[Bldg-sim] Fw: exterior convection coefficient for tilted roof

[Chris Yates]

Hi Leen,

I'm not familiar with the calculation involving the Grasshof number. The 
formula given is hc= 4+4v, where hc is the convection coefficient and v 
is the wind velocity. The formula is probably derived empirically (don't 
know for certain). Are you definitely talking about the convection 
element of heat transfer or are you talking lw radiation?

Regards

Chris

On 11/02/2012 22:51, leen peeters wrote:
> Hi Chris,
> CIBSE suggest to replace g with gcos(theta) in the Grasshof number. 
> That means, it accounts for the roof being tilted in natural 
> convection. When it comes to roofs, there is another aspects that 
> starts to be important ... the roof finishing in tiles will have a 
> 'detoriated' velocity underneath the tiles. It is mainly that one 
> directing the convective losses ...
> Am I correct on this?
> Leen
>
>
> On Sat, Feb 11, 2012 at 11:45 PM, Chris Yates 
> <chris.malcolm.yates at gmail.com <mailto:chris.malcolm.yates at gmail.com>> 
> wrote:
>
>     It's true wind does dominate external convection.
>
>     Cibse guide A will give you all the numbers that you're looking for
>
>     Chris
>
>     On 11 Feb 2012 22:40, "Paul Hay" <phcjam at yahoo.com
>     <mailto:phcjam at yahoo.com>> wrote:
>
>         I've only seen reference values of interior convection
>         coefficients for tilted surfaces, not exterior coefficients:
>         which are available for summer & winter conditions, due to the
>         difference in wind velocity.
>         Paul Hay MBA, BA(Arch)
>         Managing Partner
>         PAUL HAY Capital Projects
>
>         Caribbean Capital Project Management & Planning
>
>
>
>         P.O.Box 3367
>         Constant Springs, Kgn. 8
>         Jamaica, W.I.
>
>         tel: 1 (876) 756-0631 <tel:1%20%28876%29%20756-0631>
>         cel: 1 (876) 324-4274 <tel:1%20%28876%29%20324-4274>
>         fax: 1 (876) 756-0631
>         e-mail: paul.hay at phcjam.com <mailto:paul.hay at phcjam.com>
>         skype name: phcjam
>         twitter: http://www.twitter.com/phcjam
>         web: http://www.phcjam.com <http://www.phcjam.com/>
>         profile: http://www.linkedin.com/company/paul-hay-capital-projects
>
>
>
>
>         ----- Forwarded Message -----
>         *From:* leen peeters <l.f.r.peeters at gmail.com
>         <mailto:l.f.r.peeters at gmail.com>>
>         *To:* bldg-sim at lists.onebuilding.org
>         <mailto:bldg-sim at lists.onebuilding.org>
>         *Sent:* Saturday, February 11, 2012 2:59 PM
>         *Subject:* [Bldg-sim] exterior convection coefficient for
>         tilted roof
>
>         Dear,
>         can anyone point me to some reference values for exterior
>         convection coefficients for tilted surfaces like a roof?
>         Thanks
>         Leen
>
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-- 

Chris Yates C Eng MCIBSE

/Building Physics Consultant/

Tel:+447960731576

Email: chris.malcolm.yates at gmail.com

Skype: christopher.m.yates

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