22 replies to this topic

[AlStudent]

Hello,

I am designing a tube and shell exchanger. And to calculate the tube side heat transfer coefficient, I have to calculate first the Reynolds number.
The tube side fluid is water at 30 C with a flow rate of 6.46 liter/s and a tube diameter of 25.4 mm.
The problem is that I get a velocity of 12.7 m/s (which is to high)
I want to make sure how do we calculate the velocity of the fluid inside the tube!

Thank you very much.

[Shivshankar]

Hi,

Calculate velocity.

v = Q / A

where:

v = Velocity (m/s)

Q = Flow Rate (m³/s)

Convert liter (l) into cubic meters (m³⁾

Calculate Area

A(t) = Total Number of Tubes* Cross-sectional area of tube / Number of passes

Cross-sectional area of tube

A(i) = π*D(i)^2/ 4

Outside Heat Transfer Area

A(o) = π*Number of Tubes*D(o)*L


Where

A(t) is Total Area of Tube (m²)

D(i) is Tube Inside Diameter (m)

D(o) is Tube Outside Diameter (m)

L is Tube Length (m)

Convert millimeter (mm) into meter (m)

Hope this helps.

Regards
Shivshankar

Edited by Shivshankar, 06 July 2012 - 01:07 PM.

[Art Montemayor]

This has to be a student problem because it quickly reveals a lack of knowledge or understanding on how a heat exchanger is built and how it works. That’s OK; we have all traveled that road before.

You don’t state WHICH Reynolds Number you are referring to – the shell side or the tube side? I will assume the tube side since that is the simplest application and since that is the side that is presently giving you problems. Please refer to the attached Excel workbook. Always submit your calculations on a spreadsheet when you are making a query regarding your calculations. That allows us on the Forum to check your logic, algorithm, and math. For example, note that your answer relates to you using ONE, 1-inch O.D. tube with a 12+ BWG wall thickness. This is nothing more than a double tube heat exchanger. Is that what you are contemplating? I don’t think so.

You fail to state whether you are designing a heat exchanger from “scratch” or rating a heat exchanger. Either way, you must know the total heat transfer area before attempting to calculate the number of tubes, passes, tube size, shell diameter, and shell length. This is the normal, logical procedure of going about this calculation. You also fail to tell us all of that and, as a consequence, I believe you have made a logical and basic error. You must establish the NUMBER OF TUBES PER PASS as well as the tube dimensions. Otherwise, you are unable to proceed any further with the subsequent calculations such as the velocities, Reynolds Numbers, pressure drops, etc., etc..

That should concretely prove to you why it is essential to submit detailed spread sheet calculations – such as I have submitted.

I hope these comments help you get oriented to this calculation.

Attached Files

•  Heat ExchangerTubeside Velocity.xlsx   10.77KB   456 downloads

[AlStudent]

Thank you for your replies!

According to what you have said I have calculated the water linear velocity:

Tube cross sectional Area: Pi/4 * (InsideDiameter)² = Pi/4 *(23.3)² = 426 mm2

Tubes per pass = 217/1 (1-Pass)

Total flow area = 217 * 462 * 10^-6 = 0.1 m2

Water mass velocity = Flow rate / Total flow area = 6.46 (kg/s) / 0.1 (m²) = 64.6 Kg/s m²

Density water = 995.71 Kg/m³

Water linear velocity = 64.6/995.71 = 0.065 m/s

I believe this velocity is still too low!! I have read that water velocities inside tubes should vary between 0.6 and 2m/s! I think I have an error somewhere. If you need more details about the Shell & Tube Exchanger that I am designing from scratch, please refer to the excel sheet calculations that I made so far.
Thank you very much for your help and time.

Attached Files

•  StageSizing.xlsx   39.59KB   130 downloads

[Shivshankar]

AIstudent,

You calculated Area as 426 (mm)^2

From where did you brought 462 in following equation ?

Total flow area = 217 * 462 * 10^-6 = 0.1 m2

I believe it is typo error.

Regards
Shivshankar

Edited by Shivshankar, 09 July 2012 - 04:46 AM.

[AlStudent]

Shivshankar,

Yes, thank you, it is a typo error the Total flow area = 217 * 426 * 10^-6 = 0.092 m2. Therefore, the velocity will be 0.07 m/s
But still it doesn't solve the problem. The velocity is still too low.

