Air extract issues

[Rob 808]

Hi all

I have seen some threads with air extract issues on this platform, and thought Id give it a go to see if I can get some clarification.

We have a project with 2 sets of WC's, all with full height cubicles and an extract linear grill in each. Our mechanical contractors are struggling to achieve the design criteria on the extract during commissioning and I suspect the issue might be more serious than they are letting on!

I'm not a mechanical engineer, however I questioned why the spigot on the plenum was 100mm diameter when the duct work was sized at 150mm during the install phase. In my simplistic view, this would reduce the extract capacity by a third!! but I may be very much incorrect, it was an observation I had..

Can any one confirm if this could have the potential to have an impact on the extract rates?

Many thanks

 

[stem]

It sounds OK to me if I understand your description correctly, in that the 150mm duct is carrying the air from all four cubicles, whereas the 100mm spigots will only be carrying the air flow from one cubicle......

......like slip roads joining a motorway, the slip roads don't have the same capacity as the motorway because they are not handling the full amount of motorway traffic.

Reducing the diameter also assists with balancing. If the spigots were all 150mm the same as the duct, most of the air would flow in via the first grille, leaving little or nothing for the other three.

I would suspect it's more likely that the fan is undersized either for the required design criteria, or the design of the duct (length & number of bends) is imposing too much air resistance.

A sufficient flow of air into the WC area will also be required to replace that which is being extracted (you wouldn't suck any air out of a sealed box)

 

[Rob 808]

Hi Stem

Thanks for taking the time. It probably have been easier if I used an extract as below. Red arrows point out the 100mm diameter spigots, and its probably worth noting that the VCD's are not installed. I'm told that they can adjust on the linear grills!!

Would your view remain the same? reduced spigots not an issue?
Thanks in advance.

 

[stem]

The principle would be the same. The duct is stepped down at various places along the system to suit the airflow at that point.