Page 9 - European Energy Innovation - Winter 2014
P. 9
Winter 2014 European Energy Innovation 9


Thermosyphon reboiler Kettle type reboiler

difference between the heat Alongside the vertical heat corresponding evaporation
source and the steam, which exchanger there is also a pressure of 0.556 bar. The vertical
would result in a much larger separation tank. In that tank a pipe length is 6 metres. This 6
required heat transfer surface so water level is maintained that is metres of extra water column
that the heat exchangers would replenished by a supply of fresh produces a pressure at the
become too expensive. Moreover, water, resulting in the separation bottom of the pipes of approx.
the process medium has a high of steam and water. The water 1.156 bar, with a corresponding
viscosity, which results in the wall level is kept above the top of the evaporation temperature of
heat transfer of this medium to be pipe plate of the steam generator, 113˚C! As a consequence, the
low. That is why it is important to and because the tank and the water does not boil in a large
find an evaporation temperature heat exchanger function as part of the pipes, but takes place
as close as possible to 90˚C, but communicating vessels the pipes only though a low heat transfer
with sufficient driving force to are filled with water. The water in coefficient. This adverse boiling
realise an efficient design in terms the pipes evaporates due to the suppression gives rise to a far
of size and price - the search for heat supplied by the hot medium, from optimal heat transfer process
optimisation. so that the weight of the water and eliminates this solution.
column in the pipes reduces,
Three solutions were studied causing it to rise. The mixture of 2: THE KETTLE TYPE REBOILER
to establish how steam can be steam and water flows into the The advantage of this solution is
generated in the most efficient separation tank, the water flows that no individual separation tank
way. downward and the steam flows is necessary; see drawing. The
out through a connection at the shell has a much larger diameter
1: THE THERMOSYPHON top of the tank. than the bundle (the diameter of
REBOILER this bundle is approximately 2
This is a vertical heat exchanger The big problem with this metres), creating an area of steam
with the viscous process principle in this situation is above the bundle with natural
medium on the outside of the “boiling point suppression”. For separation of steam and water.
pipes, and the water/steam example, take an evaporation The horizontal body includes a
mixture inside ; see drawing. temperature of 84˚C with a water level that is maintained at a
few centimetres above the pipe

Here, too, the consequence is
boiling point suppression. Due
to the level of liquid, the water
pressure in the bottom pipes of
the bundle is much higher than
the 0.556 bar, with the result
that the water here does not
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