ocess of the transfer of the heat on the outside and
seems to follow the same laws. The transfer rate, therefore, will
increase with the velocity of the steam through the tube. For this
reason, internal cores are quite often used in superheaters and actually
result in an increase in the amount of superheat obtained from a given
surface. The average transfer rate in superheaters based on a difference
in mean temperature between the gas on the outside of the tubes and the
steam on the inside of the tubes is if R is the transfer rate from the
gas to the tube and r the rate from the tube to the steam:

Rr
-----
R + r

and is always less than either R or r. This rate is usually greater than
the average transfer rate for the boiler as computed in the way outlined
in the preceding paragraphs. Since, however, steam cannot, under any
imagined set of conditions, take up more heat from a tube than would
water at the same average temperature, this fact supports the contention
made that the actual transfer rate in a boiler must increase quite
rapidly with the temperatures. The actual transfer rates in superheaters
are affected by so many conditions that it has not so far been possible
to evolve any formula of practical value.

[Illustration: Iron City Brewery of the Pittsburgh Brewing Co.,
Pittsburgh, Pa, Operating in this Plant 2000 Horse Power of Babcock &
Wilcox Boilers]

INDEX

PAGE

Absolute pressure 117
Absolute zero 80
Accessibility of Babcock & Wilcox boiler 59
Acidity in boiler feed water 106
Actual evap. corresponding to boiler horse power 288
Advantages of Babcock & Wilcox boilers 61
Stoker firing 195
Water tube over fire tube boilers 61
Air, composition of 147
In boiler feed water 106
Properties of 147
Required for combustion 152, 156
Specific heat of 148
Supplied for combustion 157
Vapor in