# Air Density on the PE Exam



## StubbornRussian (Feb 9, 2016)

I have been working through the HVAC Six Minute Solutions and it is interesting that the book solutions typically use standard air density even for problems which are noticeably outside the standard range.  Number 62 for example gives you an internal heat gain and then asks you to determine the airflow needed if the outdoor air temperature is 105 degrees and you need to maintain the room below 135 degrees.  Then it asks for the amount of outdoor air that must be circulated through the room.  I guess I have three questions:

1. How do you all define outside air requirements?  When code says to bring in x cfm of outside air are you calculating that volume of air at whatever the outside air temperature is or that volume of air at room temperature or that volume of air at supply temperature?  Industry standard from my experience is that it is that volume of air at supply temperature as you would balance the fan to produce the total airflow and then balance the outside air to whatever the required percentage is.to produce x cfm of outside air.  Whatever method you use is close enough seems to be the general attitude around my firm!  Assume like the question we are talking about single zone just to keep it simple.

2.  Would you assume that air density for this problem should be 105 degrees to be the most accurate? 

3. It appears that on the PE exam prep materials, most solutions automatically use standard air density unless the problem is zoomed in on a particular piece of the system like the coil in which case the solution will find the specific volume and derive the actual air density.  What is the right approach on the exam?  I would like to avoid working these types of problems both ways on the test.

Looking forward to hearing your responses. Man I am so anal because stuff like this drives me crazy even though I know in practice it is not important otherwise code requirements would be in terms of mass flows and not volumes.


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## Audi Driver P.E. (Feb 12, 2016)

Unless I am mistaken, the difference in density isn't going to be much of a factor in your solution.  Now, if you're looking at a problem that involves a psychrometric chart (say pounds of dry air vs. pounds of humid air), you're going to want to be as specific as practical.


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## P-E (Feb 13, 2016)

Normally I wouldn't worry about air density.  PE tests problems you might encounter on the job.


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