Borosilicate glass (Pyrex) tubing has been used in heat recovery applications because it is most resistant to chemical attack and presents no corrosion problems. Fouling is minimal due to the smoothness of the surfaces. These tubes also have a low coefficient of expansion and are resistant to thermal shock, which makes them suitable as heat exchanger tubes. However, the temperature limit is about 500°F, and the pressure limit is also low, on the order of 60 psig or less. The thermal conductivity is lower than that of carbon steel, by about one — third; however, because the tube wall thickness is low, the wall resistance to heat transfer is also low. Thus, compared to carbon steel tubes the overall heat transfer coefficient is lower by only a small margin. Flue gas to water heat recovery has been accomplished by using glass exchangers.