9 replies to this topic

[Yasser]

Dear All,

 

I am assigned to design a kettle with an external wort heater (EWH)(vertical thermosyphon) by using shell and tube heat exchanger. The wort will boil from 70 C to 105 C inside the heat exchanger by using dry saturated steam at 0.7 barg with temperature of 115 C. The aim of this process is to sterilise the wort and deactivate all the enzyme. The duration of this process is set to be 1 hour and the wort will pass through the EWH 6-10 times /hr (circulation rate) (from literature) . The evaporation rate of the wort is 7%/hr. The pump will be used to start the circulation but is not required once the syphon is established which is the differences  in densities between single phase (wort) and two phase (wort and steam) . The mass flow of the wort entering the EWH is 88.4 Hl (2.45 kg/s). The outlet of EWH is going to be two phase flow (wort and steam) return to the kettle. The wort will be in kettle at the start (single phase wort ). Hops are added to the wort before boiling to deliver some characteristics to the wort such as bitterness and aroma (85 kg).

 

My problem is that i don't know from where to start. Could you please direct me to the right way. For the hops, is it possible to neglect the hops in the calculations because the amount is very small compare to the amount of wort?

 

Here is a simple sketch

 

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[Art Montemayor]

Yasser:

 

You say you don’t know where to start.  Why don’t you start by defining what you are supposed to “design”.  In other words, what are your objectives:

• Are you to produce a mechanical design or a process design?
• If you are to produce a process design, what are the details that are required as products?  Are you to calculate the volumetric batch size of the kettle?  You don’t state the size of the batch, so you either calculate it or it is given.
• - the heat transfer area of the reboiler (EWH)?
• – the size of the piping?
• - the amount (or rate) of steam required?

Our members need to know what you need help in.

 

Additionally, what work have you put into this effort?  Is the rough, scribbled sketch the only thing you’ve done to date?  Is this the quality of the work that you are going to submit to your instructor?

 

A lot more basic data is needed – such as the composition of the wort or its properties, such as density, specific heat, boiling point curve vs concentration, etc.  What are you going to do with the 7% of the wort batch that is vaporized?  I assume your kettle is vented to the atmosphere.  Therefore, the composition is changing and so are its properties – is this correct?

[Yasser]

Dear Art

 

1- I am going to produce both mechanical design and process design

2- yes, I need to calculate volumetric batch size of the kettle but i haven't done that yet 

3,4,5 - They are in the  excel sheet attached and I am still carrying out with the calculations 

 

The kettle will not be vented to the atmosphere. For 7% evaporation, mechanical vapor compressor or thermal vapor compressor both can be used to recover the vapor. The calculations in the excel sheet are not based on the mechanical and thermal vapor compressor.  

 

 Book1.xlsx   43.04KB   18 downloads

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[Art Montemayor]

Yasser:

 

Thank you for the timely reply and submittal of your workbook with some calculations.  This shows you are endeavoring to earn your grade by doing the required work.

 

However, you need to pay careful attention to your work product.  How can you propose to use a reboiler tube that has an internal diameter of only 0.075 millimeters???  This is a hypodermic needle that you are proposing.  You can't be serious about trying to flow solid hops through this diameter.  Consequently your calculations have a practical flaw and have started out on the wrong foot.   Bear in mind that if you are to tolerate any solids in the wort, you have to allow for this in the reboiler tube selected.  Usually, a 1" tube is a minimum with a 1-1/2" tube diameter being the norm.  I have not gone through the remaining calculations and I don't see any write up of the algorithm steps you are employing, so it is difficult to follow your reasoning and logic.

[Yasser]

Dear Art 

 

Sorry for the confusion, the internal diameter of the tube is 0.075 m not 0.075 mm. I am still working on the calculations, but i have some problem with the density of the two phase, I don't know how to find it. The two phase density will help me to find the pressure drop at the tube exit.

 

I really appreciate your help  

[breizh]

http://www.wlv.com/p...DataBookIII.pdf

 

You should find some information about 2 phase flow properties following this link.

