and Superheaters, Equipped with Babcock & Wilcox Chain Grate
Stokers at the Plant of the Twin City Rapid Transit Co., Minneapolis,
Minn.]

This increase in the efficiency of the boiler alone with the decrease in
the rate at which it is operated, will hold to a point where the
radiation of heat from the boiler setting is proportionately large
enough to be a governing factor in the total amount of heat absorbed.

The second reason given above for a decrease of boiler efficiency with
increase of capacity, viz., the effect of radiant heat, is to a greater
extent than the first reason dependent upon a constant furnace
temperature. Any increase in this temperature will affect enormously the
amount of heat absorbed by radiation, as this absorption will vary as
the fourth power of the temperature of the radiating body. In this way
it is seen that but a slight increase in furnace temperature will be
necessary to bring the proportional part, due to absorption by
radiation, of the total heat absorbed, up to its proper proportion at
the higher ratings. This factor of furnace temperature more properly
belongs to the consideration of furnace efficiency than of boiler
efficiency. There is a point, however, in any furnace above which the
combustion will be so poor as to actually reduce the furnace temperature
and, therefore, the proportion of heat absorbed through radiation by a
given amount of exposed heating surface.

Since it is thus true that the efficiency of the boiler considered alone
will increase with a decreased capacity, it is evident that if the
furnace conditions are constant regardless of the load, that the
combined efficiency of boiler and furnace will also decrease with
increasing loads. This fact was clearly proven in the tests of the
boilers at the Detroit Edison Company.[74] The furnace arrangement of
these boilers and the great care with which the tests were run made it
possible to secure uniformly good furnace conditions irrespective of
load, and here the maximum efficiency was obtained at a point somewhat
less than the rated capacity of the boilers.

In some cases, however, and especially in the ordinary operation of the
plant, the furnace efficiency will, up to a certain point, increase with
an increase in power. This increase in furnace efficiency is ordinarily
at a greater rate as the capacity increases than is the decrease in
boiler efficiency, with the result that the combined efficiency of
boiler and fu