Superheated steam causing a drop in exchanger heat duty?

[mjpetrag]

I have a shell and tube exchanger with product on the tubes and 50# steam on the shell. For some reason, the outlet temp of the product side fell off in a matter of a few hours and the steam valve opened wide. The condensate out has a trap on it that is working properly.

I measured the skin temp of the incoming steam and it had approximately 40 degrees F of superheat. I'm wondering if the superheat is actually hurting heat transfer instead of helping. My theory is since the steam flow is maxed out, the heat transfer is getting reduced since we have to desuperheat the incoming steam. The low sensible heat transfer is taking up room that could be used if the incoming steam was saturated. We have to wait for the steam to condense in the trap in order to get a higher mass flow. Unfortunately the steam flow meter is broken so I can't tell for sure. The outlet condensate is about 20 degrees subcooled.

I wanted to know if I am correct in my thinking or if this is a useless exercise. I can't imagine that the exchanger fouled up in a few hours. Seems off to me but stranger things have happened.

-Mike

 

[Compositepro]

Do you have a vent on the exchanger to remove non-condensibles, and is it working?

 

[mjpetrag]

I'll have to check in the morning. There is a vent on the trap but like you mentioned I'll try bleeding off the shell side and see if it improves heat transfer. I think there's a 3/4" valve on the top of the exchanger I can open up. Would non-condensibles building up affect heat transfer that quickly though?

-Mike

 

[rmw]

I suggest doing a thorough search on this forum and this site. This topic has been hacked to pieces over the years and there is a lot of good (and some bad) information for you to find.

I'd really rather not having to re-type it all. Do the search and then see if you have any specific questions after that.

The response about the non-condensables is a good one. I never trusted a trap way down below the condensate to get the non-condensables out of the shell way above it.

Your theory is right and established science. Also check your process side (fluid being heated). If the flow rate has fallen off, that can exhibit the same symptoms.

I once put a steam heated water heater Hx in a tire manufacturing plant that worked gang busters at the start of each shift when every operator on the production floor was filling his machine with water, but sucked when all the machines were full and there was no flow on the tubeside.

rmw

 

[katmar]

Something obviously changed during the short time between start up and the problem manifesting itself. Did your steam go from saturated to superheated during this time? Although superheat is bad for the heat transfer coefficient, if the steam condition did not change during this time the superheat is not the real problem.

Compositepro's suggestion of incondensibles building up would fit with the deterioration of performance with time. It will be easy to check by thoroughly venting the unit and seeing if the performance is improved.

If the steam valve is full open and the condensate is 20 degrees subcooled that could indicate a partial blockage in the steam trap or its strainer, and condensate building up and flooding the exchanger. This can easily happen at start up as debris in the piping is flushed through.

Katmar Software - Uconeer 3.0

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[mjpetrag]

bled off the condenser and heat duty did not change. We back flushed it with condensate and the steam came back. Looks like a slug of junk hit us and caked up the tubesheet.

Thank you for the replies

-Mike