he higher, the
flow per minute may be calculated from the formula:

W = 1.9AK ((P - d)d)^{.5} (50)

Where W = the weight of steam discharged in pounds per minute,
A = area of orifice in square inches,
P = the absolute initial pressure in pounds per square inch,
d = the difference in pressure between the two sides in pounds
per square inch,
K = a constant = .93 for a short pipe, and .63 for a hole in a
thin plate or a safety valve.

[Illustration: Vesta Coal Co., California, Pa., Operating at this Plant
3160 Horse Power of Babcock & Wilcox Boilers]

HEAT TRANSFER

The rate at which heat is transmitted from a hot gas to a cooler metal
surface over which the gas is flowing has been the subject of a great
deal of investigation both from the experimental and theoretical side. A
more or less complete explanation of this process is necessary for a
detailed analysis of the performance of steam boilers. Such information
at the present is almost entirely lacking and for this reason a boiler,
as a physical piece of apparatus, is not as well understood as it might
be. This, however, has had little effect in its practical development
and it is hardly possible that a more complete understanding of the
phenomena discussed will have any radical effect on the present design.

The amount of heat that is transferred across any surface is usually
expressed as a product, of which one factor is the slope or linear rate
of change in temperature and the other is the amount of heat transferred
per unit's difference in temperature in unit's length. In Fourier's
analytical theory of the conduction of heat, this second factor is taken
as a constant and is called the "conductivity" of the substance.
Following this practice, the amount of heat absorbed by any surface from
a hot gas is usually expressed as a product of the difference in
temperature between the gas and the absorbing surface into a factor
which is commonly designated the "transfer rate". There has been
considerable looseness in the writings of even the best authors as to
the way in which the gas temperature difference is to be measured. If
the gas varies in temperature across the section of the channel through
which it is assumed to flow, and most of them seem to consider that this
would be the case, there are two mean gas temperatures, one the mean of
the actual temperatures at any time across the section, and the other