of calculating an average
transfer rate. It has been used in this way for calculating the average
transfer rate from boiler tests in which the capacity has varied from an
evaporation of a little over 3 pounds per square foot of surface up to
15 pounds. When plotted against the gas weights, it was found that the
points were almost exactly on a line. This line, however, did not pass
through the zero point but started at a point corresponding to
approximately a transfer rate of 2. Checked out against many other
tests, the straight line law seems to hold generally and this is true
even though material changes are made in the method of calculating the
furnace temperature. The inclination of the line, however, varied
inversely as the average area for the passage of the gas through the
boiler. If A is the average area between all the passes of the boiler,
the heat transfer rate in Babcock & Wilcox type boilers with ordinary
clean surfaces can be determined to a rather close approximation from
the formula:

W
R = 2.00 + .0014 -
A

The manner in which A appears in this formula is the same as it would
appear in any formula in which the heat transfer rate was taken as
depending upon the product of the velocity and the density of the gas
jointly, since this product, as pointed out above, is equivalent to W/A.
Nusselt's experiments, as well as those of others, indicate that the
ratio appears in the proper way.

While the underlying principles from which the formula for this average
transfer rate was determined are questionable and at best only
approximately correct, it nevertheless follows that assuming the
transfer rate as determined experimentally, the formula can be used in
an inverse way for calculating the amount of surface required in a
boiler for cooling the gases through a range of temperature covered by
the experiments and it has been found that the results bear out this
assumption. The practical application of the theory of heat transfer, as
developed at present, seems consequently to rest on these last two
formulae, which from their nature are more or less empirical.

Through the range in the production of steam met with in boilers now in
service which in the marine type extends to the average evaporation of
12 to 15 pounds of water from and at 212 degrees Fahrenheit per square
foot of surface, the constant 2 in the approximate formula for the
average heat transfer rate constitutes