4　Shape，Size and Materials for Ducts

4.1　Ducts should be of circular cross section.Where rectangular shape is selected，the ducts should be designed to have an approximate square cross section with a ratio of the short edge length to the long edge length not less than 0.4.

4.2　The minimum thickness of panel plate shall be 5mm.

4.3　As a minimum requirement，materials of the ducts，reinforcement ribs，components and parts should be of Q235BF.

4.4　Proper material shall be selected according to the design metal temperature and the required ambient temperature.The required ambient temperature for load-bearing structures，such as unroofed supports and hangers，shall be the minimum mean daily ambient temperature.

4.5　The materials specification of commonly used insulation shall comply with the following：

a）SH/T 3115 for light weight castables；

b）GB/T 3003 and GB/T 16400 for ceramic fibre；

c）GB/T 11835 for rock wool.

4.6　Gasket material for duct flange shall be free of asbestos，and ceramic fiber cloth，ceramic fiber strip and ropes of adequate service temperature rate should be used.