Decription for the VAV Box

[mtanyer]

Dear all,
I am not not a mechanical engineer and trying to understand how the VAV Box works.

I have two questions relating to the VAV Box named "M_VAV Unit - Parallel Fan Powered --- Size 2 - 200mm Inlet". This is the VAV box mostly used in training sessions from Autodesk and others:
- Why are the sections of the input and output "supply air" different? The inlet for the supply air has a radius of 12.5 cm and the outlet the supply air has dimensions of 35*45 cm. The section of the outlet is 3.2 times bigger that the inlet. Why is that so?
- More importantly, there is one inlet for the "return air". Why isn't there an outlet for the return air as well on the same VAV Box? I was thinking that the return air will be sent to the vertical main duct VAV Box to be thrown out of the atmosphere.

Thanks

[mtanyer]

Dear all,
I am not not a mechanical engineer and trying to understand how the VAV Box works.

I have two questions relating to the VAV Box named "M_VAV Unit - Parallel Fan Powered --- Size 2 - 200mm Inlet". This is the VAV box mostly used in training sessions from Autodesk and others:
- Why are the sections of the input and output "supply air" different? The inlet for the supply air has a radius of 12.5 cm and the outlet the supply air has dimensions of 35*45 cm. The section of the outlet is 3.2 times bigger that the inlet. Why is that so?
- More importantly, there is one inlet for the "return air". Why isn't there an outlet for the return air as well on the same VAV Box? I was thinking that the return air will be sent to the vertical main duct VAV Box to be thrown out of the atmosphere.

Thanks

[HVAC-Novice · Accepted solution]

The picture posted appears to be a fan-powered VAV box that will blend return air (from the zone served) and fresh air (from AHU). That mostly is done to be able to heat without the AHU providing much air. The VAV box may have a heating coil included, or a coil is added downstream.  It also may improve air distribution for a diffuser layout that isn't good for actual variable air flow. The flow to the diffuser will be nearly constant regardless of air supplied from AHU.

The round inlet is smaller because the pitot tube air flow meter often used for these cheap devices needs a high velocity to measure. So the inlet will be made very small to create more dynamic pressure to measure. Not efficient, but cheap. 

This all based on guesses based on the picture shown. To really know, you have to ask the creator and look more into the actual family. 

[HVAC-Novice · Accepted solution]

The picture posted appears to be a fan-powered VAV box that will blend return air (from the zone served) and fresh air (from AHU). That mostly is done to be able to heat without the AHU providing much air. The VAV box may have a heating coil included, or a coil is added downstream.  It also may improve air distribution for a diffuser layout that isn't good for actual variable air flow. The flow to the diffuser will be nearly constant regardless of air supplied from AHU.

The round inlet is smaller because the pitot tube air flow meter often used for these cheap devices needs a high velocity to measure. So the inlet will be made very small to create more dynamic pressure to measure. Not efficient, but cheap. 

This all based on guesses based on the picture shown. To really know, you have to ask the creator and look more into the actual family. 

[RobDraw · Accepted solution]

These boxes can be used to downsize AHU's because the AHU only has to supply and condition the minimum amount of conditioned ventilation air. The VAV boxes can be turned on as needed for conditioning but if there is no demand, the minimal ventilation air is all that is needed. The difference in the size of the incoming supply air and the outgoing is to account for the added return air. The large size of the return is irrelevant. It just needs to be big enough for unrestricted air movement and is probably way oversized.

[RobDraw · Accepted solution]

These boxes can be used to downsize AHU's because the AHU only has to supply and condition the minimum amount of conditioned ventilation air. The VAV boxes can be turned on as needed for conditioning but if there is no demand, the minimal ventilation air is all that is needed. The difference in the size of the incoming supply air and the outgoing is to account for the added return air. The large size of the return is irrelevant. It just needs to be big enough for unrestricted air movement and is probably way oversized.