EP0735335A2

EP0735335A2 - Heat exchanger and method of manufacture, and dehydration apparatus

Info

Publication number: EP0735335A2
Application number: EP96301679A
Authority: EP
Inventors: Anthony Michael Martin
Original assignee: MDH Ltd
Current assignee: MICROFLOW LIMITED
Priority date: 1995-03-27
Filing date: 1996-03-12
Publication date: 1996-10-02
Also published as: GB9506184D0; JPH0979770A; EP0735335A3

Abstract

A heat exchanger is provided comprising a coiled duct (1) having fins (5) integrally formed with the duct. A first fluid, typically a refrigerant fluid is passed through the interior (3a, 3b, 3c, 3d) of the duct, and a second fluid, typically a sterilant gas to be dried is passed over the exterior of the duct, over the fins. A method of forming the coiled heat exchanger from a straight section of duct, formed as an extrusion with integral fins, without flattening the fins is disclosed. The invention can provide a compact efficient heat exchanger which is readily manufactured.

Description

The present invention relates to a heat exchanger. It is particularly, but not exclusively, suitable for use in a closed system, for cooling a gas vapour sterilant, such as hydrogen peroxide.
Conventional heat exchangers for cooling gases use copper tubing carrying a refrigerant fluid, with separate fins, such as aluminium fins, attached for example by soldering to the exterior of the tubing. The fins are important for cooling gases as they provide a large contact surface area. The finned sections are conventionally straight, and are joined together by short sections of curved tubing free of fins, as shown in Fig. 1a.
As an improvement on arrangements in which the fins are separate from the tubing, a one-piece straight extruded aluminium refrigerant channel having integral fins is known, and is supplied by K.M. Schmöle under the trade name Skyvefin. This is supplied as a straight length, having a section as shown in Fig. 1b, and is intended by the manufacturer for use in such an arrangement having a series of straight sections joined by curved portions having no fins. The manufacturer considers bending of the finned extrusion impossible on the grounds that it would lead to crushing of the fins.
Such heat exchangers occupy a large volume and are relatively inefficient for cooling air. It is therefore inconvenient to employ such heat exchangers for cooling gases in a closed system, as the complete apparatus is very bulky.
Furthermore, the temperature difference between the refrigerant and the gas to be cooled varies along the length of the tubing. This leads to decreased efficiency, and means that the tubing must be used in conjunction with a separate expansion coil to ensure that all of the refrigerant liquid is vaporised, since liquid entering the compressor would lead to premature failure of the compressor.
Heat exchangers employing a helical coil of tubing contained in a housing for containing a fluid to be cooled are well known. The tubing does not have fins, so the surface area exposed to the fluid to be cooled and hence the efficiency is small. For this reason, such heat exchangers are generally only suitable for use with liquids (since these generally have much higher thermal conductivity than gases), both inside and outside the tubing. The tubing is almost invariably made of copper to maximise its thermal conductivity.
Also known is a modified heat exchanger for use in a closed system, employing a helical coil of copper tubing, with a separate assembly of fins. This heat exchanger requires complex assembly of several parts, with joins between the parts. The joins between the tubing and the fins result in lower overall thermal conductivity and hence lower efficiency.
Copper tubing is not suitable for use with hydrogen peroxide or peracetic acid. For use with such gases, materials such as aluminium or stainless steel should be employed, and these have lower thermal conductivity than copper. Even if the above mentioned complicated construction could be formed from aluminium or stainless steel, the presence of joins, such as brazing or soldering between the component parts, may lead to corrosion if a heat exchanger so constructed were used to cool corrosive gases such as hydrogen peroxide vapour.
The present invention seeks to provide a heat exchanger which can be assembled easily and which can provide efficient heat transfer, whilst maintaining a compact size.
Accordingly, the present invention provides a heat exchanger comprising a conduit having an interior duct therethrough and a plurality of outwardly projecting fins integrally formed therewith, the conduit being wound into a coil; means for passing a first fluid through said duct; and means for passing a second fluid over said plurality of outwardly projecting fins. The coil has a plurality of turns, stacked in the axial direction of the coil.
The coil is most preferably substantially helical, as this simplifies manufacture, and enables a compact heat exchanger to be provided. Of course, the coil need not be exactly helical.
Most advantageously, the conduit is formed as an extrusion, as this simplifies manufacture, and ensures good thermal conductivity between the fins and the conduit.
Preferably, the conduit is formed from aluminium. This provides a lightweight heat exchanger having relatively high thermal conductivity, and resistance to corrosion by hydrogen peroxide.
Preferably, the means for passing a second fluid over said fins includes a housing in which the coil is located. This provides a compact heat exchanger which can be employed in a closed system in which the second fluid is recirculated.
Advantageously, the housing includes a first substantially cylindrical wall adjacent the exterior of said substantially helical coil, and a second substantially cylindrical wall adjacent the interior of said substantially helical coil, said first and second walls defining a flow path for said second fluid. This arrangement provides a compact heat exchanger with efficient heat transfer, and is particularly suitable when the second fluid is a gas or vapour to be dried, as it ensures that the majority of the fluid passes close to the fins, thus ensuring efficient drying.
Preferably, the heat exchanger is arranged to pass a refrigerant fluid as said first fluid through the interior of said conduit and a fluid containing a gas and/or vapour to be cooled and/or dried as said second fluid over said fins.
Advantageously, the duct is partitioned into a plurality of internal fluid channels. This may enable more efficient and even heat transfer without requiring a high flow rate for the first fluid.
Advantageously, the heat exchanger is arranged to pass a liquid refrigerant into said internal duct as said first fluid and extract said first fluid as refrigerant vapour from said internal duct, said liquid refrigerant vaporising in said heat exchanger. This can provide a very compact cooling arrangement, avoiding the need for a separate expansion coil, thereby reducing the overall space required for a refrigeration system employing such a heat exchanger.
The present invention further provides a method of operating such a heat exchanger comprising passing a refrigerant fluid as said first fluid through said internal duct, passing a warmer fluid as said second fluid over said fins, the conditions being such that said first fluid is supplied as a liquid and extracted as a vapour.
The present invention also provides the method of forming a heat exchanger comprising the steps of:

