Altitude

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drcampbell

Senior Member
Location
The Motor City, Michigan USA
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Registered Professional Engineer
Yes. A fan blows a more-or-less constant volume of air, but the density of air decreases as altitude increases. At higher altitudes, there are fewer air molecules available to pass over a heat sink and extract heat.
https://en.wikipedia.org/wiki/International_Standard_Atmosphere
https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html

The thinner air also makes high-voltage electrical apparatus more prone to corona and flashover, but that's not significant unless there are thousands of volts available.
 
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drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
I don't have a simple answer. The problem should be reviewed by someone with a flair for mechanical engineering.

It's a significant-enough problem that Los Alamos National Lab had a standard boilerplate paragraph in their purchase orders and RFQs reminding people that the lab is at a 7000-foot elevation and vendors are expected to assure that their equipment will work there.

Doing design work after the fact ("adding fans as the temperature increases") is rarely a recipe for success.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Correct

Correct

Yes. A fan blows a more-or-less constant volume of air, but the density of air decreases as altitude increases. At higher altitudes, there are fewer air molecules available to pass over a heat sink and extract heat.
https://en.wikipedia.org/wiki/International_Standard_Atmosphere
https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html

The thinner air also makes high-voltage electrical apparatus more prone to corona and flashover, but that's not significant unless there are thousands of volts available.

This is absolutely correct, and noted in Air Cooled Refrigeration condensers which are less efficient at elevation. I have noted this for sure at 4500 Ft.
 

kwired

Electron manager
Location
NE Nebraska
Another consideration though is that at 10,000 feet the outdoor ambient temp is likely lower than many other places. This same aspect applies to anything that you may be trying to sink heat away from and into the air though.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
With (per Wikipedia) the highest point in UK being 978 metres, not a problem for your local jobs, was it ... ?
Ben Nevis is 1,345 metres. We did a few ski lift drives. But let that pass.
We did drives for gold mines in the Highveld around Johannesburg, elevation close to 2,000 metres. With that and the high ambient temperatures, we fitted fans with higher flow rates.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Most if not all VFD mfrs provide an altitude de-rating formula for their drives, usually buried in the technical information somewhere. For A-B drives for example they tell you to de-rate the output current by 10% for every 3,300ft (1,000m) above the 3,300ft baseline, or looked at another way, 1% for each 330ft above 3,300ft.

So for 10,000ft elevation, that's 6,700ft over the baseline, / 330ft = 20% de-rate.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Ignorant South Carolinian here trusted Wikipedia when I asked for the highest point in UK ... I deserve a flogging.

mea culpa
Did you ask for UK or England?
Often confused by furriners.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Did you ask for UK or England?
Often confused by furriners.

I asked "highest elevation in UK", but (lacking research skills to check work) took first hit, https://en.wikipedia.org/wiki/Scafell_Pike without digging further. It says England.

The next hit was https://en.wikipedia.org/wiki/List_of_highest_points_in_the_United_Kingdom which gives Ben Nevis at 1344 metres (4409 ft.).

I'm just one of those dum sutherners from the Carolinas with the poorest school systems (and a wife who has a masters in information science who would flog me for my accepting Google and wikipedia alone ...)

Sweep me under the rug and let's get back to electrical issues ...
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Like your sense of humour - and, yes that's how we spell it........:)

A bigger challenge than altitude for us was ambient. Many of the switch rooms had aircon. But not the tin mining dredgers up the jungle in Malaysia.
But you know that going in and design the kit accordingly.
 

Timbert

Member
Location
Makawao, Hawaii
Most if not all VFD mfrs provide an altitude de-rating formula for their drives, usually buried in the technical information somewhere. For A-B drives for example they tell you to de-rate the output current by 10% for every 3,300ft (1,000m) above the 3,300ft baseline, or looked at another way, 1% for each 330ft above 3,300ft.

So for 10,000ft elevation, that's 6,700ft over the baseline, / 330ft = 20% de-rate.

I agree with Jraef, although in my experience finding this information is not as easy as he suggests. In my experience, a lot of manufacturers publish at altitude spec with no numbers for de-rating. The cooler ambient air temperatures help to some extent.

In my opinion, 20% de-rate for 3000m (10000 ft) is a good conservative number. I've been working for thirty years at sites from 3000 to 4000 meters (10000 to 13000 feet). I've haven't seen things given a proper de-rating have problems, but a plenty of issues when altitude has been overlooked. I've also seen equipment where no de-rating was applied and it worked fine because it was light, intermittent duty. But remember equipment installed at altitude tends to be on remote sites so it's generally a good idea to make sure it's more than sufficient than risk frequent breakdowns at a remote site.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
... In my opinion, 20% de-rate for 3000m (10000 ft) is a good conservative number. ...
It's certainly better than not derating at all, but I wouldn't say it's "conservative", just adequate. At 3000 m altitude, there are 26% fewer air molecules in a given volume of air than at sea level.
 
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