# Engineer Pro Guides Equations



## Luke Prinsen (Apr 18, 2022)

Some of the equations used in the Engineering Pro Guides book can't seem to be found in the HVAC and Refrigeration Reference Manual. Am I blind or is this a correct statement? Thanks!


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## justin-hawaii (Apr 25, 2022)

Hi Luke,
There are certain equations that can't be found in the NCEES Mechanical Reference Handbook like the expansion tank equation. But I still think it is important that you understand the concepts behind the sizing of an expansion tank. Here is a recent email I received from someone about a similar issue and my response in brackets. But you can also just email me and I can quickly provide to you the reference in the handbook. 

1. The solutions rarely if ever reference specific locations in the PE Handbook. [Yes, sorry about that. I have contemplated putting in the exact reference location, but for those engineers in my class, I really want them to struggle with finding things in the book. It is very important that your knowledgeable about the entire handbook. But I definitely see how it can be frustrating when you are first studying.]

2. The exam uses nomenclature and formulas that deviate from the handbook. [Yes, just be aware that the exam will have many questions that will not use the exact formula. Questions will reference deviations from the handbook. This is not like the FE exam.]

3. In the sensible heat gain formula at standard conditions, NCEES uses a coefficient of 1.1, while your exam uses 1.08. I also use 1.08 in my work, however using this coefficient will result in wrong answers in the official NCEES practice test. [I understand. It is a 1.85% difference. Most possible solutions are more than 1.85% different from each other.]

3. Solutions will use data that cannot be found or derived accurately using the handbook, such as the humidity ratio of 30F db / 27F wb air. [This actually is a trick and I wanted people to be able to use the psych table. See explanation below.
The key is to find the relative humidity.
There is an equation for this, but it isn't presented in the handbook. The way you can do this is with the Psych Table. 
The enthalpy of 30 F air at dry air = 7.206 Btu/lbm
The enthalpy of 20.3 F air at saturation = 7.206 Btu/lbm
The enthalpy values and wet bulb values line up pretty well. 
20.2 F WB - 30 F WB : 0% to 100% RH
20.2 F WB - 27 F WB : 0% to 69.4% RH

Now you have relative humidity so use this equation.

W at 0% RH = 0
W at 100% RH = .003455

So take 69% of .003455 = .0024





This was just a unit conversion to get you exposed to the other common way W is presented
7000 grains = 1 lb


4. The solution to problem 61 makes several incorrect statements about the NCEES handbook: "Navigate to the Pipe Sizing chapter and find the graph that plots Head Loss as a function of flow rate and pipe size. Use the graph for standard Schedule 40 pipe."
As far as I can tell, this chapter and graph does not exist in the current NCEES handbook. However, there are friction loss tables for standard weight steel pipe. It would have been greatly advantageous to refer to the actual section where these are located. The graph may actually exist somewhere but combined with other differences in nomenclature and lack of references, it is hard to judge what is current and whether it will be a waste of time hunting down data that may or may not exist.


Problem 61: Sorry I have a typo, it should have been table instead of graph.


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## Luke Prinsen (Jun 8, 2022)

Thank you so much for the thorough response! It's greatly appreciated, Justin.


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