Part 53

C = per cent of carbon in coal by ultimate analysis,

CO and CO{2} = per cent of CO and CO{2} by volume from flue gas analysis.

10,150 = the number of heat units generated by burning to CO_{2} one pound of carbon contained in carbon monoxide.

(E) Loss due to unconsumed carbon in the ash (it being usually assumed that all the combustible in the ash is carbon).

Loss in B. t. u. per pound = per cent C × per cent ash × B. t. u. per pound of combustible in the ash (usually taken as 14,600 B. t. u.) (36)

The loss incurred in this way is, directly, the carbon in the ash in percentage terms of the total dry coal fired, multiplied by the heat value of carbon.

To compute this item, which is of great importance in comparing the relative performances of different designs of grates, an analysis of the ash must be available.

The other losses, namely, items 2, 5 and 8 of the first classification, are ordinarily grouped under one item, as unaccounted for losses, and are obviously the difference between 100 per cent and the sum of the heat utilized and the losses accounted for as given above. Item 5, or the loss due to the moisture in the air, may be readily computed, the moisture being determined from wet and dry bulb thermometer readings, but it is usually disregarded as it is relatively small, averaging, say, one-fifth to one-half of one per cent. Lack of data may, of course, make it necessary to include certain items of the second and ordinary classification in this unaccounted for group.

TABLE 57

DATA FROM WHICH HEAT BALANCE (TABLE 58) IS COMPUTED

+------------------------------------------------------+ |+----------------------------------------------------+| ||Steam Pressure by Gauge, Pounds | 192 || ||Temperature of Feed, Degrees Fahrenheit | 180 || ||Degrees of Superheat, Degrees Fahrenheit |115.2|| ||Temperature of Boiler Room, Degrees Fahrenheit| 81 || ||Temperature of Exit Gases, Degrees Fahrenheit | 480 || ||Weight of Coal Used per Hour, Pounds | 5714|| ||Moisture, Per Cent | 1.83|| ||Dry Coal Per Hour, Pounds | 5609|| ||Ash and Refuse per Hour, Pounds | 561|| ||Ash and Refuse (of Dry Coal), Per Cent |10.00|| ||Actual Evaporation per Hour, Pounds |57036|| || .- C, Per Cent |78.57|| || | H, Per Cent | 5.60|| ||Ultimate | O, Per Cent | 7.02|| ||Analysis -+ N, Per Cent | 1.11|| ||Dry Coal | Ash, Per Cent | 6.52|| || '- Sulphur, Per Cent | 1.18|| ||Heat Value per Pound Dry Coal, B. t. u. |14225|| ||Heat Value per Pound Combustible, B. t. u. |15217|| ||Combustible in Ash by Analysis, Per Cent | 17.9|| || .- CO_{2}, Per Cent |14.33|| ||Flue Gas -+ O, Per Cent | 4.54|| ||Analysis | CO, Per Cent | 0.11|| || '- N, Per Cent |81.02|| |+----------------------------------------------+-----+| +------------------------------------------------------+

A schedule of the losses as outlined, requires an evaporative test of the boiler, an analysis of the flue gases, an ultimate analysis of the fuel, and either an ultimate or proximate analysis of the ash. As the amount of unaccounted for losses forms a basis on which to judge the accuracy of a test, such a schedule is called a "heat balance".

A heat balance is best illustrated by an example: Assume the data as given in Table 57 to be secured in an actual boiler test.

From this data the factor of evaporation is 1.1514 and the evaporation per hour from and at 212 degrees is 65,671 pounds. Hence the evaporation from and at 212 degrees per pound of dry coal is 65,671÷5609 = 11.71 pounds. The efficiency of boiler, furnace and grate is:

(11.71×970.4)÷14,225 = 79.88 per cent.

The heat losses are:

(A) Loss due to moisture in coal,

= .01831 ((212-81)+970.4+.47(480-212)) = 22. B. t. u., = 0.15 per cent.

(B) The loss due to the burning of hydrogen:

= 9×.0560((212-81)+970.4+.47(480-212)) = 618 B. t. u., = 4.34 per cent.

(C) To compute the loss in the heat carried away by dry chimney gases per pound of coal the weight of such gases must be first determined. This weight per pound of coal is:

(11CO{2}+8O+7(CO+N)) (-------------------)C ( 3(CO{2}+CO) )

where CO_{2}, O, CO and H are the percentage by volume as determined by the flue gas analysis and C is the percentage by weight of carbon in the dry fuel. Hence the weight of gas per pound of coal will be,

(11×14.33+8×4.54+7(0.11+81.02)) (-----------------------------)×78.57 = 13.7 pounds. ( 3(14.33+0.11) )

Therefore the loss of heat in the dry gases carried up the chimney =

13.7×0.24(480-81) = 1311 B. t. u., = 9.22 per cent.

(D) The loss due to incomplete combustion as evidenced by the presence of CO in the flue gas analysis is:

0.11 ----------×.7857×10,150 = 61. B. t. u., 14.33+0.11 = .43 per cent.