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272                                    APPENDIX VII

the exposure to stray air currents and variations in the cycle of
operation introduce complex variables which are difficult to
consider. When the cycle is fixed and proper allowance made for
the exposure, these losses may be reduced to mathematical

The frictional resistance in the checkers and flues may be
approximated by the formula of Mojarow.    This formula is


in which 7 = the frictional resistance expressed in inches of water
or, if the metric system is used, millimeters of
water or kilograms per square meter;
m = coefficient of friction, as determined by Mojarow =
0.003076 for English units or 0.016 for metric
units. This coefficient was determined by obser-
vations on a Cowper hot-blast stove. Murge
fixed a coefficient for mine galleries of one-third
these values  namely, 0.001026 for English and
0.0053 for metric units;
S = perimeter of passage or summation of the perimeters
of passages through which the gases flow;
L  length of the passage or flue;
SL = total surface  of passage or flue in contact with
flowing gases;
co = cross-sectional area of the passage; or, where this
varies, the mean sectional area; or, where the
stream of gas does not fill the flue, the area of the
flowing stream;
vt = average velocity of flow per second at t;
p* = specific weight of a unit volume of gas at t;
t = average temperature of the flowing gas in the section