Centrifugal Blower Performance Curves

[bduane]

It seems to me, for a centrifugal blower (compressor) handling atmospheric air with a given impeller diameter, geometry, and speed, that there should be a unique performance curve for the machine of pressure rise versus mass flow through the machine similar to a pump curve. If this is correct, then why don't compressor manufacturer's provide this information? If it's not correct, then please explain why not?

I am talking about low pressure ratio machines (i.e. pressure ratios less than 1.6) for aeration service.

 

[BigInch]

It is true, there are similar curves, but usually you must "roll your own"

There are many variables with very high sensitivity levels on the operating point (esp.) of a compressor, that even if a std curve were published for air at standard temperature of 60ºF and atmospheric pressure at sea level, similar to pumps with cool water, it wouldn't really mean anything to a specific compressor application. Gas mass flows change too much between different gas' density, reduced pressures and specific heats, and with temperatures and with pressures, so you have to make a particular curve for your specific gas MW, your specific suction pressure and the specific temperature. If you had a std curve at atmos press and a true suction pressure of 100 psia, and 100ºF, you'd have more than 6 times the mass flow rate at the "standard" condition. That would make the efficiency and power and surge limit predictions useless. If you changed from air to CH4, that's about 1/2 the MW, so your curves are blown again w/o even considering other variables.

http://virtualpipeline.spaces.msn.com

 

[pmover]

bduane,

you are correct in that mfg's of blowers should and do provide performance curves (head vs flow, etc.) for their equipment. if the mfg you are dealing with does not furnish the needed data, then it is time to deal with another mfg. perhaps you are speaking with the wrong person at the mfg as well - ask to speak with the responsible/authoritive technical person and not some sales individual.

good luck!
-pmover

 

[bduane]

BigInch:

What I'm trying to do is teach a group of civil engineers how to select a blower/compressor. Unfortunately, the curriculum for civil engineers covers only incompressible fluid dynamics and a bare minimum of thermodynmics. When you start talking density changes with compressible flow and adiabatic versus polytropic they get lost very quickly.

For the most part these compressors handle atmospheric air with ambient inlet conditions and the discharge pressure is generally less than 10 pounds. Therefore, I'm not concerned about compressibility effects and, for the temperatures and pressures I'm looking at, air behaves as an ideal gas.

If I can get a curve of pounds of air (not CFM) versus discharge pressue, then I've taken density out of the equation. A pound of air is a pound of air, but the volume it occupies is quite different.

What I'm struggling with is, if you consider a discrete molecule of gas, the centrifugal impeller is going to accelerate that molecule to a specific velocity based on the inlet and outlet curvature angles and the rotative speed of the impeller. I beleive that that velocity is a unique value and it is independant of the weight of the molecule because Avogodro says that there is an equal number of molecules in a fixed volume of gas or gas mixture (molecules in has to equal molecules out). The pressure rise is a direct function of the molecular weight of the gas molecule and the square of the discharge velocity. If this is true, then there should be a single unique curve for that impeller when it handles a gas of a known molecular weight.

What am I missing here?

Bduane

 

[BigInch]

Nothing, you have it right. I'm surprized you can't find a blower curve. I thought I've seen lots of them posted in mfgr cats on the net, that's why I thought you might have been more interested about the compressor curves, which are NOT very common at all.

The blower is just like a pump, except the medium is easily compressible. They still work on a volume basis. Each pass of the fan blade takes a volumetric chunk and it doesn't rally care what that volume is composed of, until it doesn't have enough power to move it, and since the fan blade is really a wing, until the air flowing over it is no longer at the proper angle of attack and the flow stalls.

One thing you might not be thinking about though is that the first step in a series of pressure changes is the worst in the sense that going from 2 psia to 4 psia doubles the power, while going the next 2 psi step to 6 psi only requires another 50%. The greatest relative changes occur at lowest pressures.

But that aside, you should be able to find a blower curve somewhere. Will this suit your purpose?

http://www.eere.energy.gov/building...fs/furnaces_boilers/furnace_boiler_app7_5.pdf

http://virtualpipeline.spaces.msn.com

 

[UmeshMathur]

bduane:

If you're still interested, please confirm and I'll post a generic method on how to use the vendor's polytropic head and efficiency curves for ANY gas.

 

[zdas04]

Blower curves are generally SCF/min, so it is an easy conversion from "volume" flow rate to mass flow rate since the apparent "densitiy" of an SCF of air is always the same (since SCF uses pretend pressure and temperature). That is why I refer to SCF as a surrogate for mass flow rate since SCFs behave just like lbms.

If you find the curves you want with CFM on one axis, then you can get to lbm/hr really easily.

David