Curve No. 5 shows the pounds of dry chimney gases per pound of "combustible." It is very little affected, tending to rise with the "fixed carbon." It varies inversely with furnace and flue temperatures.

Curve No. 6 shows the per cent of completeness of combustion, being the percentage of the heat of the combustible which was actually generated. It rises with increase of "fixed carbon" and its value of about 90 per cent indicates a wide margin for improvement.

RELATION OF EFFICIENCY 72* TO PER CENT OF OXYGEN IN "COMBUSTIBLE."

The chart shown in fig. 52 contains two curves plotted on the same data. The upper one was obtained by classifying the tests according to code "boiler efficiency" (72*) and the lower by classify

FIG. 52. Relation of efficiency 72* to per cent of oxygen in "combustible," classified on each as a basis. Tests 101-400.

ing them according to the per cent of oxygen in the "combustible." By the "combustible" is meant what is left of the coal after taking out the moisture, sulphur, and ash. This remnant has been designated as "pure coal," which is preferable to calling it "combustible."

Both curves show unmistakably that somehow the oxygen of the coal, which is chemically combined, exerts a harmful influence. It seems plausible that there is nothing inherent in the presence of oxygen in a coal molecule which makes it impossible to break it down and burn it as well as any molecule lacking oxygen. This assump

tion puts the blame on our furnaces.

This chart is in agreement with that of fig. 33 (p. 56), which presents a classification of several items on a smoke basis. The "unaccounted-for" loss, CO loss, and per cent of oxygen all rise with the increase of smoke and the efficiency falls very slightly.

BRITISH THERMAL UNITS PER POUND OF DRY COAL.

The curves of fig. 53 are based on 296 tests made on coals from all parts of the country. In general, the eastern coals fall on the

FIG. 53. Relations of B. t. u. value of dry coal to boiler efficiency 72* (curve No. 1); per cent rated capacity developed (curve No. 2); combustion-chamber temperature (°F.) (curve No. 3); pounds of dry chimney gases per pound of "combustible" (curve No. 4); per cent completeness of combustion (E) (curve No. 5). Tests 89-400.

right and the lignites on the left part of the curve.

On curve

No. 1 the small number near each point gives the number of tests

falling in that B. t. u. group. is based on the same number of tests in the various groups. The four-place numbers in pairs on curve No. 1 indicate the highest and lowest efficiencies 72* falling in each group. It will be noted that the highest efficiencies are nearly as good in the left groups as in the right, indicating that it is possible to burn coals of low heating value as efficiently as high-grade coals. The increase of average efficiencies is only about 7 per cent for the range of 5,000 B. t. u.

Each of the other four curves below

The average capacity, as shown by curve No. 2, rises considerably as the coal improves in heating value. Perhaps most of the low B. t. u. coals could be made to furnish as much steam if burned on a larger grate. If the coal did not clinker seriously, the use of a rocking grate would increase the capacity. On some coals the use of the automatic stoker might give better results.

Curve No. 4, pounds of dry chimney gases per pound of "combustible," descends with the better coals, partly because they are usually more manageable on the fuel bed and because they seem to break down on heating with evolution of gaseous compounds easier to burn, on which account the air supply can be reduced with safety. Curve No. 5 gives the average "furnace efficiency," or, better, the per cent of completeness of combustion. These calculated efficiencies are only approximate.

RATIO OF ASH TO SULPHUR IN DRY COAL.

It is sometimes said that although sulphur in coal does not of itself materially reduce the value of the coal for steam making, a low percentage of ash with a high percentage of sulphur is apt to make trouble. To test this possibility a classification was made of many tests on many coals, the tests being grouped according to the ratio of total ash to sulphur. The corresponding values of the averages of efficiencies 72* are practically constant.

As a check on this work a classification was also made of the ratio of ash to sulphur in dry coal on an efficiency 72* basis. As the efficiency 72* increases there is practically no change in the value of the ash to sulphur ratio. (See figs. 21, p. 30, and 28, p. 43.)

A table of efficiencies 72* classified on the basis of per cent of ash plus sulphur in dry coal shows little variation between the average efficiency values for the several groups. Tables showing the three classifications follow:

Classification of efficiency 72* on basis of ratio of ash to sulphur in dry coal.

Classification of ratio of ash to sulphur in dry coal on basis of efficiency 72*.

Classification of efficiency 72* on per cent of ash plus sulphur in dry coal (Indiana, Illinois, and western Kentucky coals).

Per cent of ash plus sulphur.

Up to 9. 9 to 11.11 to 13. 13 to 15. 15 to 17. 17 to 19. 19 to 21. 21 to 23. 23 up.

SUGGESTED STEAM-TURBINE CYCLES, HEAT ABSORPTION, AND BOILER EFFICIENCIES.

THE BOILER AND THE FURNACE AND THEIR EFFICIENCIES.

The boiler with its setting consists mainly of two parts, the furnace and the boiler proper. The furnace is the heat generator and the boiler the heat absorber. In practice both are imperfect in their functions. A perfect furnace would burn the fuel completely. A perfect boiler would absorb all the heat evolved in the furnace provided the temperature of the water in the boiler was that of the atmosphere. It follows, then, that the furnace efficiency is the ratio of the heat evolved in the furnace to the potential heat of the fuel fired, and that the boiler efficiency is the ratio of the heat absorbed by the boiler to the heat evolved in the furnace; the combined efficiency of the furnace and boiler being the ratio of the heat absorbed by the boiler to the potential heat of the fuel fired. The grate is taken as a part of the furnace. The true boiler efficiency is the ratio of the heat absorbed to the heat which is available to the boiler, this available heat being that portion of the heat in the furnace gases which is above the temperature of the steam.

If the wording in the above definitions is somewhat changed another set of efficiencies is obtained, which may be called the thermodynamic efficiencies. The efficiency of the furnace is the ratio of the heat made available to the boiler to the potential heat of the fuel fired. The efficiency of the boiler is the ratio of that portion of the heat absorbed which is available for power purposes (engine or tur

bine) to the heat available to the boiler. The combined efficiency of the furnace and boiler is the ratio of the heat made available for power purposes to the heat value of the fuel fired. The fact is that the steam engine is usually blamed for wasting heat which is not available to it.

To illustrate the above definitions let the various heat values be represented by diagrams. For the representation of the heat in the furnace a diagram such as given in A, fig. 54, can be used. This diagram has absolute temperature for the ordinates, and a factor proportional to the specific heat of the furnace gases multiplied by their weight as abscissas. In the same chart (fig. 54, B) is an ideal cycle of a steam engine or turbine plotted on temperature-entropy

FIG. 54.-Temperature-entropy diagrams for boiler: A, For air and coal; B, for water and steam.

plane. It must be understood that the abscissas of these two diagrams are in different units. The abscissa in B is taken in entropy units because the adiabatic expansion and the availability of heat to the steam engine or turbine can be best shown by temperatureentropy diagrams.

On the assumption that 1 pound of combustible is completely burned, the various quantities of heat in the furnace are represented in A as follows: The area ABCD represents the potential heat in the fuel and also the heat developed in the furnace. The heat available for the boiler, represented in the diagram by the area OBCP, is the heat above the temperature of the steam and is the limit of the heat which the boiler can absorb. The remainder of the heat, repre