Superheated Steam in Process Heat Exchanger

[learner08]

Hi!

Need some help. I've got a heat exchanger using superheated steam (10 barG at 210 deg C) to transfer heat (I know its supposed to use saturated steam but thats the way it is sadly and not able to desuperheat for the moment!!).

My problem is the system isn't functioning. I have confirmed by components to be working fine. There is a control valve before the h/ex and a steam trap after the h/ex. There is a bypass valve before the trap and a by-pass valve after the trap. The by-pass valve after the trap is connected to the condensate return line.

The whole system used to be working (might not be fine but still working!!!) but suddenly I have a problem (not enough heat transfer). Might be due to fouling but that is not my question.

My question is while I sure my steam trap is working fine and it has enough capacity (float type and about 2 times oversized), I see an improvement when I open up the by-pass line before the trap. I cannot explain this phenomena and hope someone can help.

When I open the by-pass after the trap, I can see intermittent gushes of condensate. I do not see any live steam escaping.

 

[katmar]

The behaviour of the bypass valve after the trap indicates that the trap is working properly.

I find it surprising that heat transfer improves when opening the bypass before the trap because opening this valve will lower the pressure in the exchanger, and this would lower the condensation temperature of the steam. In other words it makes your steam even more superheated.

The only reason I can see for for an improvement is that the velocity of the steam through the exchanger has increased and this would improve the gas phase heat transfer coefficient. But even this improved HTC would be lower than the HTC for condensation and you obviously had some condensation before because there was condensate after the trap. As you have about 25 deg C of superheat it may be that most of your exchanger is being used for sensible heat transfer from the gaseous phase and only a small part for condensation. It is important to remember that even though you may be getting more heat transferred when you open this valve, it is a very inefficient process because you are rejecting a lot of heat to atmosphere in the live steam passing through this valve.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[learner08]

Hi Katmar,

I certainly believe your statement that most of the heat transfer occurs in the sensible heat txf region as opposed to the latent heat region in my case as previously when we had a leaking trap in the exact system, the heat transfer was in fact better (as compared to a good trap and the by-pass before the trap closed. subsequently we replaced the trap to the one used currently but now we are facing slow hear transfer process)!! we still are unable to easily install a desuperheater. I do not think unlagging the piping would help desuperheat the steam much as the lagged portion is short.


However, I still cannot comprehend why there appears to be more heat transfer when we are working in the superheat region (as compared to the latent heat region as you have stated!!!).

Thanks for your reply!!!

 

[TBP]

In a system that's using saturated steam, you'll seem some superheat downstream of a control valve as a result of the pressure drop. This is normally of no consequence. However, if you're feeding superheated steam to a control valve, you'll have LOADS of superheat, resulting in the problems you're describing - the HX will act like it's air-bound. Speaking of air, your float trap will have a thermostatic element, intended to get the air out of the steam space on start-up. Is the thermostatic bit of your trap rated for the temps it's seeing? It could be that it's worked for a while in the high temps, and then failed. Your HX could be full of air that can't get out until you open the manual valve.

The bottom line is, if you need this HX to work, you've GOT to figure out how to feed it saturated steam.

 

[25362]

Check for reasons of condensate flooding.

 

[learner08]

Hi TBP,

The float trap I am using has a bimetal for the release. It was rightly selected for the superheated steam I am using. I understand the ramnification of thermo element in a superheated environment and have avoided using the thermo type as such.

Still no 'less painful' way of putting in desuperheater!!!

Thanks for the feedback!!!

 

[CJKruger]

learner08,

- It is not unusual to have superheated steam going into a heat exchanger.

- The exchanger must be designed for it though - a desuperheating zone (with high LMTD) and condensing zone (with lower LMTD).

- The most common process problem with the system you describe, is that the steam/condensate pressure in the exchanger is lower than the condensate pressure after the steam trap.

- To ensure that this does not happen, find the saturated steam pressure at the PROCESS side inlet temperature (steam tables). This should be higher than your condensate system pressure.

 

[rmw]

Unfortunately, unless HX's are specifically designed to have a DSH zone with high velocities good heat transfer they won't handle superheated steam well in amounts more than the small quantities caused by isenthalpic pressure drop through piping, valves and fittings.

Conversely, you don't want those high velocities required for desuperheating in the condensing zones as the moisture present will cut the thing to ribbons.

rmw

 

[learner08]

Hi CJKruger,

I am sure there isn't a back pressure problem as the condensate goes back to an open tank with minimal lift (the control valve is fully open at the time). Also the same phenomena occurs even when I open the by-pass valve immediately after the trap (which goes to atmosphere).

Thanks for your help.

 

[25362]

Visit:

http://www.armstrong-intl.com/common/allproductscatalog/consguidelines.pdf

in particular pages 29, 49 and 51.

 

[CJKruger]

learner08, it is often not the back pressure that is the problem, but the exchanger pressure.

If your process inlet is less than 100 C and your exchanger is oversized, you will have vacuum in your exchanger and condensate cannot get out even if the condensate back pressure is 0 guage.

 

[MintJulep]

I'm with CJKruger,

As your superheated steam condenses you could be developing a vacuum in portions of the exchanger.

This fits with the gushing condensate that you observe when opening the bypass valve.

 

[TBP]

On the installations I'm familiar with, a vacuum breaker is installed to allow condensate to gravity drain to a vented condensate receiver during low load conditions.

I seriously doubt that having the superheated steam condensing and pulling a vacuum is the problem. If it condensed easily, then the HX would likely work satisfactorily.

 

[learner08]

Hi CJKruger and All,

There is no way I can have vacuum currently as the control valve is in manual operations and about 100% open meaning 10barG all the time to push against any back pressure!

 

[rmw]

Do a search on this site for the relevant words. This topic has been well discussed in this forum and some of the chemical engineering fora as well as I remember.

IMHO, you are 'air' bound by the superheated steam which has already been said. And when you open the bypass you get some small amount of convective heat transfer because you increase the velocity of the SH steam though the bundle as you do.

rmw