The length of a flat circular tube with a diameter of 14mm can reach about 2 1.98 mm, and the circumference =2× radius× pi = diameter× pi. Theoretically, the flattened length is half of the circumference,14 mm× 3.14 ÷ 2 = 21.98 mm, but the specific length should be determined according to the comprehensive factors such as the material, hardness and thickness of the inner wall of the circular tube.

A circular tube is made of steel with hollow concentric cross section and open ends, and its length is greater than its circumference. It can be used in pipelines, thermal equipment, machinery industry, petroleum geological drilling, containers, chemical industry and special purposes. The specifications of circular tubes are expressed by outer diameter (such as outer diameter or side length), inner diameter and wall thickness, and their sizes range from capillary tubes with very small diameter to large circular steel tubes with a diameter of several meters.