Heat exchange – Flow passages for two confined fluids – Interdigitated plural first and plural second fluid passages
Reexamination Certificate
2003-02-20
2004-07-13
Bennett, Henry (Department: 3743)
Heat exchange
Flow passages for two confined fluids
Interdigitated plural first and plural second fluid passages
C165S165000, C165S164000
Reexamination Certificate
active
06761213
ABSTRACT:
The subject of the present invention is a reboiler/condenser heat exchanger of the bath type and a heat exchange process in a heat exchanger of the bath type.
More specifically, the invention relates to a reboiler/condenser heat exchanger of the bath type for heat exchange between a first fluid to be vaporized and a second fluid to be condensed, and to the use of this type of heat exchanger. The term “vaporization” is understood to mean partial or complete vaporization and the term “condensation” is understood to mean partial or complete condensation.
This arrangement is used especially, but not exclusively, in air distillation plants of the double-column type in which, for example, liquid oxygen at the bottom of the low-pressure column is vaporized in a bath reboiler by heat exchange with gaseous nitrogen taken from the top of the medium-pressure column.
The operation of bath heat exchangers, because of their intrinsic characteristics, imposes limitations as regards the height for exchange between the first and second fluids or as regards the temperature difference between the primary fluid and the secondary fluid.
This problem will be more clearly understood with reference to the appended
FIGS. 1 and 2
which show, on the one hand, an example of a functional diagram showing the operation of a bath heat exchanger and, on the other hand, an example of a functional diagram showing the heat exchange between the primary fluid and the secondary fluid.
FIG. 1
shows, in a simplified manner, the external vessel
10
of the bath heat exchanger, inside which vessel a number of passages
12
for the “warm” second fluid F
2
are contained, the said second fluid entering the vessel in the upper part of these passages at
14
and leaving it in the lower part at
16
. With regard to the “cold” first fluid F
1
to be vaporized, this is contained in the external vessel
10
and flows by thermosiphon effect from the lower end
12
a
of the passages for the second fluid F
2
to its upper end
12
b
, the height of this heat exchange region being equal to h.
As the diagram in
FIG. 2
shows more clearly, the first fluid F
1
at the inlet of the exchange region is at a temperature T
1-1
and at a pressure P
1-1
. This temperature T
1-1
and this pressure P
1-1
correspond to a subcooling state, that is to say correspond to a temperature below the bubble temperature T
b1
of the fluid F
1
at the pressure P
1-1
because of the hydrostatic pressure due to the head of liquid fluid F
1
. This will be shown in the above diagram in which T
b
denotes the temperature (the bubble temperature) at which the first gas bubble appears in the fluid F
1
during the heat exchange (at an intermediate pressure between P
1-1
and P
1-2
). It will be understood that the energy used to bring the primary fluid to the bubble temperature T
b
is “lost” energy, in order to vaporize the first fluid. Also shown in this
FIG. 2
is the second fluid F
2
with its entry temperature T
2-1
at which it enters the exchange region
12
and it's exit temperature T
2-2
. It can be seen that the subcooling phenomenon results in a “pinching effect” in the heat exchanges between the two fluids.
Furthermore, the thermosiphon effect, which allows the first fluid F
1
to flow, is made possible by the formation of bubbles in the first fluid. If the head in the heat exchanger corresponding to the “desubcooling” phase is too great, the thermosiphon effect will be insufficient.
It will be understood that the greater the height h of the heat exchange region the greater the hydrostatic pressure on the first fluid at the inlet of the exchange region and therefore the greater the subcooling region must be too. To sustain the thermosiphon effect which ensures flow of the first fluid, the “pinching” phenomenon must therefore be limited. In heat exchange plants of the bath type, this height is therefore limited to 2.5 metres.
Another drawback present in this type of bath heat exchanger is that the “pinching phenomenon” described above requires there to be a temperature difference between the entry temperature T
1-1
of the cold fluid F
1
to be vaporized and the temperature T
2-2
of the warm fluid F
2
of more than about 1.2° C. in order to allow the heat exchanger to operate by thermosiphon effect because of the “pinching effect”. However, it will be understood that increasing this temperature difference increases the thermodynamic irreversibilities and, consequently, reduces the energy efficiency of the entire plant. For example, in the case of the distillation of the gases contained in the air using a double column, the pressure in the column called the medium-pressure column and, consequently, the pressure in the feed air compressor, must be increased, thereby increasing the energy consumption of the entire plant.
There is therefore a real need for reboiling/condenser heat exchangers of the bath type or for heat exchange processes in a plant of the bath type which make it possible either to increase the vertical heat exchange height, in order to reduce the floor space of the plant, or to reduce the temperature difference between the first fluid and the second fluid, or else to allow a combination of these two characteristics of the reboiler/condenser heat exchanger.
According to the invention, to achieve this objective the reboiler/condenser heat exchanger of the bath type, for heat exchange between a first fluid (F
1
) to be vaporized and a second fluid (F
2
) to be condensed, the said heat exchanger, having a minimum exit pressure P
m,ex
of the said first fluid in order to allow the plant in which the said heat exchanger is mounted to operate, comprises:
means for defining a number of passages for heat exchange between the two fluids in order to make the said second fluid flow, the said second fluid having a temperature T
2-2
at the outlet of the said passages;
vessel-forming means containing the passage-forming means for making the said first fluid flow by thermosiphon effect between the said passages from the bottom upwards over a height h, the said first fluid having an entry temperature T
1-1
where T
1-1
<T
2-2
and the said vaporized first fluid having an exit pressure P
1-2
;
means for giving the entry pressure P
1-1
of the said first fluid a value such that the pressure P
1-2
is greater than the said minimum pressure P
m,ex
and means for ensuring that at least one of the two following conditions is fulfilled:
the height h of the heat exchange passages is at least equal to 2.5 m; and
the temperature T
2-2
of the said second fluid is less than T
1-1
+1.2° C.
It has in fact been demonstrated that if the exit pressure of the first fluid is,increased, the pinching effect is modified, thereby allowing either the heat exchange height h to be increased or the temperature difference between the two fluids to be decreased.
According to another aspect of the invention, the process for vaporizing a first fluid (F
1
) using a reboiler/condenser bath heat exchanger comprises the following steps:
a second fluid (F
2
) is made to flow through vertical exchange passages, the said second fluid having an exit temperature T
2-2
;
the said first fluid is made to flow from the bottom up over a height h by thermosiphon effect between the said heat exchange passages, the said first fluid having an entry temperature T
1-1
(where T
1-1
<T
2-2
) and the vaporized fraction of the said first fluid having an exit pressure P
1-2
;
the said pressure P
1-2
is given a value greater than the minimum exit pressure of the vaporized fraction of-the first fluid needed to allow the plant in which the said heat exchanger is mounted to operate; and
the height h of the heat exchange passages and the temperature T
2-2
of the said second fluid are chosen in such a way that at least one of the two following conditions is fulfilled:
the height h of the said heat exchange passages is at least equal to 2.5 m; and
the temperature T
2-2
of the said second fluid is less than T
1-1
+1.2° C.
It will be understood that this process makes it possible to im
Bennett Henry
L'Air Liquide-Societe Anonyme a Directoire et Conseil de Su
Patel Nihir
Young & Thompson
LandOfFree
Reboiler/condenser heat exchanger of the bath type does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Reboiler/condenser heat exchanger of the bath type, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Reboiler/condenser heat exchanger of the bath type will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3190186