mean temperature??

[Drew]

When ,in heat load calculations,do we use temp diff as opposed to mean temp.? With coldroom wall loss applications we just use temp difference, but with heat exchangers we use the mean temp?

Some formula :

kw = a x u x temp difference

kw = a x u x mean temp difference


When would we use the two different calc's?

[Magoo]

Drew. It is called log mean temperature difference, abbreviated to LMTD, as the temp change is not linear, this will average as a mean change the difference.
hope this helps. Magoo

[Sandro Baptista]

Why ,in heat load calculations, do we work with mean temperature differences as apposed to inlet and outlet temperature.

Some formula :

kw = q x u x temp difference

kw = q x u x mean temp difference


When would we use the two different calc's?

Inlet and outlet temperature are temperatures refering to the same fluid as opposite of the logaritmic temperature difference.
Inlet and outlet temperature can be used to get the heat received or released by the fluid > m.cp.(Tout-Tinlet) if you know their values.

m.cp.(Tout-Tinlet) must be equal to U.A.LMTD if no phase change occurs. If phase change occurs you must sum this part to the sensible heat part.

[Drew]

Thanks for the replies.

I have given it some thought and with the help of your replies now understand that when we use the formula:

Kw = a x u x LMTD it is for heat transfer through an item that doesnt have a constant temp diff and this is why we find an average.

Kw = a x u x temp diff. This is used to find the heat transfer rate when the temp diff is constant. Loke a coldroom wall.?

kw = kg/s x shc x temp diff. This is used to establish the heat gain or loss of a product.

[desA]

An excellent academic reference book to get you going in the right direction:

"Fundamentals of Heat and Mass Transfer", Incropera F.P., De Witt D.P., John Wiley & Sons

Chapter 11 - Heat Exchangers, will provide you the info you are looking for.

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For a condenser, the refrigerant side essentially comprises three sections - de-superheater, condenser, sub-cooler. These sections can theoretically be split up into 3 separate heat-exchangers. There are solid theoretical reasons for doing this. Each section has its own dTlm (log-mean temperature difference).