GB2142716A - Shell- and tube-type heat exchangers - Google Patents

Abstract

A fluid seal, for mounting on the outer periphery of a baffle in a shell- and tube-type heat exchanger, comprises a flexible annular member 48 having a base portion 51 and a pair of arm portions 54 which extend outwardly in opposite axial directions from the base portion 51. Each arm portion 54 has a free end which is curved radially inwardly of the annular member 48, for example in the form of a hook 55 or a bead 56. The arm portions 54 are preferably disposed at an included angle of between 30 DEG and 90 DEG . <IMAGE>

Description

SPECIFICATION Shell- and tube-type heat exchangers This invention relates to shell and tube type heat exchangers.

A conventional shell and tube type heat exchanger is shown in longitudinal cross-section in Figure 1 of the accompanying drawings. The heat exchanger comprises generally a tubular shell or casing 10 having a pair of inlet/outlet pipes 11 and 1 2 through which a first fluid is passed, and a tubestack 1 3 received within the casing. The tubestack is composed of generally parallel tube elements 14 through which a second fluid is passed for heat exchange with the above-mentioned first fluid, the tube elements extending between and being supported by a pair of support members or tubeplates 1 5 and 16. The tubestack 1 3 also includes a plurality of baffles 1 7 disposed between the tubeplates 15,16 and extending transversely to the tube elements 14.The second fluid is supplied to the tubestack 1 3 by means of inlet/outlet ports 18 and 19 in respective covers 20 and 21 which are secured to the casing 10 by any convenient means, such as by bolted flanges or bolt and lug arrangements (not illustrated) provided on these parts. To prevent intermixing of first and second fluids, at one end of the heat exchanger a suitable sealing arrangement 22 surrounding the tubeplate 1 5 is interposed between the cover 20 and the casing 10, whilst at the other end of the heat exchanger the tubeplate 1 6 is provided with an extended flange 23 carrying respective packings 24 and 25 likewise disposed between the cover 21 and the casing 10.The flange 23 also serves as an abutment relative to casing 10 and serves to locate the tubestack 1 3 axially within the casing.

In the above-described construction the baffles 1 7 serve to control in a desired manner the flow pattern of the first fluid over the tubes as it passes through the casing. For simplicity, the baffles are shown as plain plates though in practice first apertures are provided to receive the tube elements together with second apertures to permit passage of the first fluid from one side of the baffle to the other. In one known construction, the baffles consist of plates whose external peripheries closely conform to the internal bore of the casing and which have a centrally positioned second aperture, and plain plates of reduced external diameter with respect to the casing bore, the two types of plate being arranged alternately along the casing.For those baffle plates which closely conform to the casing bore it is important to achieve a minimum clearance between their external peripheries and the internal periphery of the casing to minimise by-pass of the first fluid between these parts, and thereby enable the maximum thermal performance of the heat exchanger to be obtained.

Customarily, this minimum clearance is achieved by known means such as machining the bore of the casing 10 to closely controlled dimensions with complementary control of the external periphery of each baffle. Usually this necessitates producing the casing by casting or manufacturing it from thick-walled tubing, both of which require a costly through-boring operation to obtain the required dimensional accuracy. Alternatively the casing can be manufactured from commercially available tubing as it stands, the tubestack 1 3 being machined to conform to the actual bore of the piece of tube used for the casing. Unfortunately this means that each tubestack is unique to its casing and therefore the tubestack cannot be used interchangeably with other similar casings and equally the casing cannot be used interchangeably with other similar tubestacks.

Accordingly if service replacements are required for either casing or tubestack these have to be specially manufactured to specific dimensions. A further alternative is to produce the casing from tubing drawn to closely specified dimensional bore tolerances but this incurs a disadvantageous costly penalty.

The provision of a close dimensional relationship between the internal periphery of the casing and the external periphery of any baffle can also cause problems where the tubestack is intended to be removable from the casing so that after a period in service it can be extracted for inspection and cleaning. In this case, the accumulation of deposits on the internal walls of the casing can seriously impede the withdrawal of the tubestack from the casing, and in extreme cases damage can result.

Should the casing 10 shown in Figure 1 made from tubing, a further disadvantage appears due to the manner in which the inlet/outlet pipes 11 and 12 are provided.

Conventionally this is done by cutting an opening of appropriate size in the casing wall and then welding a piece of pipe about or into the said opening. Normally such welding of the pipes to the casing wall causes localised distortion of the internal periphery of the casing, and requires either localised dressing by grinding or through-bore machining to be rectified.

