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Home > Forums > Mechanical Engineers > Activities > HVAC/R engineering Forum

Convective Heat output

thread403-326223
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afa13 (Mechanical)
19 Jul 12 5:30

I need some clarification regarding the following formula:

Qconv = Pn x Qs x b x ϕ

this equation is used to determine the cooking appliance's convective heat output in kw, 'b' and 'ϕ' are coefficients easily determined. I am confused whether to use the actual capacity of the appliance for 'Pn' or its power rating which is higher and will result in a big difference. When it comes to Qs is the cooking appliance convective heat output in W and sometimes its unit is W/kW.

Can anyone help clear this issue?
afa13 (Mechanical)
19 Jul 12 5:41
correction:
'Qs' is the sensible heat emitted
my bad!
willard3 (Mechanical)
19 Jul 12 8:21
Where did you find this formulaa?

They should define Pn.
afa13 (Mechanical)
19 Jul 12 9:57
go to :
http://www.scribd.com/doc/29293566/HVAC-Handbook-Kitchen-Design-Guide-HALTON

and scroll down to page 45
IRstuff (Aerospace)
19 Jul 12 10:13
Aren't they using the values listed on page 42? They show P=18kW, which is the first value in the table.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies
afa13 (Mechanical)
19 Jul 12 10:34
yea man ur right but if u refer to any catalogue for any kitchen appliance you have the power rating and you have actual capacity. I think it is the power rating but i need to be sure this is y i have it on the discussion board. And once that gets handled there is the Qs that needs to be cleared.. Thanks for the help anyway!
trashcanman (Mechanical)
19 Jul 12 13:53
Too many unknowns. If the appliance is cooking anything with water (all foods), there will be latent heat emitted. How much is the appliance used?

Look at ASHRAE Handbook of fundamentals, - internal heat gains - kitchen appliances.
afa13 (Mechanical)
20 Jul 12 5:32
do u have any idea where i can find a free pdf format?
IRstuff (Aerospace)
20 Jul 12 12:49
That would be a copyright violation, which is not condoned here.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies
Drazen (Mechanical)
20 Jul 12 16:31
if you list units of formula elements, than there is chance for us to have more certainty.

if Qs is given in W/kW, and it is electrical appliance, than it is likely that Pn is electrical power rating, but your "sometimes" provides confusion, there shouldn't be sometimes, Qs is given in W/kW or not.
afa13 (Mechanical)
21 Jul 12 6:05
yea ur right if u take unit counts it is to be W/kW but my sometimes is in its place because i encountered a document using Qs as W and another one referring to it as w/kW. I think it is best to go with the power rating since it is the main source for the convective heat loss which the power capacity doesn't affect directly! Thank you all!
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