With the cooperation and under supervision of our past president  Ir. H. Ghys
In practice heat transfer is the sum of the heat transfer that takes place through conductivity, convection and radiation. The relation below applies:
q = k x A x∆T x t
q :  The quantity of heat ( j )
k :  total heat transfer coeff. ( W/m2 x K)
A : area (m2)
∆T = temperature difference
t : time ( s )
Heat transfer frequently occurs between two bodies, separated by a wall.
For such a (clean) flat wall the relation below applies:
1/k = 1/α1  + d/λ  + 1/α2
k :  total heat transfer coeff. ( W/m2 x K)
α = heat transfer coefficient on respective
      sides of the wall ( W/m2 x K )
d = thickness of the wall ( m ) λ = coefficient of thermal conductivity
       for the wall (W/m x K)
The transferred quantity of heat in a heat exchanger, is at each point a function of the prevailing heat difference and the total heat transfer coefficient. Applicable to the entire heat transfer surface is:
Q = k x A x LMTD
Q = tranferred quantity oh heat ( Watt )
k = total heat transfer coeff. (W/m2 x K)
A  = heat transferring surface (m2) LMTD = logarithmic mean temperature
  diff. ( K )
The logarithmic mean temperature difference is defined as the relation between the temperature difference at the heat exchanger's two connection sides according to the expression:
LMTD = (Δ1- Δ2 / ( In Δ1 / Δ2 )
LMTD  =  logarithmic mean tempt. diff.(K) Δ1 =  the tempt. difference ( K ) Δ2 =  the temperature difference ( °K )
Counter flow Parallel flow