Using a fan below its minimum rated flow-rate. 1

[Abe8652]

Hi all,

We are are designing a test chamber for an HVAC system that is capable of operating in a volume flow range of 10 lt/s to 2000 lt/s. The plug fan that we have selected for our pressure head can only go to down to 10% of its maximum deliverable flow rate due to the design requirements of the motor electronic.

We had a discussion with our senior engineer and he came up with using holes in the fan support plate to cause the air to flow in the reverse direction. This way even though the fan will be operating at 10% or more of its maximum flow rate, the actual flow rate through the system will be quite less. It is a closed system so we cannot let the air to leave the chamber. I have attached an diagram to illustrate what I mean.

I have: Fan performance curves and system curve for system without any holes/perforations in the fan plate.
I need: The size of the perforations that will give us the required flow rate by causing reverse flow through them.

 

[LittleInch]

Without knowing what the pressure is either side I doubt you will be able to come up with a figure. I would go for a variable slot or hole whereby you raise or lower a sliding plate up to 50% of the open area. The issue is that to generate flow through your HVAC system you will need a certain pressure across the right hand element of your system. This may well mean that the fan you're using will be subject to stalling and flow past the blades - should be OK, but you would need to make sure.

Going for a 200:1 turndown with a single unit is just asking for trouble though...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Use the orifice equations in Crain's technical paper.

 

[Abe8652]

Thanks for the replies guys. I do realize that using the same fan to cover such a large range of flows is just asking for trouble. However, that's what our senior engineer wants to do.
Peace,
Abe

 

[georgeverghese]

As Little Inch says, some means of adjusting the recycle slot area is the way to go - that would limit your choice to some kind of manual or remotely operated damper.
From your description, the pressure upstream of the test device will vary depending on the flow through it, so obviously you cannot base the setting of this damper based on pressure - do you have a flow meter somewhere in the path of this air stream ?

 

[Abe8652]

Yes, there is flow meter between the fan and the test device.

 

[georgeverghese]

Okay, but this is not shown in your sketch - does the location of this flow element measure the net flow that goes through the test device, or does it measure total air throughput from the fan ?

Else consider relocating this flow device to (a) upstream of this fan - some where near where you've got the upstream arrow or (b) on the air exit from this test device - near the downstream arrow. (a) may be better, since it is not prone to errors in flow readout resulting from temp drop through the test device which will be the case with location (b).