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How to find Change in Temperature for sheet metal

How to find Change in Temperature for sheet metal

How to find Change in Temperature for sheet metal

(OP)
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.   

RE: How to find Change in Temperature for sheet metal

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

In the summer it may approach 100 C.

RE: How to find Change in Temperature for sheet metal

(OP)
Thanks.

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

Is there a way to calculate it from these values.   

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

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

        

             
 

RE: How to find Change in Temperature for sheet metal

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.



 

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

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.

RE: How to find Change in Temperature for sheet metal

Try searching "ASHRAE"

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

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.

 

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

Hi briand2

Can't get your link to work sorry.

desertfox

RE: How to find Change in Temperature for sheet metal

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.

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal

From briand2's linked paper:

Quote:

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.

RE: How to find Change in Temperature for sheet metal

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

RE: How to find Change in Temperature for sheet metal



The coefficient of thermal expansion for steel is 0.00000645in/in/deg F.

In plastic injection molds we always used this calculation for expansion in our hot runner systems.
based on 68f you could apply it to any temp range you wish.

Lou

  
 

RE: How to find Change in Temperature for sheet metal

desertfox,

The roof covering (or more specifically its attachment to the deck) has to cope with the full range of possible movement over the full seasonal temperature swing.

 

RE: How to find Change in Temperature for sheet metal

hi MintJulep

I realise that the roof as to cope with the full temperature swing from summer to winter which may well be +100 deg C to -10 deg C however if we make a steel frame in a factory at 15 deg C then the temperature range or ranges would be (100-15) summer and (15-(-15)) winter so in the summer months to work out expansion of the frame it would be based 85 deg C range and in the winter the contraction would be based on a 30 deg range.
I suppose if you assumed 115 deg C range for both expansion and contraction it would be very conservative but I was never taught to do it that way.
Well I assume the OP as got the information he requires now at least.

regards

desertfox

RE: How to find Change in Temperature for sheet metal

Is the sheet metal insulated? What would the temperatures in the space below the sheet metal be in summer and winter?
Can we assume a wind velocity at 15mph? Is the roof of corrugated sheet metal?  So many questions!!

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