Plate Heat Exchanger with Exchanging Structure Forming Several Channels in a Passage

Abstract

A heat exchanger with brazed plates including a stack of parallel plates defining a plurality of generally flat fluid circulating passages, closure bars which delimit the passages and distributing means for distributing a fluid to each passage of a first series of passages and means for conveying another fluid to a second series of passages where at least one passage contains organized exchanging structures which form a plurality of channels in the width of the passage and also at least three channels in the height of the passage is provided. The invention is useful for air separation by cryogenic distillation.

Description

BACKGROUND

1. Field of the Invention

The present invention relates to a plate and fin heat exchanger.

2. Related Art

There are various types of plate and fin heat exchanger, each suited to a particular field of use. In particular, the invention applies advantageously to a heat exchanger of a unit for separating air or H₂/CO (hydrogen/carbon) mixtures using cryogenic distillation.

This exchanger may be a main exchange line of an air separation apparatus which cools the incoming air by indirect exchange of heat with the cold products originating from the distillation column, a supercooler or a vaporizer/condenser.

The technology often used in these exchangers is that of brazed plate and fin aluminum exchangers, making it possible to obtain very compact components with a large heat-exchange area.

These exchangers are made up of plates between which corrugated sheets or fins are inserted, thus forming a stack of passages known as “cold” passages and passages known as “hot” passages.

The heat-exchange fins commonly used are straight fins, perforated fins and serrated fins.

These corrugated fins are characterized using the following parameters:

• h(mm): height of the corrugated fin (from 3 to 10 mm)
• e(mm): thickness of the corrugated fin (from 0.2 to 0.6 mm)
• n(m⁻¹ or inch⁻¹): number of corrugated fins per unit length (from 177 to 1102 corrugations/m)
• perf(%): level of perforation (5% for perforated fins)
• l_s(mm): serration length (in the case of serrated fins)

Thus, the hydraulic diameters (Dh) of the fins conventionally used in brazed plate and fin heat exchangers range between 1 and 6 mm. These corrugated heat-exchange fins are currently formed using a press.

There are various ways of increasing the heat-exchange area.

The heat-exchange area which separates two fluids is made up of an area known as the “primary area” which corresponds to the flat area between the two fluids and of an area known as the “secondary area” which generally consists of fins perpendicular to the primary area and thus forming a corrugated heat-exchange fin. It is the number of fins inserted (the fin density) and the height of the fins which increase the heat-exchange area.

The denser the set of fins, the larger the heat-exchange area. However, there is a manufacturing limit and there are constraints associated with the method. The press tool used to manufacture the corrugated set of fins is able to obtain the maximum densities of 1023 to 1102 corrugations per meter. The selected fin density may be lower when it is preferable to limit pressure drops. In addition, under certain operation conditions such as in bath-type vaporizer/condensers, constraints associated with safety limit the number of corrugations per meter to values well below the maximum values that can be achieved in the manufacture.

The fins have a temperature gradient. Beyond a certain fin height, the region in the middle of the fin does not exchange heat anywhere near as well. There is therefore an optimum fin height corresponding to an optimum fin coefficient. The fin heights commonly used vary from 3 to 10 mm.

It is also possible to increase the heat-exchange coefficient.

The more turbulent the fluid, the better the heat-exchange coefficient. This turbulence can be generated by altering the shape of the channels or by inserting turbulence-generating obstacles (e.g. perforated straight fins, serrated fins, herringbone fins, louvered fins, or by inserting mini fins, apertures, etc.).

When a fluid is being vaporized, a surface which has a higher number of nucleation sites exhibits a better heat-exchange coefficient. These nucleation sites are micro-cavities of various sizes and shapes (re-entrant cavities) present at the surface or through a porous layer.

When a fluid is being condensed, the thickness of the liquid film has an adverse effect on the heat-exchange coefficient. It is therefore advantageous to drain the liquid away using grooves, perforations or reliefs.

