2008 Sample Exam

[heath014]

Hello everyone,

I took the HVAC afternoon section of the 2008 exam and had a few questions:

For the first few problems an enthaly pressure diagram was given for ammonia R-717. I am more familiar with the diagrams that show the process curve, so I used one out of ASHRAE fundamentals. Big mistake. I got every answer wrong. Does anyone know why they are so different? How do I use the one they gave in the exam booklet?

520. Do you have to use the Q=mc(p) change in T equation because we are dealing with different densities?

530. States you have a compressor volumetric eff of 75 percent. So why don't you multiply the answer by .75 and then the answer would be a, not B? I think this is because the motor efficience is actual/ theoratical, but for a compressor it is the amount of gas compressed.

537. Does anyone know where to get enthalpy wheel equations? I only have what I have gotten from the sample exam.

For any mach design experts,

Is there any designation on what formulas to use according to the situation? I get confused on if I should use Von Misses, eulers, shear stresses, max stresses, mean and alternating stresses etc

Thanks again everybody! All of your input is very helpful!!!!! Thank you!

 

[mechgirl]

Hey Heather,

I saw your questions you sent to the yahoo groups.

I don't care for these diagrams either, but I didn't work the problems both with ASHRAE and NCEES, so I can't say if ASHRAE charts will yield an incorrect answer. I am more inclined to use the charts given on the exam. I usually start with the property chart before going to the pressure enthalpy diagram.

#520: You need lb/hr, not cfm. So you can use density, or I just used the rule of thumb Q = 1.08*cfm*delta T , and I got close enough.

#530: This is volumetric efficiency, not motor efficiency. You use the volumetric efficiency to calc the pressure before the compressor, so that you have the pressure on either side of the evaporator, and can get the enthalpy for each pressure state point, use the load on the evaporator, and solve for the mass flow.

#537 check chapter 25 of ASHRAE Systems handbook. Also, I google searched to see how these worked. I think any problems will be pretty simple. Like, when looking for the load, you just need the change from one air side (exhaust) to calculate. What threw me on the practice test, was that I was looking at too much information and didn't read carefully to see what the problem was asking.

I'm certainly not the machine design expert, so hopefully someone else will chime in. I would recommend a second read through the MERM failure theories chapter (50?) because that helped clarify some things for me. I asked a question about this a few days ago (I think the post is titled failure theories) so you might want to check that out.

 

[mechgirl]

I need to take another look at #530, I just looked at my work for the problem, and I can't follow how I did it. So, for now, ignore my answer to that. Maybe the volumetric efficiency was just excess info trickery from NCEES, because I can't see now that I used it at all.

 

[mechgirl]

I'm glad I rechecked this, because I 'm not sure I worked it correctly on the practice exam, but I happened to get the right answer.

Use MERM eqn 33.7, mass flow = Qin / delta h evap .Liquid enthalpy at 260 psig is h-in (45.694), and vapor enthalpy at 20 deg is h-out (106.2). The compressor efficiency is not needed, because we're just calc'ing across the evap, and we have all the info to find hin and hout.. I found that in most of the problems that I got wrong, or that took me too long, it was because of excess info.

 

[BrianC]

Regarding the failure theories and alternating stress question...

For ductile materials (elongation >5%) use max shear stress or von Mises (distortion energy) criterion. Max shear stress theory is more conservative than von Mises, but von Mises has better correlation with test data.

For brittle materials uses max normal stress or Mohr's theory. Max normal stress theory is more conservative than Mohr's theory.

efunda.com gives a bit more detail.

http://www.efunda.com/formulae/solid_mecha...re_criteria.cfm

Use Soderberg and Goodman diagrams for alternating stress situations (fatigue loading). Goodman diagram is less conservative than Soderberg, but is generally preferred none the less. When using these two methods, be sure to include the fatigue strength reduction factor (Kf) on the alternating stress portion only. Kf is not applied to the static stress (mean stress). Additional stress concentration factors may be required depending on the problem.

Hope this helps and is what you were looking for. I know it helped me refreshh a bit.

 

[SMTx]

"#537 check chapter 25 of ASHRAE Systems handbook. Also, I google searched to see how these worked. I think any problems will be pretty simple. Like, when looking for the load, you just need the change from one air side (exhaust) to calculate. What threw me on the practice test, was that I was looking at too much information and didn't read carefully to see what the problem was asking."

These problems are simple, so don't over complicate them.

This a total heat wheel.

1. Use the rule of thumb Q=4.5 x cfm x delta h

2. Multiply Q by 0.98 (for 2% losses) and divide by 12,000 btu/hr - ton

These are the basic formulas that you will need on most enthalpy wheel problems

 

[MechGuy]

I found that in most of the problems that I got wrong, or that took me too long, it was because of excess info.

This is a good thing to have learned. The key to any exm question is, what exactley are they asking for? You have to be able to identify the problem before you can solve it.

That and UNITS UNITS UNITS!