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

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

"Bugs" wrote in message
oups.com...
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


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..

Said fan curves are measured, not calculated. Fan design is a black art!!

Bugs wrote:
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

On Fri, 29 Jul 2005 15:01:17 GMT, "Rick" wrote:
"Bugs" wrote in message
roups.com...
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


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..

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.

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

On 29 Jul 2005 06:12:00 -0700, "smitha4u"
wrote:

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

See
http://tinyurl.com/7b9lz

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.

"RoyJ" wrote in message
link.net...
Said fan curves are measured, not calculated. Fan design is a black
art!!

Bugs wrote:
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


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...

RoyJ wrote:
Said fan curves are measured, not calculated. Fan design is a black art!!

Indeed, and it is amazing the efficiency of fans today (some at least,
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.

On Sat, 30 Jul 2005 09:28:15 -0500, Don Stauffer
wrote:

RoyJ wrote:
Said fan curves are measured, not calculated. Fan design is a black art!!

Indeed, and it is amazing the efficiency of fans today (some at least,
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.

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

What amazes (or is it irritates??) me is something simple like a
standard forward curve centrifical blower wheel like the many million of
them used in a house furnace, etc. Look up the tables in Grainger or the
manufacturer, then compare to the one that you already have installed.
Just try and estimate the output of the installed one!!! Especially
when it is not behaving itself.

Don Stauffer wrote:

RoyJ wrote:

Said fan curves are measured, not calculated. Fan design is a black art!!

Indeed, and it is amazing the efficiency of fans today (some at least,

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.

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

On 3 Aug 2005 12:34:17 -0700, smitha4u wrote:

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

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 http://www.fantech.com.au of their one.
Geoff