3 replies to this topic

[rbel038]

Hi All,

 

I am evaluating a software package that will hopefully enable us to large design compressible steam flow systems alot faster than currently by spreadsheet.

 

One issue has arisen which i am not sure is the result of my knowledge gap or that of the softwares inherent design. It relates to steam flow in pipelines.

 

Consider a long steam pipeline in the order of 1-1.5km. The velocities in the pipeline are in the order of 40-45m/s and the insulation is perfect. As the steam flows along the pipe, pressure is reduced via frictional losses with the pipe wall and minor losses through flow direction changes. We can consider the pipeline adiabatic because there is no heat transfer to the outside environment through perfect insulation, we can also consider the pipeline isenthalpic expansion since the velocity component of enthalpy (relating to internal energy) is much lower.

 

Given the above, which is my current understanding of steam pipe thermodynamics , should a drop in pressure along this pipeline not involve superheat being generated? (in order to conserve enthalpy).

 

I appreciate all your thoughtful responses.

[Bobby Strain]

The heat loss and friction loss are important, too. Your software should address all aspects of friction flow and heat transfer. So, maybe you want to pose your question in a different manner.

 

Bobby

[rbel038]

Hi Bobby , Thanks for your reply. The heat loss is definitely important and something that must be included in the design , however my issue is with how the enthalpy is handled along the pipe.. It seems that for "adiabatic" model that enthalpy is being calculated from temperature and pressure along the pipe , when in my understanding it should be a constant enthalpy along the pipe which should give rise to some superheat at a lower pressure near the end of the pipe if we have started with saturated steam near the inlet.

 

The actual conditions in the line would be a combination of this 'drying' effect due to pressure drop and heat loss through insulation.

 

Im just trying to establish whether or not my understanding of adiabatic pipeflow is wrong.

[Bobby Strain]

In an adiabatic operation, given inlet temperature and pressure, and outlet pressure, then the temperature is calculated such that the enthalpy is the same as inlet conditions. This process is typical of pressure loss across a sudden restriction that is not a nozzle. So, if you have steam tables, you can compare the calculation in question with the enthalpy values in the steam tables. If you have a process simulator of some sort, you should be able to reproduce the results of the software. Probably Katmar or Steve Hall can give you more precise answers in terms of thermodynamics. If I were undertaking such a calculation, I would use increments to calculate pressure drop and heat loss, and that should be good enough for engineering work.

 

Bobby