There are many ways to make a 90 elbow with iron pipes, depending on the materials and needs. There are usually the following types: seamless pipes are not too thick, with a diameter of about 30 or less, and can be cold-bent, that is, directly bent into the required shape with a pipe bender; The nozzle is easy to crack when it is cold-bent, and can be hot-bent, that is, heated to about 500℃ and directly bent; The diameter is large, about 30 to 80, so it is best to fill dry sand first and then hot bend; No matter how big the diameter is, the pipe wall is thinner, so it is suggested to cut and weld, which can be cut into two 45 angles and welded in turn, just like chimneys used in the past; It can also be cut into a variety of shapes with a big head and a small head, and then paired into a "shrimp bend" to make the bend look like a prawn shell, one section at a time. The former has a simple structure, but it has a large resistance to fluid, while the latter has a small resistance to fluid.