Book - School of Science and Technology

Ductwork components and auxiliaries 443Shoe Angled BellmouthFigure 16.7 Typical ductwork branch connections15ºMax30ºMax15ºMaxConcentric Eccentric With splittersFigure 16.8 Typical ductwork expansion and contraction sectionsduct contractions the angle of taper is less critical, but 40 should be considered a limit forgood design.Fan and plant connectionsCatalogue fan performance figures will not be obtained in practice if the inlet air stream isother than uniform and without turbulence. At the fan discharge, all transitions should begradual, and bends closer than three duct diameters should be arranged to turn in such amanner that the velocity profile from the fan is maintained. Air flow upstream of plantitems such as filters, humidifiers, heating and cooling coils must be uniform across thewhole cross-section of the equipment, otherwise performance will be reduced to below therated duty.There are too many possible combinations of duct configuration to show here, andreference should be made to CIBSE TM 8.Data have been established to enable the reduction in fan performance arising frompoor intake or discharge ductwork configurations to be calculated. This is expressed as anadditional system resistance to be added to the ductwork and component pressure lossand is termed system effect.system effect (Pa) ˆ system effect factor velocity pressure at the fan intake ordischarge (Pa)Some typical system effect factors are given in Figure 16.9.