Heat transfer characteristics and friction factor in the horizontal double pipes of a flat tube with full and varied spacer length helical screws at various rotational speeds are investigated for Reynolds numbers ranging from 580 to 1,582. The heat transfer and friction factor of the inserted tube are significantly increased compared to those of the plain tube. The Nusselt number, friction factors, and thermal enhancement efficiency were increased with decreasing spacer length at increasing rotational speed under the same operating conditions.