12 replies to this topic

[jeaojt]

I am currently designing a shell and tube heat exchanger used to condense hydrocarbon gas from 200C to 40C with river water as the coolant. From an energy balance the cold fluid output comes out to be bout 350C. Is the cold fluid output allowed to be greater than the hot fluid input and if not does this indicate I need more than one HE unit?

[SilverShaded]

 

Jeaojt:

 

That only works if you have a magic heat exchanger than transfers heat from cold to hot.

 

Why dont you use an air cooler?

[jeaojt]

Silver Shaded:

 

Thank you so much for the reply,  I did think the water outlet temp was a bit crazy! My though behind using the shell and tube w water coolant was there was more info out there on how to design it and I was hoping to use the extracted thermal energy as an energy input downstream. How would you advise I move forward?

[breizh]

Hi,

Consider these documents to support your work .

Probably something is wrong with your calculation .

Note : a good reference "Process heat transfer" by D.Kern 

Good luck

Breizh 

[SilverShaded]

You could condense the vapours directly against the down stream energy requirement,  generally using some intermediate stream for energy transfer (e.g. water) is less efficient.

 

if you really want to use water you need a much bigger flow of water to maintain a minimum DT.

For a simple solution (not the best energy wise) air cooler and trim water cooler would be better.
 

[Art Montemayor]

jeaojt

 

You are obviously doing something wrong with what you call your calculations.

As Silver Shaded points out, common sense dictates you consider what your calculations tell you.  You obviously can't create heat in a heat exchanger; you can only transfer some of it.

 

You should follow Breizh’s recommendation.  Read Chapter 7 in Don Kern’s classic text book, Process Heat Transfer – specifically pages 145-175 and learn how a temperature cross occurs in a TEMA 1-2 type heat exchanger.  Your answer is there.

 

You should never attempt to use a heat exchanger without knowing the physical characteristics of the exchanger.  Kern clearly points this out and explains in detail the advantages and disadvantages of each type.  Each type has its benefits and trade offs.  You should supply all your basic data and detailed calculations if you need serious and accurate recommendations from our experienced Forum members.  

[Pilesar]

There are some practical reasons against using river water to cool a very high temperature stream. River water will contain a lot of dissolved minerals. Water at high temperature does not keep minerals in solution very well. When precipitates form in the water, they will tend to deposit on the heat exchanger tube wall which will reduce heat transfer and may restrict water flow. If there are any areas of the tube where the wall temperature is hot enough, the water may boil right next to the tube leaving a film of vapor. Since the heat transfer to a vapor is poor, heat exchanger performance suffers. The continual drying and wetting of the tube wall can be a factor in tube failure. Air cooling does not have these problems so is often used when cooling very hot streams.

  Cooling with a process stream is more energy efficient than using a utility stream. Process heat integration can make the difference in profitability. The trade off between capital cost and energy efficiency is not always easy to evaluate.

[breizh]

Hi,

If you want to use an air cooler , let you consider the brochure attached .

Breizh 

[jeaojt]

Hello,

 

Thank you for the resources, they have been very helpful.

 

Hi,

Consider these documents to support your work .

Probably something is wrong with your calculation .

Note : a good reference "Process heat transfer" by D.Kern 

Good luck

Breizh 

[jeaojt]

Having double checked my calculations I vas using a very small volume of water. Having increased the flowrate to 15L/s I am getting a cold side output temp of around 50C which is a lot more reasonable.

Thank you so much for taking the time to help me!

You could condense the vapours directly against the down stream energy requirement,  generally using some intermediate stream for energy transfer (e.g. water) is less efficient.

 

if you really want to use water you need a much bigger flow of water to maintain a minimum DT.

For a simple solution (not the best energy wise) air cooler and trim water cooler would be better.
 

Edited by jeaojt, 11 March 2022 - 05:36 AM.

[jeaojt]

I was concerned about fouling from the water so thank you for confirming that. I will re-design the process using an air cooler to discuss pros and cons in my report.

 

There are some practical reasons against using river water to cool a very high temperature stream. River water will contain a lot of dissolved minerals. Water at high temperature does not keep minerals in solution very well. When precipitates form in the water, they will tend to deposit on the heat exchanger tube wall which will reduce heat transfer and may restrict water flow. If there are any areas of the tube where the wall temperature is hot enough, the water may boil right next to the tube leaving a film of vapor. Since the heat transfer to a vapor is poor, heat exchanger performance suffers. The continual drying and wetting of the tube wall can be a factor in tube failure. Air cooling does not have these problems so is often used when cooling very hot streams.

  Cooling with a process stream is more energy efficient than using a utility stream. Process heat integration can make the difference in profitability. The trade off between capital cost and energy efficiency is not always easy to evaluate.

[Pilesar]

Air cooling will not be suitable all the way down to your 40 C process target. A solution may be to use air cooling to bring the process temperature down and then water cooling the rest of the way.

[breizh]

Hi,

Pay attention at the outlet temperature  water side (50C) , check whether it's possible to discharge to the river . This temperature is quite high and regulation may apply .

 

Good luck

Breizh