A type of heat exchanger known as a micro-scale heat exchanger has recently appeared.

This is an exchanger which has channels with hydraulic diameters smaller than one millimeter. Reducing the size of the channels makes it possible to expand the heat-exchange area (making the apparatus more compact). The heat-exchange coefficient then becomes practically inversely proportional to the hydraulic diameter.

S. Kandlikar in “First International Conference on Microchannels and Minichannels 2003, <<Extending the applicability of the flow boiling correlation to low Reynolds number flows in microchannels>>” proposes the following classification, based on the hydraulic diameter of the channels:

• • mini-channels such that: 1 mm<Dh<3 mm (corresponding to the Dh values of current corrugated fin sets)
  • mini-channels such that: 200 μm<Dh<1 mm
  • micro-channels such that: Dh<200 μm

For mini-channels (200 μm<Dh<3 mm): the laws of fluid dynamics for conventional pipes still apply.

For micro-channels (Dh<200 μm): the surface effects take on a considerable importance and the conventional laws of fluid dynamics no longer apply.

EP-A-1008826 describes a plate-type heat exchanger in which at least one of the passages contains tube-shaped closed auxiliary passages the maximum width of which is greater than 50% of the distance between two adjacent plates.

The amount of flux exchanged across an exchanger is given by the following equation:

φ=k×S×ΔT

For a given ΔT, exchanges can be improved only by increasing the heat-exchange coefficient (k) and/or by increasing the heat-exchange area (S).

In the case of brazed plate and fin heat exchangers, increasing the heat-exchange area using a so-called “secondary” area reaches its limits because of manufacture and/or constraints involved in the method. Increasing the heat-exchange coefficient by creating turbulence is advantageous, but has two main pitfalls:

• • increasing the turbulence increases the pressure drops;
  • it increases the cost of manufacture because of the complexity of the geometry involved.

Thus, creating a new shape of corrugated fin set cannot increase the heat-exchange coefficient markedly beyond the levels achieved in existing fin sets. As to creating nucleation and liquid drainage sites, these two methods relate only to a particular type of heat-exchange, mainly vaporization or condensation.

It would therefore appear to be difficult to make a substantial improvement to brazed plate and fin heat exchangers by pursuing development along the same lines as described hereinabove.

Furthermore, technology of the micro-channel type is very expensive (micro-machining of the channels) and is currently reserved for very small-size heat exchangers: it does not at the present time apply to applications such as the separation of air in which the throughput and the temperature difference are high.

SUMMARY OF THE INVENTION

The proposed solution aims to increase the heat-exchange area by incorporating a third heat-exchange area known as the “tertiary area” into the already existing (“primary” and “secondary”) areas.

We are proposing three devices which make it possible to add a “tertiary” area to the corrugated heat-exchange fin sets currently used in brazed plate and fin heat exchangers:

• • a “multiple corrugated fin set” exchange passage;
  • “mini-channel” heat-exchange fin sets, extruded fin sets;
  • “mini-channel” heat-exchange fin sets, capillary tubes.

One subject of the invention relates to a brazed-plate heat exchanger, of the type comprising a stack of parallel plates which define a plurality of fluid-circulation passages of flat overall shape, closure bars which delimit these passages and distributing means for distributing a fluid to each passage of a first series of passages and means for sending another fluid to a second series of passages, in which exchanger at least one passage contains at least one organized exchange structure which forms a plurality of channels in the width of the passage, each channel being in contact with either at least two other channels or at least one other channel and one plate, the exchanger being characterized in that the structure also forms at least three channels, and preferably at least five channels, in the height of the passage.

As a preference, each channel is in contact with at least three other channels or one plate and two other channels. The plate may be a plate defining a passage or a secondary plate located in the passage.

