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NOTES ON STEAM. 453
maintained at 95 pounds per square inch, the temperature of
the water from the condensed steam will be 324 degrees, the
yame as the temperature of the steam, but each pound of steam
as it is condensing will give out 885.8 British thermal units of
heat.
The fifth column gives the number of cubic feet of satu-
rated steam which will weigh one pound at the given pressure
and temperature. The sixth column gives the weight of one
cubic foot of saturated steam of corresponding given tempera-
ture. For instance, one cubic foot of steam at 95 pounds per
square inch absolute pressure will weigh 0.2198 pounds, and 100
cubic feet of steam of 95 pounds per square inch absolute pres-
sure will weigh 100 X 0.2198 = 21.98 pounds. In other words, it
will take 0.2198 pounds of water to give one cubic foot of steam
at 95 pounds absolute pressure, and it will require 21.98 pounds
of water to make 100 cubic feet of steam of 95 pounds absolute
pressure.
The seventh or last column gives the relative volume of
steam at the given pressure as compared with water at 32
degrees F. For instance, one cubic foot of water will give
1646 cubic feet of steam at atmospheric pressure, but one cubic
foot of water gives only 219 cubic feet of steam at 125 pounds
absolute pressure.
Steam Heating.
In the ordinary practice of heating buildings by direct
radiation the quantity of heat given off by the radiators or steam
pipes will vary from \% to 3 heat units per hour per square foot
of radiating surface for each degree of difference in tempera-
ture; an average of from 2 to 2X is a fair estimate.
One pound of steam at about atmospheric pressure contains
1146 heat units, and if the temperature in the room is to be
maintained at 70°, while the temperature of the pipes is
212°, the difference in temperature will be 142 degrees. Multi-
plying this by 2%, the emission of heat will be 319)^ heat units
per hour per square foot of radiating surface. Dividing 319^
by 1146 gives 0. 3 pounds of steam condensed per hour, per square
foot of radiating surface. From this may be estimated the re-
quired size of boiler, as the boiler must always be capable of
generating as much steam as the radiators are condensing. A
rule frequently given is to have one square foot of heating sur-
face in the boiler for every 8 to 10 square feet of radiating sur-
face and one square foot of grate surface for every 350 to 500
square feet of radiating surface.
One pound of coal is required per hour per 30 to 40 square
feet of radiating surface.
When steam is used for heating dwelling-houses, one square
foot of radiating surface is required per 40 to 80 cubic feet of
space, according to location, number of windows, etc. As a
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