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Heat Exchanger Cleaning Issues

369 days ago, I made entitled "Learning Process From Cleaning Plate Heat Exchanger" which was a follow up entry from "Some Updates". You can refer to the 2 posts for reference. From that post, I received some interesting response and questions from few engineers asking more detail about the cleaning of heat exchangers. The questions are taken from the comment section of that Learning Process From Cleaning Plate Heat Exchanger post without any editing. I answered the questions but I add more of my answers here after thinking about it...

Questions:

I'm a process engineer and your article is very useful and interesting. Could you say (if it possible) how long time did this plate heat exchanger work good without cleaning? And was the concentration of caustic solution high?

Answer:

Thanks for your kind words Olga.

How long time did this plate heat exchanger work good without cleaning?

That depends on how you use the plate heat exchanger and the types and quality of fluid that passes the plate heat exchanger. From my experiences, the plate heat exchanger can operate effectively up to 1.5 years without cleaning, but that is because the feed oil is clean and other combining parameters are good. There are also cases where we have to clean the heat exchanger after 4-5 months... There's no straight answer to this. It depends on a lot of factors. You need to really sit down and monitor the processing parameters and the quality / condition of fluid entering it. I have about 16 plate heat exchangers which I monitored and all of them have different records. Those who belong at the same section in the plant will have almost similar cleaning track record. All of them have different classification of problems too. So, we need to really look at the heat exchanger(s) and make a proper inspection, evaluation and analysis.

The caustic concentration was 3-5%. This also depends on how severe the scale build up is inside the plates. You can have lower concentration if the scale is lesser. You can add up more of the caustic concentration, but it may be not good for your plates (of the plate heat exchanger) or the tubes (of the shell and tube heat exchanger).

Questions:

How heavy and fooling is that oil?

Does it really worth the trouble to use a plate exchanger respect to a shell and tube for such fluids? After all a shell and tube is much easier to clean.

Answer:

It depends on your process and application. What is the type of flow? what is the pressure and temperature? You have to use a shell and tube heat exchanger if you have a high pressure and high temperature. A shell and tube heat exchanger is more expensive. A plate heat exchanger is cheaper and can be used for lower temperature and lower pressure. The main constraint of the plate heat exchanger is because of the gasket used cannot cope with temperature higher then 200oC. so, it's a matter of the effect of process parameters and not the easiness to clean the heat exchanger. A shell and tube heat exchanger 2 pass (or U tube) is also sometimes very difficult to clear especially at the U bend. You need a special equipment with high pressure of jet water to clear the scale, fouling. In worse cases, you need to introduce a small drill combined with the high jet water, preferably up to 20,000 psi to ensure you eliminate the stubborn scale.

Hmmm...maybe those of you who have other experiences on dealing with heat exchanger cleaning can share it with us here...TQ!

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posted by zaki yamani @ 12:20 PM,

4 Comments:

At Sunday, August 17, 2008, Blogger baley said...

Sure a U-type of rear end will be hard to clean, but you don't use them with high fooling liquids.

AFAIK Plate exchangers do not have a good fire resistance and they should not be used if the liquid contains solids. Normally the fooling factors considered on design are 4 times over the shell and tube ones.

Otherwise if you are dealing with a relatively clean fluids low temperatures, very high thermal effectiveness a plate exchanger (or a plate fin ) can be the way to go.

 
At Sunday, August 17, 2008, Blogger alzack said...

Hi Baley,

Thanks for your response.

Yes, we don't use a U-tube for a high fouling fluid. From my humble experience, there are cases where we use fluids (oil) that has lower fouling tendency. Fouling is not suppose to occur. But, due to some operational issue, the shell and tube heat exchanger which operates at 270oC normal day have to be stopped once in a while - mainly because no / lack of raw material, different customer requirement, running different product(s), shut down / turn around etc. Therefore, the shell and tube heat exchanger have to be stopped and cooled down. As a result, there will be remaining fluid/oil which could not be easily drained contributed to this the fouling / scaling effect - and it harden and in worse cases form carbon deposit... There maybe other reasons / factors, which I haven't yet notice. Maybe, other process engineers can share their experiences as well.

I have no experience using AFAIK plate heat exchangers and haven't considered or studied the fire resistance. However, I think the fire resistance features are directly related to the type of gasket used for the plate heat exchangers. I would not attempt to use a fluid mixed with solid traces to enter the PHE. I fear it may block or create high pressure at the plate heat exchanger inlet.

Yep, I totally agree with you that if the fluids are clean and require low temperature, a plate heat exchanger is the best option.

Just my 2 cents...

Hey Joe Wong...any idea...?

 
At Monday, August 18, 2008, Blogger baley said...

Thanks for the reply Zaki , AFAIK means As Far As I know, sorry for the Internet slung ;)

 
At Monday, August 18, 2008, Blogger alzack said...

Haha.... No wonder!!! I thought it was a brand of a plate heat exchanger. Thanks for teaching me about AFAIK... :)

 

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The Author

zyz

I’m Zaki. I used to be a project, process and chemical engineer. Few years ago I successfully became a Chartered Engineer (IChemE) and Professional Engineer (BEM). I'm now employed as a chemical engineering educator/researcher/consultant. Hope you like reading my blog. I welcome any feedback from you. My email: zaki.yz[alias]gmail.com. TQ!


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