[QUOTE=RogGoetsch]Just a reminder that "U" is the overall coefficient of heat transfer, so the thermal conductivity of a given material is usually only a small factor in heat exchanger design. In my experience, surface film factor coefficients rule.

Rog,
With regards to surface film factor coefficients ruling we know how oil logging has a significant impact on the operation of a remote, low-temperature refrigeration unit in addition, a similar situation exists when we have laminar oil flow which results in poor oil return to the compressor and poor heat transfer so a concept to remedy this situation is called Altered Bi-phase Flow (ABF) regime; an annular (ring-like) film maximizes heat transfer throughout the evaporator, increasing the useable evaporator tube surface area. The result is a cooler tube wall with a higher evaporator pressure. The technology’s primary benefits are the elimination of laminar refrigerant flow and the resulting poor heat transfer coefficients.
The present method has been focused on having liquid at the entry to the evaporator to ensure that the maximum heat transfer capacity was available. This new approach focuses more on the rate of heat transfer. The method has been described as “distributed enthalpy,” because the slow-moving refrigerant at the evaporator entry is more evenly distributed throughout the length of the evaporator. It develops a multistage pressure drop in that it separates the liquid from the vapor and entrains the liquid into the higher vapor velocity. The resulting temperature and pressure uniformity within the evaporator improves frost formation, refrigerant feed stability, compressor ratios, and net cooling rate. That directly impacts product temperature recovery and the annular flow that is derived from distributed enthalpy encourages oil to return to the crankcase and provides proper lubrication — even after extended periods of operation.