3 replies to this topic

[lance21]

Hi! I've been working as a process engineer for 2 years now and currently involved in the thermal design of shell and tube heat exchangers. As I am relatively new to this field and no experience in operations, I would like to ask from experienced members of this forum to help enlighten me on this topic.

 

Generally, we size shell and tube heat exchangers for maximum flowrate (e.g. normal flowrate x 1.2 or as specified in the process datasheet). I am thinking that the shell and tube heat exchanger is always filled with liquid (assuming no phase change, just sensible heating/cooling) during this maximum flowrate operation. but how about during normal operation (without the multiplier) or turndown operation (like half or so of the normal flowrate). will the shell side still be fully filled with liquid just moving at a low velocity?

 

Thank you in advance!

 

Edit: I think I should've posted this in the process heat transfer section. Will post in the right section in the future.

Edited by lance21, 24 August 2013 - 11:11 PM.

[Raj Mehta]

Well according to me, STHE is usually designed for 10-20 % margin over your normal operating conditions and the line sizing is based on this design conditions. Now when you operate it under turndown conditions, your line size being fixed, so a change in your flowrate will directly lead to change in your velocity, which means under turndown case, your velocity will decrease and will ensure that the STHE is always fully filled on the shell side.

 

The moment you increase the flow, your velocity will again increase, which means the entering rate of fluid in STHE has increased, which has to match the exit rate or accumulation will take place and pressure in shell side will increase, so the velocity adjusts accordingly (increases accordingly, with the increase in flow), but the shell side will always be fully filled with liquid. 

 

Hope things are clear for you now.   

[lance21]

Thank you Sir Raj Mehta, you have made it very clear for me.

[curious_cat]

Your basic confusion arises from a mis-match between flow and inventory. 

 

You can have a trickle flow into a massive inventory too. Those variables are unrelated.