An internally grooved heat transfer tube for a cross fin tube type heat
exchanger of a refrigerating air-conditioning water supply apparatus
using a high-pressure refrigerant, wherein an intra-tubular heat transfer
rate is improved while maintaining a sufficient strength for pressure
resistance.A heat transfer tube formed of copper or copper alloy has
internal fins between internal grooves. In the tube, t/D is not smaller
than 0.041 and not greater than 0.146, d.sup.2/A is not smaller than 0.75
and not greater than 1.5, where D [mm] is an outside diameter of the
tube, t [mm] is a groove bottom thickness, d [mm] is a depth of each
groove, and A [mm.sup.2] is a cross sectional area of each groove. N/Di
is not smaller than 8 and not greater than 24 where N is a number of
grooves, and Di is a maximum inside diameter corresponding to an inside
diameter of the tube.