ARRANGEMENT OF BOILER FURNACES The only object of burning fuel under a boiler is to convey heat to the water inside the boiler. Any furnace arrangement which allows the heat provided by the combustion of the fuel to escape up the stack is an inefficient arrangement. It is, of course, impossible to attain 100 percent efficiency in the burning of fuel in furnaces. It should be emphasized, however, that the furnace is simply a means of transferring to the water the heat units contained in the fuel.
In burning fuel oil under boilers, all of the oil should be con sumed before it reaches the boiler surface because the impinge ment of the flame upon the boiler surface retards or arrests com bustion. Practically all of the modern oil burners introduce the oil into the furnace in finely divided particles for the purpose of shortening the duration of the burning and the _oil spray is thor oughly mixed with air before it is raised to the furnace tempera ture. Careful attention to the design of the furnace is of much more importance than is the selection of a burner.
Incandescent brick work around the flame is, of course, de sirable, but in many cases a satisfactory compromise is effected by using a flat flame burning close to the white-hot floor through which air is steadily flowing. Even in a cold furnace a good burner will maintain a suspended clear and smokeless flame. The path of the flame should be such that heat is uniformly dis tributed over the boiler heat-absorbing surface without direct flame impingement. The linings of furnaces should be kept tight and there should be no openings except those necessary for the in troduction of the mixture of fuel oil and air. Improper insulation results in radiation of heat from a furnace. Each square foot of exposed wall or arch surface represents a loss of heat through radiation. The refractories used should be the best of uniform thickness, and as mechanically perfect as possible. Under ordinary firing the first pass of the boiler should be located directly over the furnace in order that the heating surface may absorb the radiant heat from the incandescent -fire brick. Gen erally speaking, it is not desirable to have fire brick arches and target walls because they localize the heat with a resultant burn ing out of tubes or bagging of shell on account of the limited overload capacity.
The velocity of the gases in their passage through the fur nace should not be so high that complete combustion of the oil does not take place. The problem of obtaining complete coin: bastion is comparatively simple. Sufficient oxygen must be sup plied to burn the hydrocarbons contained in the fuel oil and excess air must be avoided.
The following statement in the Report of the U. S. Naval "Liquid Fuel" Board gives concisely the fundamentals of fur nace design: "A liquid fuel such as crude petroleum requires an ample combustion space, more indeed than does almost any other sort of combustible material. The relative dimensions—length, breadth, and depth—of the combustion spaces are of minor im portance. The primary requisite is volume, and that alone, pro vided all parts of it are traversed by the same quantity of gas in a given time ; in other words, provided the gases are not short circuited through or across some parts of the space to the neglect of others. Thus, if a current of gas flows through a cubic foot of space at the rate of 1 cubic foot per second, each particle of gas will spend one second within the space, regardless of whether the space is long and narrow or short and wide. In a long and narrow space there is less chance of the gases taking a short cut, and herein lies the sole utility of introducing baffles in the combustion space. Indeed, there is a strong objection to their introduction arising from the fact that the narrower the passage the greater will be the velocity of flow and the greater the distance to be traversed. Since the resistance that the draft pressure must ovrcc,ine is proportional to the square of the velocity of flow and to the length of the passage, it follows, that for a given volume of combustion space the draft resistance will be propor tional to the cube of its length. The advantages are, therefore, in favor of the combustion space of large cross section and short in the direction of the flow of the gases.