quite a large proportion of the
total. The comparative increase in the transfer rate due to a change in
weight of the gases is not as great consequently as it would be if this
constant were zero. For this reason, with the same temperature of the
gases entering the boiler surface, there will be a gradual increase in
the temperature of the gases leaving the surface as the velocity or
weight of flow increases and the proportion of the heat contained in the
gases entering the boiler which is absorbed by it is gradually reduced.
It is, of course, possible that the weight of the gases could be
increased to such an amount or the area for their passage through the
boiler reduced by additional baffles until the constant term in the heat
transfer formula would be relatively unimportant. Under such conditions,
as pointed out previously, the final gas temperature would be unaffected
by a further increase in the velocity of the flow and the fraction of
the heat carried by the gases removed by the boiler would be constant.
Actual tests of waste heat boilers in which the weight of gas per square
foot of sectional area for its passage is many times more than in
ordinary installations show, however, that this condition has not been
attained and it will probably never be attained in any practical
installation. It is for this reason that the conclusions of Dr.
Nicholson in the paper referred to and of Messrs. Kreisinger and Ray in
the pamphlet "The Transmission of Heat into Steam Boilers", published by
the Department of the Interior in 1912, are not applicable without
modification to boiler design.

In superheaters the heat transfer is effected in two different stages;
the first transfer is from the hot gas to the metal of the superheater
tube and the second transfer is from the metal of the tube to the steam
on the inside. There is, theoretically, an intermediate stage in the
transfer of the heat from the outside to the inside surface of the tube.
The conductivity of steel is sufficient, however, to keep the
temperatures of the two sides of the tube very nearly equal to each
other so that the effect of the transfer in the tube itself can be
neglected. The transfer from the hot gas to the metal of the tube takes
place in the same way as with the boiler tubes proper, regard being paid
to the temperature of the tube which increases as the steam is heated.
The transfer from the inside surface of the tube to the steam is the
inverse of the pr