providing a former having means defining a substantially helical groove on the exterior thereof;
supplying a substantially straight conduit having an internal duct therein and a plurality of outwardly projecting fins integrally formed therewith;
securing a portion of said conduit to said former;
winding said conduit around the groove of said former to form a substantially helical coil;
removing said former from said coil by rotating said former relative to said coil.

Preferably, the conduit is provided with at least one attachment portion substantially free of fins and said attachment portion is secured to said former prior to winding said conduit around said former. This facilitates securing of the conduit to the former, for example by clamping.
Preferably, said attachment portion is provided adjacent an end of said conduit, as this may further facilitate securing of the conduit to the former.
Preferably, said conduit has at least one supporting surface substantially free of fins, for example ridges or lugs, desirably at the sides thereof, and said former is provided with projections arranged to abut said at least one supporting surface. This may facilitate winding of the helical coil around the former, and alleviate the problem of damage to the projecting fins.
Preferably, the conduit comprises a substantially rectangular tube having an internal duct partitioned into a plurality of fluid channels, said fins projecting from at least major portions of the long sides of said rectangular tube, at least one portion of said long sides being free of said fins and serving as a supporting surface for supporting the conduit on said former during winding.
More preferably, two portions of said long sides adjacent the edges thereof serve as supporting surfaces, the former being provided with corresponding substantially helical projecting ridges for supporting said supporting surfaces, and having a space therebetween into which a plurality of the fins project during said winding.
The present invention also provides a former for use in such a method, a gas sterilising apparatus incorporating such a heat exchanger, and the use of such a heat exchanger for drying a sterilising gas, or other gas containing hydrogen peroxide or peracetic acid.
An embodiment of the present invention will now be described, with reference to the following drawings in which:

Fig. 1a shows a conventional refrigerant expansion coil employing a straight conduit with integrally formed fins;
Fig. 1b shows a section on A-A of Fig. 1a;
Fig. 2 is a cross-section of an embodiment of the present invention;
Fig. 3 shows a former used in the manufacture of the embodiment of Fig. 2 with a conduit attached; and
Fig. 4 is a view in the direction X of the former of Fig. 3.

