Compressor Hp help

[Audi Driver P.E.]

It's been a while since I took the FE, and my memory is rusty on a lot of topics, but one thing I know we hardly (if at all) covered was power to run pumps and compressors, so I'm quickly trying to get up to speed.

In one of my practice tests, there is a problem that involves a multi-stage compressor, and the question asks about one of the stages. In the worked solution, the answer is:

Hp = Mdot (deltaH)/const

All of the worked solution makes sense but the const. I can't find a similar formula nor anthing that would make this constant make sense in the MERM.

They give the solution as (going from memory):

Hp= [(the value for Mdot from the question)*(Cp)*(delta T in Rankine)* (conversion factor for conversion of hours to seconds)]/ some strange three digit value with units of BTU/hr-lbm

I can supply more details if needed, but I can't find a single reference that uses that formula, nor has the werid const.

Anyone here know of a good source?

 

[Ramnares P.E.]

Post the problem/solution. If it's from an NCEES sample exam or SMS tell us which and what problem number.

 

[Audi Driver P.E.]

Ok, the problem is from MERM Sample questions (came with the 12th edition). It's problem 128.

The given data for the problem is Ti=100 deg. F, Pi=56.3 Psia, To=424 deg. F, Po=225.2 psia, Mdot = 200lbm/sec air, Cp=0.24 BTU/lbm*R, R=53.35 ft*lbf/lbm*R

Find the Hp of the compressor:

The given solution is

Hp=Mdot(deltah)/const

Hp=[(200lbm/sec)*(0.24)(884-560)(3600sec/hr)]/2,545 BTU/Hp*Hr

Its the 2,545 value that I don't know where it comes from. Is that just a conversion factor?* If so, why do they call it a "Const"?

*I think I just answered my own question

 

[Ramnares P.E.]

2545 is just a conversion factor. 1 Hp = 2545 BTU/hr. Don't let the nomenclature used confuse you.

If you don't have a Lindeberg conversion manual already, I strongly recommend getting one.

 

[Audi Driver P.E.]

Thanks! Makes a lot more sense. Does that conversion manual come in handy on the PE exam very often?

 

[Ramnares P.E.]

Yes. I found it to be very helpful. While the MERM cover has the vast majority of standard conversions, the conversion manual is much easier/faster to use and will save you some time - particularly if you have to do several conversions to get the units required.

If you plan on using it, I highly recommend getting it now and practicing with it.

 

[Audi Driver P.E.]

Ok. Many thanks!

 

[lundy]

2545 is just a conversion factor. 1 Hp = 2545 BTU/hr. Don't let the nomenclature used confuse you.

If you don't have a Lindeberg conversion manual already, I strongly recommend getting one.

That conversion manual is gold. Saved me a lot of time for sure! It's relatively cheap to other references as well.

 

[Audi Driver P.E.]

I got the conversion book. I love it and am thankful for the recommendation. You guys are great!

 

[TXtoCA]

I got the conversion book. I love it and am thankful for the recommendation. You guys are great!

same here. Thanks guys for the recommendation!

 

[Ramnares P.E.]

Glad to help. Best of luck.