Engineer Boards

# Slay the P.E.

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Mechanical Engineering Exam Prep

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1. ## Critical insulation thickness

Exactly. Its impossible to tell what will happen unless you specify: 1. Current thickness of insulating layer. 2. Thermal conductivity of insulating layer. 3. Convective coefficient.
2. ## Critical insulation thickness

The addition of insulation makes it thicker, so the resistance due to conduction increases. But, at the same time, as you add insulation the outer surface area of the insulation increases, so the resistance due to convection decreases. So as you add insulation from zero thickness you have two competing effects: increasing conduction resistance versus decreasing convective resistance. Imagine you have a bare pipe and add a very thin layer of insulation. For very thin layers, there is actually an increase of heat transfer because the heat transfer increasing effect of the bigger surface area is greater than the heat transfer reducing (insulating) effect due to the insulation layer thickness. If you add another thin layer you make it worse, and another thin layer also makes it worse until at a certain thickness the insulating effect of the added thickness becomes dominant. After this point, adding layers contributes more to the increase the conductive resistance than it does to decrease the convective resistance. That point where this crossover occurs is the "critical insulation thickness"
3. ## When to use psychrometric quick equations

Hi NH, In those occasions in which you have used both approaches, have you found a significant difference? My guess is no. For test day, its quicker and best to just use the simplified equation. Here's a screenshot of what ASHRAE says in their textbook by Howell, Coad & Sauer: The key is that "in the range at which air usually passes through coils, fans, ducts and other equipment, its density is close to standard and is not likely to require correction" - so unless you have some crazy unusual application, the approximate equations should be good enough. Also, the constants 1.08 and 4.5 will change if you are dealing with high altitude. For 5,000 ft instead of 1.08 you use 0.92 and instead of 4.5 you use 3.73
4. ## What is effective eccentricity?

ezzieyguywuf, Here's whats happening: You have two forces applied, the concentric one of 100kip and the eccentric one of 150kip. The eccentric one is applied at a distance of 3.33 inches from the neutral axis. In order to use the secant formula, you need a single equivalent force, not two. The magnitude of this equivalent force is of course, 250kip. But where is it applied? It will be applied at a distance e from the neutral axis so that the moment about the neutral axis due to this equivalent force matches the moment about the neutral axis due to the original system of forces. The moment due to the original force system is (150kip x 3.33in). The moment due to the equivalent force is (250kip x e). Set those two equal and you get the value of e. Does that help? If not, let me know. I can create a sketch that will illustrate it better.
5. ## Globe control valve

We have uploaded a short write-up that further illustrates the issue. EngineerBoards Question 001.pdf
6. ## Variable specific heat & constant specific heat

Perhaps the key to this (as it applies only to the PE exam) is found by looking at problems 533, 534, and 535 of NCEES's TFS practice exam. In problem 533 they explicitly tell you to use constant specific heat (and they even tell you which value to use). In problem 535 they explicitly tell you to use variable specific heats (i.e. use the air tables). In problem 534 they don't tell you what to use. If you look at their solution, they went with the air table. What would happen if you'd gone with constant specific heat? If you do, and use Cp=0.24 Btu/lbm (not the best value to use, but.. lets play along) you get an answer of 46,264 hp. Their answer is 45,800 so you'd be off by 1%. The other answer choices are off by WAY more than that. You'd still pick the right answer, (D). So, they're not that "evil" to give you answer choices that are within 1% of each other. You're safe going with constant Cp.
7. ## Gas turbine

Using gas turbine exhaust gas to generate steam in a HRSG and then injecting that steam into the gas turbine is known as the Cheng cycle. The theoretical analysis is presented here http://www.chengpower.com/cheng_cycle® Here's a case study where it was implemented: http://intpower.com/wp-content/uploads/2012/10/repr_jul_1986_prepared_foods.pdf
8. ## Did any one try the SlaythePE free PE Practice Exam?

Hi Eric Thanks for linking to our site! We're glad you found some of the free resources useful. We agree that the graphical form of the compressible flow tables is neat. Remember that when you purchase access to the solutions to our exam, you get email access to help on any of the problems. This means that if the written solution is not clear enough we can provide more thorough explanation/discussion via email conversation. Cheers!
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