1 reply to this topic

[Kentucky08]

Main Calculation : https://drive.google...?usp=drive_link

Summary: https://drive.google...?usp=drive_link

Good day, Sir / Ma’am,

My manager assigned me to design a new condenser for the Methanol refining section. This is my first condenser design, and would like to respectfully ask for your advices and guidance. This is not the main condenser but a safety/vent condenser that will receive methanol from the first condenser. Attached are my sample calculations and a summary of trial cases.

Summary of my approach and assumptions:

• I used Kern’s method for the shell-and-tube design.

• I aimed to utilize the allowable tube-side pressure drop to increase the tube-side velocity and thereby improve the overall heat transfer coefficient.

• Cooling medium: refrigerated (soft) water — I assumed a lower dirt factor because of the softer water and expected lower fouling.

• I included trial cases and a short summary of results in the attached file.

Request for guidance:

1. Please confirm whether using Kern’s method is acceptable for this safety condenser design, or advise if an alternative method is preferred.

2. Is it acceptable to exploit the allowable tube-side pressure drop to increase U (overall heat transfer coefficient), or should we limit velocity for erosion/corrosion considerations?

3. Please confirm or advise on the dirt/fouling factor assumption for refrigerated soft water in our service.

4. My resources are Kern, Ludwig and Coulson for my calculation.

5. Any additional comments on materials, tube layout, or special requirements for a vent/safety condenser (e.g., two-phase handling, venting, drains, instrumentation) would be appreciated.

I have attached the calculation workbook and the trial summary. I welcome any corrections or recommendations so I can update the design accordingly.

Thank you for your time and guidance.

[Pilesar]

Your links are a nice way to provide documentation. 

Is this an academic exercise or for a real working plant? Are you using exchanger design software?

 

I challenge the assumptions.

Assumption 1) Assume first condenser can only condense 85% of the total MeOH load.

There is a problem with the main condenser. Troubleshoot. Rate the main condenser for existing conditions. Fix the main condenser.

Assumption 2) New condenser will be receiving pure MeOH with no other mixture.

If this were pure MeOH, then the main condenser would condense it and there would be no vapor! There is probably additional uncondensed gas in this stream as well. Design the new exchanger for the extra vapor load including the inerts!

 

If I were designing this exchanger from scratch, I would look at:

a) Horizontal orientation, probably an E shell, large shellside inlet nozzle at the top of the unit, smaller outlet nozzle at the bottom and another for venting inerts. 

b ) Baffles vertically cut. I try to maximize baffle pitch and maximize baffle cut to reduce shellside pressure drop. The computer program can help optimize. The shell side is operating under vacuum (about 12 psia if the condensing temperature is correct) so perhaps shellside pressure drop may be important.

c) Try 60 degree pitch as it can give higher heat transfer. With a 60 degree pitch, the distance between vertical tube rows is greater so there may be less condensate loading as the liquid drops down the tube bundle.

d) There will be significant sub cooling whether you ask for it or not. Your summary shows about 70F subcooling. Size outlet nozzle for maximum 3 ft/s velocity to reduce the chance of backing up the condensate.

e) If 149 F saturation temperature is true, using refrigerated water to condense seems inefficient. If refrigerated water is needed, then this proves the original assumptions are incorrect.

 

Not to ignore your specific request for guidance -- I will comment on each:

"Please confirm whether using Kern’s method is acceptable for this safety condenser design, or advise if an alternative method is preferred."

Kern is a standard that many use successfully. If it works for you, use it. I have only looked at it briefly and found it did not address many of the questions I had about exchanger design.

"Is it acceptable to exploit the allowable tube-side pressure drop to increase U (overall heat transfer coefficient), or should we limit velocity for erosion/corrosion considerations?"

Your tubeside velocity of 8 ft/s seems fine. Refrigerated water is a limited resource and complicates the economic analysis.

"Please confirm or advise on the dirt/fouling factor assumption for refrigerated soft water in our service."

You should be able to calculate fouling factor from the existing refrigerated water exchangers!

"My resources are Kern, Ludwig and Coulson for my calculation."

Try to find access to specialized exchanger design computer software. What did you use to generate the calculation summary?

"Any additional comments on materials, tube layout, or special requirements for a vent/safety condenser (e.g., two-phase handling, venting, drains, instrumentation) would be appreciated."

See my comments above. Many decisions depend on information not supplied with the forum question. Exchanger design is great fun for an engineer since there are so many different ways to get a good design.