### Basic constitutive equations

With known individual heat transfer coefficients, wall thickness, and wall thermal conductivity the overall heat transfer coefficient for plane walls is calculated as:

where

k = overall heat transfer coefficient [W/m^{2}·K]

α = individual heat transfer coefficient [W/m^{2}·K]

w = wall thickness [m]

α
= wall thermal conductivity [W/m·K]

If the wall consists of several layers of varying thickness and thermal conductivity
this equation becomes:

For a circular wall the equation becomes:

where

A = heating surface [m^{2}]

r_{o} = tube outer radius [m]

r_{i} = tube inner radius [m]

L = tube length [m]

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