I have a complex mathematical model (about 2000 lines of code) which calculates heat exchanger performance. Using Q=UxAxLMTD I want to iterate the entering temperature until I find that the installed surface satisfies a target duty. At present I 1 guess an entering temperature and from the duty calculate the leaving and mean temperature. 2 Calculate various fluid properties, Reynolds No, Prandtl No etc and so a U-value 3 Using the target duty, I calculate the required surface 4 I compare the required surface with the actual surface and make a correction factor ie k x ((Required-Actual)/Actual) where k is say 0.25 5 New entering temperature = previous entering temp x (1- correction) after playing about with k I generally get convergence at between 5 and 40 iterations but sometimes I get oscillation about the solution. The books I have seem to use first and second order derivatives but my process is too complex for that. Any simple solutions?