# MERM App 17.C Pressure drop column



## navyasw02 (Oct 14, 2010)

I use App 17.C all the time to quickly convert gpm to ft3/min for different pipe sizes, but I've never used the pressure drop column. In fact, I dont even know how to use it and I dont think I've even seen any example problems that use it. Anybody know?


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## Bman (Oct 15, 2010)

Navy,

I never thought of using the table to convert units, not a bad idea! I believe it is a shortcut table to find the pressure drop vs. using the equations (finding darcy friction factor, etc.). I used it once or twice while studying, but felt more comfortable working the equations... I didn't use it during the exam at all.


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## navyasw02 (Oct 15, 2010)

Yea, this is probably one of my most used tables when I have a fluids problem. It allows me to go back and forth between gpm and ft3/min in about a second by just looking up the pipe size from the table. I thought the same thing that it might account for Darcy friction factors, but I dont think it does. I tried it yesterday taking the pressure drop, scaling it to the length of pipe that was given in the problem, converting it to a head loss and comparing it to what I calculated manually. It didnt work, but maybe I just made a mistake somewhere. Calculating the friction factor isnt hard, but it requires calculating the reynolds number which requires you to look up the kinematic viscosity or absolute viscosity and density which can be a time waster.

Oh on another note, besides bringing a second book that has a table of Prandtl numbers, is it listed in a table in the book somewhere for various fluids other than water? For some reason the sample exams like to use liquid sodium.


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## Bman (Oct 15, 2010)

I'm not sure about tables for Prandtl numbers, I don't recall them coming up much during my studying or during the exam, but I took the HVAC depth. FWIW, I brought a Heat Transfer text book with me that had really good steam tables (much better than MERM), but I didn't end up even needing it, they usually provide these values as givens along with a lot of other useless information in the problem statement. My impression was that they weren't checking to see if you could look up a value in a table as much as they were trying to determine if you knew the useful vs. wasteful information given in the problem...


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