The behavior of discrete heat exchangers as a function of temperature, heat transfer area, and 3He flow rate is shown for a general case by means of some machine calculated curves. From these curves, the appropriate heat exchanger size can be determined for the various steps of a discrete heat exchanger. Also, curves for the mixing chamber temperature Tm as a function of total surface area divided by flow rate for various amounts of external heat to the mixer are shown for the continuous exchanger. From these curves the optimum 3He flow rate for a given heat input and exchanger size can be determined. Viscous heating has been included in the calculation of one typical coaxial tube exchanger. It is found that heating in the dilute stream has negligible effect on Tm, whereas the viscous heating in the concentrated side, though several times smaller than that in the dilute side, has considerably more effect on Tm. Based on this behavior, an improved method of ``heat sinking'' heat leak paths to the mixer is suggested.