RE: U-value in Prebid

[Arie Kalkman <a.kalkman@xxxxxxx>]

Dear Marco,

In the TRNSYS manual on TYPE80 (convective heat transfer coefficient calculation) you will find equations
and references to the Nusselt-theory which is the background of the equations and values you mention.
The values of  7-8 for inside - and 23-25 for outside heat transfercoefficients are widely used, and can be
used in most situations; this holds for both walls and windows. You may recalculate the outside value using
hourly wind speed data, but usually a fixed value is sufficient since it represents a minor contribution in the heat
resistance of modern - well isolated- buildings.

Arie



-----Oorspronkelijk bericht-----
Van:	Marco Sangiorgi [SMTP:sangiorgim@inta.es]
Verzonden:	donderdag 21 november 2002 13:09
Aan:	trnsys@relay.doit.wisc.edu
Onderwerp:	U-value in Prebid

Dear TRNSYS users,

I'm a little perplexed about the calculation of the final u-value of walls
in Prebid. The resulting u-value is not just the "sum" of the thermal
conductivity of the layers that composes the wall, but it additionally
combines heat transfer coefficients of 7.7 W/(m? K) inside and 25 W/(m? K)
outside. ??

What is it for? Is it possible to change these coefficients?

Moreover, the Prebid asks for the convective heat transfer coefficients of
the walls. So those additional coefficients in the calculation of the
u-value don't represent the heat transfer due to convection. So, what do
they represent physically? And how have they been calculated?

Thank you,

Marco