3.4.3. ConstructionFan blades shall

3.4.3. ConstructionFan blades shall be of high efficiency backward curved aerofoil design or profiled backward curvedplate design. Where replaceable rotor side plates are used it shall be possible to readily replacethese plates through the access doors in the fan casing.The ID fan casing shall be fabricated to resist any corrosion due to the flue gas, according to the fluegas conditions (pollutants, gas components, humidity, temperature…). Adequate drainage possibilitywith ball valve shall be provided at the bottom of the casing.To prevent the stack natural draft, the run down rate of the ID fan or rotating it whilst out of service,an approved braking arrangement or a system providing the same function shall be fitted. This brakeshall be arranged to operate at run-down of the fan and be automatically released as part of the fanstarting sequence.The pedestals and base-plates supporting the bearings adjacent to the side casings of the ID fanshall be so designed that they can be removed to enable repairs to be made to the lower portion ofthe fan casing.The joint bolts for the fan casings shall be bigger than 6 mm in diameter with a pitch of 100 mm atthe maximum. All bolted joints shall be made with PTFE sealant band.Fan shafts shall be designed so that the first critical speed is at least 50% above the maximumrunning speed.Fan shaft shall be connected to the drive shaft by means of direct flexible coupling of proven design.The fan impeller hub shall be of mild or cast steel of an approved grade and shall be fitted and keyedor welded to the shaft.Where water cooled bearings are used they shall be suitable for operating from the cooling watersystem. Integral cooling water pipework, including all necessary valves, supports and water flowindicator on the discharge side, shall be supplied. The bearings shall be fitted to bed-plates ofapproved construction which shall also carry the motor’s coupling and they shall be so designed that