×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

altitude vs. temperature

altitude vs. temperature

altitude vs. temperature

(OP)
I am modeling an electronic heat sink specified to 10,000 ft. How should I change the pressure to handle this? Will lowering the environmental pressure correct the elevation change?
-R

RE: altitude vs. temperature

Lowering the environmental pressure will correct for it.
However are you considering the effect of temperature changes?

RE: altitude vs. temperature

(OP)
That is the main question I am asking. Should I change the temperature effects? So say I am looking for 10k ft and 65 C.

RE: altitude vs. temperature

So what's the issue?  You're given the temperature.

TTFN



RE: altitude vs. temperature

Temperature gets colder higher up, that is the reason to check temperature effects.

RE: altitude vs. temperature

Electronics in military equipment are not necessarily operated outside at 10,000ft.  Additionally, solar load can bring the temperature up to 65ÂșC regardless of external temperature.

TTFN



RE: altitude vs. temperature

My point was that if temperature at altitude affected the design, it would be from colder temps.  If the widget was heated or insulated, sea level conditions would control, and temp at altitude wouldn't matter.

RE: altitude vs. temperature

Reducing air pressure in your model may not give the correct results (depends on your analysis tool).  

You'll probably need to enter the correct air density.  This can be calculated from temperature and altitude. If you don't have the formulas, you can try this little calculator: http://www.denysschen.com/catalogue/density.asp  

ko  (www.ecooling.biz)

RE: altitude vs. temperature

(OP)
That is really what I wanted. I needed to know the air density.. Now do you have one that adds humidity?
Thanks,
Rodney

RE: altitude vs. temperature

Tables are available of temperature, pressure, and density at increasing altitudes. They are standard values and are not constant as they fluctuate with weather changes. I saw them in an aeronautical handbook and wish my memory was better so I could tell you the name. The data comes from one of the Federal agencies, maybe NWS or NOAA.

RE: altitude vs. temperature

.
The relationships between temperature, pressure and altitude that are used in air pollution dispersion modeling are presented in the book "Fundamentals of Stack Gas Dispersion", Fourth Edition, 2005 available at www.air-dispersion.com, and the following is a direct quote from that book:

Atmospheric pressure decreases with increasing altitude, and any air rising from the warm surface of the earth will expand as it rises to lower atmospheric pressure levels.  Taking the atmospheric pressure at sea level to be 14.696 psia, we can obtain the atmospheric pressure at any altitude from this approximation:

(1) Pa = 14.696 (0.963)a

We can obtain the temperature which will be acquired by dry air rising from sea level to any given altitude from this expression which assumes that the rising air expands adiabatically:

(2) Ta =Ts (Pa/14.696)(k-1)/k

where:
a = altitude in 1000's of feet
Pa = atmospheric pressure at altitude a, in psia
Ta = air temperature at altitude a, in degrees R
Ts = sea level ambient temperature, in degrees R
k = 1.4 for air


Using the above relationships, one finds that from sea level to 8000 feet, the atmospheric pressure decreases 0.5 psi per 1000 feet ... and the temperature of dry air will decrease 5.5 degrees F per 1000 feet.

Milton Beychok

RE: altitude vs. temperature

Rodney,

You need a psychrometric chart to find the relation between humidity and density.  In most environments (5-95% humidity, 15-50C) humidity has a negligble effect on heat sink performance.

A tutorial on psychrometric charts: https://www.uwsp.edu/it/tlrn/LOs2003/paperlo/

ko  (www.ecooling.biz)

RE: altitude vs. temperature

(OP)
Thanks,
R

RE: altitude vs. temperature

I think that although temperature decreases as altitude increases, the drop in air density has a negative effect on heatsinks.  MIL-W-5088 has info on this in relation to current ratings in electrical wiring which relys on ambient air cooling and it applies a 5% derating factor at 10,000ft.

RE: altitude vs. temperature

Rodney mentioned 10,000 feet at 65C, so I'm guessing his heat sink is not airborne.  

His worst-case scenario might be a heat sink at the top of an electronic cabinet on a hot day with an A/C failure in a crowded computer room in La Paz, Bolivia...

ko  (www.ecooling.biz)

RE: altitude vs. temperature

Point taken, but the efficiency of heatsinks will be less than at sea level and I would start with at least a 5% derating factor.

RE: altitude vs. temperature

Rather than a straight derating, Rodney should be able to change his model to simulate density at 10,000 feet & 65C, which is 40% less than sea level at 20C.  

ko  (www.ecooling.biz)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources