Nitrogen......... | 0.1045 | --- | 0.18 | | | Ash.............. | 0.0540 | --- | --- | | | +----------+------------+ | | 85.5 | | | | | | | Total........... | 1.0000 | 2.572 | --- | | | 9-1/3.lb nitrogen | --- | --- | 118.9 | | | 6 lb. excess of air. | --- | --- | 71.4 | _| | +----------+------------+--------+------------+ Total cubic feet of | | | | | products per 1 lb. | | | | | of coal........... | -- | -- | 226.4 | 100.0 | ---------------------+----------+------------+--------+-----
-------+

The temperature of the furnace not having been determined, we must calculate it on the supposition, which will be justified later on, that 50 per cent more air was admitted than was theoretically necessary to supply the oxygen required for perfect combustion.  This would make 18 lb. of air per 1 lb. of coal; consequently 19 lb. of gases would be heated by 14,727 units of heat.  Hence: 

14,727 u. 
T = ---------------- = 3,257 deg.
19 lb. x 0.238

above the temperatures of the air, or 3,777 deg. absolute.  The temperature of the smoke, t, was 849 deg. absolute; hence the maximum duty would be

3,777 deg. — 849 deg.
--------------- = 0.7752.
3,777 deg.

The specific heat of coal is very nearly that of gases at constant pressure, and may, without sensible error, be taken as such.  The potential energy of 1 lb. of coal, therefore, with reference to the oxygen with which it will combine, and calculated from absolute zero, is: 

Units.
19 lb. of coal and air at the temperature
of the air contained 19 lb. x 520 deg. x 0.238 2,350
Heat of combustion 14,727
-------
17,078
Deduct heat expended in displacing atmosphere 151 cubic feet — 422
------
Total potential energy 16,656

Hence work to be expected from the boiler: 

/ 3,777 deg. — 849 deg. \
= 17,078 units X ( --------------- ) - 422 units
\     3,777 deg.    /
---------------------------------------------- = 13.27 lb.
966 units

of water evaporated from and at 212 deg., corresponding to 12,819 units.  The actual result obtained was 11.83 lb.; hence the efficiency of this boiler was

11.83
------- = 0.892.
13.27

I have already claimed for a boiler that it is a veritable heat engine, and I have ventured to construct an indicator diagram to illustrate its working.  The rate of transfer of heat from the furnace to the water in the boiler, at any given point, is some way proportional to the difference of temperature, and the quantity of heat in the gases is proportional to their temperatures.  Draw a base line representing -460 deg.  Fahr.,