RELATION BETWEEN THE HEAD AND PRESSURE 35
1. The friction of the gaseous currents against the walls of the
chambers and flues, which has not been considered in these
computations, as sufficient data have not been available to estab-
lish the value of the coefficient.(1)
2. The effect of changes in the direction of flow.
In the computations made for ventilating ducts it is considered
that a change in direction of 90° introduces a resistance equal to
the velocity head, or that all of the velocity of the flowing gases
will be lost or absorbed. If this is correct a change in direction
of 180° may be considered as equivalent to two changes in direction
of 90°, that is to say, it may be assumed that the resistance offered
will be equal to twice the velocity head. A change in direction
of 45° may be assumed as offering a resistance equivalent to 0.6
of the velocity head/2)
The following method is employed in approximating the resist-
ance occasioned by changes in the velocity of the gases by reason
of their passing from a larger to a smaller or a smaller to a larger
flue or passage.
If a gas flows from a small channel into a larger channel or
chamber there will be a change in velocity from Fmax to Ymin, and
there will be no loss of head; but, when the gas passes from a
larger channel into a smaller channel a sufficient velocity head will
be required to impress upon the gas the increase in velocity
The computation of the resistance offered by the checkerwork
of a regenerator is given below as an example. This checkerwork
is composed of vertical channels, those in one tier being 120 mmX
60 mm and in the alternate tiers 60 mm X 60 mm.
The velocity of the gas in the smaller channels is" Fmax and
in the larger channels Vmm, so that
__ V max
C1) This lack has recently been supplied, in part by W. A. Mojarow, in a
paper on the "Friction of gases in brickwork flues" in the Rev. de la Societe
russe de Metallurgie, 1913, No. 3, pp. 325-370; Revue de Metallurgie, XI bis,
May, 1914, P> 820. Appendix IV.
(2) Considerable data covering the flow of air may be found in, Le Chauflage
Industrielle by H. Le Chatelier, p. 47? & seq. D. Murgue "On the loss of head
in Air Currents in Underground Workings. Trans. A.I.M.E., 1898, Vol.
XXIII. Appendix V. (Notes et Formules p. 1090 et seq.)