I have carried out duct sizing for a Mechanical ventilation system with six supply outlets each having 0.2 m3/s flow rate. The ductwork is largest close to the system marker (AHU) and decreases downstream of the flow as would be expected. The expectation is for the duct size to keep on decreasing or at least stay the same as the flow rate decreases (due to subsequent duct sections serving less number of supply outlets). However the result showed an increase in duct size as well as a negative pressure drop at a particular section which does not seem right. Could anyone please explain. Thanks
Hi Izunna - a negative pressure is possible due to static regain. Is the duct section connected to an outlet?Regards,
Duncan Brown
Hi Duncan,
It is actually a negative pressure drop, not a negative pressure. The section is not connected directly to an outlet, it is just a branch run. Could you please let me know why the increase. Than you
Izunna - I meant a negative pressure drop. And this is likely in this scenario.
I would need to see your project to comment further. Please exit the software, go into your project folder and zip up all of the files. Once that is done, use this link to get the project to me. regards,
Thank you Duncan. I have sent the project file as requested.
Ok - thanks - can you let me know which section no. I should be looking at? that would help if its a complicated layout.
Section 5
Hi Izunna - This has been a tricky one to track down. During the initial sizing, section 5 is exceeding the velocity constraints set at the system marker. This is causing the section to be sized as shown. To allow for this, you should modify the velocity constraints a little.From the duct menu, select System Marker > Change System Marker, select your marker. When the dialog appears, click onto Sizing criteria and change the Max velocity for 500mm ducts to 2.6. This will adjust the duct size next time you size the system.
You should note that you will still get some static regain in section 5. If you wish to remove this, fix section 3 to 600x500.regards,