cooling of hot oil

[jlmiller]

I have a small plate exchanger that I am using to cool an oil stream from 222 degC to 100 degC using cooling tower water at say 20 degC. Initially the exchanger had temperature control on the water side and worked great but scaled up on the water side. It ws replaced with identical unit, the water flow was set to full on without control to prevent scaling. It worked for about a month and again scaled up on the water side.

My question is: any suggestions to a better or more appropriate method of cooling the hot oil?

Thank you.

 

[pmover]

1) what quantities (oil primarily, water can be computed) are to be cooled?
2) this service has a large temperature difference, thus i am not surprised at the fouling that has occurred. try using demineralized water, but even this water needs to be cooled. it may be possible to increase water flow?
3) air cooler
-pmover

 

[TD2K]

What sort of film temperatures on the water side do you back calculate you could be seeing?

Cooling water can start to create scaling problems at temperatures much over 140F depending on your water composition so given this oil temperature, you might be well under a 'conventional' bulk water outlet temperature limit of 120F but be really cooking right at the heat transfer surface.

is it worthwhile to consider a small air cooled heat exchanger to precool the coil? Another option might be a small cooling water booster pump to further increase water side flow through the unit.

 

[JOM]

Will water velocity really have any effect on fouling rate? This isn't a sediment that needs to be kept in suspension, is it? It's the chemistry of dissolved minerals at work. (Just thinking out loud.)

Why was it necessary to replace the original unit? One of the features of PHEs is the ability to strip them down and scrub them clean, regasket if needed, reassemble and it's good as new. Cheers,
John.

 

[25362]

A possible solution is to use tempered (soft) water in a closed circuit. The tempered water would work, say, between 50C and 70C and be itself cooled by circulating cooling water. The heat exchanger surfaces should be recalculated in view of the "new" LMTDs, linear velocities and wall temperatures should be so designed as to avoid hardness precipitation.

I hope there is some value in my suggestion.

 

[jlmiller]

Thank you all for your thoughts and suggestions. I believe that a secondary loop as suggested by 25362 will be the best solution.

Just to answer a few queries of other responders:
- the oil is a product and is hot coming from another process.
- the original exchanger was replaced because these are welded sealed units (our plant standard when dealing with these high temperatures). We tried descaling it but were unsuccessfull.

Thanks again.

 

[lilling]

My guess is that the scaling is caused by boiling of the water which means that the residuals could remain on the surfaces. Could it be worth the effort to try to increase the water flow so that the water does not boil?

 

[ardilesd]

Think that the problem is in the wall temperature, not in the bulk temperature.
This can be calculated as shown in the thread <A HREF=&quot;thread391-39755