I tend to look at the major variables first, and then drill down to the finer details if and when necessary.

The published charts are, at best, an approximation. The Friction Loss Chart assumes exhaust air to be a theoretical, incompressible gas of density 1.2 kg/m3 (normal temperature and pressure) running in average, clean galvanised metal ducts.

When you are working with real gases and products of combustion the major effect will be the assumed gas temperature and density.

Apart from geographic elevation you can also look at the effects of static pressure changes at different floor levels. Mostly these are minor.

If you add a dust load of 100 gm/m3 you increase the density to 1.3 kg/m3. The dominant effect will be on your fan motor. But the major load on the fan is dirty filters, so the change will hardly be noticed by the motor.

If you want to get deeper into the problem I suggest you start studying Dilute Phase pneumatic conveying, and the properties of real gases. There is a pile of information to go through - not all of it relevant to dust extraction - but interesting none the less. ■

Mall,

For ordinary dust collection systems you do not need to take density of dust or dust concentration into account. The guidelines in ACGIH will tell you all. It is only when dealing with pneumatic conveying that material friction becomes significant.

Cheers,

Michael Reid. ■