Abstract: An internally enhanced heat transfer tube comprising a heat transfer tube including an internal surface and an internal diameter (D); a plurality of roughness elements on the internal surface of the heat transfer tube, each roughness element having a height (e) above the internal surface where the ratio of the height (e) to the internal diameter (D) falls within the range 0.004.ltoreq.e/D.ltoreq.0.045.

Abstract: A system is disclosed for the transfer of heat from an ambient heat sink of variable temperature to a conditioned space, said system being operable in a first mode when the temperature of the heat sink is at a relatively high level, and in a second mode when its temperature is at a relatively low level. The system includes a first refrigerant circuit which is operable in said first mode as a heat pump of the vapor compression type to absorb heat from the ambient heat sink and transfer same to the conditioned space. A second refrigerant circuit is provided which is operable in the second mode, when the temperature of the ambient heat sink is at a relatively low level, to absorb heat from the ambient heat sink and transfer same to the first refrigerant circuit, which then transfers the heat to the conditioned space. Thus, in second mode operation, the first and second refrigerant circuits operate in a "cascade" relationship.