Settled sediments in plate exchanger

[JuanMC]

Hi!

We have several plates heat exchangers in our facility. In a closed circuit, runs treated water that cools an electric generator, and in an open circuit runs non-treated water, on counter flow.
Due to the high quantity of sediments in the open circuit, the thermal flow decreases almost every month and it is necessary to dismount the heat exchanger to clean all the settled sediments in the plates.
¿Is there any difference between stainless steel and titanium, just talking about the sediment settling? ¿Is there another way to control the water quality, besides chemical products or filters (¿coatings, etc?)?

The chemical products implies more, and strict ambiental controls, and about the filters, instead of dismounting the exchangers, we would need to replace filters continuously.

Thanks,

JuanMC

 

[btrueblood]

"instead of dismounting the exchangers, we would need to replace filters continuously."

But, if designed well, changing filters can be cheap, quick and easy. Or, at least, easier than dismounting HX, seperating and cleaning plates, reassembling, leak checking, disassembling, repairing leaks, reassembling....

 

[JuanMC]

You are right btrueblood.

But we're thinking about changing the characteristics of the plates, looking for a decrease of the "adhesiveness" (sorry, i'm not sure if it is the right word to say it) of the sediments, so we wouldn't need to change or clean nothing at all.

Thanks!

 

[pmover]

be advised that plate heat exchgrs are mainly used in applications with clean fluids. otherwise, the fouling or deposit of sediments will reduce the exchanger's effectiveness.

i suspect that someone, likely a salesman, decided that compactness (i.e. exchanger size) was best. sorry, i should not guess . . .

best to determine particle size of the sediment material and then design and install a filter to remove all sediment material before entering exchanger. depending upon fluid temperatures, suspended minerals may precipitate from the fluid, thus be deposited onto exchanger surface.

treating the exchanger surface is a bandaid approach and may very well not resolve the problem.

either replace the exchanger, install a filter, or continue cleaning as is currently practiced. best choice is to install a filter as replacing the exchanger will only continue the same problem.

good luck!
-pmover

 

[unclesyd]

We use both of the following type of filters in conjunction with plate type heat exchangers. We use the self cleaning feature on both types.

http://www.asmfab.com/Automatic-Backwash-Filters.aspx

Take a look at the Auto-Klean line of metal edge filters.

http://www.cuno.com/industrial/gas.shtml

 

[JuanMC]

Thanks for your answers Uncle, Pmover.
I have heard about magnetic conditioners, so I thought there could be something similar to deal with our sediments.
Now, after i read more about the mag-conditioners, i've realized that those would be useless for our intentions.
I guess we must use the traditional cleaning ways.

Thanks a lot!

 

[kenvlach]

What is the nature of the sediments -- rust, carbonates? If the exchangers are titanium or stainless steel, backwash them with nitric acid solution (25-35 vol%).

 

[rmw]

Your heat exchangers may be improperly sized. PHE's need a high velocity to perform well and if the velocity is maintained in the plates the sedimentation won't take place, no matter what is in the water.

I suspect that someone sized the PHE's for a design with a lots of safety factor in the duty and the flows and in normal service they don't have enough water flow on the open water circuit side to maintain adequate velocity.

Check your design and see what can be done to maintain the design water flow on both sides of the plate.

On the other hand, PHE's make the best filters in the world if your water has particles in it larger than the flow passages between plates. If that is your problem, look at the links UncleSyd gave you and start there and get a good self cleaning filter to protect your Hx's.

rmw

 

[Deltathx]

Next time you disassemble these unit for cleaning, try to establish what is causing the fouling. Is it suspended solids in the water or is it crystalization, is it chemical or is organic growth. Once you determine the source of the fouling they you can determine whether a filter will help or whether the heat exchanger needs to be re-designed. Either case the problem can be corrected. This may help;

http://www.deltathx.com/uploadsDocs/FoulingFactors.pdf

 

[EdStainless]

It has been said, but I'll repeat it.
Syd and rwm have it. Good self flushing pre-filters and higher flow velocities.
Stainless is almost immune to flow erosion, unless you have a lot of debris. Get the flow speed up.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[DYV1973]

As mentioned by rmw the PHE could be oversized. If you have too many passages in parallel, the velocity decreases, and the PHE looses its self-claning effect.

When calculating for PHE type of heat exchangers, please do not use a fouling factor (as is done for shell and tube). But it might be better to take an overdesign of just a couple of percent on HTC (5-10%). In your situation, it might even help to take out some plates, if you have a lot of overdesign (Overdesign makes happen what you try too avoid, in case of a PHE).
The best way would be to calculate the exact right PHE, and then choose a PHE with same number of plates, inlet dimensions, width ect, but only with a longer plate (for example look at the APV or Sondex series).

As a solution for filtering, you can look at

http://www.bernoulli.se/Eng/

This company manufactures self cleaning filters, which are used in combination with PHE's.

 

[JuanMC]

Thanks Deltathx, Ed and DYV.

Let me explain a little further our problem:

I'm talking about heat exchangers of a little hydroelectric plant (50 Mw). As I said before, there is treated water running by one path, and non-treated water on the other side.
The non-treated water cools the refrigeration water, which runs in a closed circuit, unlike the non-treated water. This one (non-treated) is taken from the discharge of the hydroturbine, so, the intake of this water is the same than the used for the turbine. There is a little dam, to provide the water, so, according to the surface-level variation, we have seasons with poor water quality and sometimes, we have very clean water (on high levels).
It's hard to have a "proper" HEx size, because the operation conditions are changing all the time (despite we have almost constant temperatures during the year, the capacity of the HEx changes).

The exchangers has Titanium plates, with nitrile gaskets. We're waiting for hydrologic studies of the sediments to know if there is biological growth or simple sediments over the plates.
The manufacturer suggest the use of sodium hydroxide or sodium carbonate to control the biological growth, or nitric, citric or phosphoric acid to control the common sediments (metal oxides, corrosion products, silt, alumina). Yet, the non-treated circuit is open to a river, so we can't use chemical products without having a proper environmental control, which is almost as expensive as dismounting the plates and wash them manually.
Finally, that's the problem!.

Thanks again to all of you, i'll keep on reading to find the best option, and looking for what would we need to use the chemical products.


JuanMC