What are acceptable roof discharge arrangements?
THE COOLIE HAT COWL SHOULD NOT BE USED
It reduces dispersion, deflects discharged air down for re-entry to the building, and increases the effects of wind on performance: A JET COWL IS PREFERABLE
HSG 37 specifies that the discharge terminal should be designed so that the cleaned air from the LEV extraction equipment is discharged to the outside atmosphere without re-entering the building (b) & (c). To achieve this, discharge stacks on roofs may need to be extended to a height of 3 metres or more above the roof level. For some work activities involving the use of potentially harmful or odorous contaminants [e.g., soft/hardwood dusts, solder fumes], increased discharge heights may be necessary to ensure that the public and the environment are protected.
Ductwork outside the building should be robust and well-supported to withstand high wind loadings, and will need a high standard of maintenance.
DfEE Bulletin 88 specifies that fumes should be discharged in a vertical direction, at a velocity of at least 5ms-1, ideally the point of discharge should be 1.5 x the height of the building or 3 metres above the highest point of the building whichever is the higher. Where this is not practicable, designers should use their discretion: discharge must be as high as possible, and the siting of the discharge stack relative to the building protrusions and neighbouring buildings is critical.
The point of discharge should be at least 1 metre above the highest point of the building, this avoids the effects of wind variations and reduces the possibility of obstruction by vegetation, footballs etc.
The stack discharge must be situated so that the prevailing winds, natural down draughts, eddies caused by building protrusions etc do not cause the entrainment of fumes in any fresh air entering the building, or other buildings.
Attention must be paid to the position of windows and ventilation inlets, particularly any on higher buildings close to the building and also to any air inlets on the roof on which the stack is to be mounted. Arrangements to prevent significant quantities of rain passing into the system when the fan is not functioning should be incorporated; any small quantities of rain reaching a centrifugal fan should pass out through a drain hole in the fan scroll.