Referring to Fig. 2, and also to the section shown in Fig. 1b, an extruded aluminium conduit 1 having four channels 3a, 3b, 3c, 3d therein and a plurality of integrally formed exterior fins 5 is provided in the shape of a helical coil. Instead of aluminium, another material which is resistant to corrosion by the refrigerant and the fluid to be cooled, and has good thermal conductivity may be used. An example of another suitable material is stainless steel. Aluminium is, however, preferred due to its extrudability relatively light weight, relatively high strength, good thermal conductivity, and resistance to corrosion by fluids such as hydrogen peroxide gas.
The channels 3a-3d are connected at one end of the coil respectively to a refrigerant inlet connector 7 and at the other end to a refrigerant outlet connector 9 for passing refrigerant fluid through the conduit. In this embodiment, the inlet connector 7 connects all four channels 3a-3d to a single inlet, and the outlet 9 connects all four channels 3a-3d to a single outlet.
A housing 11 having a generally cylindrical section surrounds the coil 1 in close proximity to the exterior edges of the fins 5. An internal generally cylindrical plug 13 is provided in close proximity to the interior edges of the fins 5 occupying the void within the coil 1, the space 12 between the housing 11 and plug 13 providing a flow path for a gas and/or vapour, such as hydrogen peroxide gas around the fins 5 of the coil from an inlet 15 adjacent the bottom of the coil to an outlet 17 adjacent the top of the coil. As can be seen, the majority of the space 12 is filled by the coil 1, thereby ensuring intimate contact between the gas and fins 5.
A drain outlet 19 is provided at the bottom of the housing 11 for draining liquid such as water condensed from the gas or vapour. The housing 11 is made of a material which is not harmed by the gas, typically plastics material to provide heat insulation, but other materials, e.g. metal such as aluminium may also be used.
The cooled gas outlet 17 is at the top of the coil, adjacent the refrigerant inlet 7, and the gas inlet 15 is at the bottom adjacent the refrigerant outlet 9. This ensures a maximum temperature difference between the gas to be cooled and the refrigerant over the length of the coil, and provides progressive cooling. Although it is conventional to have a cooled fluid exiting from the bottom of a heat exchanger, the present arrangement provides efficient progressive cooling combined with good separation of gas and condensed liquid. Of course, this connection arrangement can be altered if desired.
Referring to Figs. 3 and 4, the method of construction of the helical coil will now be described. A portion 20 of the conduit 1 free of fins 5 is secured to a rigid former 22 with a clamp 24 by bolts. The former 22 is made of any suitable rigid material, e.g. steel. Edges 1a, 1b of the conduit rest on respective ridge portions 26a, 26b defining a helical groove 28 on the exterior of the former 22. In this embodiment, a single ridge 26 provides both ridge portions 26a, 26b. The conduit 1 is wound around this groove 28 to form a helical coil. This may be achieved by rotating the former 22 slowly, for example on a lathe, while feeding the conduit 1, e.g. manually, to rest on the ridge portions 26a, 26b. The groove 28 is of sufficient size to accommodate the fins 5 and the radius of curvature and pitch of the helix are chosen so that the conduit 1 may be bent into a helix without the fins 5 being damaged or crushed against the former. It can be seen, for example in Fig. 2, that fins are provided on portions of the conduit having substantial curvature. This can be contrasted with Fig. la, in which the curved portions are free of fins.
In the present example, the conduit 1 has a rectangular outer section 28 mm by 5 mm, with four sets of slightly curved fins 5 projecting approximately 1 cm away from each long side. Unfinned supporting surfaces 1a, 1b each 1 mm wide are provided at each edge of each long side. The former has a circular section and has a 10 mm wide ridge 26 providing ridge portions 26a, 26b. The external diameter of the ridge 26 is 70 mm, and the diameter of the former between the ridge portions 26a, 26b is 40 mm, the spacing of the ridge portions 26a, 26b being 27 mm, the groove 28 thus being 18 mm deep by 27 mm wide to accommodate the fins 5. This gives a fairly closely wound coil in which each turn is close to (about 1 cm away from) the previous turn, giving a compact overall size, and ensuring good heat transfer efficiency.
Although in this embodiment the former is circular, it will be understood that embodiments having other shapes, e.g. ellipsoidal or even polygonal may be used in certain circumstances.
After winding an appropriate length of the conduit into a helical coil, in this embodiment at least about 4 or 5 turns, the clamp 24 is removed, and the former may be removed from the interior of the coil by relative rotation of the two - the former is effectively unscrewed from inside the coil. The conduit 1, which in this embodiment is made of aluminium, has some resilience, which causes it to spring outwards slightly when the tension present during winding is released, thereby alleviating the problem of friction between the former 22 and the coil 1. After removing the coil 1, the fins 5 may be re-aligned as necessary.
This arrangement allows a compact heat exchanger to be formed relatively easily from a single piece extrusion, without damaging the fins 5 projecting from the conduit 1. The conduit 1 need not necessarily be formed as an extrusion, but could, for example, be formed by casting or other processes provided that the fins 5 are integrally formed as a one-piece whole with the conduit prior to the conduit 1 being wound into a coil.
The heat exchanger of the present embodiment has been found particularly suitable for use in a sterilisation apparatus, for drying a recirculating stream of hydrogen peroxide gas and/or peracetic acid. A single heat exchanger may be used, or two or three or more in sequence. The heat exchanger may be provided with means for defrosting the coil. Water vapour condenses on the fins and is removed from the bottom of the heat exchanger via drain outlet 19. Refrigerant fluid is supplied as liquid to the interior channels 3a-3d of the conduit 1, and vaporises in the heat exchanger before being passed to a compressor. Because of the high efficiency of the heat exchanger, all of the refrigerant can be vaporised, and the risk of liquid refrigerant entering the compressor and causing damage can be avoided. This avoids the need for a separate expansion coil, thereby further reducing the space required for the refrigeration system. Previously it had not been possible to provide positive cooling for such gases efficiently in a compact apparatus.
The foregoing description discussed use of the heat exchanger to cool or dry a gas passing over the fins by passing a refrigerant through the interior. It is of course apparent to one skilled in the art that the heat exchanger may be employed for a number of purposes with liquids, gases or combinations of the two. The dimensions of the heat exchanger may be varied as necessary to suit the viscosities, flow rates, and thermal conductivities of the fluids concerned.

