Log Mean Temperature Difference, ∆ The log mean temperature difference, ∆ , given in Equation S39, is an average effective temperature difference between the two fluids within the heat exchanger. It is a function of the temperature differences at arbitrarily assigned ends "A", ∆ , and "B", ∆ . The log mean temperature difference makes the assumption of constant heat capacity in the fluids; thus it can't be used across heat exchangers which transition between phase changing and non-phase changing processes. It must be applied separately to each process.