Heat transfer coefficient - Coolpack
What tabels are used to determine m_WALL (the thermal resistance of the wall)? I cannot believe the variances in the tables that I have come accross recently. I previously did my calculations using the ASHRAE handbooks from the 1970's or Modern refrigeration & Airconditioning 1975. These are not metric/SI and converting is cumbersome.
Re: Heat transfer coefficient - Coolpack
Quote:
Originally Posted by
Graham
What tabels are used to determine m_WALL (the thermal resistance of the wall)? I cannot believe the variances in the tables that I have come accross recently. I previously did my calculations using the ASHRAE handbooks from the 1970's or Modern refrigeration & Airconditioning 1975. These are not metric/SI and converting is cumbersome.
This is from help in CoolPack:
Quote:
As a guideline for the thermal conductivity of prefabricated walls (walls, cieling og floor) the following can be used (Ref.: W.F. Stoecker, "Industrial Refrigeration Handbook", page. 610, table 18.4. McGraw-Hill, 1998. ISBN 0-07-061623-X)
Re: Heat transfer coefficient - Coolpack
Yes. I can read the help page. Where the question started to form was from a technical article in a magazine where ASHRAE tables were quoted and the resultant calc supplied the answer of 0.55 meter squared K/W for a 50mm wall (expanded polystrene). If you visit engineers toolbox the value for polystyrene .03 W/m K and no matter how I play with the 50mm thickness I cannot get anywhere near the .34 that W.F. Stoecker achieves. I also have some hand me down tables that alledgedly originate from CIBSE and AIRAH the one has a value of 25.68 W/m K for which if multiplied by 0.05 (the wall thickness) = 1.284 which then divided into 1 = 0.77 as the coeficient which is way different to the 0.34. Who's tables do I use when I want to have a 230mm Brick wall plastered with 15mm perlite and then 50mm Polystrene ?
Re: Heat transfer coefficient - Coolpack
Re: Heat transfer coefficient - Coolpack
Quote:
Originally Posted by
Graham
... question started from a technical article in a magazine where ASHRAE tables ...the resultant calc supplied the answer of 0.55 meter squared K/W for a 50mm wall (expanded polystrene). ...engineers toolbox value polystyrene .03 W/m K and no matter how I play with the 50mm thickness I cannot get anywhere near the .34 that W.F. Stoecker achieves. I also have some hand me down tables that alledgedly originate from CIBSE and AIRAH the one has a value of 25.68 W/m K for which if multiplied by 0.05 (the wall thickness) = 1.284 which then divided into 1 = 0.77 as the coeficient which is way different to the 0.34. ...I want to have a 230mm Brick wall plastered with 15mm perlite and then 50mm Polystrene ?
Graham, 0.03W/m.K seems for me a a little bit low value, but this figures doesn't change if you have a thin or a thick wall, This because you said you played with a thickness of 50 mm to achive some result.
Perhaps I understood it wrong.
There's also a difference between expanded and extruded polystyrene. Perhaps Eng. toolbox gave another sort of polystyrene.
I haven't looked in tables but I can remember out of my head that you need +/- double thickness for expanded polystyrene if you want the same U-value for a PU wall.
http://en.wikipedia.org/wiki/Polystyrene
http://www.texasfoam.com/technical-data.htm
Don't confuse conductivity with isolation factor (don't know if these are the right English expressions) because the first is the opposite of the second.
The 25.68 you mention seems to be 1/25.68 = 0.038 W/m.K. I think you or the article didn't used the right units.
Re: Heat transfer coefficient - Coolpack
Quote:
Originally Posted by Peter
Don't confuse conductivity with isolation factor (don't know if these are the right English expressions) because the first is the opposite of the second.
Hi Peter. I think the word/phrase you are looking for is Thermal Resistance, which is shown by R. Therefore R= 1/C where C equals the Conductivity of one material of the wall section. In other words, R equals the inverse of C.
It's important to remember the difference between K & C. K = conductivity and C = conductance.
C is defined as the heat transfer factor for a specific building material at a specified thickness.
K is the heat transfer factor for a specific building material at an actual thickness.
So....
R (total) = (X1/K1) + (X2/K2) + (X3/K3) + (1/fi) + (1/fo)
X = actual thickness of material
fi = inside wall film coefficient
fo = outside wall coefficient
and the numbers indicate the different building materials.
So, 1/R (total) = U, or the overall heat transfer coefficient of the entire wall cross-section.
I think this is what you were hinting at.;)
Re: Heat transfer coefficient - Coolpack
Peter,thanks but Wikipedea links to The engineering toolbox, Texas foam is Imperial, I have been using similar numbers for 20 years now. Iceman, thanks you have all the letters in the right place, they make sense and I can use them. My problem is the different pieces of paper that I have, contain column headings that are the same however the numbers opposite the various materials differ greatly. I do not know what chart/table to use. Consider this a poll.
Re: Heat transfer coefficient - Coolpack
Graham,
Look at the fine print on the pages you have. They may list the values at different specific temperatures or they may be based on new material versus aged material.
Or, they may reference different standards (ASTM, etc)
Or..... they could be wrong!:eek: (just kidding). There must be a reason, but off the cuff I don't know what it would be without doing some homework.
Re: Heat transfer coefficient - Coolpack
Quote:
Who's tables do I use when I want to have a 230mm Brick wall plastered with 15mm perlite and then 50mm Polystrene ?
Quote:
Look at the fine print on the pages you have. They may list the values at different specific temperatures or they may be based on new material versus aged material.
Hi . . . how are you all ???
For the polysterene you should use the manufacturer value for the convinient range of tempreture , many densities of polysterene boards exists and each one with a different value.(maybe that's why your not getting good resolution results).
Respects and regards