It is an object of the present invention to obviate or mitigate the problems and disadvantages described above.

According to the present invention, there is provided a heat exchanger which comprises a hollow casing through which a first fluid is passed, and a tubestack received within the casing and including a plurality of tube elements through which a second fluid is passed for heat exchange with said first fluid, the tubestack also including at least one baffle extending transversely to the tube elements and a flexible annular member provided on the baffle or on each of at least one of the baffles, the flexible annular member having a base portion which engages an outer periphery of the baffle and a pair of arm portions which extend in opposite axial directions from the base portion and which engage an internal surface of the hollow casing, a free end of each arm portion being curved radially inwardly of the baffle.

Conveniently, the free end of each arm portion has the form of a hook or a bead. The arm portions extend at an included angle of between 30" and 90".

The invention will now be further described, by way of example, with reference to the remaining figures of the accompanying drawings, in which: Figure 2 is a longitudinal sectional view of a first embodiment of a heat exchanger according to the present invention which includes a plurality of fluid seals; Figure 3 is a sectional view of a baffle of the heat exchanger shown in Figure 2, with one of the fluid seals mounted on its periphery; Figure 4 is a similar view to Figure 3, but showing a modified version of the fluid seal; and Figure 5 is a similar view to Figure 2, but showing an alternative form of heat exchanger.

Referring first to Figure 2, the heat exchanger shown therein like the conventional construction described above comprises a hollow tubular casing 26 through which a first (shellnside) fluid is passed by means of inlet/outlet pipes 27 and 28. A tubestack 29 is received within the casing 26 and includes a plurality of generally parallel tube elements 30 through which a second (tube-side) fluid is passed for heat exchange with the shell-side fluid. The tube elements 30 extend between and are supported by a pair of tubeplates 31 and 32, the tubeplate 32 having a flange 33 on its external periphery which engages an end of the casing 26 and thereby locates the tubestack 29 axially within the casing.

A plurality of baffles 34 are disposed between the tubeplates 31,32 and extend transversely to the tube elements 30 to control the flow pattern of the shell-side fluid through the interior of the casing 26. In the embodiment actually illustrated, alternate ones of the baffles have a central aperture therein (not shown) through which the shell-side fluid flows in use, while the intermediate baffles (referenced 34') each have an external peri phery which is rather smaller than the internal periphery of the casing, such that an annular space is defined between each baffle 34' and the casing through which the shell-side fluid can flow. In this way, a convoluted flow pattern of the shell-side fluid is obtained to increase the heat transfer efficiency of the heat exchanger.

The casing 26 is formed from tubing, which may be seamless or may be seamed by either longitudinal or helical welding. In one embodiment, the tubing is in a readily available commercial form and its internal bore is subject to known dimensional tolerances. In an alternative embodiment, the tubing has an internal bore whose diameter is accurately sized. During manufacture of the casing 26, a tubular portion 36 adjacent one end of the tubing is deformed inwardly by a conventional rolling, swaging or flow forming technique such that its internal diameter d, is accurately sized to a value less than the minimum possible diameter of the tubing bore, while a tubular portion 37 adjacent the other end of the tubing is similarly deformed outwardly such that its internal diameter d2 is accurately sized to a value greater than the maximum possible diameter of the tubing bore.A tubular intermediate portion 38 disposed axially between the portions 36 and 37 retains the natural bore diameter of the tubing. Preferably, the portions 36, 37 and 38 are all coaxial, as illustrated. It will be appreciated from Figures 2 and 3 that smooth transitions are achieved between the various different internal diameters of the casing. An outboard end 39 of the portion 36 is flared outwardly at an included angle of 1, while an outboard end 40 of the portion 37 is similarly flared outwardly at an included angle of 2 Oi and 02 being substantially 90" in the illustrated construction. Such flaring of the end portions 39 and 40 serves to stiffen the casing ends and also enables sealing packings to be accommodated in a manner to be described later.The axial lengths of the portions 36,37 and their respective ends 39,40 accord with the overall design requirements of the heat exchanger.

The ends of the casing 26 are closed by respective covers 41 and 42 having therein respective inlet/outlet ports 43 and 44 for the tube-side fluid. A sealing packing 45 is axially interposed between the cover 41 and the flared end 39 of the casing 26, and also seals against the external periphery of the tubeplate 31. Another sealing packing 46 is received between the flared end 40 of the casing and the flange 33 on the tubeplate 32, while a further sealing packing 47 is received between the flange 33 and the cover 42.

