Fan curve data points make no sense?

evcelica · April 22, 2020, 22:41

I'm looking at a duct fan, 18" diameter, and one of the flow curve points is 4000 CFM air at 0.125" H2O SP (31.1 Pa). How is this possible, when the dynamic head of that volume flow of air through this area is 0.324 "H2O. (81 Pa)

In fact, I made a simple axisymmetric model of an 18" Diameter tube, 40" long in a large recirculating free space with a 81 Pa domain interface in the center, and I got results of 2775 CFM of flow.
So the K value of this assembly is around 2 using average velocity.
The average dynamic pressure @ the interface with that flow rate areaAve((Density*Velocity^2)/2)@Inlet = 53Pa.
hand calcs with average velocity give 39 Pa.

Why would that give you an impossible data point of 4000 CFM @ 31 Pa?
If you placed a DP transmitter on each side of this duct fan during testing you could not measure that data point, so what gives?
What are they ignoring or omitting with this data point? I'm not sure how to interpret this fan curve data?

evcelica · April 22, 2020, 22:54

Thinking about this more, the only way you could have a DP that low is if this were used as some booster fan in a closed system. And did not have any inlet or outlet losses. So if your flow in that system, (from some other pumping source) was 4000 CFM, this would boost the pressure by 0.125 inH2O, or 31Pa.

Is that really how they do it? Seems pretty messed up to list flow rates for DP's which are nearly impossible to achieve in real configurations. This fan, sitting in free air with nothing else attached, couldn't provide that flow rate!

Or am I missing something with how these are rated?

ghorrocks · April 23, 2020, 19:32

I am no expert on this, but this reference appears to confirm my understanding: https://www.tcf.com/wp-content/uploa...es-FE-2000.pdf

I understand that the fan is tested in a long-ish duct with pressure tappings on either side of the fan. The pressure tappings are commonly static pressure, but can be total pressure. It appears to be static pressure in your case. As the duct is the same size on either side of the fan then the flow rate is the same and the dynamic pressure is the same. So the pressure point you quote with 31Pa is simply the increase in static pressure over the fan with dynamic pressure constant (at 81 Pa you state).

Does that help?

evcelica · April 29, 2020, 22:12

Thank for the Reply Glenn,
Yes, that makes sense, and if you calculate the system curve and intersect it with the fan curve, it will never intersect at those low and zero SP points, so I guess it doesn't matter too much. Just weird that they show those values.
I found the AMCA 210 standard, but didn't look into it too much yet, as it is pretty lengthy: https://law.resource.org/pub/us/cfr/...a.210.1999.pdf

April 22, 2020, 22:41	Fan curve data points make no sense?	#1
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	I'm looking at a duct fan, 18" diameter, and one of the flow curve points is 4000 CFM air at 0.125" H2O SP (31.1 Pa). How is this possible, when the dynamic head of that volume flow of air through this area is 0.324 "H2O. (81 Pa) In fact, I made a simple axisymmetric model of an 18" Diameter tube, 40" long in a large recirculating free space with a 81 Pa domain interface in the center, and I got results of 2775 CFM of flow. So the K value of this assembly is around 2 using average velocity. The average dynamic pressure @ the interface with that flow rate areaAve((Density*Velocity^2)/2)@Inlet = 53Pa. hand calcs with average velocity give 39 Pa. Why would that give you an impossible data point of 4000 CFM @ 31 Pa? If you placed a DP transmitter on each side of this duct fan during testing you could not measure that data point, so what gives? What are they ignoring or omitting with this data point? I'm not sure how to interpret this fan curve data?

April 23, 2020, 19:32		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I am no expert on this, but this reference appears to confirm my understanding: https://www.tcf.com/wp-content/uploa...es-FE-2000.pdf I understand that the fan is tested in a long-ish duct with pressure tappings on either side of the fan. The pressure tappings are commonly static pressure, but can be total pressure. It appears to be static pressure in your case. As the duct is the same size on either side of the fan then the flow rate is the same and the dynamic pressure is the same. So the pressure point you quote with 31Pa is simply the increase in static pressure over the fan with dynamic pressure constant (at 81 Pa you state). Does that help? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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April 22, 2020, 22:54		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	Thinking about this more, the only way you could have a DP that low is if this were used as some booster fan in a closed system. And did not have any inlet or outlet losses. So if your flow in that system, (from some other pumping source) was 4000 CFM, this would boost the pressure by 0.125 inH2O, or 31Pa. Is that really how they do it? Seems pretty messed up to list flow rates for DP's which are nearly impossible to achieve in real configurations. This fan, sitting in free air with nothing else attached, couldn't provide that flow rate! Or am I missing something with how these are rated?

April 29, 2020, 22:12		#4
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	Thank for the Reply Glenn, Yes, that makes sense, and if you calculate the system curve and intersect it with the fan curve, it will never intersect at those low and zero SP points, so I guess it doesn't matter too much. Just weird that they show those values. I found the AMCA 210 standard, but didn't look into it too much yet, as it is pretty lengthy: https://law.resource.org/pub/us/cfr/...a.210.1999.pdf