The Locomotive - Lighthouse Survival Blog

76 THE LOCOMOTIVE. [Mat,
weeks amounts to, say, 54 pounds. At 5 cents a pound tliis would cost $2.70. If tannate
of soda were used iu place of common soda ash, the cost of the solvent would be
somewhat greater ; but we shall assume that soda ash does the work satisfactorily, and
shall take $2.70 to rejoreseut the cost of the solvent for the run of three weeks.
At the end of this time the boiler is opened and cleaned. This involves several
items of expense, which we proceed to consider. As a general rule it will be necessary to renew
the gaskets ; and we may fairly estimate the cost of these at $1.25. In the absence Uf
an agreement to the contrary, it will probably be necessary to pay the engineer for eight
hours' work, and as he will need some help, this item may safely be estimated at $3.00.
We next come to the heat-loss involved in blowing off and shutting down. If the
water is bad it may be necessary to blow down 3 inches every day. This is equal to
about 17 cubic feet per day, or, say, 300 cubic feet in three weeks. In addition, we must
blow out a boiler-full of hot water at the end of the three weeks. Now a boiler such
as the one described will contain about 150 cubic feet of water, which added to the 300
cubic feet blown out during the three weeks, makes 450 cubic feet, or about 28,000
pounds. The blowiug-off pressure being, say, 20 pounds, the temperature of the water
will be about 260° ; and if the temperature of the feed is 60°, this means a loss of 200'
heat units per pound of water, or 5,600,000 heat units in all. The boiler, weighing, say,
16,000 pounds, is also cooled by the same amount, and as the specific heat of iron is-
.114 the loss from this cause is 16,000 x 200° x .114=365,000 heat units. We have also to
consider the loss of heat from the brickwork of the setting. To calculate this we shall
assume that a layer of brick 8 inches thick, extending all around the inside walls of the
setting, cools, say, 600°. To calculate the weight of this layer of brick we first estimate
the inner surface of the setting. In the present case this is about 262 square feet. A
layer 8 inches thick and having this surface will have a volume of approximately
262 X 1 = 175 cubic feet (the f is the thickness of the layer, expressed in feet). Now a
cubic foot of brickwork weighs about 112 pounds, so that the weight of the layer under
consideration would be 175x112 = 19,600 pounds. Now the specific heat of brickwork
is about .20, so that this 19.600 pounds, in cooling 600°, gives up 19,600x600°x.20 =
2,352,000 units of heat. The heat account therefore stands as follows :
Heat units in the hot water blown off, = 5,600,000
Heat units lost by cooling the boiler itself, = 365,000
Heat units lost by cooling the setting, = ....... 2,352,000'
Total heat units lost, = 8,317,000'
Now in good average good practice, a pound of coal evaporates about 8 pounds of
water; which is equivalent to saying that we can obtain about 8,900 heat units from
each pound of coal. Hence, to obtain 8,317,000 heat units we should require 8,317,000
^ 8,900 = 934 pounds of coal. At $4.50 a ton this coal would cost $2.10.
The expense account for three weeks' run now stands thus:
(1.) — RlNNING WITH THE WeLL WaTER.
Cost of scale solvent, = . . . . . $2.70
" gaskets, = . . . . . . . . . . • • 1-25
" labor, = 3.00
" the heat lost, = 2.10
Total cost with the well water, = $9.05