4 replies to this topic

[The MANUFAN]

I am new to heat exchanger. Although I have already taken heat transfer, we did not cover exchanger chapter thoroughly.
I am trying to find the area and heat duty of my shell and tube heat exchanger.
Shell side = process stream
Tube side = utility stream
Utility stream -= mps ( 10 bar and 184 degree c)
process stream ( methanol + water) = 1570532 lbmol/hr and temp= 30 degree c and pressure = 43.5 psi
Methanol flow rate = 1479211 lbmol/hr
Water flowrate = 91320.99 lbmol/hr
I do not know the flowrate of my utility stream
I do not know how to approach this problem!!!

[Art Montemayor]

Manufan:

It is apparent that you did not cover the subject of heat transfer very much.

You also fail to remember the “golden rule” in Chemical Engineering: When in doubt, make a heat and material balance. If you follow this criteria, you will arrive at your heat duty – which is the first and foremost value you require before attempting to find size of heat transfer area in an exchanger.

Another note: What is “mps”? If you are going to use acronyms or symbols, first identify them. We can’t be expected to know what is on your mind.

Still another note: Don’t bother us or waste our time and effort by giving us numbers that are not significant. Use common sense and call your water flow rate 91,321 lbmol/hr and be done with it. No one is going to seriously believe that you have to have accuracy on the water stream to 1/100th decimal place. You are wasting your time with decimal places when you should be concentrating on the mass and heat balance issue in order to resolve your “utility” stream.

[The MANUFAN]

Art Montemayor:

Thank you for your reply.

Here mps means medium pressure steam, which is used here to heat up my process stream. I was using Q = (mdot). (sp. heat ) (delta T), and try to equate heat gained = heat loss, but I do not have the flow rate of medium pressure steam and the outlet temperature of the steam.

I was confused on that part.
Any suggestion would be highly appreciated.

[breizh]

Hi ,
If I understand well, the steam is saturated ( 11 bars absolute and 184C ) , refer to excel file , thus the heat duty should be the product of the latent heat of steam * steam flowrate .

Hope this helps
Breizh

[Art Montemayor]

Manufan:

I am trying to help you. But you won't help me help you.

I have already given you the method to resolve your problem. Please pay attention to what I write because I am telling you exactly what you should do in order to resolve this simple problem. Your lack of attention or of not reading my advice in detail is, perhaps, the cause of you not succeeding in understanding heat transfer problems. You must prepare yourself before each lesson or lecture with the subject matter and pay very close and concentrated attention to the instructor. Let me repeat once again what you have to do to resolve this problem:

• Make a heat and mass balance around the heat exchanger. Do this by first drawing a sketch of the heat exchanger with all the incoming flows and the outgoing flows.
• Next, make a mass balance around the heat exchanger;
• Make a heat balance around the heat exchanger. You should make note that all the heat captured by your tube-side methanol solution will be coming from the steam that is condensing in the Shell-side.
• You know that you want to heat up your methanol solution. You give us 30 oC - that must be the methanol solution inlet; you fail to give us the methanol solution outlet. You MUST KNOW WHAT YOU WANT TO HEAT THE METHANOL SOLUTION TO. You have to know that. You don't give it to us - probably because you failed to pay attention to the common sense question: what do you want to heat the solution to??? Once you know how much you want to heat the solution, you know the amount of heat it takes to heat it by using the heat capacity equation.
• Once you have the amount of heat transferred to the methanol solution, you equate it to the equation for latent heat in the shell-side. (The condensation of steam involves only latent heat - that's all). The equation for the latent heat transfer is the mass of steam multiplied by its latent heat value - which you know since you know the pressure and condition of the steam.
• You solve for the mass of steam required to furnish the heat to the methanol solution.

Now that I've outlined all the steps involved, go out and successfully do it. And next time, always pay carefull attention in lecture, ask questions, and take detailed notes of all the Prof says.

But first, always study the textbook carefully days before the actual lecture so you will know beforehand what the Prof is going to talk about and what he is going to say.