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# Over - design

## Over - design

(OP)
Hi there, wish you a great Year for everybody.

Hope that somebody could clarify me following issue.

When dealing with surface overdesing in an heat exchanger sizing, I understand as follows.

If I suppose to have, x% overdesign, it means as follows:

I will realize the calculation of the heat exchanger considering not the real area A, but a new area A1 = A*(1-x).

Therefore capacity won't be

Q = U*A*LMTD

but

Q = U*A1*LMTD

(and therefore lower)

Is my understanding correct ?

Many thanks

### RE: Over - design

over design (or margin if you like) should not lead to a smaller HX - that does not sound logical. Normally you add the margin to compensate for fouling.

If you want to maintain your Q with a large UA then the LMTD will have to go down

Best regards

Morten

### RE: Over - design

(OP)
Many thanks mr. MortenA

think that I have been not clear, apologizes for this.

In shorts, what you are saying me is:
if I consider an heat exchanger with surface overdimensioned at same conditions I would get an higher capacity with respect of the one with no overdimensioned surface.

Is it correct ?

Many thanks

overdesign!

A1 = A*(1+x)

x=20% , A1=1.2A

### RE: Over - design

yes - that is correct.

Best regards

Morten

### RE: Over - design

Clean that is... Once it becomes fouled, it will eat up that overdesign fast.

rmw

???

### RE: Over - design

AO1958,

I don't think there is a recognized standard or method for calculating overdesign.  However, I have some customers who usually have such a requirement.  Here's how I deal with it.  Say the requirement is for a 10% over design.  Then the actual Q * 1.1 = U * A * LMTD

Sometime it is expressed as a de-rate.  If it's a 10 % derate, then actual Q = .9 * U * A * LMTD.

These are almost the same at 10%.  At higher values the two methods are very different in the amount of impact on the design.  For example a 20% de-rate is the same as a 25% overdesign.

I always use the a calculated U value that includes any required fouling factors.

Regards,

Speco (www.stoneprocess.com)

### RE: Over - design

You are using "overdesign" so that you have head room built into the design in case it isn't operating at optimum efficiency. The requirement is probably given based on proven fluctuations in operating conditions.

StrykerTECH Engineering Staff
http://www.stryker-tech.com/

### RE: Over - design

(OP)
Hello,

I'd like go get back to this topic, thanking all of you for the interesting and helpful topics.
Now I have to design an heat exchanger with 15% of surface over design to get a capacity of  410000 Btu/h.

My question is as follows:

Mr Speco,please, how would you proceed ?
Not all of the temperatures are known, but I can apply epsilon NTU method.
Shall I apply this method decreasing heat exchange area by 15%.
In this case there is not a direct relationship between capacity decreasing and area decreasing.

Please, could you send me a link where to find an example of a calculation with surface over design ?

Or, considering the problem in a revers way, how to link an over capacity to a over surface ?
Imposing same LMTD and considering them the ratio

Q/Q1 = U*A/U1*A1 determining in this way the ratio between A and A1 ?

where
Q = U*A*LMTD  = design capacity
Q1 = U1*A1*LMTD  = capacity found

Many thanks

### RE: Over - design

AO1958,

First, you need to nail down the specifics of your process.  If mass flows on on both sides are fixed, it's a little easier.  Then you come up with a heat exchanger design that meets the performance requirement with those flows and temperatures.

Now you need to add another 15% to the surface.  If your design is such the additional surface reduces the overall heat transfer coefficient, then you need to add still more surface such that the overall product of surface (area), U (heat transfer coefficient), and LMTD (corrected for the configuration) is at least equal to 115% of the required heat load.

For example, if your exchanger is a simple plate and frame exchanger (let's assume this is a straight liquid-liquid exchanger) , this could be done in one of two ways:

#1.  If you have a design that requires, say 100 ft^2 of surface, you could add another 15% to the number of plates.  But then,  both fluid velocities would be reduced, also reducing the overall heat transfer coefficient.  So, you would need still more plates.  You may end up with a unit that has 120 ft^ as a result, depending on the what happens to the overall U value.

#2.  You may have an option of adding more surface by using a taller plate instead of adding more plates.  If you keep the same number of plates, the overall coefficent would stay the same.  However, say the taller plates have more than 15% in height (flow channel length).  This may allow you to actually reduce the number of plates required, as long as your design is within the required pressure drop limits.  This would be analogous to increasing the tube length in a shell and tube exchanger to get additional surface.

In some types of exchangers, the flow of one of the fluids is not fixed.  Then you simply alter the configuration of the exchanger, combined with the variable fluid flow to achieve the 15% overdesign.

Regards,

speco (www.stoneprocess.com)

### RE: Over - design

It's worth noting that overdesign in terms of actual heat transfer is often significantly less than the overdesign based on surface area. Typically I see requirements for 10% additional surface area which might tranlate into perhaps 3-4% additional heat transfer. Why even bother?

Also keep in mind that a larger exchanger is not only more likely to foul, and it's more difficult to clean once it does.

-Christine

### RE: Over - design

Christine74,

You are right about just adding surface.  I've seen cases where someone would design a finned tube exchanger with 10 fins per inch, then just add a fin to get more surface.  The end result would be more on the order of a 3-4% actual overdesign.

I'm not sure I totally agree with you about the overdesign being more likely to foul.  I think it really depends on the type of exchanger.

Regards,

Speco (www.stoneprocess.com)

### RE: Over - design

Adding excess area is an attempt to compensate for the fact that the correlations used in heat exchanger design are not all that precise, nor is the thermo used to calculate mixture properties.  It's not a substitute for fouling factors.

Whether the extra area appreciably increases fouling risk depends entirely on the design, as does the necessity for adding it up front in the first place.

### RE: Over - design

Over design will always create a larger surface. But make sure that this area will in series (extra pass) or longer plate / pipes. You will have the same shear and thus reduce the risk of increased fouling. Adding more plates in parallel for a PHE will make that the PHE will foul more easily and so you improve what you try to avoid.

DYV

### RE: Over - design

Add the 15 or 20 % over design tot he calculated heat exchanger surface.  Then go back and recalculate the heat transfer cofficient based on the new surface area.

The fouling overdesign is taken care of in the fouling factor as opposed to clean tubes.

Ken

Ken
KE5DFR

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