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amount o. heating surface required to maintain a temperature of 70° F. in A, B, and C, assuming that the radiators will be heated by steam having a pressure of 5 lb. by the gauge, the outside temperature being 10° below zero. Let us suppose that the windows are each 6 ft. by 3 ft. and that the exposed walls are built of good, ordinary brick, lathed and plastered inside.
When lathed and plastered brick walls are used, as in the figure it is safe to estimate that about 10 sq. ft. of wall sur. face will be equivalent in cooling power to 1 sq. ft. of glass; consequently, in this case wall surface
equivalent glass surface.
10 Let us commence with room A; the amount of glass surface here is 6 X 3 X 4 = 72 sq. ft. To this must be added the exposed wall surface reduced to a glass equivalent; thus, 10 (25 + 20) — 72
37.8 sq. ft.
10 Since we assume that the inner walls, floors, and ceilings are not cooling surfaces, the total cooling surface of the room A is 72 + 37.8 = 109.8 sq. ft.
The temperature of steam at 5 lb. gauge pressure is 227°, and the difference between 70° above zero and 100 below zero is 700 + 10°; therefore, substituting in the formuia, we have
70 + 10 S
X 109.8 56 sq. ft., nearly.
227 70 This, however, counteracts only the cooling effect of the walls and windows, and to make reasonable allowance for air leakage, add 25 per cent. of the above amount, or 14 sq. ft., which gives 56 + 14 70 sq. ft. of direct radiating surface.
Suppose that we allow 20 per cent. of the direct radiating surface (70 sq. ft. in this case) for a moderate exposure to winds; the size of the radiator we would place in A would then be 70 + 14, or 84 sq. ft.
For convenience, we may divide this into two radiators, a and b, a having an area of 56 sq. ft. and b an area of 28 sq. ft. This will so divide the radiator surface that one-third,
or 28 sq. ft., may be used during mild weather; two-thirds, or 56 sq. ft., for moderate cold weather; and the whole, or 84 sq. ft., for use during severe weather.
In like manner and under the same conditions, we find, for the room C, that the sizes of the radiators c, d, and e should be, respectively, 40, 82, and 42 sq. ft.
As the coldest winds blow in the direction of the arrow, the radiator having 82 sq. ft. is placed in the left-hand exposed corner of the room C. A better distribution of the radiating surface in this room would be to make d 42 sq. ft. only and place a radiator having 40 sq. ft. between the windows toward which the arrow points; this would insure a more uniform temperature in the room.
It will be observed that A, B, and C, three rooms having the same shape and cubical contents require 84, 40, and 124 sq. ft. of heating surface, respectively, in order to maintain a temperature of 70° F. in each when the outer atmosphere is 10° below zero. This example shows how imperfect must be the rule-of-thumb method of proportioning radiators to the cubical contents of rooms.
Tables Useful in Radiator Installation.-A number of tables that will prove of use in proportioning radiation for various purposes are given in the succeeding pages.
Surface. Size. Surface. Steam. Return.
DIRECT RADIATORS. Heat units emitted per hour, per square foot of external surface, per degree of difference in temperature.
50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240
1.29 1.33 1.36 1.39 1.41 1.46 1.49 1.52 1.56 1.59 1.63 1.66 1.69 1.73 1.76 1.80 1.83 1.86 1.90 1.93
1.54 1.58 1.62 1.66 1.70 1.74 1.78 1.82 1.86 1.90 1.94 1.98 2.02 2.06 2.10 2.14 2.18 2.22 2.27 2.31
1.46 1.50 1.54 1.58 1.62 1.65 1.69 1.73 1.77 1.81 1.85 1.88 1.92 1.96 2.00 2.03 2.07 2.11 2.15 2.19
2.01 2.06 2.12 2.17 2.22 2.27 2.32 2.38 2.13 2.48 2.53 2.59 2.61 2.70 2.75 2.80 2.85 2.90 2.96 3.01
6.40 63.10 7.18
4.24 41.20 4.50
1.65 2.05 1.39 1.90
INDIRECT RADIATORS. Heat units emitted per square foot of actual surface per hour, per degree of difference in temperature.
NATURAL DRAFT, EXTENDED SURFACES.
Velocity of Air per Second. Feet.
Extended-surface indirect radiators are frequently used for small installations of a blower heating system, but their efficiency is not nearly as high as that of pipe heaters. When extended-surface indirect radiators are thus used, the surface should about be doubled, and the sections must be staggered. Extended-surface indirect cast-iron radiators are used extensively for gravity air circulation, being supplied with a coldair duct to the outer atmosphere and a warm-air pipe to a register in the room to be warmed. Both the plain-surface indirect and the pipe heaters are used extensively for blower systems of heating. They are usually set about 16 to 24 pipes deep, and are supplied with exhaust steam from engines, or live steam through a pressure regulator.
APPROXIMATE AMOUNT OF RADIATING SURFACE REQUIRED FOR HEATING GREEN
HOUSES BY STEAM *
* For heating by hot water, the required amount of radiating surface may be found by multiplying the values above tabulated by 1.5.
TESTING HEATING SYSTEMS.
The foilowing table will be found useful in determining at any time whether a given heating apparatus is capable of maintaining a temperature of 70° throughout the building in zero weather. As an illustration, suppose the outdoor temperature to be 50° at the time of testing; then, if the temperature within the rooms is equal to or in excess of 99° F., as given by the table for such condition it may safely be assumed that sufficient heating surface has been provided