465
OF ENGINEERING FOKMULÆ.
Evaporative Power of Locomotive Boilers.
(Longridge, ‘ Min. Inst. Civ. Eng.,’ vol. lii.)
S = Surface of fire-box in square feet.
« = Surface of tubes „ „
N — Number of units required to evaporate 1 cubic foot of
water from 60L to temperature of water In boiler
• = say 71,000.
m, = Units of heat transmitted per hour through 1 square
foot of surface = say 11 for each degree Fahr.
æ = Temperature of gases (before entering tubes from fire-
box) above temperature of water = say 2100.
h = W (1 a w.
W — lbs. of fuel consumed per square foot fire-grate per
hour.
G = Surface of fire-grate in square feet.
w = Weight of gases and unconsuiued air arising from the
combustion of 1 lb. of fuel / __
<f = Specific heat of this mixture = ’237 J '
e = Base of hyp. log. = 2-718.
U — Units of heat arising from the combustion of 1 lb. fuel.
Heat generated per hour = U W G.
Smx
Evaporation from fire-box per cube foot per hour == - ■ „ - •
hx
Evaporation from tubes = —
Conclusions.—(1) No fixed rule can be established as to the
best relative proportions of grate, fire-box, and tube surfaces.
(2) Length of tube does not affect economic result. (3)
Diameter of the tube is a matter of indifference. (4) When
the quantity of fuel burnt is 60 or 60 lbs. per square foot of
grate per hour, the combustion is nearly perfect; but loss
resxilts from carbonic oxide passing away unconsumed with
hard firing. (5) A large increase in heating surface in pro-
portion to coal burnt only slightly increases the economical
effect. . , .
In locomotives the economic effect is in proportion to ths
fourth root of tha heating surface.
2 H