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NOTES ON STEAM 457
x = Weight of steam required.
H= Number of heat units above 32° in one pound of steam,
as given in Table No. 82.
tx = Temperature of water before heating.
/2 = Temperature of water after heating.
Expansion of Steam in Steam Engines.
When steam is expanded without doing work and prac-
tically without losing heat by radiation, it will become super-
heated, but if it is doing work, as in a steam engine, it will lose
heat during expansion.
According to the best authorities, the pressure is inversely
proportioned to the volume raised to the –^- power if heat is
neither added nor taken away by any outside source during the
time the steam is being expanded in the steam engine cylinder.
This is called adiabatic expansion of steam. Thus: PV1 * 111 = a
constant.
The pressure is inversely proportional to the volume raised
to the If power if the steam is kept dry at the temperature of
saturation, during expansion, by means of a steam jacket out-
side the cylinder. Thus: PV1 - 0625 = a constant.
When the pressure is considered to vary inversely as the vol-
ume it is called isothermal expansion. Thus: PV = a constant.
The isothermal curve is not exactly the correct curve to
represent the expansion of steam, but it is the theoretical curve
usually drawn on the indicator diagram, because it is so easy to
handle and is also very nearly correct.
The following formula gives the mean effective pressure
according to isothermal expansion.
M.E.P.= PX X (
’ +^/y- l-P,
-0
Absolute terminal pressure = Pi X —
Pi = Absolute initial pressure.
r = Ratio of expansion.
P2 = Absolute back pressure.
M. E. P. = Mean effective pressure.
Hyp. log. (hyperbolic logarithm), see page 126.
Table No. 83 gives the terminal and the mean effective
pressure of steam expanded under any of these three different
conditions.
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