5 replies to this topic

[Lien]

Hi,

 I am a chemical engineering student. I am working on one of Valero project which ask me to determine the length of an 8" carbon steeling pipe in the blowdown relief service. I had been given the ambient temperature is 60F, the air velocity is 15mph. and the inlet temperature changing by time. I have to determine the temperature out of the pipe, and the length of the pipe so that the range of the outlet temperature is around -20F. there is no insulator with the pipe. Please help!!! I need it. thank you very much. 

[Pilesar]

As a student engineer, this seems a good time to consult your Heat Transfer textbook if you have had that course. I recommend you go through the calculations yourself and set up your own spreadsheet for your problem. While there may be quicker ways to find a "canned" answer, doing it yourself will help you truly understand it and will put you on the road to being an expert in heat transfer.

First you must determine the mass flow rate and the relief temperature at several discrete time intervals. If you are lucky, the relief temperature will be above -20F at each time step because vapor to vapor heat transfer through a bare pipe is not very efficient. You can calculate the cooling duty rate required at each time step by multiplying together the vapor mass flow & the heat capacity & the temperature difference required. Now you have the heat flow (Q). Look to fill in the other parameters in the equation Q=U*A*LMTD. Calculate the LMTD assuming the air is constant temperature. You can calculate an overall heat transfer coefficient (U) or pick a reasonable value guided by experience. Now you can solve for the heat transfer area required. Since you know the pipe diameter, you can solve for the length needed. Do this for each time step and find the maximum length required. Add some extra length to cover the simplifying assumptions in your calculations.

Chris Haslego offers a useful spreadsheet in the files section of cheresources:

http://www.cheresour...ure-prediction/

The formulas are viewable and you can use his spreadsheet as a sanity check for your calculations. Use that spreadsheet also to investigate the effects of pipe wall thickness and insulation.

[Lien]

Thank you very much for your help Pilesar. I went over the heat transfer textbook and did my own spreadsheet, but it came out a little different from what I expected. Your answer helped a lot. Thank you very much again.

[gegio1960]

Lien,

if you share your spreadsheet, maybe someone will check what you've done 

Good luck

[Lien]

Yeah, this is my spreadsheet. i asked my boss if it was corrected but he was not sure either. Please check. Thank you very much.

The data in yellow is the given data that i got from Hysys. All others are calculated.

Attached Files

•  final 2 extra radiation.xlsx   313.6KB   50 downloads

Edited by Lien, 19 March 2013 - 09:42 AM.

[Lien]

I think there are some mistake in my spreadsheet. I have to calculate the heat transfer and the overall heat loss of the system. but I do not have the thermal conductivity and thermal diffusivity of the vapor inside the tube. Since then I got stuck and could not move on to the next step. I had been asked to calculate the overall heat loss too, but I do not know what is that for. I am totally lost. In my spreadsheet, I have all the information in yellow color with the vapor out temperature given. Since the vapor out of the blowdown temperature is also a temperature in to the pipe, and it's a little weird, I got a weird vapor out of the pipe temperature too. I am not sure if I did that correctly, but my boss just ask me to check the temperature out EVERY FT to see what is the different of the temperature with time and distance. Please, help me. Thank you very much.