MERM Practice Problems 20.3 and 20.4

[pike482]

I have a question on both if someone can help me out.

For 20.3 in the MERM practice problems, why can you just say n2/n1 = 1/2. Why don't you have to use the affinity laws to get the cfm amd pressure for half speed at the scale size to find your scale size AHP and then use the similarity equations to calculate your full size AHP? I know doing it this way must be wrong because I can't get the correct answer, but don't know why you can just use the similarity equation for AHP saying the speed is half but using the given parameters to find the scale AHP.

For 20.4, the solutions use the equivalent length to find the pressure drop for the elbows. Using the loss coefficient method gives me a different pressure drop than equivalent length. Is there a reason why?

Thanks!

 

[gaidox]

I dont have my practice prob but you have to remember the limits of using affinity vs similar equations. If fan size is constant while speed varies use " affinity". If the fan size varies " use

similarity eqns". Scale size to actual size comparison have size variation so use similarity equations.

Hope it answer your question.

For your other question, I still need to check it out.