10 replies to this topic

[jordan1111]

Hello Everyone,

 

Thanks in advance for helping me in this matter.

 

I am a process engineer at a salt evaporating plant. We have a multiple effect evaporators here. The general heat exchanger and evaporator's drawing is attached. We are basically using the steam generated from the previous evaporator to heat the brine in the next evaporator. This steam will have some non-condensable gas in it. Currently, we have been fighting with a heat exchanger that constantly have an increase in its LMTD, whereas other heat exchangers are not. 

 

We have been looking at the possibility of fouling tubes, non-condensible gas blocking the heat transfer surface, condensate flooding in the shell side and steam side scale formation. 

 

Fouling tubes: Since it is a mixure of CaSO4 and salt slurry going through the tubes, I expect there would be plugged tubes or scale fomation inside the tubes, but each time during the shutdown, we open the manway to check, there are minmum amount of plugged tubes and I do not really see any scale on the tubes. 

 

Non-condensable gas: We have two nozzles locating at the top and bottom of the heat exchanger (shell side) to get rid of the non-condensable gas. 

 

Condensate flooding: The condensate drain line is just located a few inches above the bottom of the tube sheet, so whatever it is condensed, it should be removed. There should be minimum amount of condensate in the shell side. The condensate drain line is connected to the side of a condensate pot. where the condensate level within the pot is higher than the drain line.

 

Steam side scale formation: Never considered before. Not sure if this is causing the problem.

 

Another fact is that this evaporator and circulation loop is much smaller than the other 3 evaporators. I know the circulation loop is 24'' whereas others are 30''. I am wondering if the vapor release area (evaporation surface area) is restricted that it cannot release the steam in the evaporator that caused this increased LMTD?

 

 

I am trying to figure out what really causing this increased LMTD and I am wondering if anyone here has some experience on solving this issue?

 

Thanks a lot in advance.

Attached Files

•  Capture.PNG   125.82KB   3 downloads

[Bobby Strain]

You need to provide at least a sketch of the evaporators in the train. And identify the one that is giving problems.

 

Bobby

[jordan1111]

Please see attached. The problem is the #3 heat exchanger. 

 

Let me know if you need anything else.

 

Thanks in advance.

Attached Files

•  Flow sheet screenshot.jpg   92.28KB   2 downloads

[breizh]

Hi,

Is it a new problem or from the start up ?

Did you talk to the equipment's supplier , he may have ideas ? 

Have you ever considered to use a pump to improve the circulation (Forced circulation) and heat transfer ?

 

Breizh 

[jordan1111]

Hi Breizh,

 

It has been a problem for some years. 

All our evaporators are forced circulation evaporators, the only difference is that the #3 evaporator is a smaller pan compared to the other 3 evaporators and the #3 evaporator's forced circulation pump is 24'', whereas others are 30''. 

 

Let me know if you need anything.

Thanks.

[breizh]

Hi,

can you increase the load on the first 2 effects , check whether you can increase the circulation on them , do you have enough power available ? Check whether the impellers on those effects (pumps) are optimized? Can you increase the size of the wheels ?

It's weird , you have 4 effects and the 3rd one is smaller ?   thanks to explain the reason .

More information are needed , as I said you should contact the vendor  for clarification.

Note : can you share a PID of the unit? 

 

Good luck

Breizh  

[horatorres]

That is a good problem to solve.

 

Handle slurry is a real headache, I have the experience,, working as contracted process specialist,  with a Heater,steam in shell side, slurry in tubes side that never reach design process condition,,  we made several test and found noting with the heat exchanger,

 

Finally we decided to increase the slurry velocity and the heat exchanger improved the heat transfer, gotcha  slurry at some velocity trens to plug the tubes, reduced the heat transfer and the troubles began. When we opened the HX nothing was on the tube side,  We plug  5 % tubes and the troubles were gone

 

Check how you can increased the slurry velocity in this step of these evaporator and check carefully because any mechanical change have to be compared with the others

 

Horacio

[jordan1111]

Hi Breizh,

 

Thank you for taking the time to answer my questions. 

The Delta T across the heat exchangers are all within 1-3 degree, which I believe that the pumps are at its optimum setting. 

This #3 evaporator was a calendria type back in 1949, but we modified it in the 80s to a forced circulation. 

Unfortunately, the vendors for both heat exchanger and evaporator are not around anymore. 

And I would love to share the PID with you, but I do not have one with me.

 

Just for your information, the brine inlet and outlet temperature does not really change within time in the #3 heat exchanger, but the steam temperature at the shell inlet keeps increasing from 170F to 190F, so there is a resistance of heat passing to the brine, but what could be the reasons?

 

Question: what are the reasons to increase circulation in the first 2 effects? And enough power available for?

 

Thanks again.

[jordan1111]

Hi Horatorres, 

 

Thanks for the response.

 

Are you intentionally plugging 5% of the tubes? May I ask what velocity range do you have? We have roughly 6-8 ft/s (in average) of velocity going through the tubes.

 

Thanks again.

[breizh]

Hi ,

My thought was to evaporate more on the two first effects if possible . When the product is becoming more concentrated, more viscous the heat transfer coefficient tends to drop.

In other words let you try to increase the velocity inside the effects to improve the heat transfer by increasing the circulation rate , the draw back could be the breakage of the crystals !

 

Breizh

[jordan1111]

Breizh,

 

We are using parallel feed system and the product/slurry from each pans will flow to a slurry tank and pump to a dryer. 

I understand that more viscous fluid could impact the heat transfer coefficient and more vacuum, more viscous. The quetion is that our #4 evaporator is operating at a higher vacuum than #3 evaporator, which means the fluid will be more viscous, but the LMTD for #4 heat exchanger is fine.

We would think about the idea of increasing the velocity through the tubes. Breakage of crystals seems less of a concern compared to constant increase of LMTD.

 

Thank you for taking the time to answer my questions.