NCEES 2008 #110 vs NCEES 2001 #108

[Krakosky]

Similar types of problems but I got the 2008 problem right and the 2001 problem wrong. I think it has to do with full load vs rated load (whether or not to include the motor efficiency) and line vs phase values (MERM pg 67-9, whether or not to include a sqrt(3)). Anyone else have trouble with these problems?

 

[Outlaw44]

If you look at table 19.1 on page 19-5 of the MERM, there are equations you can use to solve for amps, given various things. Sorta similar to table 18.4 (I think) that helps you solve for hydraulic HP given the variables in various forms.

Hope this helps! I'm about to explode!

 

[MetsFan]

You use the pump efficiency to calculate the brake horsepower. You use the pump and motor efficiency when you're looking at the overall motor horsepower. The true energy imparted to the water requires no efficiency corrections. The energy imparted to the water plus pump losses requires pump efficiency. The energy used by the overall pump requires pump and motor efficiency.

So:

Water Horsepower (whp) = (Flow, gpm)(Head, ft) / (3,960)

Brake horsepower (bhp) = (Flow, gpm)(Head, ft) / (3,960)(Pump Efficiency)

Motor Horsepower (HP) =(Flow, gpm)(Head, ft) / (3,960)(Pump Efficiency)(Motor Efficiency)

Remember to look on a horsepower motor chart since not all sizes are available (Table 18-6).

I hope that makes sense. Outlaw44, I know the feeling. I am so ready to get this over with.

Good luck to all of us!

 

[ksprayberry]

For what its worth, take a good look at the fan law equations and the pump laws. You can derive them several different ways and substutute different quantites in.

Like cfm1/cfm2 = rpm1/rpm2 ....But sp1/sp2 = (rpm1/rpm2)^2 so since the ratio of cfm is equal to the ratio of rpm, you can replace one with the other, might save some time if you know how one relates to the other. Know what I'm saying?

Ok, I'm off to Macon, Ga. I wish everyone the best.

KS

 

[Krakosky]

Thanks everyone. Table 19.1 cleared things up. I was looking in ch 67. Same info but that table makes it more clear.