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Slay the P.E.

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About Slay the P.E.

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    Project Engineer

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    Prospect Heights, IL
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    Mechanical Engineering Exam Prep

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    CBT exam will not be open book. Reference material will be provided onscreen. Looks like NCEES will write a reference handbook like for the FE. The ME PE exam will transition to CBT somewhere in the 2019-2021 timeframe.

    Here’s an oldie but really good story about a costly mistake because of a units mishap: http://www.cnn.com/TECH/space/9909/30/mars.metric.02/

    I am convinced that one of the biggest hindrances to success with the PE exam is indeed being sloppy with units (NEVER write a number down without its units) and also, not practicing unit conversions to become quick at it. Additionally, be mindful of the units you are required to answer in. Through your calculations, you might arrive at an answer of 0.5 feet, but the question might have been something like: ... the required thickness (inches) is most nearly: (A) 0.5 (B) 4.0 (C) 6.0 (7) 8.0 If you’re not careful you might choose (A); 0.5 — because it’s the number you got in your calculations, however in this case the correct answer would be (C). This is unfortunate, but it happens. You might have the concept down cold, but can still get the answer “wrong”.
  4. Pump curve problem

    Yes, efficiency. The affinity laws are based on the assumption that efficiency remains constant. So, in that figure (assuming it is to scale) if you pick (240,75) which is on the 77% efficiency line you would get (216, 61) which is the point on the 4.5" curve with that same efficiency. But... there are many other points one can pick with a known efficiency -- for example (130, 100) -- that don't fall on the corresponding point on the other curve; you in fact get (117, 81). Could be that equation 18.48 would have to come into play. Interestingly enough, if you pick (200,90) the point of peak efficiency for 5" you get (180,73)...roughly the point of peak efficiency for 4.5" At least if you pick (280, 60) you can get something approximate like (251, 49). Seems like the whole thing falls apart for lower efficiency.
  5. Pump curve problem

    I agree. Tomorrow you are more likely to encounter correctly designed problems.
  6. Pump curve problem

    Here's further illustration of why picking two points with the same flow rate and using only equation 18.45 is wrong. The answer you get depends on the flow rate you pick: If you pick Q2 = 410 qpm then set h1 = 131 ft and h2 = 84 ft, use eq. 18.45 to get D2 = 9.6" If you pick Q2 = 380 qpm then set h1 = 140 ft and h2 = 108 ft, use eq. 18.45 to get D2 = 10.5" If you pick Q2 = 300 qpm then set h1 = 148 ft and h2 = 123 ft, use eq. 18.45 to get D2 = 10.9"
  7. Pump curve problem

  8. Pump curve problem

    Curses! I posted the wrong figure in the original problem. That green curve in the top post is drawn incorrectly. This is the right one: So my response to you should have been (edited numbers in bold): The key is that when diameter changes, 18.46 and 18.47 both apply. If we pick a point in the blue curve say (380, 140) and pick one of the choices as D2 (say, 10.5 as a first attempt) we get Q2 = 333 qpm and h2 = 107 ft. This point is not on the green curve, so (A) would be incorrect. Next we try D2 = 11" and we get Q2 = 348 qpm and h2 = 118 ft. This point is on the green curve, so (B) is correct
  9. Pump curve problem

    That's the correct answer, but I disagree with the steps. Why pick points with the same flow rate? and why not points with the same head added instead? I could pick Q1 = 438 gpm, Q2 = 320 (both have h = 110 ft) use D1 = 12" and use equation 18.45 to solve for D1 which yields 8.8" The key is that when diameter changes, 18.46 and 18.47 both apply. If we pick a point in the blue curve say (380, 140) and pick one of the choices as D2 (say, 10.5 as a first attempt) we get Q2 = 319 qpm and h2 = 99 ft. This point is not on the green curve, so (A) would be incorrect. Next we try D2 = 11" and we get Q2 = 334 qpm and h2 = 108 ft. This point is on the green curve, so (B) is correct
  10. Pump curve problem

    Unfortunately, that is incorrect. First of all, the "P" in equation 18.47 refers to power, not head. I believe you meant to use equation 18.46 instead. However, applying 18.46 with h1 = 140 ft, h2 = 90 ft, D1 = 12 in and solving for D2 will still be incorrect. Since speed is held constant, both 18.45 and 18.46 apply. If you look at 18.45 you see that the flow rate will also change (not remain constant as you have assumed)
  11. Pump curve problem

    A recent exchange here on the boards inspired me to create a cool problem. Here goes: An old copy of a pump curve from a manufacturer shows the curve corresponding to an impeller diameter of 12 inches (shown here in blue). Another curve is shown in the graph (shown here in green), but a coffee stain on the graph covered the impeller diameter corresponding to this curve. The impeller diameter (inches) corresponding to the green curve is most nearly: (A) 10.5 (B) 11.0 (C) 11.5 (D) 12.5 Post your solution and explain your reasoning.
  12. PPI Six minute HVAC - #30

    This is a bad problem and you should just ignore it. The way they have written it makes it impossible to solve, and their solution is wrong. Here's why: Pump affinity laws essentially take one point (Q1, h1) on a pump curve (for a given diameter, D1) and assign to it one and only one point (Q2, h2) on a different pump curve (which corresponds to a different diameter D2). If you do this for enough points on the original curve, you can get enough points on the new curve that you can draw it. Note that since Q2 =( D2 /D1 )Q1 and h2 =(( D2 /D1 )^2)xh1 the flow rate and head between corresponding points are never the same. In graphic form (see below), pump affinity laws take each blue point in the original curve, and assign to it a red point on the new curve. Applying pump affinity laws is like using the green arrows in this graph: The issue with the problem you've posted is that you are not given the whole pump curve for the original diameter. You are only given a single point of the original curve. The problem is not solvable, and as you've noted, there are issues with it. In simpler (and easier to understand) terms, the problem you posted can be re-written as: “A pump curve for a known diameter passes through the blue point in the graph below. Find the impeller diameter that will make the curve pass through the red point”. Hint: Can't be done. You can use h2 =(( D2 /D1 )^2)xh1 to solve for D2 with h2 = 115 ft and h1 = 140 ft. This gives you D2 = 11.3 in. However, this does not correspond to the red point. The flow rate would have to change. It will be Q2 =( 11.3 / 12.5 ) x 380 = 344 gpm. In graphical form (see below), applying affinity laws takes the blue point, along the green arrow and gives you the green point. . From the graph above, its clear that trimming down the impeller to 11.3 inches will not make the pump add a head of 115 ft for a flow rate of 380 gpm. Their solution is wrong. The problem as presented cannot be solved. As always, I could be mistaken and welcome any critique of this write-up.
  13. set of psychrometric charts?

    I would say the real issue is with whoever wrote the problems you’re working on. Getting an enthalpy of 30.6 BTU/lbm instead of 30 BTU/lbm (a 2% difference) should not lead you to picking a wrong answer choice. Please post an example of a problem where this has happened...
  14. set of psychrometric charts?

    Are the deviations bad enough that they lead you to wrong answer choices? The answer choices typically wouldn’t be that close. Can you provide an example?
  15. set of psychrometric charts?

    There’s a high altitude one (5,000 ft) in the NCEES exam booklet. We have extended range (from 20F to 120F) for sea level for free download here: https://www.slaythepe.com/free-resources.html