Fouling Factors - Effect of Regular Cleaning 1

[tlpchris]

Is anyone aware of any quantitative studies showing the effects of regular tube cleaning on the tube side fouling factor? I would like to increase the plugging limit on a heat exchanger from 5% to 10% by reducing the design fouling factor. The heat exhanger uses treated river water on the tube side (R=0.002) and a closed loop treated water (R-0.0005) on the shell side.

 

[speco]

Tipchris,

If you look at the TEMA standards for fouling factors, the ones you are using are pretty conservative, especially the one on the cooling water side. In an typical shell and tube exchanger with these fouling factors, your actual safety factor should be in the range of 100%. That is, your Uclean/Ufouled is probably in the range of 2.0.

I am presuming that the fouling factors you have given are in English units, such as .002 hr-ft^2-degF/BTU.

Fouling factors are actually the artificial resistances included in the DESIGN of an exchanger. They are basically little more than safety factors to cover eventual fouling as well as other uncertainties in the design.

What I think you really mean is fouling RESISTANCE. That is, the actual reduction in overall heat transfer coefficient due to fouling. This is a bit more difficult to get a handle on. If you have actual operational data on your exchanger, you should be able to back-calculate an effective fouling resistance. If you start with the design Uclean, and use the actual terminal temperatures of the exchager, you should be able to calculate an effective fouling resistance, assuming that the flows on both sides of the exchanger are approximately the same as those for which it was designed.

However, in any case, you can determine a point where the actual fouling becomes an operational problem.

 

[JohnFLund]

The following link provides an article related to the cleaning of heat exchangers.

http://www.cepmagazine.org/pdf/060156.pdf

 

[Spunar]

Speco great response. I recommend 0.001 hr-ft^2-F/BTU for the treated river water. This is in line with a Carrier Condenser Theory and Selection Aplication Data from 1979 and 0.0001 hr-ft^2-F/BTU for the treated fresh water loop. This is in line with the latest numbers out of ARI.

Good luck,

Chris

 

[235zzzo]

Hi Fellows!

And how about fouling resistance on the air-side of a condenser gas coil working in industrial environments with a losts of dust and high temperatures?

Has anyboby have some studies about the cleaning processes and frequencies? How much dust do I permit versus minus some efficiency decrease percentage...

In common words: - The point is to clean the coil, but not every two ~ three days, (it's too expensive) but rather once a week. How much do I lost in efficiency ?

Thanks.
zzzo

 

[speco]

235zzzo,

I'll take a stab at this one, too. If I undersand your question, you are asking about fouling on the air side of a FINNED tube coil, correct?

I have not seen any studies on this, only have some design experience and have seen a few field problems with finned tube coils/bundles. From this limited exposure to actual field problems, I would conclude that dust is seldom a problem. It should just blow through. Normally, air-side fouling of a finned tube bundle occurs when there is fibrous or stringy material floating around in the air. The most common problem is from Cottonwood trees. The effect is to plug the finned tubes, blocking air flow.

Mathematically, an air-side fouling factor makes very little difference in finned tubes unless the fouling resistance is huge. If you are working on an extended surface (finned surface) basis, the tube side fouling factor is multiplied by the ratio of outside to inside surface in calculating the overall U (heat trasfer coefficient). Typically, this ratio is around 20-25 to 1.

Regards,

Speco

 

[235zzzo]

Hi Speco!

Thank you for you reply!

You mean that the surface ratio you refer, can accept "grosso modo", some quantity of dust.

As a matter of fact, we have a lots of thin and very dry dust.

What you say, suggests me to go deeper and take a look about the parcial internal and external heat tranfer coefficients and not only the overall U, in order to realise if the ratio effect plays the influency, as much as you said, let me use these terms.

We have to be sure, we don't have here any counterbalance phenomena...

And thinking about air side of a FINNED tube coil, why the design applies such ratio, I believe that is to balance those different parcial heat transfer coefficients values, isn't it?

The point is to check which design margins we have, in order to estimate how much foulig factor can be allowed in each particular case, it seems to me!?

Any second thougts about this? By the way, which "field problems with finned tube coils/bundles" you refer, for instance? Can you be more specific?

Best Regards,
zzzo

 

[speco]

zzzo,

OK. I'll come clean (pardon the pun!). My area of specialty is air-cooled exchangers, which are essentially finned tube bundles with fans. I have seen many of these in the field, some lasting as long as 50 years in service. I have never seen one fouled from dust. The only air-side fouling I have ever witnessed was in the South (Louisiana)where the finned tubes became clogged with the junk from Cottonwood trees. Because of it's fibrous nature, the the stuff accumulated to the point where air flow was seriously reduced. The static pressure on the air side is also increased adding further to the air-flow problem.

The usual solution to this is to wash it backwards out of the finned tubes. That is, in the opposite direction from the air flow. Sometimes screens are used to catch the stuff before it enters the tube bundle. Obviously, this doesn't work with dry dust.

There are several ways to calculate heat transfer in finned tubes. Some methods base everything on the bare tube surface. Others use the finned surface to achieve the same result. If your calculation method uses the extended (finned) surface, then all the resistances must be adjusted to this surface. This means that the inside film resistance (inverse of the inside coefficient) must be multiplied by the ratio of surfaces of outside finned area to inside to inside tube area. The same thing goes for the inside fouling factor.

If you have specific data on your condenser, I will be happy to take a look at it and offer my opinion as to what the problem might be. For this, I would need all the details of both the process and finned tube bundle arrangement (like a data sheet). You can send it to jim@stoneprocess.com

Thanks,

Speco

 

[tickle]

Going back to original q.

When starting up a new or clean HE the HE actually encourages fouling because heat transfer is more efficient so less of the service fluid is required. Therefore lower velocities are experienced and more fouling. As the fouling increases more service fluid is required, the velocities increase and the fouling rate is reduced. Some of the fouling factors given are based on the velocity of the fluid.

A HE is also upto 200% over designed with the fouling factors, end users safety factors, designers safety factors, manufacturers safety factors and conservatism in the design. Additionally, flowrates and heat duties are rarely at the design conditions.

I remember seeing, but not reading, an article or a section in a book about the quantification of fouling based on the thicknesses of fouling layers and the material thermal conductivity. I suggest an internet search for it.

 

[235zzzo]

Hi Fellows!

I'm sorry, if I could suggest to skip to main initial question, nevertheless, these matters are correlated, so we can discuss them transverse way, may I say.

Speco: Once more, thank you for your great feedback and your will to help, as well. I will prepare next week some updated data, and I'll send to you.
Cheers!
zzzo