Heat Transfer Coefficient

To calculate the coefficient α we need the relative easy accessible values of the temperature and the volume flow:
The transferred heat (Q) in an exchanger:
Q = α x A x (T2 - T1) x Δt
with: α = heat transfer coefficient, A = heat transfer surface, (T2 -T1)= temperature difference, Δt = examined time frame
The transferred heat (Q) of one of the liquids:
Q = cp x dm x (T2 - T1) x Δt
with: cp = heat capacity of the fluid, dm= volume flow of the fluid, (T2 - T1) = temperature difference, Δt = examined time frame

equalizing the both heat transfers as well as the average temperatures ΔTm
                  (T_Hot_In -T_Coldt_Out) - (T_Hot _Out- T_Coldt_In)
       ΔTm =                                                                                                 
                                         T_Hot_In - T_Coldt_Out
                                         T-Hot_Out - T_Coldt_In

results in the heat transfer coefficient (α):

                   cp x dm x (T2 - T1)
       α =                                      
                        A x ΔTm

For the control (development of heat transfer coefficient) are no absolute values required, so it is possible to set A as constant (=1), and assuming always the same liquid cp and hence the same density ρ and set both as well as constant(=1). Doing like this we will get the "qualitative" Heat transfer coefficient αm .
αm = 1 x dv x (T2 - T1) / 1 x ΔTm

             1 x dv x (T2 - T1)
αm =                             
                  1 x ΔTm

             dv x (T2 - T1)
αm =                             

using: T2, T1 the inlet and outlet temperature of the fluids and the volume flow.
The heat loss is also taken as constant.