Supercritical units usually work at a pressure of 24 MPa and above, which is higher than the critical water pressure of 22.09 MPa. A subcritical cylindrical steam generator usually works at about 13 MPa or 18 MPa. Many of the steam generators purchased in the 1970s and 1980s are cylindrical water tube types that operate at 18 MPa, produce superheated steam at 540°C, and have one or two stages of steam reheating. These generators have the ability to burn pulverized coal and petroleum fuels, although petroleum fuels gradually abandoned due to the increase in price and problems related to their supply. Although natural gas still used in power plants in some parts of the world, due to its high cost, it now mostly used for domestic purposes in the United States of America. Anyway, natural gas is a clean burning fuel and relatively pollution free. The steaming capacity of modern power plant steam generators is high and its value can vary from 125 to 1250 . Power plants are between 125 and 1300 megawatts. On the other hand, industrial steam generators are those that used in industrial companies and other institutions and include different types. These generators can be pulverized coal-fired water tube type steam generators, although they are powered by lump coal, oil or natural gas, and often a combination of them, as well as municipal waste, process waste energy, or other by-products also be used. In some of them even electric heating is used. Some of them are of the heat recovery type, in which the waste heat of industrial processes is used. These generators can also be of the fire tube type. Industrial steam generators usually do not produce superheated steam, but produce saturated steam or even only hot water.