Usually it's 58*1800, 58*2100! Outer diameter + length, basically three types of pipes are used: galvanized pipe, pex, ppr. North: The three are the most used because of their good resistance to frost. In the south, because the frost problem is not solved and installation is easy, galvanized pipes are mainly used.
It is mainly divided into two types: all-glass vacuum solar collector tubes and glass-metal vacuum solar collector tubes. Needless to say, the all-glass models are divided into 58mm and 47mm in terms of diameter (outer diameter), and the length ranges from 1200mm to 2100mm. The all-glass vacuum collector tubes can be used in direct plug type, U-tube and heat pipe, and can be adapted to almost any type of project. Glass-metal vacuum solar collector tubes are divided into diameters of 70 and 100mm, and the lengths are also numerous, but the longest seems to be 2000mm. Glass and metal currently use types of heat pipes. Use is limited and the price is relatively high. I don't know if you are satisfied!