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Dunkin (Mechanical) (OP)
13 Aug 08 13:30
Reference: thread403-82316: Fan Horsepower equation derivation
I seem to lose out on this subject.  Please note the following:

Fan BHP = (cfm x static press "w.c.) / (6356 x fan efficiency)

Fam Motor HP = fan bhp / motor efficiency

Following are values from a propeller fan manufacturer's catalog for a particular fan:

        Static press. = 0.125"w.c. , cfm = 11,207   Fan motor HP = 1.437

By using the above formula  fan BHP = (11,207 x 0.125) / (6356 x .7) = 0.31
                                              Motor HP = 0.3148 / 0.9 = 0.35

There is enormous discrepancy between calculated Fan Motor HP of 0.35 and the fan OEM catalog value of 1.437 HP.  This is typical of all fan OEM catalogs I see.

Can somebody shed some light?

Thank you and have a great day!

quark (Mechanical)
14 Aug 08 2:11
Propeller fans have overloading type performance characteristic and the motor selection is based upon the maximum power the fan can draw, if the resistance is less than the design value, to avoid overloading of the motor. Check where your duty point lies.

Dunkin (Mechanical) (OP)
14 Aug 08 8:43
quark:  Thanks very much.  I am looking into fluid coolers (often called dry coolers) with direct drive motors, which mostly have cont. duty motors and steady air resistance.  The 1.437 BHP is from the standard (common) cfm vs sp & cfm vs BHP fan performance curves.  I will think out your comment, but frankly I still need to understand the matter.  If you can elaborate, that will be great!

have a good day!

ChrisConley (Mechanical)
14 Aug 08 16:48
For a fluid cooler, inlet air conditions (turbulent) may be a large factor.

A perfect installation with space up, and downstream of the fan would require only .35hp to move 11,000 cfm, under actual installation it requires 1.437 brake horsepower.

Is the actual motor 1.5hp or 2hp? Quark's comment has more to do with the size of the installed motor, not the published brake horsepower.
Dunkin (Mechanical) (OP)
15 Aug 08 10:38

thanks for the response.  The 1.437 BHP is off of actual fan performance curves.  It corresponds to 11,200 cfm at 0.125"wc static pressure.  The design engineer then has to allow for motor efficiency and pick a suitable motor that will have at least 1.437/0.8 = 1.79 HP power.  The 1.437 BHP is a laboratory reading, which allows for smooth air flow upstream and downstream of fan.  The above figures are from

thanks again

quark (Mechanical)
18 Aug 08 6:19
what did the manufacturer say? There may be built in safety factors. I saw comments like "the actual bhp may be significantly less than the quoted value" etc.

Dunkin (Mechanical) (OP)
18 Aug 08 18:01
I am not a customer of these fans yet, but expect to be shortly.  This situation is fairly uniform for all the OEMs I browsed.  The engineers select a motor hp that is a little higher than the manufacturer recommended for obvious reasons:  OEM is putting his best foot forward, motors come in certain multiples of hp (can't always get hp to match exactly your number)and other.  The OEM bhp numbers are actual test numbers from wind tunnel (code tester) tests and are as reliable as they come.  So, there is something here that does not jive.  well, hopefully I will discover it.

thanks for the input!

have a great day!

quark (Mechanical)
20 Aug 08 0:16

Please do educate us on your findings (including your actual observation of power consumption).


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