The present invention relates generally to heat exchanger tubes. In particular, the invention relates to the external surface configuration of a heat exchanger tube that is used for evaporation of a liquid in which the tube is submerged.
Many types of air conditioning and refrigeration systems contain shell and tube type evaporators. A shell and tube evaporator is a heat exchanger in which a plurality of tubes are contained within a single shell. The tubes are customarily arranged to provide a multiplicity of parallel flow paths through the heat exchanger for a fluid to be cooled. The tubes are immersed in a refrigerant that flows through the heat exchanger shell. The fluid is cooled by heat transfer through the walls of the tubes. The transferred heat vaporizes the refrigerant in contact with the exterior surface of the tubes. The heat transfer capability of such an evaporator is largely determined by the heat transfer characteristics of the individual tubes. The external configuration of an individual tube is important in establishing its overall heat transfer characteristics.
There are several generally known methods of improving the heat transfer performance of a heat exchanger tube. Among these are (1) increasing the heat transfer area of the tube surface and (2) promoting nucleate boiling on the surface of the tube that is in contact with the boiling fluid. In the nucleate boiling process, heat transferred from the heated surface vaporizes liquid in contact with the surface and the vapor forms into bubbles. Heat from the surface evaporates the liquid surrounding the vapor bubble and the bubble grows in size. When the bubble size is sufficient, surface tension is overcome and the bubble breaks free of the surface. As the bubble leaves the surface, liquid enters the volume vacated by the bubble and vapor remaining in the volume has a source of additional liquid to vaporize to form another bubble. The continual forming of bubbles at the surface, the release of the bubbles from the surface and the rewetting of the surface together with the convective effect of the vapor bubbles rising through and mixing the liquid result in an improved heat transfer rate for the heat transfer surface.
It is also well known that the nucleate boiling process can be enhanced by configuring the heat transfer surface so that it has nucleation sites that provide locations for the entrapment of vapor and promote the formation of vapor bubbles. Simply toughening a heat transfer surface, for example, will provide nucleation sites that can improve the heat transfer characteristics of the surface over an otherwise similar smooth surface.
In boiling liquid refrigerants, for example in the evaporator of an air conditioning or refrigeration system, nucleation sites of the re-entrant type produce bubble columns and good surface heat transfer characteristics. A re-entrant type nucleation site is a surface cavity in which the opening of the cavity is smaller than the width of subsurface cavity. An excessive influx of the surrounding liquid can flood a re-entrant type nucleation site and deactivate it. By configuring the heat transfer surface so that it has relatively larger communicating subsurface channels with relatively smaller openings to the surface, flooding of the vapor entrapment or nucleation sites can be reduced or prevented and the heat transfer performance of the surface improved.