Super heated steam to blame?

[bmemmott]

We have a HX at our plant that has both 600# and 160# steam attached. We have been running on 600# steam and I am currently looking at the economic implications of switching to 160# steam. My problem is that when looking at historical data, the required flowrate for 160# steam is 33% higher than when 600# steam is used. I am not sure what is going on. I would think that as 160# steam has a higher heat of vaporization, less steam would be required. I just wondered if anyone had some ideas on why this is the case.
Thanks

 

[TBP]

Superheat is sensible heat. It is necessary to get past this relatively small amount of sensible heat, to get to the latent heat, which is where all of the "action" is, in steam HXs.

Until the superheat is gone, the HX will behave as if it is airbound.

 

[rmw]

As it is well explained by joesteam. It is a phenomenom that is able to be observed in first effect sugar evaporators that take their heating steam from turbine exhaust that contains superheat by turning the DSH upstream of the evaporator on and off. These are vessels with tubesheets that can be up to 20 ft in diameter. A clear change can be seen in the vigor of the boiling where the superheat is present at the inlet side of the calendria and where it finally changes to latent heat transfer toward the rear of the vessel. Vigerous boiling at the inlet side of the evaporator is virtually non existant. Once the DSH is started, that line separating the vigor of the boiling disappears and the entire upper tubesheet (vertical tubes, rising film boiling) becomes engulfed in vigerous boiling.

It was a clear and unambiguous visual observation of what that I had known theorhetically for years. I'll never believe otherwise.

rmw

 

[Latexman]

When the DSH is off, most of the tube is above the saturation temperature; and when it's on, most is not.

Good luck,
Latexman

 

[rmw]

If that is true (I'll have to keep thinking about that one based on the liquid temperatures on the shell side) then it makes the point. The heat transfer rate in the superheated steam affected zone is so slow that the outer tube wall temperature is allowed to be heated above the saturation temperature by the SH in the steam (sugar juice is the same temp on the other side) while in the zone where the steam is saturated, the heat transfer rate is sufficient to keep the temperature profile across the tube low enough to keep it below saturation.

Did I mention that once the DSH is started the overall performance of the heater took off like a rocket?

Good Point Latexman. I think it makes my case.

rmw

 

[Latexman]

Yep, it makes my case too.

Good luck,
Latexman