Design of a ventilation system

[ChrisF11]

I am currently reviewing a ventilations system design for a 6 floor office building.

The original design was medium to high velocity (5-6m/s for an office) which has now been amended to try and convince us that we can use the existing risers.

The new ventilation flow diagrams show velocities of 1-2m/s second in the risers and 4-6m/s in the branches. Additionally the pressure drops range from 0.2pa/m to 1.9pa/m.

I am not alone in thinking that no design methodology ahs been applied to this (equal pressure drop/velocity method) and that these wide ranging values for pressure drop and velocity are not good practise. In fact would it be correct of me to assume we will have fan, noise and flow problems throughout the system?

A bit of confirmation would be appreciated in this because I am trying to convince building management the contractor is trying to do it on the cheap to save money by using the existing risers!!!!

[frank]

When designing ductwork, it normally falls into 1 of 3 classes:

0 - 5m/s - Low Velocity
5 - 10m/s - Medium Velocity
10m/s and above - High Velocity

On Low velocity systems (i.e. Offices) it is normal to size the main ducts for a velocity between 4 & 5 m/s and branch ducts at 2.5 m/s. At these velocities, duct borne noise from velocity is quite low. Breakout noise is something different and can be transmitted noise from the fan if it is not isolated (flexible connections & AV's).

To see how you particular existing ductwork suits, you need to know the air volumes and duct sizes (main duct and branch ducts) to work out the velocities.

[ChrisF11]

Thanks Frank,

My main concern is that the pressure drops and velocites along the branches varies from 1.17 to 5.62m/s then back to 2.06. and the pressure drops from 0.22 to 1.9pa/m. This is not normal is it?

Regards

Chris

[frank]

It doesn't sound normal but so long as you are getting the correct volumes at the grilles and the fan can cope with the pressure drop I can't see why the existing ductwork can't be used.

[ChrisF11]

thanks Frank

[r.bartlett]

who holds design liability?