5.3　Air Ducts

5.3.1　Air intakes of forced draught fans shall be located to avoid suction of stormwater and contaminated air.The lowest point of the air intake shall be more than 4.5m above the ground，and shall be more than 1.5m above the platform surface.

5.3.2　Silencing provisions shall be made in the air intake ducts of the forced-draught fan.

5.3.3　Air intake bypass ducts should be designed if air-warmer is located upstream of the forced-draught fans.Air-warmer located downstream of the forced-draught fans for short-duration service should be removed easily.

5.3.4　Where two forced draught fans are installed，the connecting ducts should be arranged symmetrically to enable uniform distribution of air flow and avoid airflow impingement.An isolation damper should be installed at the outlet of each forced draught fan.

5.3.5　Manholes should be provided in the inlet air ducts of the air preheater.

5.3.6　Arrangement of the air ducts adjacent to the tubular heater shall meet the following requirements：

a）To provide uniform air flow into each burner；

b）To provide proper space for burners piping；

c）The plenum design and layout should be such that there is a clearance around and under the plenum to permit withdrawal of burner parts without dismantling the plenum；

d）The plenum should not enclose the structural supports of the fired process heater without providing for structural integrity.

5.3.7　Where the forced-draught tubular heater is required to operate under natural draft conditions，a quick-response natural-draught air doors shall be designed in the air ducts adjacent to the tubular heater.