Statements have been made that from 50 to 100 per cent of air is required, depending upon the system and type of furnace used.
Some authorities have stated that not more than 2 to 3 pounds should be consumed per square foot of combustion space in order to insure smokeless com bustion, but in practice more will be found necessary.
An excess amount of air will so lower the tem perature of the furnace gases as to delay combustion to an extent that will cause carbon monoxide to pass off unburnt from the furnace. A sufficient amount of carbon monoxide in the gases may cause the action known as secondary combustion, by igniting or mingling with the air after leaving the furnace. This causes the breachings and doors to shake, also fires in the stack.
If less than the required amount of air is sup plied, the carbon burns to monoxide instead of dioxide, and its full heat value is not developed, and will read ily be detected by the appearance of smoke. The ab sence of smoke does not necessarily demonstrate or prove that the combustion is perfect, but a clear flame consumes less oil than a smoky one.
A dark red flame, slightly smoking, is nearer to complete combustion. The chemical combination of a combustible with oxygen disengages energy in the form of heat.
The quantity or measure of this heat may be ex pressed in British thermal units (B.t.u.), or the quart tity of heat required to raise the temperature of one pound of water, one degree Fahrenheit.
The number of British thermal units released by the combustion of one pound of the following sub stances, and the resultant temperatures are : Hydrogen burned to 11.0, 62,032 B.t.u. Temp. 5,898° F. Carbon burned to CO., 14,500 B.t.u. Temp. 4,939° F. Carbon burned to CO, 4,452 B.t.u. Temp. 2,358° F.
The great loss of heat due to the incomplete com bustion of carbon is shown by the difference between the total heat of perfect combustion of carbon to CO. (14,500 B.t.u.), and that of carbon to CO (4,452 B.t.u.).
One pound of carbon, when imperfectly burned produces 12 + 16 = 2 1-3 pounds of carbon monoxide. 12 If this quantity of gas is burned to carbon monox ide, the total amount of heat released will be 14,500 — 4,452 = 10,048 B.t.u. ; therefore the calorific value of 048 one pound of carbon monoxide is 10, = 4312 B.t.u.
2 1-3 A typical analysis of petroleum is as follows : Carbon 84 Hydrogen .14 Oxygen 02 1 00Allowing the oxygen to unite with its equivalent of hydrogen to form water, the calorific value would be based upon the corrected quantities as follows : Carbon .84 X 14,500 = 12,180
Hydrogen .1375 X 62,032 = 8,520 20,700 B.t.u.—the calorific value of one pound of oil when its carbon is burned to CO..
If, however, combustion is imperfect and the car bon be burned to CO, then the total heat would be : .84 X 4,452 = 3,739 .1375 X 62,032 = 8,529 12,268 B.t.u., or about 59 per cent of that obtained in the first instance. In fuel oils, hav ing a less percentage of hydrogen, the proportionate loss would be greater.
The following report was recently made on board a large ocean going steamer in order to determine the furnace efficiency of the boilers.
"In accordance with our agreement with you we have made investigations regarding the furnace effi ciency in the boilers of your steamship Queen and beg to submit the following report : Flue gas samples were obtained from the front con nection of each furnace in all of the boilers to avoid possible air leaks in the uptake and to obtain individual furnace conditions and analyzed to determine the per centage of carbon dioxide contained therein with a view to increasing same to the point of maximum efficiency. Our initial determinations revealed the fact that an unusually high percentage of CO. prevailed under ordinary running conditions. This was found to be 10.0 per cent, which was soon raised to 11.1 per cent by a few minor changes indicated by the analyses. In one particular case the furnace only indicated 7.0 per cent of CO2 and an investigation revealed a dirty burner. The burner was immediately changed and the furnace then produced 11.4 per cent of CO. with out alteration of draft. In another case similar con ditions prevailed and the CO. content was raised from 9.0 per cent to 13.0 per cent. Two analyses were taken from each furnace in succession to check up any error or inaccuracy in the work and to avoid false observations. Except in a few isolated places the fire men seemed to be getting exceptional results and with the aid of the analyses the inferior fires were soon brought up to normal. The only guide for the fire men, however, is the expansion pyrometer in the stack and indications of smoke in the stack and the latter case is deceiving because one furnace might easily pro duce all of the smoke while, in an effort to better con ditions the firemen would probably alter each furnace. This, you can see, would ordinarily result in an excess of air in every furnace but one or two.