Calculating Volumetric Flow Using Motor Sizing Formula

[MAPower]

Hi,

I'm trying to get some help doing a rough estimate calculation of the volumetric flow through and induced draft fan on a balanced draft unit without doing a performance test. Here is the data that I have available to me in our historian:

- fan inlet & outlet pressures
- fan inlet temperature
- fan speed
- fan motor (VFD driven) operating amps
- fan motor (VFD driven) operating kW

I have used a motor sizing equation to give me a rough estimate, but am not sure if this formula can be used to calculate the CFM?

Horsepower = {CFM*Pressure(lbs/sq ft)}/{33,000*efficiency}

I have a few questions regarding this method:
- is this a valid equation to use for a rough estimate?
- is the efficiency the efficiency of the motor or the fan?
- solving for CFM, will it give me ACFM or (S)CFM?
- I am using pressure rise across the fan for the pressure

Any help would be great.

Is there a different equation I could use to determine a rough estimate of the vol. flow?

 

[burnt2x]

MAPower,
You need to have the performance curve of the ID fan. You can get it from the manufacturer of the fan. In it, you get the volumetric flow at specific speed of the fan as well as the outlet pressure. Then you apply the Affinity Laws of Fans.

   Q1      N1    D1[sup]3[/sup]     where: Q = volume flow
   ---  =  --- X ----           N = RPM
   Q2      N2    D2[sup]3[/sup]            D = fan diameter 

   DP1     N1[sup]2[/sup]   D1[sup]2[/sup]     
   ---  =  --- X ----    where: DP = pressure head
   DP2     N2[sup]2[/sup]   D2[sup]2[/sup]             

   P1      N1[sup]3[/sup]   D1[sup]5[/sup] 
   ---  =  --- X ---     where: P = fan power input
   P2      N2[sup]3[/sup]   D2[sup]5[/sup]

Hope this helps.

 

[7anoter4]

As IDF is a centrifugal fan see:

http://www.cincinnatifan.com/catalogs/EngData-203-internet.pdf

page no. 4, mainly.
According to this:
AHP = CFM x TP/6356
AHP. – Air Horsepower, is work done by the fan expressed as horsepower.
CFM. – Cubic Feet per Minute, is the volume of air moved per minute
TP. – Total Pressure, is the sum of the static pressure (SP), and velocity pressure (VP) at any given point in a ventilating system.
If TP is calculated with actual data then CFM will be ACFM
BHP = CFM x TP/6356 / TE
BHP. – Brake Horsepower, is the horsepower absorbed by the fan
TE = Total Efficiency of the fan [from the Manufacturer curve as burnt2x indicated].
If TE is calculated with actual data then CFM will be ACFM.
Required motor power will be:
P[kW] =BHP*0.746/eff
eff=motor efficiency
SCFM is normally used to designate flow in terms of some base or reference pressure, temperature and relative humidity. Many standards are used, the most common being the Compressed Air & Gas Institute (CAGI) and the American Society of Mechanical Engineers (ASME) standards, which are 14.7 PSIA, 68°F and 36% relative humidity (RH). This converts to a density of 0.075 lbs/cu.ft. for air.
If what do you need is SCFM see:

http://www.pdblowers.com/t6-scfm-standard-cfm-vs-acfm-actual-cfm.php

in order to covert to SCFM.[Standard cfm]