Thank you very much.

[Shivshankar]

AIstudent,

In first post you gave flow 6.46 l/s (liter per second)

and post no. 4, you mentioned it Kg/s (Kilogram per Second) .....?

Water mass velocity = Flow rate / Total flow area = 6.46 (kg/s) / 0.1 (m²) = 64.6 Kg/s m²

Who will convert this ?

There is difference between Mass Flow Rate and Volumetric Flow Rate.

I believe again it is typo error.

http://en.wikipedia..../Mass_flow_rate

http://en.wikipedia....etric_flow_rate

Regards
Shivshankar

Edited by Shivshankar, 09 July 2012 - 07:45 AM.

[AlStudent]

6.47 Kg/s is the mass flow rate to consider and not the first post.

Thank you.

[Shivshankar]

AIstudent

For while go through this documents.

Regards
Shivshankar

Attached Files

•  exchanger.pdf   241.91KB   163 downloads
•  shelltube.pdf   77.72KB   149 downloads
•  shellandtube.zip   6.84MB   144 downloads
•  designshelltube.pdf   30.03KB   150 downloads

Edited by Shivshankar, 09 July 2012 - 11:27 AM.

[Art Montemayor]

AIstudent:

You are working your problem in the wrong manner and procedure.

Your dilemma has a simple and practical answer. Please refer to the attached modified version of your workbook and read my comments.

The answer to determining the number of tubes that can reasonably be used in a TEMA configuration lies in the type of heat transfer that you want to carry out. You specifically call for pure counter-current heat transfer - which is usually more expensive to apply to a heat exchanger, but that's OK. The calculation that you make to "determine" the number of tubes is not a valid calculation for the results expected. You must apply horse sense to every calculation and step that you apply to your design. Knowing full well that the inside film coefficient is very sensitive to the amount of forced convection currents that take place, you also know that this is dependent on velocity: the higher the velocity, the better the film coefficient and the better the heat transfer. However, you also know there are practical, empirical known values for velocities inside tubes. This is not only due to pressure drop, but also to metal errosion effects.

You should be using a "reasonable" tube fluid velocity and calculating what number of tubes that will take, using "reasonable tube lengths" and, if necessary, a different TEMA configuration - like an "F" shell (although I despise them) - or multiple shells stacked in series.

There is more than one way to skin a cat.

Attached Files

•  Heat Exchanger StageSizing.xlsx   19.98KB   163 downloads

[AlStudent]

Thank you Shivshankar for your readings. I've already gone through some of them.

Art Montemayor,

Thank you very much for your comments and remarks. It is very helpful. However, I have some more questions if you might allow me.

The tube length of 3.6m used is required from my supervisor since the exchanger will be held in a limited space.

Do you think a BFM design would work? Ok, lets assume it will.

Using a 25.4 outside tube diameter, a tube side velocity desired of 1.2 m/s , and 3.6 m tube length assigned bu the supervisor.

We will have a number of tubes of 12 approximately.

The heat transfer area will be 4.07 m²/pass

My calculated heat transfer area is 63.4 m²

Therefore 63.4/4.07 = 16 tube passes required in one heat exchanger shell !!!

Which is not possible!! So BFM is not recommended in my case.

How do I proceed then knowing that my design will be given to my supervisor in order to purchase the heat exchanger?

Thank you very much for your help.

[AlStudent]

Please anyone?

I would really appreciate. Thank you.

[Art Montemayor]

AIstudent:

This thread is taking different twists and turns and taking a lot of your time (as well as ours) simply because we haven’t received ALL the basic data and background in the original post. Now we are told that you must adhere to a counter-current heat exchanger with a tube length of 3.6 meters. That is very restrictive and makes the design an impossibility when combined with the other restriction of a very small driving force (LMTD = 2.5 ^oC).

Study the calculations I have revised on Rev1 of my workbook and you will see how this works out. Note that you can vary the presumed allowable tubeside velocity and come out with different tube passes for the 3.65 meter long tubes. The very small LMTD of 2.5 ^oC makes the required heat transfer area come out very large. The large area involves more tubes and you simply run out of options because of the number of passes.

You can use multiple BFM exchangers stacked one onto another, but the number is impractical. The heat transfer can be carried out, if required. But a TEMA type shell and tube is not the way to go on this application.

