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RELATION BETWEEN THE HEAD AND PRESSURE 33
From this it may be seen that without regard to the kind or
density of the liquid which is in motion, any particular velocity
will correspond to a velocity head, which may be absolutely
determined and which will be obtained by the formula
But the pressure necessary to impress motion upon a liquid
will be different for each liquid and is proportional to its density.
In considering the movement of gases, the same fundamental
principles must be observed:
1. Motion or velocity of flow cannot be initiated or maintained
without a corresponding loss of head or pressure, as the loss of
head and the velocity impressed have the relationship of a cause
and an effect.
2. Any particular velocity of flow corresponds to a velocity
head expressed in terms of the gas which is in motion, which may
be determined by the formula
3. Equal velocities will be impressed upon gases of different
densities by equal velocity heads, but by different pressures.
These pressures are proportional to the specific weight of these
gases or to the weight of a unit volume.
As the gases appear to act as a form of liquid having the
particular property of considerable variation in its specific weight
according to changes in its temperature, the third principle is of
great importance in all computations relative to the circulation
of the flame or hot gases.
The following examples will illustrate this point:
A. In the heating chamber of an open-hearth furnace the hot
air emerges from the port with a velocity of 18 m per second at
a temperature of 1000°. What is the pressure required to impress
this velocity upon the air?
To impress a velocity of 18 m per second upon a fluid will
require a velocity head h—16 m 51.^
In this case it is air at 1000° which is in motion; the weight
of 1 cu m of this gas is
Aiooo = 1+Vy^ = 0 kg 277.
(1) Refer to Appendix III.