I think it really depend on what you want to extract from your model, and what type of heat exchanger you are interested in (shell-and-tube, plate, etc.). I mostly worked on shell-and-tube heat exchangers, so my answer might be influenced by that. From the simplest to the more complex, you might consider:

-Epsilon-NTU relations could help you in simple problems, but they are not meant to reveal how the details of the internal design of the heat exchanger (e.g., baffle cut, baffle spacing, etc.) affect the heat transfer coefficients and pressure drops, for example.

-Empirical relations (such as those shown in the attached paper for shell-and-tubes) could help you to evaluate the heat transfer coefficients or pressure drops as a function of the heat exchanger design.  You can consult the book by Shah and Sekulic (2003) to obtain some of these empirical relations.

-CFD simulations could also be considered, but they are much more time-consuming.

I hope these comments help you!

Article Minimizing shell‐and‐tube heat exchanger cost with genetic a...

Thanks indeed, sure, it was helpful. Reza

Thanks Ali Akbar

For me that I am working on energy integration system where you dont look at the details and only you follow control volume concept, Item (1) you mention is major criterion. But, I was thinking that for example we all do like this: (consider a heat pump cycle) we develop all thermodynamic equations in EES and when it comes to evaporator or condenser, we only write the easiest first law equation, for me it doesnt make sense and I cant trust it. I was asking about a way to make this modeling more accurate, still not go to heat transfer and fluid mechanics details.

Regards

Reza

Hi Reza

For more accuracy I suggest to use the second law of the thermodynamic with this you will discover how the exergy is destroyed in your system and how to improve its effiency. As you mentionned you're working on energy integretion system, I think you also need to define the pinch of your system and to investigate its effect. Combining the first and the second law should be helpful for your work.

To determine the efficiency according to the second law you can use the definition made by Sorin et al. cause the general expression of the efficiency from Grassman doesn't give more information about how the system work and wich parameters effect the performance.

Thanks Oumar

Yes you are right, exergy analysis is powerful tool getting help from entropy generation concept. We in UOIT with Dr. Dincer and Dr. Rosen focus on exergy analysis of integrated systems and optimization.

But, about pinch, can you just give me more details, how? any references or model papers to show the modelling steps, I dont care about results. Thanks

Thanks

This one may be helpful, it deals many integrated systems and also describs the modelling steps. 

Regards

Oumar

Thanks Oumar

Hi Reza

The complete simulation and design is divided into 2 steps:

first the mass and heat balances has to be fulfilled. This process outcome will be mass flow and transferred. In this step if there are a number of unknowns that won't match the equations. Guess overall heat transfer coefficient and area. Then you can find Q and LMTD in try and error attempt(as EES solve all equations simultaneously  no need for the try and error is observed) the other method is NUT-epsilon which doesn't require try-error process. 

The second one is design step in which details  tube diameter shell diameter length of the tubes and baffle shape and distances ... are chosen based on recommended mechanical criteria and existing condition(for example for tubes standard tubes are in inch; considering schedule the inter diameter is clear ) in a way that the calculated area is satisfied. If the parameters are logical (for example the tubes are not too long), it is time to recalculate the U(overall heat transfer coefficient) according to the details obtained and if required, find pressure drop to recalculate the properties for the heat transfer simulation.

Best Regards

 Rasool

Thanks Rasool for your such detailed help. I will consider them nicely for my next works.

Hi Reza,

It depends on the type of the heat exchange and  which kind of optimization you are gonna apply. However, if you just want to optimize the heat exchange not in a cycle, most of the studies considered, Fin pitch, fin height, fin offset length, cold stream flow length, no-flow length, and hot stream flow length are considered as six decision variables.

If you are going to optimize a Heat Recovery Steam generator (HRSG), that could be pinch point temperatures, approach point temperatures, mass fraction to each pressure level and it could be either live steam to the turbine although in huge CCPP it is almost fixed. If you need more information, read these articles. 

Hi Pouria, Thanks

Actually, I asked question to address my personal issue regarding system modelling for instance in EES. When it comes to heat exchnagers, we easily assume perfect heat exchange between two streams and we apply 1st law nicely, but, it does not make sense for me. 

From the other aspect, consider a condenser where hot stream undergoes three steps of desuperheating, condensing, and subcooling, then how can we propoerly model and estimate cold stream inlet and outlet temperatures? we have data on inlet and outlet temperature of hot stream and also mass flow rate of it. In order to determine mass flow rate of cold stream via 1st law statement, we need to have inlet and outlet temperatures of cold stream. This is my main concern. Please let me know about it, thank you. What should I do to determine cold stream's temperatures properly with out considerations of those 6 decision variables that you mention. What would be the easiest, but almost precise method?

Thanks indeed

Reza

Reza just use your engineering sense

If you know the both inlet temperature and flow rates (hot and cold) to determine outlet temperature you need to know U(overall heat transfer coefficient) and A (heating surface). It is obvious that different heat exchangers have difference performance. 

U can be estimated based on the streams circulating in the heat exchanger. for example liquid-liquid streams varies from liquid gas or boiling and condensing ones. The variation of temperature (as you said the phase changes) effects on the LMTD.

But if one of the streams out let is known too. Then the the Energy equation is fully defined. So the other stream out let can be calculated and then if required A can be determined.

Thanks Rasool, I did the same, with assumption of a pinch level, I determine the temperatures but I was wondering if there is any method to make it more precise other than employing heat exchangers equations for overal heat transfer coeff.

Thanks

Rzea

You know U is a function of all details of the heat exchanger including tubes arrangement, shell size and arrangement, baffles and their size and positions, the martial, using finned or bare tubes, the size and spacing of fins the streams velocity.  The more accurate this parameter, the result will get closer to the exact solution. In fact the estimation mentioned in the previous comment is totally rough.

Even after exact mathematical solution, I think the result may have 10 percent of deviation from real experimental results due to the complicity of the problem. 

I think Aspen Bjack and HTFS can help you since they can simulate your HX via the exact mathematical solution and at the same time considering the process variation. More exact solution may be achieved by CFD. 

Thanks for your productive answers Rasool. 

Regards

Reza