NECEES Mechanical PE Reference Manual Released

[Tiger15]

The unit conversion table in page 2 erroneously has the viscosity unit "reyn" as equal to 1 lb-ft/s^2 where it should be 1 lbf-ft/s^2. 

Well. I'm old school solid mechanics guy....
lb ALWAYS means lbf. lbm is a unit for mass that I personally hate and never use because gc is evil. 
SLUGS 4 LIFE!!!!

But I know that's not how yall thermal-fluids engineers think.

 

[Dr. Barber]

Well. I'm old school solid mechanics guy....
lb ALWAYS means lbf. lbm is a unit for mass that I personally hate and never use because gc is evil. 
SLUGS 4 LIFE!!!!

But I know that's not how yall thermal-fluids engineers think.

 That's the kicker with this manual. They use lb, lbf and lbm. Sometimes by "lb" they mean "lbm" and sometimes they mean "lbf". I mean, this thing is just a real hoot.

 

[Audi Driver P.E.]

Ok, I welcome any clarification and pointing out what I'm doing wrong here. If I'm not making any dumb mistakes, then this is further proof that this handbook is an unmitigated disaster.

Consider this simple question: What is the dynamic viscosity of SAE 50 oil at 50C?

If we use the graph in page 10, we get... what do we get? I have no idea. is this 4.5? 45? 450?

Note there is a 10^2 below the horizontal red line and another 10^2 above the red line. At first I thought the 10^2 below was a typo and should be just 10, but there is already a 10 further below. So, ¯\_(ツ)_/¯

View attachment 14551

Then I thought I could use the imperial units graph of page 9. Going in there at 122F (50C) I get 16 reyn, which is 1.1x10^8 mPa*s which is nowhere near anything close you could get from the SI chart.  So, yeah, unless I'm doing something really wrong, this is REALLY screwed up.

View attachment 14552

What a Charlie Foxtrot.  First of all, at 50C, SAE 50 Oil would be about 97 mPa-s, which is 14.1E-6 Reyn. So the first chart would (apparently???) indicate a value of 45 (to be in the correct ballpark for factors of 10), but that corresponds to the actual value for SAE 30 oil (from other references I have). If you look at the Imperial units graph, you will note that it properly has a mu symbol (indicated on the Y axis label) but for some reason there is a comma between that and reyn. That chart is closer to correct, value-wise, for micro reyn. If you use a value of 16 micro reyn, you get about 110 mPa-s, which is closer to the actual value. I don't know how to read that first chart. On your exam, I would use the second chart, convert, and call it a day.

 

[OldSquaw]

Here's another issue I found:

 

[Dr. Barber]

What a Charlie Foxtrot.  First of all, at 50C, SAE 50 Oil would be about 97 mPa-s, which is 14.1E-6 Reyn. So the first chart would (apparently???) indicate a value of 45 (to be in the correct ballpark for factors of 10), but that corresponds to the actual value for SAE 30 oil (from other references I have). If you look at the Imperial units graph, you will note that it properly has a mu symbol (indicated on the Y axis label) but for some reason there is a comma between that and reyn. That chart is closer to correct, value-wise, for micro reyn. If you use a value of 16 micro reyn, you get about 110 mPa-s, which is closer to the actual value. I don't know how to read that first chart. On your exam, I would use the second chart, convert, and call it a day.

Thanks. I hadn't noticed the "mu" -- good catch. Yeah, what a sh*t show. That first chart is useless. In their defense, they do provide a reference to where the chart came from. Maybe it's messed up in the original source and they just copied it.  It shows, however, that nobody who is participating in the creation of this manual has used the dang chart. 

 

[Dr. Barber]

Here's another issue I found:

View attachment 14678

LOL

JFC

 

[OldSquaw]

Dr. Barber,

Are you reporting all of this to NCEES? I haven't reported anything in hopes that you were, lol! If you're already doing this then I won't bother.

I'm one of the poor people that is taking the exam in April with this reference.

 

[Dr. Barber]

Yes.

I have reported most of the things in this thread to them. Not this latest set of findings, but I will get around to it probably this weekend.

 

[OldSquaw]

Awesome! Thanks!

