Notations:
b  is the pitch of the furnace corrugations (in
mm);
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 crosssectional area of a longitudinal
section of the corrugated furnace wall, of length
b and thickness (ec) (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 mm^{4});
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 mm^{4}) (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)
S_{1}  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.
S_{2}  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.
