Induction Vs. Pressure Independent Type of VAV box

[Buildtech2]

Can anybody guide me on the induction type VAV box and pressure independent type VAV box characteristics related to the uniform air distribution. Our client commented that, when we use pressure independent type of VAV box, the nearest diffusers from VAV box will supply more air then furthest diffusers during part load conditions. He further advised me that by using induction type VAV boxes, this problem can be avoided because the pressure available downstream of induction VAV box is more than pressure independent type of VAV box. I don't think these comments are technically justifiable.

In my understanding, the only advantage we can have with the induction type VAV box is the cold air dumping can be avoided by mixing cool primary air with the return air during part load condition and I think air distribution characteristics for the both induction type and pressure independent type will be the same. Appreciate to receive few comments on client comments.

 

[BronYrAur]

The minimum air flow on the induction box will be higher than the minimum of the traditional VAV since you are blending return air and increasing the box discharge temperature. Therefore, the downstream flow profile will be more uniform, and more air will reach the remote diffusers.

 

[Buildtech2]

Thanks for the response. Will there be any major difference in the flow pattern downstream of VAV box in induction and pressure independent types. Is there any duct design modifications required if we change induction type to pressure independent types of VAV boxes.
As per CIBSE guide H, the pressure drop in the induction VAV box is higher then PI VAV box.

 

[willard3]

That's likely because the velocity with the induction units is higher which would indicate different flow patterns.