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In the particular situation of heat transfer across a plane wall of uniform thickness, k
is related to the individual film heat transfer coefficients, α 1 and α 2, of the two fluids by

the equation (2)

1 W (2.3)
k  ( ),
2
1  1 m K
 
 l 
1 2
where:

2
 k = the overall heat transfer coefficient (W/(m •K))
 λ = the thermal conductivity of the material (W/(m·K))

2
 α = the individual convection heat transfer coefficient for each fluid W/(m •K)
 δ = the wall thickness (m).

Heat transfer coefficient is the inverse of a thermal insulance. This is used for

building materials (R-value) and for clothing insulation.
1 1  1
R     (2.4)
k  1   2

Temperatures t S1 and t S2 on the surfaces of a plane wall are determined from equations:

1  1  
t  t  q , t  t   q   (2.5)
1 s 1 s 2 1  
  
1  1 
For multi-layered cylinder plane wall

1
k (2.6)
1  1
  i 
  
1 i 2

For the special but very important case of heat transfer through the wall of a plain
round tube, the different heat transfer areas on the inside and outside surfaces of the

tube need to be considered:

1
k  (2.7)
l
1 1 d 2 1
 ln 
 d 2 d  d
1 1 1 2 2

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