SECT. II.]
PROPERTIES OF STEAM.
93
147. A knowledge of this cause of the reduction of the force of steam to
atmospheric elastic force, and of the importance of not losing force where either
economy of heat or of space is desirable, creates a strong desire to know its amount,
knowing that the most esteemed manufacturers of steam-boat engines cause the
steam to pass round between the jacket and the cylinder; as if to expose it
as much as possible to the cooling effect of the atmosphere, to reduce its elastic
force before it enters the cylinder to exert its power.
148. The reduction of the temperature of steam reduces its elastic force to
that of a lower temperature, and during this reduction a portion of the steam
becomes water. If/denote the elastic force in the boiler, and f that after the heat
has been lost,
f-f
f
will be the quantity reduced to water, and this multiplied by its heat of conversion
into steam must be equal to the heat the whole has lost by cooling; therefore
And here it will be remarked, that when t'" is equal to the whole heat of con
version, /' will be nothing; or the whole will be cooled into water as it is in an
apparatus for warming buildings. We are now in a condition to give an answer
to the question of what is the loss of force in any particular case. Let the tempe
rature of the steam be 220°, and its force 35 inches of mercury, the length of the
steam pipe 12 feet, its diameter 6 inches, the velocity of the steam in the pipe
80 feet per second, and the temperature of the air 60°. Then by art. 145.
we have
T = 220 - — = 209°
20
1*71 (T—f") _ 1-7 x 1? - /onQ - m '
d v
and therefore by the equation above we have
consequently there is in this case a loss of force equivalent to 0*23 inches of
mercury, or tht of the force; but this is one of the most favourable of the cases
that usually occur in practice. In steam-boat engines where the steam has to
pass round the cylinder, the force in the cylinder is stated, from observation,
1 The number 967° is here taken as the heat of conversion into steam, but in general I use
1000° as more accurate. (See art. 82.)