4 replies to this topic

[Art Montemayor]

Universal:

I am moving this thread to the Simulation Forum. I frankly don't know what it is that you are trying to do with a simulation program and a heat exchanger design, but I can offer the following advice: if you are trying to actually design the details of a heat exchanger, then you are wasting your time (& simulation time) on a simulation program. No one in real life does this. However, if you want to see how the overall heat exchange requirements will be affected in a simulation program studying a variety of process requirements, then you are using the right software.

I am finding that the majority of chemical engineering students coming to our Forums either don't really understand what a Simulation Program is, what it is designed to do, and what it is used for in real life - or they simply are being taught all the wrong things about simulation in their classrooms. I would like to think it is the former. No heat exchanger fabricator or designer is going to look at a simulation program output and seriously take it as a heat exchanger design. If they receive it from you as such, they will laugh in your face.

If you want to try to seriously design a heat exchanger in detail (size of tubes, number of tubes, number of tube passes, orientation, size of shell, shell passes, nozzle sizing, tube spacing, etc., etc., then you need a program like HTRI. In my opinion, I can obtain the same degree of accuracy - or better - if I calculate a heat exchanger by hand as compared by letting a simulation program do it. I know this because I have done it many times in the past.

As I stated previously, I hope that you are merely trying to see the effects of heat transfer (or the effects on the heat transfer) when you vary conditions on a simulation program (run a "what-if?"). That, in my opinion, is the true value of a heat exchanger estimation as spit out by Aspen, HySys, etc. I would have thought that your heat transfer teachers would have advised you as such.

[maitreyee ghosh]

My only advice to you, please go through textbook like D.Q.Kern for reading about shell & tube heat exchanger deign. As you don't know that a 2-8 exchanger means shell side pass 2 and tube side pass 8, you are wasting your time on simulation. Neither have you done the calculation in an excel spreadsheet or by hand.

Regards
Maitreyee

[Art Montemayor]

Universal:

Perhaps I made a mistake by moving your thread to the Simulation Forum and should have kept it in the Student Forum where other un-informed and inexperienced students could contribute to your query.

As Maitreyee has pointed out, you've failed to do your basic homework before diving into a sophisticated and specialized computer application like Aspen. It is obvious to anyone that you haven't read Don Kern's "Process Heat Transfer" – especially pages 139 to 144. If you had, you would know how the Ft (true temperature difference/LMTD) factor is derived and employed in heat exchanger design and temperature estimation. This is basic shell & tube heat transfer and something that must be mastered and totally understood before undertaking any analysis of a shell & tube unit. Otherwise, you are wasting your time as well as everybody else's – especially Aspen's.

The only output you can obtain from Aspen on a heat exchanger operation is:

1) The total heat transferred;
2) The estimated temperatures of the inlet and outlet streams;
3) The estimated heat transfer area required;
4) The estimated tube side pressure drop; and,
5) The estimated shell side pressure drop.

Aspen can give you the above if you input the correct data, such as flow rates, a minimum of 3 fluid temperatures, the configuration of the shell & tube unit, and all the required physical and chemical properties required to do the calculations. Of the above results, the only one that is mildly credible and reliable is the first one.

Item 2 is dependent on accurate input of the configuration, the leakage rate, and fabrication tolerances; as long as these are accurate, you will obtain a credible estimate.

Item 3 is only a reliable estimate insofar as the input is accurate – including the configuration, the leakage and fabrication tolerances.

Items 4 and 5 are merely "best estimates" and not only are dependent on the configuration, leakage, and tolerances but also are based on semi-theoretical relationships that sometimes can yield bad results. This is a very undeveloped and unperfected area of shell & tube heat exchanger design that has been more tolerated than dominated. If you have ever attempted to calculate and predict the pressure drops in a shell & tube, you will know what I speak of.

Of the above listing, I would only contemplate considering Aspen's output on the first 2 Items – and these are simple, spreadsheet calculations quickly generated and checked for correctness. Therefore, that is why I criticize the use of Aspen (or any Simulator) for this type of calculation. Simulators are great tools in the hands of professionally experienced and capable Process Engineers for the purposes of designing and studying a process in certain key areas of design – quickly, effectively, and with relative accuracy. Simulators should not be expected to calculate and predict what we ourselves still have not mastered nor totally understand how to define.

The physical configuration of a shell & tube heat exchanger is a subject that can only be appreciated by those engineers who have had to work in the field operating, controlling, and maintaining these units. A relatively un-experienced student cannot appreciate what the significance of 1 to 2 mm of excess clearance can do to the operation of an internal floating head, 2-pass shell & 8-pass tube heat exchanger. Neither can one appreciate what it takes to disassemble a tube bundle from its shell until one has to spend several days trying to do so – under a blazing sun and through a moon-lit night. The effects of a couple of millimeters of clearance and shell side fouling can ruin your week – and often the results of the heat exchanger.

I have gone to lengths to try to explain to you the significance of what you are trying to do and the proper way to understand it in order to give you further depth and knowledge – in spite of your inexperience. As a student, you cannot be expected to have the physical experience yet; however, if you are aware of this, you will try to understand what will be expected of you when you graduate and how to better cope with the expectation of accumulating all the great experiences that lie in front of you during your career.

Good Luck.