How to find Change in Temperature for sheet metal

[McQSE]

I am trying to determine the change in temperature for a standing seam metal roof in order to complete thermal expansion calculations. I would appreciate any help.

 

[MintJulep]

In the winter it will be near the record cold temperature for your location.

In the summer it may approach 100 C.

 

[McQSE]

Thanks.

My record temps are:
High 107 deg F
Low 4 deg F

Is there a way to calculate it from these values.

 

[MintJulep]

It will get much hotter than the reported record dry bulb temperature.

 

[desertfox]

Hi McQSE

If you know the ambient temperature for the area at the different times of year you take the difference between the ambient and the max and min values and put them into this formula:-

[Δ]= L * [α] * [T1-T2]

so for example if your ambient was 68F

then the expansion of the seam roof in one plane would be

[Δ]= L * [α] * [107-68]

desertfox

where [Δ]= change in length

[α]= Coeff of linear expansion for metal roof

T1 & T2 = temperature difference

 

[hacksaw]

MintJulip may speak from experience on this, how about a bit more detail

is the protected space ventilated, is the sheet galv. etc., what is the orientation relative and deg lat., etc, etc.

 

[MintJulep]

I just know that metal left out in the blazing sun all day gets stinkin' hot.

 

[Bribyk]

Asphalt will get up to 50-60 degC (measured during road racing) so that wouldn't be surprising for a low-reflectivity metal roof. As a former roofer I can tell you it gets hot enough to burn bare skin so I'd think 60 degC (140 degF) would be about in the ballpark.

 

[sailoday28]

Try searching "ASHRAE"

 

[briand2]

In the UK, for example, when carrying out air conditioning load calculations, we make use of the concept of "sol-air" temperature for opaque building fabric materials. This takes account of the fact that solar radiation increases the outer surface temperature to a value higher than the ambient air temperature. These sol-air temperatures are tabulated (for manual look-up / calculation) by CIBSE (a UK organisation).

For instance, on 21/june, at 13:00, the air temperature is tabulated as 23.6degC whereas the the sol-air temperature for an opaque roof is tabulated as either 49.3degC (dark material) or 36.8degC (light material).

I'm not suggesting for a minute that you use these UK values (I assume you are in the US), but you may care to post your question on the Eng-Tips "HVAC/R engineering" forum and the US-based guys there will no doubt answer your question with reference to ASHRAE methods.

Regards,

Brian

 

[desertfox]

Hi briand2

Thanks for the information, so if you were asked to workout the change in length due to a temperature rise on what do you base your temperature range on? do you base it on 49.3 - 23.6 degrees C or some other figure.

regards

desertfox

 

[briand2]

desertfox,

If I were to do that sort of calculation (which I don't), I would probably use minimum surface temperature in winter (which will be less than typical minimum air temperature) at one extreme, and maximum surface temperature in summer (say sol-air temperature, as I previously described). You may find this link (to a Canadian document) informative:

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd047_e.html

Regards,
Brian

 

[desertfox]

hi briand2

Tried the link but it doesn't appear to work at present.
My philosophy would be to take the ambient and subtract it
from from the known max and min temperatures for the time of year thereby giving a temperature range to enable one to workout the elongation.

I'll give the link another go thanks

desertfox

 

[MintJulep]

The minimum temperature seems pretty straight forward. Use the record low ambient temperature (or the minimum design dry bulb if you don't want to be so conservative).

I suppose you could make a case that at night the roof could radiate heat to the sky and maybe get a bit colder than ambient air.

The maximum temperature is more difficult to make an obvious assumption.

I had thought about sol-air, but I'm not sure it's appropriate. The concept of sol-air temperature is that it is a fictitiousair temperature that allows the use of simple conduction equations for building envelope heat gain to account for the effects of solar radiation incident on the envelope outer surface. I'm not sure that it is necessarily representative of an actual physical surface temperature.

Fundamentally, you could do a heat balance calculation on a unit area of roof and derive the surface temperature. Seems like there would be lots of assumptions made in such a calculation - so in reality I don't know how useful the exercise would be.

 

[briand2]

The sol-air temperature may indeed by "fictitious", but it's nevertheless a reasonable estimate of maximum temperatue in the absence of a much more detailed analysis. In calculations it is is used as an assumed air temperature, so the assumed material surface temperature would be a bit lower than this (due to temperature drop across the outside air surface film).

A reasonable value to use for the minimum temperature will most certainly be lower than the expected minimum outside air dry bulb temperature, because the effect of radiation to the clear sky (as experienced / demonstrated by car windscreens, etc) is significant.

Both of these points are discussed at the link in my earlier post.

Regards,

Brian

 

[desertfox]

Hi briand2

Can't get your link to work sorry.

desertfox

 

[MintJulep]

Ok, you made me actually get out my ASHRAE handbook.

Sol-air would actually over-estimate the surface temperature.

Sol-air temperature is defined as the air temperature, in the absence of all radiation exchange, that would produce the same heat flux into a surface as the combined radiation exchange.

If you assume a perfectly insulated roof structure below the metal roof, and steady-state conditions, then sol-air temperature would equal surface temperature.

In a real roof, there will be often be heat transfer through the roof into the building, so there will always be a net heat flux into the roofing material on the sunny side, and hence sol-air temperature must always be higher than physical surface temperature.

Therefore, using sol-air temperature for an expansion calculation would be conservative.

 

[briand2]

desertfox,

However I try the link, it works, so I don't know why it won't work for you; perhaps you can just copy paste it into your browser address bar?

Anyway, I've tried providing it again on this post, this time using the "Step 3 Attachment" (which I've not used previously!).

Regards,

Brian

 

[MintJulep]

From briand2's linked paper:

Analysis has shown that dark roof surfaces may reach temperatures of the order of 230°F in summer and fall a few degrees below the minimum air temperature in winter.

I'm surprised that my initial 100C swag is actually a bit low.

 

[desertfox]

Hi Again

Sorry briand2 the link still doesn't work even copy and paste maybe its my browser.

Hi Mint your 100 deg C isn't far out from what you have posted but how do you get the temperature range? ie if the roof is manufactured and installed at say 20 deg C then the temperature rise is only 80 deg C if it reaches 100 deg C in the summer, or have I missed something.
Searching on here I can find standard air temperatures for the uk like around 20 deg C at a given altitude and pressure and summer and winter temps but yours are probably different anyway.

regards

desertfox