6 replies to this topic

[sheiko]

Dear all,

I have attached an Excel spreadsheet that describes my problem and the solution i propose. Could you please let me know if my approach is sensible and correct?

To put it in a nutshell, i have a 2 shell and tube heaters in parallel that i want to rate after a capacity increase on process side (natural gas) but, as shown in the spreadsheet, some data of the initial design are missing...so i used a shortcut that i would like you to check.

Thank you in advance for you help and please let me know if something is unclear.

Attached Files

•  reheaters.XLS   40KB   156 downloads

[narendrasony]

Dear Sheiko,
I think your assumption that product of U*A is constant is not correct, since "U" is a function of flow rate. You have to solve the following equations simultaneously:

U.A.(dt)m = M.Cp. (To-Ti) = m.cp. (ti-to) ...........(1)

Where M = Natural gas flow (new condition)
Ti, To = Inlet & outlet temp. of natural gas, Here "Ti" is known & "To" is to be
determined

m = Cooling water flow (here fixed as for old case)
ti, to = Inlet & outlet temperatures of cooling water, "to" is unknown

A = Heat transfer area, fixed for the given exchanger
U = Heat transfer coefficient for new flow conditions.
(dt)m = LMTD with corr. factor. This is also an unknown function of "To" & "to"

So these equations are to be solved simultaneously for determining To, to & (dt)m. This will be a trial and error solution. You can start by assuming a value of "To", then determine to and (dt)m. Then for given (dt)m and "to" find new value of "To" and compare with old "To". Then continue till required accuracy of results.

Regards
Narendra Kumar

[sheiko]

Narendra, thank you for your contribution.

First, there is no cooling water but hot water that heat the natural gas.

After, i agree that during the rating step, it is better to fix the water flowrate instead of the water outlet temperature. However, as the U will increase when the gas flowrate increase (the gas tubeside heat transfer coefficient will vary with approximately the 0.8 power), is it not conservative to consider UA constant? I mean, the lower the product UA, the more difficult the heat transfer...right?

[djack77494]

sheiko,
I agree with your assessment that considering UA to be constant should be a conservative assumption. Obviously the area truly is a constant. U will increase with increased flowrates, so I think you're fine. Check that you're happy with all incoming and outgoing temperatures. The one concern I would have is that fouling will increase as temperature increases. Make sure your water return temperature doesn't get too high. Should be good for screening purposes.

[narendrasony]

Sheiko,
Sorry for taking water stream as cooling water instead of hot water. Silly mistake.
I agree with your conservative approach. But later on if required, advantage of increased "U" along with slight increased hot water flow which I think should be possible, you may very well achieve the earlier temperatures.

I would like to know how the maximum duty of an exchanger is fixed. Is it done through HTRI ? For a given exchanger, can it be done by Excel Calculations ?

Narendra

[Muhammad Adnan]

Dear all,

I have attached an Excel spreadsheet that describes my problem and the solution i propose. Could you please let me know if my approach is sensible and correct?

To put it in a nutshell, i have a 2 shell and tube heaters in parallel that i want to rate after a capacity increase on process side (natural gas) but, as shown in the spreadsheet, some data of the initial design are missing...so i used a shortcut that i would like you to check.

Thank you in advance for you help and please let me know if something is unclear.

Dear Sheiko,

In my opinion you get heat duty required by natural gas by following equation instead of the eqution you used,

Q=M*Cp*(Delta T)

from the calculated heat duty get the required mass flow rate required of hot fluid by using the same above equation.

After that use HTRI or B-jack or Aspen Tasc or Tasc+ or Aspen HTFS to rate your heater.

I hope this may resolve your problem.

Now, one advice for you; Try to explain your problem in more efficient way. As people just need what you require not what you have.

Take care and have a nice day.

[sheiko]

QUOTE (Muhammad Adnan @ Apr 26 2009, 07:27 AM) Dear Sheiko,
In my opinion you get heat duty required by natural gas by following equation instead of the eqution you used,
Q=M*Cp*(Delta T)
I agree.

QUOTE (Muhammad Adnan @ Apr 26 2009, 07:27 AM) from the calculated heat duty get the required mass flow rate required of hot fluid by using the same above equation.
Why? the hot fluid (process side) flowrate is a constraint not a variable.

QUOTE (Muhammad Adnan @ Apr 26 2009, 07:27 AM) Now, one advice for you; Try to explain your problem in more efficient way. As people just need what you require not what you have.
Take care and have a nice day.
I think it is also important to provide the available basic data and assumptions.

Thanks.

Edited by sheiko, 07 May 2011 - 07:57 PM.