When to use psychrometric quick equations

[NHEngineer037]

I am not sure how to best attack psychrometric equations on exam day. 

Q=1.08 * CFM * deltaT  -or-  Q= 1/spVol * 60 * 0.24 * CFM* delta T

Q=4.5 * CFM * deltaH  -or-  Q = 1/spVol * 60 * CFM* deltaH

My go to method is the more detailed approach and taking the extra 3 seconds to get the specific volume.  I've found that some practice problems, most actually, have solutions based off of the quick equations.  Its not always the case though.   

Any advice for a best strategy on test day? 

Thank you,

Ryan

 

[NHEngineer037]

This question also applies to mixing problems.

I was taught in school that you use mass flow rates of the air, not CFM, to determine how to weigh the T, W, or H or mixing airstreams. 

A bunch of solutions seems to favor just using CFMs, which is OK if the solutions have a large spread, but when they don't, I'm torn on what calc to do!  Sometimes ill calculate it both ways in a sample problems, which would be a ridiculous time consuming strategy on test day. 

Looking forward to hearing some thoughts..

 

[Slay the P.E.]

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

​

 

[JHW 3d]

On exam day, do it using both methods, then average your results.

⋮

:joke:

 

[namod65]

I have gone through the HVAC 6MS three times now and both NCEES 2016 and 2001 practice exams a few times, plus Justin Kauwale's materials. I'm taking the test in Oct.

In the NCEES practice tests they mostly use the quick formulas in their solutions. I noticed each practice test had one problem where the problem goal is to determine if you know how to use the correct density rather than the quick equations. You had to use the correct density to get the right answer on this problem since the solutions were close enough and the temperature conditions were far enough from normal. 

In the 6MS there was only one problem where you can tell it's goal was to see if you can figure out to use the correct density and not the quick equations. Most of the 6MS problem solutions use the long formula, but the answers are far enough apart that it didn't matter if you used quick equations. There was even another problem later on that dealt with outside air temperatures of like 130 degrees, which clearly has a different density. But the solution just used the standard .075 lbm/ft^3, which pissed me off. The goal of that problem wasn't to test you on determining what density to use, but something else, so the answers were far enough apart to where it didn't matter.

My theory is that of the 80 problems, they'll put one in there that will try to trick you on the use of the quick equations. The rest we should be fine using the 1.08 and 4.5 equations. We just need to be able to recognize that one problem. The outdoor air will probably be either really hot or really cold and the answers will probably be close together.

There's so many topics and they only have 80 questions. So each question is carefully chosen to test you on a specific topic/trick of the trade/unit conversion/etc... There's just too many topics that build on top of the use of the Q=mCpDT, Q=mDh equations, and they want to see if you know those topics without the variable of tripping you up on the density each time. So it's a waste of a question after they do it once or twice.

That's my theory at least hahaha.

 

[justin-hawaii]

namod65,  I really like your theory!  I second it.  

My theory is that of the 80 problems, they'll put one in there that will try to trick you on the use of the quick equations. The rest we should be fine using the 1.08 and 4.5 equations. We just need to be able to recognize that one problem. The outdoor air will probably be either really hot or really cold and the answers will probably be close together.