3 replies to this topic

[csr24]

Hi,

 

I am a student and I have to design a shell-tube heat exchanger for an OTEC plant.

 

the mass flow rate of the ammonia is 215 Kg/s while the water flow rate is 20000 kg/s

 

the water temperature is 27°C and the ammonia is 23,5°C liquid at the inlet and saturated vapor 23,5°C at the outlet.

 

I think it is better to put ammonia inside the tubes because it is toxic and the saltwater in the shell because the flow rate is higher than ammonia and fouling should be higher  as well. Moreover, the shell is probably easier to clean, do you think it is a good decision?

 

 

[srfish]

If the ammonia is being 100% vaporized, then the normal type of heat exchanger used is the kettle type. Also saltwater has a fouling factor of 0.002 and the Ammonia 0.001.

[Saml]

Tubes are easier to clean. You remove the cover and high pressure wash. To clean the shell side you need to remove the tube bundle or do a cleaning in place with some cleaning agent (that may be as simple as water or a more complicated sequence of chemicals)

 

Also, for seawater you may need special metallurgy (titanium, duplex) that is easier/less costly to implement on tube side. 

 

Ammonia is generally non fouling. However, 100% evaporation will lead to accumulation of small amount of contaminants like oil.

 

Depending where it is produced, the purity of ammonia, and your process, this may be significant or not. In cases where it is, you provide a way to remove a portion of the boiling liquid as a purge.

 

I will guess that your process involves a closed ammonia loop, filled with refrigerant grade ammonia,  so this should not be a problem and you may go to a fully welded design on the shell side (N-Type front head) without many concern for contaminants accumulation.

 

Plaase, be aware that I wrote in potential terms like "may" or  "should". It is you that must study your case, the information you have, and make your design decision.

Edited by Saml, 06 March 2017 - 07:19 AM.

[Pilesar]

A shell large enough to handle 20000 kg/s water flow will likely be uneconomical. Consider reducing your water flow rate or submerging your tubes directly in the ocean without a shell.