According to other optional aspects:

• • the structure is made up of a plurality of cylinders;
  • inside a passage, there is at least a secondary plate of flat overall shape parallel to the plates that define the passages;
  • the structure is formed of a superposition of corrugated heat-exchange fin sets, each pair of adjacent corrugated heat-exchange fin sets possibly being separated by a secondary plate;
  • the structure is formed of a single body containing a plurality of channels;
  • a channel has a hydraulic diameter of between 1 and 6 mm;
  • a channel has a hydraulic diameter of between 200 μm and 1 mm;
  • a channel has a hydraulic diameter of less than 200 μm;
  • a passage has a height of between 3 and 18 mm;
  • the channels have a circular, oval, square, rectangular, triangular or diamond-shaped cross section.

Another subject of the invention is a cryogenic separation apparatus comprising at least one exchanger as defined hereinabove.

Another subject of the invention is an air separation apparatus in which a main heat-exchange line and/or a vaporizer-condenser and/or a supercooler is a heat-exchanger as described hereinabove.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in greater detail with reference to the drawings in which:

FIG. 1 illustrates a generic channel with an illustration of the relevant variables.

FIG. 2 of the attached drawings depicts, in perspective with partial cutaway, one example of such a heat exchanger, of conventional structure, to which the invention applies.

FIGS. 3A, 4A and 5A depict a heat-exchanger passage viewed in the direction of flow of the fluids according to the prior art and FIGS. 3B, 4B, 4C and 5B depict a heat-exchanger passage viewed in the direction of flow of fluids according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 2, the heat exchanger 1 depicted consists of a stack of parallel rectangular plates 2, all identical, which between them define a plurality of passages for the fluids to be placed in an indirect heat-exchange relationship. In the example depicted, these passages are, in succession and cyclically, passages 3 for a first fluid, 4 for a second fluid and 5 for a third fluid. It will be understood that the invention covers heat exchangers involving just two fluids or heat exchangers involving any number of fluids.

Each passage 3 to 5 is flanked by closure bars 6 which delimit it, leaving inlet/outlet apertures 7 open for the corresponding fluid. In each passage, there are spacer corrugations or corrugated fins 8 which act as thermal fins, as spacers between the plates, particularly at the time of brazing, and as a way to prevent any deformation of the plates when using fluids under pressure, and as guides to guide the flow of the fluids.

The stack of plates, closure bars and spacer corrugations is generally made of aluminum or aluminum alloy and is assembled in a single operation by furnace brazing.

Fluid inlet/outlet boxes 9 of semicylindrical overall shape are then welded to the exchanger body thus produced to fit over the corresponding rows of inlet/outlet apertures and are connected to pipes 10 supplying and removing the fluids.

The channels can be formed using various techniques such as those described in “Micro échangeurs thermiques” by Anton GRUSS in “Techniques de l'Ingénieur, 06-2002”.

The solution in FIG. 3B is to replace the conventional corrugated heat-exchange fin set used in FIG. 3A with several corrugated heat-exchange fin sets 13 of the same type, but with a shorter fin height. These new sets inserted in one and the same passage of the heat exchanger are assembled using thin sheets covered in braze metal 13. These sheets, termed “tertiary area sheets”, constitute the so-called “tertiary” added area. In the example, there are two sheets separating three fin sets.

All types of corrugated fin set that are commercially available can be used, merely by modifying and adapting the fin height. As a result, all the parameters that make up the geometry of a type of corrugated fin set can be adjusted (the thickness, density, perforation of the fin, etc.). The other parameters are:

• • the passage height,
  • the number of exchanger fins per passage,
  • the thickness of the tertiary area sheet (theoretically equal to the thickness of the corrugated fin set),
  • the shape of the tertiary area sheet: solid or with carefully positioned perforations.

For this “multiple corrugated fin set” technology, the hydraulic diameters are of the same order of magnitude as the width of the channel in a conventional corrugated fin (1/n−e).

