A boiler furnace is primarily a chemical reaction chamber, designed to promote and control the oxidation of hydrocarbon fuels. A secondary function is to absorb a portion of the reaction heat evolved. The temperatures within a boiler furnace can be controlled, to a large extent by the amount of effective Radiant Heat Absorbing Surface (RHASJ) present in the furnace. Heat is radiated from the flames (a cloud of flaming particles of carbon, ash, and molecules of gas) as a primary source, to the heat absorbing tube surfaces, with the uncooled refractory surfaces acting as an intermediary, receiving heat from the flames and reradiating this heat to the absorbing surfaces and back to the flames. Refractory surfaces reradiate heat by virtue of their own temperature, not by a reflecting ability. Refractory must, therefore, have a temperature intermediate between the flames and the heat-absorbing surfaces. The smaller the area of refractory present within the furnace, compared to the total surface, the lower will be the refractory temperature. For a given heat input or firing rate, the heat absorbed per unit area decreases with an increase in total RHAS. However, the greater the RHAS the greater will be the total amount of heat absorbed by the furnace and therefore the temperature of the gases leaving the furnace will be lower. The absorption rate, BTU per hour per square foot of RHAS is therefore an excellent criterion of furnace loading. Calculation of this absorption is, however, a very complex problem. Each designer employs his own semi-empirical methods for this computation so that these methods are proprietary. All designers commence with the same primary variable, namely, the heat input, BTU per hour input to the furnace per square foot of RHAS, and it is recommended that this be used as an approach to furnace design and performance criteria. Since marine boilers are normally fired with residual oils of nearly uniform heating values, this criterion can be reduced to lbs, of fuel oil per hour per square foot of RHAS. Such a figure is significant if used in conjunction with the standard reference fuel recommended in the T&R Bulletin No. 3-11. The present trend is toward more extensive water cooling in the form of tangent tube walls in the side, roof and rear, and in some cases in the front wall. This trend reflects the demand for less refractory maintenance, accomplished by lowering the operating temperature of the refractory. For some years, specifications for marine boilers have set forth a maximum heat release per cubic foot of furnace volume per hour, usually in the range from 65000 to 85000 BTU/cu. ft. hr. at normal load. As a performance criterion, this measures only the ability of firing equipment to maintain a satisfactory combustion condition. It is particularly misleading as a measure of refractory temperature and RHAS loading since furnace volume increases as the cube while the area increases only as of the square of the linear size increase. In calculating this fuel firing rate per square foot of RHAS, certain factors in calculating the area must be considered as given in the following sections.