blower motor power???? 2

[etchyHA]

guys,,

how can i determine the blower shaft power?

 

[MikeHalloran]

Look it up?

Meaure current and voltage?

Strain gages on the shaft?

Force transducers on the motor mounts?



Mike Halloran
Pembroke Pines, FL, USA

 

[etchyHA]

no i mean a law like in shaft power of
pump=Q(gpm)*H(ft)/pump eff.*3960

 

[MikeHalloran]

Section 1 here may help:

http://www.dep.state.pa.us/dep/deputate/minres/dms/website/training/ventilation.htm

Mike Halloran
Pembroke Pines, FL, USA

 

[Zapster]

BHP=(cfm)*(static pressure + velocity pressure{in. of water))/4100

The above “rule of thumb” will need to be adjusted for fan efficiency.

 

[HVAC68]

HP = CFM x SP
__________________

6355 x Efficiency

HVAC68

 

[Zapster]

HVAC68

I see that you have a different conversion factor in the denominator than I had. 6356 vs. 4100

This will make a big difference. So, I looked in some references:

“HVAC Equations, Data, and Rules of Thumb” by Arthur A. Bell gives 6356 on page 38

“Air-Conditioning Systems Design Manual” by ASHRAE gives 4100 on page 5-31

I haven’t cranked the numbers to find which factor is correct; however, this just shows how it is important to personally verify these cookbook rules of thumbs. I will drop a note after I make the calculations to verify which factor is correct.

Regards,
Zapster

 

[imok2]

Shaft power is calculated as a product of input power and motor efficiency or H = BE×? motor

Where: H = shaft power (W)
BE = input power(W)
and ? motor = motor efficiency

 

[25362]

The formula for fans' power input is:

HP = (CFM)(in. wg)(K)/(6354[×][η])​

[η] is the efficiency as a ratio of fan output power to fan input power which includes losses due to skin friction, leakage and mechanical friction.

K is the compressibility coefficient, itself a function of the polytropic exponent, and the absolute total pressures at the inlet and outlet.

As to velocity head, in. wg, it is estimated from the air velocity in fpm, assuming air incompressible, 1 in. wg = 5.193 lb/ft², g_c = 32.174 ft.lbm/lb.s², the formula is:

in. wg = (V/4005)²​

,

The divisor 4005 results from assuminmg dry air at 70^oF and a pressure of 29.92 in. Hg with a density of 0.075 lb/ft³. It becomes 4100 when assuming moist-saturated air at 25^oC.