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NOTES ON STEAM. 455
taken as 8T
3
o pounds. Therefore 69.21 pounds divided by 62.5
gives 1.1 cubic feet, or 69.21 pounds divided by 8.3 gives 8.34
gallons.
At atmospheric pressure one cubic foot of steam has nearly
the weight of one cubic inch of water, and the weight increases
very nearly as the pressure ; therefore, for an approximate estima-
tion, if no steam tables are at hand, it is well to remember the
rule
:
Multiply the number of cubic feet of steam by the absolute
pressure in atmospheres, and the product is the number of cubic
inches of water required to give the steam.
Note.—In all such calculations for practical purposes, a
liberal allowance must be made for loss and leakage.
Weight of Water Required to Condense One
Pound of Steam.
The following formula gives the theoretical amount of water
required to condense one pound of steam
:
h — h
W= Weight of water required per pound of steam con-
densed.
H= Number of heat units above 32° in one pound of steam
at the pressure of exhaust. This temperature is
obtained from Table No. 82.
t\ = Temperature of water when entering the condenser.
fa = Temperature of water when leaving the condenser.
tz = Temperature of the condensed steam when leaving
the condenser and entering the air-pump.
Example.
Steam of four pounds absolute pressure is exhausted into a
surface condenser. The temperature of the condensed steam
when leaving the condenser and entering the air-pump is 120°.
The temperature of the cold water when entering the con-
denser is 65°.
The temperature when leaving the condenser is 105°.
How many pounds of condensing water is needed per pound of
steam condensed ?
Note.—In the steam table, page 451, the total number of
heat units above 32° per pound of steam of four pounds absolute
pressure is given as 1128.
Solution
:
„, 1128 + 32 — 120
105 — 65
W= —— = 26 pounds of water per pound of steam.
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