The increases in heat-exchange area for various fin heights and by comparison with a conventional fin set of equivalent density n are given below:

Conventional configuration
h				h		h
passage	e fin	n	w	channel	h fin	channel	increase
(mm)	(mm)	(m−1)	(mm)	(mm)	(mm)	(mm)	in area
	n* = 2
5.1	0.2	551.18	1.61	4.9	2.45	2.25	19%
5.1	0.3	393.7	2.26	4.8	2.45	2.15	25%
	n* = 3
7.13	0.2	944.88	0.86	6.93	2.24	2.04	12%
7.13	0.2	629.92	1.39	6.93	2.24	2.04	24%
	n* = 4
9.63	0.2	944.88	0.86	9.43	2.26	2.06	13%
9.63	0.2	629.92	1.39	9.43	2.26	2.06	27%
n* = number of fins over the height of a passage (with tertiary area sheet thicknesses of 0.2 mm)
w = channel width
h channel = channel height

We are restricted here to channel heights (h channel) of a minimum of 2 mm (for brazing reasons).

For the same volume, increasing the number of fins stacked up in the heat exchanger increases the cost of manufacture thereof. However, the installation cost remains the same.

The solution in FIG. 4B is to replace the conventional corrugated heat-exchange fin set used in FIG. 4A with a structured corrugated fin set 17 comprising numerous mini-channels 19 of square cross section. This corrugated fin set can be produced by extrusion.

The extrusion manufacturing method means that it is possible to conceive of any channel cross section (rectangular, triangular, round, diamond-shaped, etc.). FIG. 4C shows channels of triangular cross section.

The main parameters are the height of the passage, the number of channels per passage height, the number of channels per meter width of passage and all the parameters involved in the geometric shape of the channels used (channel height, width, diameter, etc.).

This method of manufacture also allows the possibility of inserting micro-fins or mini-fins inside the channels in order further to increase the heat-exchange area and/or to drain away a liquid.

The length of the channels (fluid exchange length) can be divided into several extruded corrugated fin set modules spaced a few millimeters apart so as to allow inter-channel communication.

There are three categories of channel geometry differentiated in terms of the hydraulic diameter (Dh) of the channels:

• • channels such that Dh is of the same order of magnitude as the width of the channels in conventional corrugated fin sets (w=1/n−e);
  • channels such that Dh ranges between 200 microns and 1 mm (mini-channels);
  • channels such that Dh is less than 200 microns (micro-channels).

The increases in heat-exchange area obtained for the three categories mentioned hereinabove are as follows:

For channels such that Dh is of the same order of magnitude as the width of the channels in conventional corrugated fin sets (w=1/n−e), we are here quoting the increases in heat-exchange area (se) for various fin heights and with respect to a conventional fin set of the same height and equivalent density n.

Conventional corrugated fin set
h = 5.1 mm	Extruded structure
n	w	se	h channel
(m−1)	(mm)	(m2/m2)	(mm)	increase
551.18	1.61	7.18	2.25	19%
393.7	2.26	5.51	2.25	33%
Conventional corrugated fin set
h = 7.13 mm	Extruded structure
n	w	se	h channel
(m−1)	(mm)	(m2/m2)	(mm)	increase
944.88	0.86	14.73	0.96	40%
629.92	1.39	10.48	1.53	40%
Conventional corrugated fin set
h = 9.63 mm	Extruded structure
n	w	se	h channel
(m−1)	(mm)	(m2/m2)	(mm)	increase
944.88	0.86	19.44	0.97	43%
629.92	1.39	13.63	1.69	42%

For the channels such that Dh ranges between 200 microns and 1 mm (mini-channels), we are here quoting the increases in heat-exchange area (se) for various fin heights and with respect to a conventional corrugated fin set of the same height and with a high density n.

	Conventional corrugated fin set
	h = 5.1 mm		Extruded structure
	n	se	h channel
	(m−1)	(m2/m2)	(mm)	increase
	1 102.36	12.36	0.2	161%
	Conventional corrugated fin set
	h = 7.13 mm		Extruded structure
	n	se	h channel
	(m−1)	(m2/m2)	(mm)	increase
	1 102.36	16.84	0.2	171%
	Conventional corrugated fin set
	h = 9.63 mm		Extruded structure
	n	se	h channel
	(m−1)	(m2/m2)	(mm)	increase
	1 123.62	20.86	0.2	197%

For the channels such that Dh is less than 200 microns (micro-channels), we are here quoting the increases in heat-exchange area (se) for various fin heights and with respect to a conventional corrugated fin set of the same height and with a high density n.

