AW: U-value in Prebid

["Jochen Wriske" <wriske@xxxxxxxxxxxxxxxxxx>]

Hi Marco,

The U-Value that is shown in the Prebid-Formular (and in the *.inf-file,
written from Prebid)
is just for your information. It is used when calculating the DIN4701
energy-demand.
And you're right: It is calculated by using the (hardcoded) alphas: 7.7
W/(m2 K) inside and 25 W/(m2 K) outside (This correspondends to the fact
that the U-Value does not change by entering different values for the
"Convective Heat Transfer Coefficient of Wall" in the formular).

During the simulation the heat transfer coefficient is calculated each
timestep, regarding two things:
	1) Convective Part - Your entered values for alpha ("front" and "back").
	2) Radiative Part - Internal calculation by using the surface-temperatures.
I suppose the PREBID-"Wall type manager" shows the U_Value according to the
DIN4701-rules for calculating, because the "real" U-Value is varying during
the simulation.

Hope this helps

	Jochen

--
Dipl.-Ing. Jochen Wriske
Lehrstuhl für Technische Thermodynamik
RWTH Aachen      www.ltt.rwth-aachen.de
Tel. 0241 / 80-98 153  Fax 80-92 255



> -----Ursprüngliche Nachricht-----
> Von: owner-trnsys@relay.doit.wisc.edu
> [mailto:owner-trnsys@relay.doit.wisc.edu]Im Auftrag von Marco Sangiorgi
> Gesendet: Donnerstag, 21. November 2002 14:48
> An: trnsys@relay.doit.wisc.edu
> Betreff: RE: U-value in Prebid
>
>
> 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.
>
>