Whose responsibility?

A measure which has been introduced in legislation by a num­ber of countries is to limit the value of Watts/litre/sec for the fans in a particular building. It is usual to take the total flow of air — supply or extract, whichever is the greater—and divide this into the total installed motor power. The value is usually between 1 and 1.5. It should be noted that motors are invariably rated in kW whilst fans have flowrates above fractional defined in m3/s.


KW/m3/s =w/l/s

As 1000 litres = 1m3

And 1000 Watts 1 kW


KW = m3/sxkPa



….. 3 , kPa kW/m Is =


Whilst many system designers, when confronted with such specifications, attempt to pass it on to the fan supplier, they must realise that they are the most important party in comply­ing. It is primarily a question of reducing system pressures by lowering duct velocities, decreasing the number of bends, lengths of straight, etc. Increasing fan and motor efficiency is of lesser importance.

Posted in Fans Ventilation A Practical Guide