Increasing the diameter of a steam line for a steam coil inside a batch still

[tmengineer]

Hello,

I'm designing a steam coil to be used in a batch process still - we are manufacturing the still and the steam coil and the P&ID is being done by others.

The steam inlet line is DN65 and in order to get a sufficient heating area* inside the still a double layered coil is going to be used. The steam line is DN65 then T's inside the still to two parallel coils with DN 100 - the concern is that increasing the cross sectional area of the steam line by approx. 5 times the steam inside will condense too quickly.

*The heating area is determined by the cycle times for the process, which cannot be altered as this will affect the product.

The steam is a 2.5barg saturated feed. The steam coil is SCH 10 304 SS. From an energy balance the steam will leave the coil as a slightly subcooled condensate (however the degree of sub-cooling is negligible). I know that with the increased cross sectional area the flow velocity will be significantly less, and so the heat transfer coefficient will decrease, but are there any other major concerns that I should worry about?

Any feedback would be greatly appreciated!

 

[StoneCold]

If it was my project I would be worried about pressure drop in the DN65 line for the load generated by the two DN 100 coils.
FYI we do batch distillation all the time. We use jacketed stills with agitators. Jacket styles vary, some dimple jackets, some half pipe coils.
None of them are actually in the still. If I wanted additional heat I think I would use a pump around loop with an exchanger.
Just my two cents.

Regards
StoneCold

 

[StoneCold]

On second though you might want to make the tie in for both coils outside of the vessel. Then if it turns out you need a bigger header that can be done with out modifying the vessel later.

 

[sshep]

Hello tmengineer,

I think your concern about the lower steam velocity impairing the heat transfer rate may be not be real. You are assuming that the heat transfer rate is limited due to film resistance on the steam side. Someone has estimated a heat transfer rate to calculate the area needed. The heat transfer rate calculated from wall thickness and conductivity, fouling factors, and film coefficients.

It is highly likely that the process side resistance will dictate the heat transfer, in which case you may be fine. Someone has done this work for you when they calculated the required area. All you need to do is find it.

good luck,
sshep

 

[georgeverghese]

Steam condensing heat transfer coeff is not affected by steam velocity as would be the case for single pahse sensible heating - take a look at heat transfer textbooks on this topic.

 

[tmengineer]

Hello,

Thanks for your replies!

Stonecold, we do make (dimple) jackets for some process vessels but unfortunately the still body has already been made so I'd much prefer to find a solution without putting a jacket on the side.

Closed loop heater systems with a re-circulation line and heat exchangers are sometimes used in the industry but again, I'd rather find an immersed coil / steam pan design that would be appropriate.

The tie in outside of the vessel may be an option - when you say that this would allow for a bigger header if required, how would it be possible to increase the pressure in the line if the tie in was accessible outside?

Sshep, I took the overall heat transfer coefficient from Towler and Sinnot (Chemical Engineering Design: Principles, practice and economics of plant design and operation) as they have quoted values for heat transfer coefficients for steam coils immersed in dilute aqueous solutions. They quote a range of 500 - 1000 W/(m^2.K), so I took an value of 750 W/(m^2.K) for the initial calculations. For a water heated coil transferring to dilute aqueous solutions the overall heat transfer coefficient range is quoted as 200 - 500 W/(m^2.K) - so my worry was that by increasing the flow area the steam would condensate faster which would give a lower heat transfer coefficient.

Georgeverghese, thanks for that! I guess that answers my question about the heat transfer coefficient.

I'm going to do some work on estimating the pressure drop caused by increasing the flow area. Currently the steam is fed at 2.5barg saturated - if there is a significant pressure drop then the coil temperature would decrease and so the temperature differential available would be less and so the coil area would have to increase.