You ask: “How do I proceed then knowing that my design will be given to my supervisor in order to purchase the heat exchanger?” My response is that the specifications of tube length, exchanger type, and small driving force make this application impractical and certainly not recommended.

Attached Files

•  Heat ExchangerTubeside VelocityRev1.xls   67.5KB   152 downloads

[Shivshankar]

AIstudent,

Now you are left with only one option that you should Optimize your design of heat exchanger in MATLAB with limited data available, you can only help yourself.

Look at, attached documents.

*Good Luck

Regards
Shivshankar

Attached Files

•  6SHeatExchangerProject-100506.pdf   136.75KB   108 downloads
•  HeatExchangerProject-Spring2010.ppt   158.5KB   89 downloads
•  Team1-HeatExchangerPowerPoint.pdf   166.44KB   94 downloads
•  Team5-Project2Heat_Exchanger_Design.pdf   352.85KB   92 downloads
•  ME414HEPresentation.pdf   190.81KB   72 downloads
•  HeatExchangePresentation.pdf   3.79MB   92 downloads

Edited by Shivshankar, 12 July 2012 - 04:28 AM.

[Steve Hall]

Isn't the big issue in this specification the temperature cross between the two fluids? By specifying that the cooling water outlet temperature is higher than the process water outlet temperature, a single-shell design must use single-pass tubes.

Conceptually, the number of tubes is set (within a range) by the chosen tube diameter and process water flow rate, aiming for a velocity in the 0.5 to 2.0 m/s range. Again conceptually, with the overall tube length defined by the problem statement, the heat transfer calculations can be performed to determine the required area and therefore the number of shells.

With a complete and careful analysis, the answer might not (probably won't) be that each shell is identical. In other words - and I'm just speculating here - a portion of the heat transfer might be possible in a mult-pass unit (tube side) with another portion in single-pass units. Although the overall heat balance indicates a temperature cross, when the problem is broken up into sub-problems (one for each shell), the temperature cross issue might go away.

If I were faced with this problem, I would ask questions about the cooling water. Is the flow restricted such that the temperature cross is required? If using multiple shells, I would divide the cooling water supply so that the coldest temperature (21 C) is fed to each shell, rather than having the warming water cascading from one shell to the next. But I would also explore alternatives to a shell-and-tube unit including plate-and-frame and spiral exchangers. Having "fixed" the number of tubes, I would turn my attention to the shell and estimate the maximum allowable flowrate of the cooling water based on pressure drop through the shell. I'd use that value to determine a new Outlet Temperature. This would tell me if multiple tube passes are possible and, if so, I'd iterate through the analysis to eventually find a workable solution. Then, I'd look at that solution with a practical eye: does it fit in the space, is it simple enough to install and pipe, can it be controlled, etc.

Regards,
Steve

[AlStudent]

Thank you all for your replies.

Do you think this design would work? However, I still have a high shell-side pressure drop in exchanger 1. Do you think I should use a double segmental baffle to lower it a little bit?

But, looking at this design, do you think it is a practical design, knowing the specifications?

Also, I have another question please. If I implement a 2-pass shell in my design, will it have any influence on the calculations? If yes, what are the equations that are going to be modified?

Thank you very much.

Attached Files

•  StageSizingExchanger.xlsx   42.18KB   88 downloads

[srfish]

I think your design would work if both water streams are very clean. I ran the design through our software and it shows that very little fouling factor is used in the calculated overall heat transfer coefficient.

We show much lower calculated pressure drops.

Your design now has a good water velocity.

A slight change is in the inside shell diameter. A more conventional shell I.D. is 387 mm.

[AlStudent]

For which exchanger are you talking about? exchanger 1 or 2 ? because I made a sketch in the excel sheets to show in details how it will be designed.

Thank you again.

[srfish]

It is the one you posted today.

[AlStudent]

Yes. It has a sketch and calculations for both exchangers that are implemented in series.

[srfish]

Exchanger number one(1).

[AlStudent]

In the excel file I sent there are 3 sheets: The first sheet is a sketch representing the design, the second one is the calculations for exchanger number 1 of the sketch and the third is the calculations for exchanger number 2.

[AlStudent]

What would happen if I have a pressure drop between 1 and 2 Bar in the tube and shell sides of my exchanger?
In other words, if you consider that 2 bar is a high pressure drop, what are its consequences on the exchanger?

Thank you.