12 replies to this topic

[Dawood]

Dear All

 

I am currently trying to design water - water Shell and Tube Heat Exchanger, the data available to me is:

 

Hot Stream:                    T in = 105 ^oC                    T out = 40 ^oC                  Flowrate = 111.5 m³/ hr

Cold Stream                   T in = 35 ^oC                      T out = 95 ^oC                  Flowrate = 110.0 m³/ hr

 

Using  the book "Process Heat Transfer" by Kern, while desiging i found the Number of tubes to be 5000, each 6m long having 1.25in OD. I believe this number is wrong because the datasheets availabel at the back side of the back has the Maximum shell ID to be 39in and can maximum accomodate around 1000 tubes.

 

please help me on this and let me know what is the mistake. also, if anyone of you has a good spreadsheer to calculate and desing heat exchangers i would be very thnakful if you could share it with me.

 

Thanks

[Art Montemayor]

Dawood:

 

First, ask yourself:

• How  can anyone help me find my mistakes if I don't even show them my calculations?
• How can anyone have an idea of the size of the heat exchanger if you don't state the capacity that you have calculated - Btu/hr or kcal/hr?

 

The answer to both questions is it isn't possible.  Therefore, furnish your detailed calculations in a format where they can be checked easily and commented upon.  The format to use for this purpose is an Excel spreadsheet, not a scanned .pdf or photo of your pencil notes.

[meetyourmaker]

dawood,

There is a huge temperature cross in this exchanger. Are you using 1-1 configuration for this exchanger? Please furnish more details.

[Dawood]

Dear Art Montemayor

I have stated the information that I used to design the Heat Exchanger. I thought that might be sufficient. if you require my calculations they are attached herewith.

 

Dear Meetyourmaker

The Huge Temperature Cross is the problem I am facing. Will it effect if we switch Hot side and cold side?

The type of SHell and Tube HEat Exchanger is not finalized we can select it based on our design. but I was thinking of usinge 1-2 or 2-2, whichever fits the best.

 

I have attached my calculations. please review them and help me to design this heat exchanger.

 

Thanks

Regards

 

 

Attached Files

•  Shell and Tube Heat Exchanger Design.xls   39.5KB   204 downloads

[breizh]

Your heat transfer  coefficient seems low to me and you may think to install more than one HEX !

my 2 cents

 

Breizh

[Dawood]

Dear Mr. Breizh
as it is a water-water HX. so according to literature the assumed heat transfer coefficient for water is between 250-500 W/m²K. If  my overall heat transfer co-efficient is less, please guide me from where to extract suitable data for this.

 

Besides this there is a temperature cross in my design how can i avoid that?

 

Thanks and regards

[breizh]

Consider this resource , you may find what you are looking for.

 

http://www.engineeri...ng/thermal.html

 

Breizh

[Pingue2008]

Dawood,

 

I did a quick calculation using your assumed pipe diameter and length and have different number of tubes. see the attached for my calculations it is just under yours. possible mistake: I think the calculation of the log mean temperature in your spreadsheet is wrong also, the way you defined the overall heat transfer area might be a problem and U seems low as previously indicated by breizh. you will see that my calculations are in English units.

 

Thank you,

Attached Files

•  Shell & Tube Heat exchange design.xlsx   26.67KB   133 downloads

[srfish]

To avoid a temperature cross and keep the given heat load,  you will need a lot more water flow in the tube-side. A small help would be to put the larger flow in the tubes.

 

If you cannot change the operating temperatures, it is going to take multiple heat exchangers.

[Dawood]

Thank you

Pingue2008, Breizh and Srfish for all your suggestions.

 

pingue2008 i have reviewed your calculation and truely they were helpful in correcting my design. as my number of tubes were almost double.

 

Gentlemen! as I am unable avoid temperature cross, so I am planning to design series of HX. possibly 2. but I am unable to decide how to design both of them. Shall I breakup the temperature range between two HX? or what is the best way to design two HX in series??

[srfish]

In this case where both sides of the heat exchanger have sensible heat transfer, you do not need to design each individual heat exchanger of the exchangers in series. Since you have Donald Kerns book, I suggest you read chapters 7 & 8 and use the LMTD correction factors in back of the book. I believe you will find this will require numerous heat exchangers in series. You can cut down on the number in series by using a counter-flow arrangement such as a long baffle in the shells.

[Bobby Strain]

You can download a useful calculator for exchanger effectiveness factor, F, here.

http://www.mediafire...uc97pl64ye2o94c

 

Bobby

[breizh]

Thanks Bobby,

Similar resource can be found here ( if like me you cannot get access to mediafire) :  

 

http://www.chemsof.com/lmtd/lmtd.htm

 

Breizh