 

[Audi Driver P.E.]

@EB NCEES REP, not sure if you have any sway over this or not (I kiiiiiiinda doubt it), but worth the tag, probably. This exam booklet is a total Charlie Foxtrot.

 

[OldSquaw]

5.2.2 Transient conduction using lumped capacitance model:

The reference manual says to use units for thermal conductivity in Btu-in/hr-ft^2-F in the equation for Biot number. If you use the rest of the units they suggest, the units don't work out. Should be in Btu/hr-ft-F.

It would be better if they just said nothing regarding the units and let the test taker work out the units on their own. When they state which units to use, it makes you think that you can use those units...but they yield the wrong answer.

 

[OldSquaw]

Guys,

FYI, I just logged into my account and noticed there is a version 1.1 out. I haven't looked through it yet but I'm thinking they've probably addressed some of the issues.

I just skimmed through the new reference manual and all the refrigerant properties charts still have units of Btu/lb-F for enthalpy...

Rather than fixing the equation for two stage refrigeration cycle COP they just put "FOR EQUAL MASS FLOW RATES IN STAGES 1 AND 2"

 

[Dr. Barber]

Rather than fixing the equation for two stage refrigeration cycle COP they just put "FOR EQUAL MASS FLOW RATES IN STAGES 1 AND 2"

LOL WHAT?!?!?!?!?! 

OMG the mass flow rates in the two stages can never be the same.

Energy conservation in the intercooler requires: 

m6h6 + m4h4 = m1h1 + m7h7. (assuming a well-insulated chamber)

Now let m_highP = m1 = m4 and m_lowP = m6 = m7. Put these in the energy balance to obtain:

m_lowP (h6 - h7) = m_highP (h1 - h4) thus the mass flow ratio is:

m_lowP/m_highP = (h1-h4)/(h6-h7)

Look now in the P-h diagram for the cycle to see that the ratio (h1-h4)/(h6-h7) is ALWAYS < 1 and can never be 1.

I'm speechless.

 

[Dr. Barber]

Looks like in the new version 1.1, they fixed the efficiency of the Otto cycle and the COP of the gas refrigeration cycle.

 

[OldSquaw]

LOL WHAT?!?!?!?!?! 

OMG the mass flow rates in the two stages can never be the same.

Energy conservation in the intercooler requires: 

m6h6 + m4h4 = m1h1 + m7h7. (assuming a well-insulated chamber)

Now let m_highP = m1 = m4 and m_lowP = m6 = m7. Put these in the energy balance to obtain:

m_lowP (h6 - h7) = m_highP (h1 - h4) thus the mass flow ratio is:

m_lowP/m_highP = (h1-h4)/(h6-h7)

Look now in the P-h diagram for the cycle to see that the ratio (h1-h4)/(h6-h7) is ALWAYS < 1 and can never be 1.

I'm speechless.

View attachment 14809

I agree. This is pretty concerning to me as a test taker.

 

[Dr. Barber]

I hope CBT takers never are asked to find the entropy of a saturated liquid-vapor mixture of water at 100 psia.

View attachment 12668​

They fixed this too.

 

[Dr. Barber]

Definition of hydraulic radius, RH, and hydraulic diameter, DH, are wrong. They are saying RH = (1/2)*A/P where A=flow area and P=wetted perimeter. They are also saying DH, = RH/2)

 ​

But in reality, RH = A/P and RH, = DH/4 as shown in these 2 different sources: 

1. MERM13:



2. Fluid Mechanics by Cengel and Cimbala:

 

[Dr. Barber]

So, this should be a "4" instead of a "2"

 

[Dr. Barber]

After shitting on them so much, I figured it's time to throw them some love.  The Moody chart in the manual has a secondary horizontal axis for the product of the flow velocity in fps and the pipe ID in inches.  This is nice because with this you don't have to look up viscosity and calculate the Reynolds number.  It's a time saver for sure...

I spot-checked a couple of values and it works.  Now, this works only for 60F water so if a problems says its another fluid or its water at like 150F then this shortcut will not work well.

 

[Dr. Barber]

The line for f=0.022 is partially missing.