Hope this helps

Breizh

[Art Montemayor]

Yasser:

 

What you propose is using a 0.075 m (3") diameter tube size.  This is an exaggerated size of reboiler tube and you will probably not find it in the marketplace.  As I said, what is usually employed is a 1.5" tube size (which is an available, standard size).

 

I doubt if you will have a perceptible pressure drop across the 3" tube.

[Yasser]

Dear Art,

The process is an unsteady state heat transfer.  Is that because of the mixing between the hot wort coming from external boiler at 100 ^oC, and the relatively colder wort at 70 ^oC?  Because i said before that the wort will be in the kettle at the beginning at 70 ^oC?

If that is the case, Is it possible to make the process a steady state batch one by charging the wort directly to the external heater from the beginning rather than in the kettle?  The cold wort will be charged to the external boiler by a centrifugal pump.  Then the hot wort will be sent to the stainless steel kettle to store the heat in the wort.  The hot wort will not be circulated directly to the external heat exchanger in order to prevent mixing between hot and cold wort.  The circulation starts when the hot wort reaches the maximum working level in the kettle ( maximum amount of wort).

[Art Montemayor]

Yasser:

 

It is extremely important, in my opinion, to first advise you (and all other chemical engineering students reading this thread) on how important it is to identify and communicate the total facts of your assignment or project in an accurate and detailed manner.  Unfortunately, this is not taught in many universities and it deprives the student of entering the engineering profession with full knowledge of what is expected of him and how to best utilize his tools in an actual industrial environment.  The subject here is a SCOPE OF WORK (SOW).  When you learn and employ a SOW, you are practicing engineering as a true professional does it.  The SOW defines the problem at hand and all the pertinent and related details that must enter or be part of the total solution.  This is what has been missing from this thread and what must be ultimately outlined in order to accomplish what you need to do.  Otherwise, you cause a lot of wasted time and effort on your part as well as on all of our members by not establishing a firm and definite SOW for your assignment.

 

You have not furnished a detailed explanation of what it is that you have been told to do.  If you are to calculate the size of a required steam heating device that will take a batch load of wort and heat it to a pre-determined temperature within a given amount of time in a kettle, then what you have before you is an UNSTEADY STATE HEAT TRANSFER operation.  Unsteady state heat transfer is an operation where heat transfer is taking place due to a temperature difference within the system which is changing with time.  Although seemingly simple and direct, unfortunately study of heat flow under these conditions is complicated.  It is one of the toughest unit operations to understand and to resolve.  In fact, it is the subject for study in a substantial branch of applied mathematics, involving finding solutions for the Fourier equation written in terms of partial differentials in three dimensions.  Nevertheless, there are ways to resolve specific heat transfer done in an unsteady state.  One engineer who devised and published a calculation method is Donald Q. Kern, author of “Process Heat Transfer”, McGraw-Hill (1950) – a classic in engineering texts.  An article explaining how to apply this method was published in 1963 and you can obtain it as per the description in the attached document titled “Nomographs for Unsteady State Heat Transfer”.  That should help you to resolve your problem as you have indicated it without details.  You would be smart to obtain and study Don Kern’s book on heat transfer.  Every engineering student should have a copy.

 

However, if you now want to change your SOW on your assignment (which I don’t understand nor condone), then you can employ a steady state heat transfer operation by pumping the wort charge into your kettle one time through an in-line heat exchanger heated with steam.  You have to make up your mind – or have your assignment changed for you.  Which SOW do you want to discuss?  But before discussing it, write it out in clear, detailed English.

 Nomographs for Unsteady State Heat Transfer.docx   147.58KB   22 downloads  1-Unsteady-Transient-Conduction.docx   51.04KB   18 downloads  Unsteady State Heat Transfer.docx   98.65KB   13 downloads  Unsteady State Heating & Cooling.pdf   119.45KB   23 downloads

[jamisrinu]

Yasser:

I think the amount of steam needed calculation might be wrong in your excel work book.You just divided steam sensible heat with total  heat. In fact you have to use latent heat (sensible heat will be negligible). You have to take latent heat at 0.7 bar & then you can calculate steam requirement. I may be wrong.

Please check.