c - is the corrosion allowance = 0.75mm

d - is the mean diameter of furnace (in mm) (see note 1);

d_max - is the maximum mean diameter of the furnace (in mm)

d_min - is the minimum mean diameter of the furnace (in mm);

e - is the furnace plate thickness (in mm);
E - is Young’s modulus of elasticity (in kg/cm²) at the design temperature.
Et - is the specified minimum elevated temperature yield stress or the 0.2% proof stress at the design temperature (in kg/cm²);
F - is the cross-sectional area of a longitudinal section of the corrugated furnace wall, of length b and thickness (e-c) (in mm²) (see Figs. XII/94A to C, XII/94D to E, XII/94F to G).

I - is the second moment of area of one complete corrugation about its neutral axis excluding the corrosion allowance (in mm⁴);
Is - is the second moment of area of plain stiffener section(see Fig. XII/21 & XII/22) about its neutral axis, including a length of the furnace of 0.55 (de)^1/2 on each side of the stiffener (in mm⁴) (see note 2);
L - is the distance between the centre of two effective points of support (in mm) (see note 3);
P - Is the design pressure (in kg/cm²)
S₁ - is the factor of safety:
     =2.5 for furnaces in class I and class II boilers
    =3.5 for furnaces in class III boilers
    =2.0 for furnaces and wrapper plates not exposed to flame.
S₂ - is the factor of safety:
    =3.0 for class I and class II boilers
    =3.9 for class III boilers
u - is the percentage out of roundness to be taken as 1.5 for plain furnaces      and1.0 for corrugated furnaces;
w - is the depth of corrugation (in mm).

NOTE 1: For corrugated furnaces, mean diameter is equal to inside diameter plus full depth of one corrugation referred to in Fig. XII/94  which is equal to the inside diameter + e + W.

NOTE 2: When calculating Is, it is only necessary to take into account a corrosion allowance on the furnace gas side.

NOTE 3: Stiffeners complying with figure XII/21 and XII/22 boiler and reversal chamber end plates are considered to be effective points of support.

NOTE 4: The design temperature of the furnace = ts +4e + 15 saturation temperature plus four times the thickness of the furnace plus 15.