Designing a heat exchanger has two aspects: Rating and Sizing.
If you are referring to the rating problem where the shell and tube side configuration is known, feed stream conditions are known and you want to calculate exchanger area, overall heat transfer coefficient, thermal efficiency and pressure drop, then that can be done by selecting Calculation Type as "Shell and Tubes Exchanger Rating". Spoken tutorial titled Shell and Tube Heat Exchanger also deals with the same.
If you are referring to the sizing problem where the heat exchange requirement is specified and the designer must calculate the heat exchanger size. Normally, pressure drop limits are given for each fluid stream. Since the entering flow rates, temperatures and pressures are given, and the heat duty (or leaving temperatures) is specified, the thermal effectiveness, NTU are directly calculable. A true sizing problem is considerably more complex than a rating problem. A number of decisions must be made prior to making the thermal performance calculations. This is not possible to simulate in DWSIM.
However, we are working on the sizing of the heat exchanger in OpenModelica. Once completed, we will also add it to the Chemical Simulator library in OpenModelica.