Fan system curves

Hello all I am new to this group and found some interesting discussions iin this so i thought i could get soome help from you. My question may be sort of silly but i am coming out. Can any one explain what are system curves for an electrical fan and what are the fan curves. what is the difference between those. If we are given an electrical with motor specification how can we draw system curves and fan curves to defiine a operating point for the fan. what data should i need to collect for that. I have the el.motor spec as 2Kw, 200rpm variable speed DC motor. I was given a question in the class that " if you are asked to draw sytem curves and fan curves and find the operating point of the fan with the given motor spec. assume a cooling scenario in the car and think of a radiator fan to be designed" so this is my actual queest. I appreciate if any one could help me in understanding this in detail or help me with some websites where i could get better info Regards Smitha

Reply to
smitha4u
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I assume that you are talking about fan output curves. They correspond to water pump output curves. It is simply a plot of the output pressure vs. output volume for any given impeller/fan style. At zero flow with the outlet blocked the pressure will be at a maximum. At maximum flow with the outlet fully open, the pressure will be near zero. The curve traced betweeen these points shows the operating point for any condition of discharge resistance, ie. friction. Somewhere on the curve is a point that requires the maximum horsepower for that particular fan, which produces the most volume at the greatest pressure possible. There is usually a horsepower curve associated with the discharge curves which shows the motor HP required at any point on the curve. These are used to select the proper fan motor/impeller/HP combination for design of a system. Bugs

Reply to
Bugs

correspond

pressure

curve

possible.

combination

And once you have the characteristic curves (flow vs pressure) of the fan (or pump) you plot the system restriction curve on top of that. The intersection(s) will be the operating point..

Reply to
Rick

Reply to
RoyJ

In English: You want to make sure the fan blade is big enough to do the intended work (known CFM at a known back pressure), and that the motor HP is big enough to swing the fan blade at it's maximum horsepower input needs without stalling or overheating.

And you have to make sure that also holds when you switch in the slower fan speeds on a multiple speed system, which for automobile cooling systems is usually accomplished by switching a series wirewound resistor in the motor +12V power lead. The motor needs to start reliably every time you switch on the power, even if you try starting it at the slowest speed.

You want to leave a safety margin when you specify the motor HP, but you can't just 'go way big' and leave the motor very lightly loaded. Because when you build thousands (or millions) of systems for resale every extra dime you spend comes out of the profits.

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Reply to
Bruce L. Bergman

See

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Buffalo Forge used to publish a very good book entitled "Fan Engineering", copyright 1970 by Buffalo Forge. No ISBN number, apparantly only available from Buffalo Forge. 729 pages. It thoroughly treats fan curves and related issues.

Fan curves show the relationship between pressure, flow, speed and power *for a given fan*. Designing a fan to produce a given behavior is a very complex matter. You would also need to know (or define, or measure) the flow vs pressure behavior of your radiator.

Reply to
Don Foreman

correspond

particular

possible.

curve.

combination

Yes, and in the case of a radiator fan, sometimes at more than one speed. The fan laws can also be used to calculate the various parameters at other conditions...

Reply to
Rick

anyway) compared to a hundred, or even fifty years ago. I suspect the growth of computational fluid dynamics has helped to advance that black art, however.

What is amazing to me are these model gas turbine engines with such dinky centrifugal compressors. I have some old books on GT design that indicate it would be impossible to design a GT with a compressor smaller than about 12 inches and even have it sustain itself. What is even more amazing to me is that these were originally designed by hobbyists.

Reply to
Don Stauffer

Do you think it has anything to do with the rev's these things run at? IIRC, idle is 80,000 rpm or so, full bore is more like 200,000 or so. Don't modern turbochargers run at similar speeds? They aren't nearly

12" around either.

They're amazing to watch (and hear), but most turbine models I've seen move too fast for me. They are nearly out of sight before the pilot gets them turned around, then come by in the blink of an eye. I'm a lot more comfortable at 50 mph than 200 for models.

Pete Keillor

Reply to
Pete Keillor

Reply to
RoyJ

Thank you all. I have a question, Are these fan curves and system curves are drawn before the fan is designed or after the fan is designed. Are these curves used for the design of fan blades?

Regards Smitha

Reply to
smitha4u

They are done as a result of testing the fan to one of a number of different standards, using a specially designed test rig. I think there is a pic at

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of their one. Geoff

Reply to
Geoff M

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