The Steam Injector
A theoretical and practical treatise on the design and operation of injectors and on the flow of fluids through and the design of nozzles.
Forfatter: V. A. B. Hughes
År: 1912
Forlag: The Technical Publishing Company Limited
Sted: London
Sider: 145
UDK: 621.176
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THE STEAM NOZZLE.
13
end of a diverging steam nozzle depends upon the degree
of expansion of the steam within the nozzle. If the steam
is not expande-d within the nozzle down to the pressure of
the medium into which it is fiowing, then it leaves the
nozzle with some of its heat pressure energy unconverted
into kinetic energy, and such energy will be absorbed by the
feed water, which, in the case of the injector, surrounds the
exit end of the steam nozzle. In this way, though the heat
energy is not lost, yet it does not assist in increasing the
velocity of discharge of the steam from the steam nozzle.
On the othei’ liand, if the steam is over-expanded within
the steam nozzle, the velocity of the jet will be sacrificed
in order to increase the jet’s cross sectional area.* It has
F1GS. 11 AND 12.
been found by experiment in connection with the diverging
nozzles used in certain types of steam turbine that the
velocity losses due to slight over-expansion within the
nozzle are greater than those due to slight under-expansion,
so that it is better to have the nozzle too short than too
long. In faet, it is recommended that the nozzle be de-
signed foi- slight under-expansion.
The following table, in which Am and At respectively
indicate area of nozzle mouth and throat, and pv and p2
boiler and exhaust steam pressure (absolute), gives
sufficiently accurate values (for practical purposes) for the
ratio of areas of nozzle mouth and throat for different
ratios of initial and final steam pressures.
* The valne of the ratio of the velocity of the jet at the point where the proper
degree of expansion is attained to that of the jet at the mouth of the nozzle may
be taken as approximately equal to the ratio of the cross-sectional area of the
nozzle at the mouth to that of the nozzle at the said point of correct expansion ;
in other words, — = —, where V denotes velocity, A area, m nozzle mouth,
’ V«« Ax ’
and x the section of the nozzle where correct expansion is attained.