Design Considerations for Roof-Mounted Ventilation Systems

A methodology is presented for determining the air flow rate through a stack-ventilated single-spacedenclosure bearing a roof-mounted ventilation tower. We develop a "system discharge coefficient" which takes into account the pressure losses that occur at the intake opening of the enclosure, inside the tower and at the outlet opening. The system discharge coefficient is interpreted as a reduction in the area of the path that the air flow takes. Based on this reduced area the air flow rate is then determined. The proposed methodology may be applied to a broad range of ventilation tower geometries which house typical components. To illustrate how to apply this technique, we determine the volume flow rate through a tower connected to a ventilated space; the tower houses a number of common components, e.g. insect screens, acoustic baffles and louvres. For each component the pressure losses and the pertinent resistance coefficients are presented and the most pressure consuming components identified. Once identified their design may then be altered so as to improve air flow rates.