air . .
Radiation per pound of coal.....
Heat units given off 1 square foot radiating surface B.T.U.2 for 70° Temp, diff. . .
For instance, to maintain the temperature of a greenhouse 70 at zero weather, there should be 1 square foot of radiating surface for 4.0 square feet of glass for hot-water heating, in which the maximum temperature of the water is maintained at 180°; or there should be 1 square foot of radiating surface for 5 square feet of glass for low-pressure (under 5 pounds) steam. These numbers are given somewhat greater by some authorities, and there is no doubt that if the house is not much exposed, higher proportions will give satisfactory results.
The preceding table gives more exact values for these quantities, and will be found to accord with the best practice in heating of greenhouses, either by steam or hot water. Each pound of coal burned on the grate will transfer to the water or steam in the heater about 9000 B.T.U. As the amount of coal consumed can be varied with the draft or firing conditions, it is evident that no fixed rule can be given for the proportion of grate to radiation.
1 From Carpenter's work on "Heating and Ventilating Buildings."
2 British Thermal Unit, — heat required to raise 1 lb. of water 1 degree.
Size of pipes connecting radiating surface and the boiler or heater.
Various empirical rules have been given for proportioning main-supply and return pipes, which have proved quite satisfactory in practice. George A. Babcock gives the following rule, which will be found very satisfactory for greenhouse heating, whether with low-pressure steam or with water: —
The diameter of main pipe leading to the radiating surface should be equal in inches to 0.1 the square root of radiating surface in square feet. The main pipes should not be less than \\ inches in diameter, return pipes for water heating the same size as mains, and, for steam heating, one size less than mains, but never less than 3/4 inch in diameter. The following table shows the radiating surface supplied by various sizes of main pipe.