Claims

A heat exchanger comprising a conduit(1) having an interior duct(3a,3b,3c,3d) therethrough, means for passing a first fluid through said duct and means for passing a second fluid over said conduit(1);
characterised in that the conduit has a plurality of outwardly projecting fins(5) integrally formed therewith and is wound into a coil having a plurality of turns stacked in the axial direction of the coil.
A heat exchanger according to claim 1, whereinthe coil is substantially helical.
A heat exchanger according to claim 1 or claim 2, wherein the conduit is formed as an extrusion.
A heat exchanger according to any one of the preceding claims wherein the conduit is formed from aluminium.
A heat exchanger according to any one of the preceding claims wherein the means for passing a second fluid over said fins includes a housing(11) in which the coil is located.
A heat exchanger according to claim 5 wherein the housing(11) includes a first substantially cylindrical wall adjacent the exterior of said substantially helical coil(1), and a second substantially cylindrical wall(13) adjacent the interior of said substantially helical coil, said first and second walls defining a flow path(12) for said second fluid.
A heat exchanger according to any one of the preceding claims arranged to pass a refrigerant fluid as said first fluid through the interior of said conduit and a fluid containing a gas and/or vapour to be cooled and/or dried as said second fluid over said fins(5).
A heat exchanger according to any one of the preceding claims wherein the duct is partitioned into a plurality of internal fluid channels(3a, 3b, 3c, 3d).
A heat exchanger according to any one of the preceding claims arranged to pass a liquid refrigerant into said internal duct (3a, 3b, 3c, 3d) as said first fluid and extract said first fluid as refrigerant vapour from said internal duct, said liquid refrigerant vaporising in said heat exchanger.
A method of operating a heat exchanger according to any one of the preceding claims, the method comprising passing a refrigerant fluid as said first fluid through said internal duct, passing a warmer fluid as said second fluid over said fins, the operating conditions being such that said first fluid is supplied as a liquid and extracted as a vapour.
A method of forming a heat exchanger comprising the steps of:
providing a former(22) having means(26a, 26b) defining a substantially helical groove(28) on the exterior thereof;

supplying a substantially straight conduit(1) having an internal duct therein(3a, 3b, 3c, 3d) and a plurality of outwardly projecting fins(5) integrally formed therewith;

securing a portion(20) of said conduit(1) to said former (22);

winding said conduit(1) around the groove(28) of said former(22) to form a substantially helical coil;

removing said former(22) from said coil by rotating said former(22) relative to said coil.
A method according to claim 11 wherein the conduit(1) is provided with at least one attachment portion(20) substantially free of fins(5) and said attachment portion(20) is secured to said former(22) prior to winding said conduit(1) around said former(22).
A method according to claim 11 or claim 12 wherein said conduit(1) has at least one supporting surface (1a, 1b) substantially free of fins(5) said former(22) being provided with projections(26a, 26b) arranged to abut said at least one supporting surface (1a, 1b).
Use of a heat exchanger according to any one of claims 1 to 10 to dry a gas in a sterilisation apparatus.