Stevens coils. This system resembles a cooling crstllizer with hot water in the rotating elements. Generally it is not possible to achieve the high heating areas with this equipment, which means that higher approach temperatures are necessary. Because of the higher relaive speed between heating surface and massecuite, a somewhat higher temperature may be countenanced. They heve generally been phased out in favor of stationary tubular finned reaters. However they do have the advantage of achieving some mixing of the massecuite, which is a more efficient method of transferring heat in masse cuites than conduction