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Evaporator Trouble Shooting

heat transfer

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#1 August

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Posted 19 June 2017 - 05:14 AM

The evaporator is designed for vaporizing liquid N2 to gas N2. Tube is N2 and shell is warm water.

 

It is not a standard type shell & tube HX. There is a cap collecting all the tubes at the outlet, but cap is not connecting with the shell. So small piece of tube is exposured in atmosphere. This small piece is covered with ice on several tubes. The outlet gas temperature should be around 20 C.

 

Anybody has encountered this kind of problem before?

Any possible causes?

 

Thanks.



#2 Mahdi1980

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Posted 19 June 2017 - 03:53 PM

Hi,

It is obvious that such a phenomenon happens since N2 has a very low dew point which means that the liquid nitrogen has a low temperature, much lower than zero ( depend on pressure the dew point is different) . So, when the tubes are exposed to the ambient, they convert the moisture of the ambient air into the ice and it forms just on the tubes.

 

To avoid forming of the ice around the tubes, the best way is insulation of that part of the cap tubes which are in contact with atmosphere.

 

Please let me know the moisture content of the air ( relative humidity ) and the pressure of the N2 in the tubes.



#3 Bobby Strain

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Posted 19 June 2017 - 04:17 PM

You don't mention any problem. Ice accumulation is probably normal for this type of unit. The tubes in the water bath are probably covered with ice, too.

 

Bobby



#4 breizh

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Posted 20 June 2017 - 06:26 PM

Hi ,

Nothing really abnormal there considering the process ,vaporization, and the ambient condition (air moisture ).

In my facility in Thailand deicing was taking place for this reason .

 

Breizh


Edited by breizh, 20 June 2017 - 06:31 PM.


#5 IGC

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Posted 21 June 2017 - 03:59 AM

I'm a bit confused here...

 

From your description, to me it seems like a single straight tube pass heat exchanger.  That is the inlet is at one side and the outlet at the other, with the ice growth at the outlet.

 

If the gas outlet temperature is +20degC, and the ice is forming on the tubes right before they are collected in the end cap for the gas outlet, then it must be significantly cold ambient air temperature for the outside skin metal temperature to be below 0degC...  If that is the case, then the ice growth can't be that severe either as the water content in the air will be minimal!

 

Is the heat exchanger actually delivering GAN at +20 degC?

Do you have a better description of what type of heat exchanger it is?



#6 August

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Posted 21 June 2017 - 06:21 AM

I try to upload a picture but didn't succeed.

 

This is a customized the HX with strange configuration.  It can be imagined as a normal S&T HX, only the head is not connected with the body of shell. In total there are 40 tubes. 3-5 of them covered with ice at the exposure part. The rests are OK. The outlet gas temperatue is 20C. So there shouldn't be any ice. If there is ice formed, it must be due the low T in the tube.






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