Theory

The shell is the first of the planar layers between the gas-side wall face and the surroundings (Thermal condition of a wall); its conductivity $k$ enters the conduction resistance $t/k$ with the shell thickness (Shell thickness). The library holds one conductivity per material (the solver's layer model takes a constant conductivity) and the emissivity used when Emissivity is on auto:

material	k [W/(m K)]	emissivity (range)	service limit
Carbon / aluminised steel	42	0.8	-
Stainless steel 304 / 409	20	0.5 (0.4-0.6)	-
Aluminium	165	0.15 (0.1-0.2)	250 °C
User defined	Shell conductivity	0.8	-

For a steel shell of 1.5 mm the shell resistance is about $10^{-4}$ m² K/W, far below the outer film resistance (about 0.1 m² K/W at $h \approx 10$ ): the choice of steel hardly changes the heat loss, the emissivity and any insulation do. The library is shown read-only on the Material step.

Equations

R_{shell} = \frac{t}{k}

(1)

where	meaning	unit
$R_{shell}$	Conduction resistance of the shell per unit area	m² K/W
$t$	Shell thickness	m
$k$	Conductivity of the material	W/(m K)

Default and rule

Default: Stainless steel 304 / 409 (k 20 W/(m K), emissivity 0.5)
Rule: Stainless steel 304 / 409.
Source: The library values are the program's design values at about 400 °C (one value per material; the default emissivity is the middle of the stated range). They are not taken from a cited handbook; check them against your material data.

Valid range Carbon / aluminised steel | Stainless steel 304 / 409 | Aluminium | User defined

Service limit

A case whose gas inlet is hotter than the material's service limit (aluminium above 250 °C) gets a warning.

Constant conductivity

The conductivity does not vary with temperature.

Theory

Equations

Default and rule

Valid range Carbon / aluminised steel | Stainless steel 304 / 409 | Aluminium | User defined

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