About the return air of VAV system

[lzh007]

if there are lots of VAV box at the supply terminal, how to design the return duct? There will be a return fan because return ducts reach to each room.

 

[remp]

Your return air fan quantity will be the same volume capacity as the supply roughly, less say toilet exhasut and any other tgeneral exhasut from the floor.

You can minimise the duct by running into the ceiling plenum and stoping at the shaft wall. Size approx 5 m/s , keep the velocity down for noise reasons. if you have full height wall at each room, you may wan to extend the duct to this areas or eles have transfer ducts. Size the ducts the same 0.8pa/m. suck from the ceiling space is cheapest and easiest.

 

[willard3]

Any fan system with ducted supply and plenum return is impossible to balance properly.

Duct all supplies, duct all returns.

Return volume is what max supply volume is minus building exhausts and whatever % OA you are using to maintain slightly positive pressure in what I will guess are office spaces.

 

[AbbyNormal]

Some times you do not have the luxury/space to duct the return.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[lzh007]

While, when the VAVs reduce the S/A volume, how will the supply and return fans correspond?

 

[atlas06]

lzh007 (Mechanical) 23 Nov 06 0:07
While, when the VAVs reduce the S/A volume, how will the supply and return fans correspond?

With Return fan tracking and using air flow measuring stations (AFMS), one on RA fan, one on SA fan (in fan scrolls), and one in OA intake duct.