losses various types of extended surfaces such as longitudinal latitudinal and or corrugated fins may be used to increase the heat transfer surface on the sand side furthermore the fins may have additional features to improve the contact between the flowing sand and the base tubes in addition the tubes may have elongated or elliptical shapes to improve the contact and heat transfer with the sand indeed the preferred u201c tube u201d cross section may be more plate like or similar to an elongated rectangular passage than a generally circular u201c tube u201d these additional features enhance the contact between tube and fins with the sand and heat transfer to or from the sand may be included various additional features to enhance heat transfer to or from the sand or from the tube to the internal heat transfer fluid may be included in some situations for example it may be advantageous for the granular material to travel in overall cocurrent flow to the flow of internal heat transfer fluid fig3 b illustrates an exemplary embodiment in which the sand or similar granular material 15 is moved by the conveyor in a direction 35 roughly in cocurrent flow to the flow 38 of the heat transfer fluid 28 an overhead view of the supply and return piping including the heat transfer tube bundle 8 is shown in fig4 the heat transfer tube bundle 8 is integrated.