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Heat Exchanger %Overdesign

Heat Exchanger %Overdesign

Heat Exchanger %Overdesign

(OP)
Hi, I'm currently designing Heat exchanger and found some confusing spec regarding design margin.
on specification it is stated as follows:

"Overall heat transfer coefficient (U) calculation divided by U design shall be more than 1.05 in additional to design margin which is specified by the Process Licensor."

is it common to add design margin on U value? as far as i know, we usually add margin on heat transfer area.

RE: Heat Exchanger %Overdesign

Targeting 5-10% overdesign for heat exchangers is common, even though it doesn't actually mean much as the exchanger is unlikely to operate exactly as described on the data sheet with the thermal resistances matching the specified fouling factors exactly.


-Christine

RE: Heat Exchanger %Overdesign

Be careful with terms here; you've got them the wrong way round:-
U_design is higher than U_actual. U_actual may incorporate a min design margin, as in your case, so U_actual/U_design > 1.05, which is done by including excess effective heat transfer area. U_design must include fouling factors for both shellside and tubeside as appropriate i.e U_design < U_clean.
Effective heat transfer area < actual heat transfer area.

RE: Heat Exchanger %Overdesign

There is a fine art to this, and different people approach it differently.
They are talking about the total U for the unit, so it ends up being and increase in area.
Since this is part of a licensed process the process variables should be very well defined.
(I am presuming that they dozens to hundreds of these units in service)
You want some margin, but as little as possible.
Heat exchangers that are too oversized are always going to underperform.
This cushion is likely to allow for local variations in fouling, flow, effects of ambient temp, and even such things as variations in tube wall thickness.
Tubes purchased to the same specification may have a 10% difference in wall thickness and that can translate to about 0.5% reduction in heat transfer.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Heat Exchanger %Overdesign

Error in my previous post :

U_design is higher than U_actual. U_actual may incorporate a min design margin, as in your case, so U_actual/U_designU_design/U_actual > 1.05(1+0.05+process licensor margin) ,

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