	Conventional corrugated fin set
	h = 5.1 mm		Extruded structure
	n	se	h channel
	(m−1)	(m2/m2)	(mm)	increase
	1 102.36	12.36	0.05	717%
	Conventional corrugated fin set
	h = 7.13 mm		Extruded Structure
	n	se	h channel
	(m−1)	(m2/m2)	(mm)	increase
	1 102.36	16.84	0.05	741%
	Conventional corrugated fin set
	h = 9.63 mm		Extruded structure
	n	se	h channel
	(m−1)	(m2/m2)	(mm)	increase
	1 123.62	20.86	0.05	818%

The solution in FIG. 5B is to replace the conventional corrugated heat-exchange fin set used in FIG. 5A with a suitable number of capillary tubes. The capillary tubes can be easily arranged in an ordered fashion because of their shape. The capillary tubes are covered with braze metal to mechanically assemble the whole.

The adjustable parameters are the height of the passage, the diameter of the capillary tubes, the thickness of the capillary tubes or the number of capillary tubes per m².

We are here quoting the increases in heat-exchange area (se) for various fin heights and with respect to a conventional corrugated fin set of equivalent density. D_ext is the external diameter of the capillary tube.

	Solution: 7 capillary tubes per
Conventional corrugated fin set	passage height
h passage	n	Dext	increase
(mm)	(m−1)	(mm)	in se
5.1	551.18	1.4	50%
5.1	393.7	1.4	96%
7.13	944.88	1.2	23%
7.13	629.72	1.2	73%
9.63	944.88	1.4	10%
9.63	629.72	1.4	57%

In each example, the diameter of the capillary tube corresponds to the maximum diameter in order to obtain an increase in heat-exchange area with respect to the conventional solution; a smaller diameter will give a far less pronounced increase in heat-exchange area.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.

Claims

1. A brazed-plate heat exchanger, of the type comprising a stack of parallel plates which define a plurality of fluid-circulation passages of flat overall shape, closure bars which delimit these passages and distributing means for distributing a fluid to each passage of a first series of passages and means for sending another fluid to a second series of passages, in which exchanger at least one passage contains at least one organized exchange structure which forms a plurality of channels in the width of the passage, each channel being in contact with either at least two other channels or at least one other channel and one plate, the exchanger being wherein the structure also forms at least three channels in the height of the passage.

2. The exchanger of claim 1, in which the structure is made up of a plurality of cylinders.

3. The exchanger of claim 1, in which the structure is formed of a single body containing a plurality of channels.

4. The exchanger of claim 1, in which a channel has a hydraulic diameter of between 1 and 6 mm.

5. The exchanger of claim 1, in which a channel has a hydraulic diameter of between 200 μm and 1 mm.

6. The exchanger of claim 1, in which a channel has a hydraulic diameter of less than 200 μm.

7. A cryogenic separation apparatus comprising at least one exchanger of as claimed in claim 1.

8. The cryogenic separation apparatus of claim 7, in which a main heat-exchange line and/or a vaporizer-condenser and/or a supercooler is said brazed-plate heat exchanger, of the type comprising a stack of parallel plates which define a plurality of fluid-circulation passages of flat overall shape, closure bars which delimit these passages and distributing means for distributing a fluid to each passage of a first series of passages and means for sending another fluid to a second series of passages, in which exchanger at least one passage contains at least one organized exchange structure which forms a plurality of channels in the width of the passage, each channel being in contact with either at least two other channels or at least one other channel and one plate, the exchanger being wherein the structure also forms at least three channels in the height of the passage.