Centrifugal Type Fan Backpressure

[RJB32482]

I'm new with vacuum conveying and vacuum producers.

We have a system where there is a pressure transducer on the discharge side of 2 fans (one rated for a higher flow than the other). The fans both tie into one header before the transducer. We have a control valve also on the discharge side of the duct (opens if P is above setpoint, closes is P is below). Setpoint is 0" w.c.

Questions I have are:
1. High pressure alarm is 8" w.c. What happens when too much backpressure is on the discharge side of the fan (lower vacuum, hurt fan parts)? How bout if discharge pressure was too low (too much flow?)

2.We see more stable pressure whenever two fans are running or when the higher flow rated one is than just the lower flow rated one. My theory is more flow stabalizes the pressure reading better. Does this theory hold up in practice?

Also the system airveys material from checkbins (RA's on bottom) through a cyclone into another bin (FR also after cyclone). Would plugged filter receivers or cyclones cause higher or lower static pressure?

Thanks.

 

[thermcool]

I dont think back pressure of 8 inch of water column or low pressure will have any effect on the performance or parts of the fan.

Yes, the pressure at the exit of the fan (high or low) will effect the volumetric flow rate of the air output by the fan.

About using two fans, My experience is that it produces more turbulence. Its something like mixing of two water streams into one, you will always expect more turbulence in the flow. if you want uniform flow, you can put some flow straighteners (layer of coarse mesh ) or just give enough entry length (length of the duct) for the flow to develop (smoothen) before it approaches the intended application.

 

[RJB32482]

OK so the material being airveyed has pentane in it and goes to an oxidizer. So would 8" w.c. build too much pentane in the lines which could cause an over LEL environment? Or should the alarm be set higher?

Thanks.

 

[thermcool]

Well, As you know every fan have a Pressure Drop(or pressure difference at the inlet and outlet) versus flow rate curve. For the particular fan that you are using, If you have the specification for the fan you can easily calculate the flow at the exit of the fan.

Now the water column of 8 inches, will produce a pressure of approximately 2 Kpa at the exit. If you look at the performance curve for the fan (Static pressure versus CFM (flow rate)) you can make out how this pressure at the exit can effect the flow rate. Building pressure at the exit will decrease the static pressure and the flow rate for sure. but for the type of the fam that you are using, the alarm setting that you want to set will be decide by the static pressure of that model. If the fan is operating at high static pressure (enough margin for 2 Kpa back pressure) may be you can keep the present alarm setting. If not you can reduce the alarm setting accordingly. I hope this will help.