RE: U-value in Prebid

["Marco Sangiorgi" <sangiorgim@xxxxxxx>]

Thank you Arie for your answer, but I think we are talking about 2 different
things.

TYPE80 is for the calculation of the CONVECTIVE heat transfer coefficient
and it should be independent of the Prebid.

I'm talking about the Prebid u-value that should be the "global CONDUCTION
heat transfer coefficient" of a surface. It's explained in the Prebid manual
"4.3.2 Definition of a New Wall Type".

The question is: Why is this standard u-value determined with FIXED combined
heat transfer coefficients of 7.7 W/(m2 K) inside and 25 W/(m2 K) outside?

Beside this u-value, it is possible to introduce the CONVECTIVE heat
transfer coefficients that are for default 11 Kj/h(m2 K)=3 W/(m2 K) inside
and 64 Kj/h(m2 K)=18 W/(m2 K) outside. This is what you said it is derived
from the Nusselt therory, isn't it?

It seems to me that the Prebid is taking 2 times account of the CONVECTION:
1 time using those fixed values and 1 time using this convective heat
transfer coefficients.

Am I misunderstanding something?

Thank you again,

Marco

-----Mensaje original-----
De: owner-trnsys@relay.doit.wisc.edu
[mailto:owner-trnsys@relay.doit.wisc.edu]En nombre de Arie Kalkman
Enviado el: jueves, 21 de noviembre de 2002 13:24
Para: 'trnsys@relay.doit.wisc.edu'
Asunto: RE: U-value in Prebid



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.