Some caveats: this technique is for shell and tube exchangers, using English units of inches, feet, and square feet. It does not work well for other designs, although a clever engineer can probably develop similar quick estimating procedures. Also, for highly viscous fluids, shorter tubes are desirable to minimize pressure drop. There are also various tube inserts to increase turbulence in low-velocity but shorter tubes.
Roger E. Sowell, Esq.
Marina del Rey, California