heat transfer differences between superheated steam and saturated 1

[corvus]

Im trying to calculate the difference in heat transfer between a pipe carrying saturated steam and a pipe carrying superheated steam. I know there is a difference in heat transfer due to the properties and because saturated steam has a wetted perimeter, but im at a loss as to the particular calculation. Could anyone point me in the right direction?

 

[Montemayor]

The subject of transferring steam's sensible superheat has been discussed in many prior threads on this Forum. Superheat is not generated for heat transfer through walls because it is so inefficient and can't be justified economically. No engineer that I have worked with or discussed this same subject over the last 45 years would try to do it on a practical basis. Superheated steam is generated for purposes other than conventional heat transfer.

Perhaps you can share your reasons for trying to quantify the amount of inefficiency involved. Is this an academic pondering? Or is it a serious industrial project need?

Kern discusses this in page 281 of his "Process Heat Transfer" and cites one of the obvious reasons for not giving superheat any importance in heat transfer: the desuperheat in atmospheric pressure steam with 100 of of superheat represents only 5% of the total heat load. I've never known anyone to take credit for the superheat transfer in steam heaters/condensers. Maybe you have a better system or method.

 

[corvus]

The purpose of the question is to determine the heat loss in an insulated pipe for a distribution network. The system is superheated up to the building walls, then desuperheated and then sent through a heat exchanger for process work. In doing the distribution analysis, i have only a saturated steam program STEAM2000, but i can do superheat calculations if i have an acurate idea of the heat loss from the pipe, acutally wrote a database to do just that, but the heat loss calcs are based on saturated. Im trying to determine the amount of error im introducing into the calculations by not adjusting for the heat transfer properties of superheated steam verses saturated.

 

[25362]

As far as I'm given to understand, spreadsheet models are generally based on the assumption that the fluid and the thermally insulated pipe wall temperatures are equal whether one deals with superheated or saturated steam.

The assumption doesn't introduce major errors, not greater, anyway, than those in estimating the convective and radiative HTC from the outer surface, and at the same time simplifies the analysis.

 

[corvus]

And this was my initial assumption, however, my client has a doctorate and insists that I take into account the differences between saturated and superheated steam. I'll admit im curious to see the differences myself, but otherwise I would be more than happy to make that assumption and move on to the actual flow analysis!

 

[25362]

I wonder... If the "conservative" assumption of taking the pipe wall temperature equal that of superheated steam renders a result of a temperature drop of, say, 0.5^oF per 100 ft of pipe length, what purpose would have producing more sophisticated estimations ?

 

[rmw]

First, would it matter that most desuperheaters do not desuperheat right down to saturation? So you will always have 5-10-20° of superheat in your process steam (assuming your post-desuperheater runs are not so long so as to have enough temperature loss to get you down into saturation) depending upon your method of desuperheating.

Second, if you did your heat loss calculations based on a saturated condition, which infers the benefit of latent heat transfer on the steam side (and produces moisture I am sure you don't want in the piping) then you would be safer for the superheated portion of the piping, and would have to only deal with the driving force of the delta T of the superheated steam.

Sounds like the best of both worlds to me, unless, of course, there is a real possibility of getting into saturation.

rmw

 

[sailoday28]

For the region of superheat, I would do the following:

Treat the superheated flow as a non condensible gas. From Reynolds number, etc Determine a heat transf coef and the resulting heat flux thru that region of pipe metal and insulation.

With the known heat flux and inside film coef determine the pipe wall inside temp. If the temp is above saturation, the original calc is fine. If temp of inside wall is sat or less, then condensation will occur and heat trans coef should be that of wet steam.