The tubeplates 31 and 32 of the tubestack 29 are dimensioned so that they are received with suitable clearance within the portions 36 and 37 respectively when the tubestack is fully inserted in the casing 26, any resultant gaps being sealed by the sealing packings 45, 46 and 47. The baffles 34 and 34' on the other hand are received within the intermedi ate portion 38 of the casing. Where the tubestack is to be interchangeable between different casings, if the baffles 34 were to take the conventional form shown in Figure 1 then their external dimensions would have to be no greater than the minimum possible diameter of the tubing bore, or else it might not be possible for the baffles to be accommodated within the casing portion 38.However, in the event that the diameter of the tubing bore is near its maximum possible value, there will be substantial gaps between the external peripheries of the baffles and the internal periphery of the casing portion 38, with the result that substantial by-pass of the shell-side fluid will be possible.

In order to overcome this problem, each baffle 34 isprnvided with a flexible rim which enables it to conform to the bore diameter of the tubing from which the casing is produced.

One example of such a baffle is shown in Figure 4, wherein an elastomeric tyre 48 is mounted on the periphery of a baffle plate 49. Apertures in the baffle plate 49 through which the tube elements 30 respectively pass are referenced 50 in this Figure. The tyre 48 comprises an annular base portion 51 having a groove 52 in its internal periphery within which the outer edge of the baffle plate 49 is received, the base portion 51 also having a radially outwardly facing peripheral surface 53. A pair of flexible arms 54 extend radially outwardly from the base portion 51 in opposite transverse directions with respect to the baffle plate 49 at a desired included angle, usually between 30" and 90", and terminate at their free ends in respective rounded hook formations 55 which are directed generally radially inwardly of the baffle.

The tyre is dimensioned so that, when it is fitted on the baffle plate 49 the diameter D, of the surface 53 is less than the minimum possible diameter of the tubing bore and approximately the same as the diameter of the tubeplate 31, and the outermost diameter D2 of the flexible arms 54 when in their relaxed state is not less than the maximum possible diameter of the tubing bore, preferably slightly more than the latter. In addition, the proportions of the base portion 51 are such that it does not obscure the outermost tube apertures 50 when fitted to the baffle plate 49, and such that under any combination of induced vibration or gravitational effects (such as are encountered with a long heat exchanger mounted horizontally) there is no likelihood of the baffle plate shearing through the tyre.

The tyre 48 may be formed by an extruded section cut to length with its ends joined to form a ring, or may be moulded as a ring in the first instance.

An alternative form of baffle is shown in Figure 5, being generally similar to that described above with reference to Figure 4, except that the hook formations 55 at the free ends of the flexible arms 54 are replaced by rounded heads 56 again directed generally radially inwardly of the baffle.

Typically, during insertion of the tubestack 29 into the casing 26, the flexible rim of each baffle in turn first engages the flared end 40 and then engages within the enlarged casing portion 37 and subsequently within the casing portion 38. The flared nature of the end 40 together with the smooth transition between the casing portions causes the flexible arms 54 of the tyre 48 to deform radially inwardly of the baffle without posing any substantial resistance to the insertion of the tubestack.

The rounded ends of the arms 54 provided by the hook formations 55 or the beads 56 greatly assists such insertion because it prevents the advancing edge of each tyre 48 from digging into the side walls of the casing portions 37 and 38.

When the tubestack 29 is fully inserted within the casing 26, the natural resilience of the tyres 48 maintains the arms 54 in contact with the internal periphery of the casing portion 38, thereby substantially or entirely eliminating by-pass of the shell-side fluid. Moreover, as the shell-side fluid flows through the interior of the casing it creates a higher pressure on one side of each baffle 34 than on the other side thereof, which causes the arm 54 on the higher pressure side of the respective tyre 48 to be pushed outwardly against the internal wall of the casing portion 38, thereby assisting the conforming action of the tyre. The arm 54 is, however, sufficiently rigid to prevent its being forced between the base portion 51 of the tyre and the casing wall.

If in service fouling deposits accumulate on the internal walls of the casing 26, withdrawal of the tubestack 29 for inspection or replacement is not impeded because the rounded ends of the arms 54 enable the latter to ride up and over any such deposits. The arms 54 will similarly ride across the various openings in the casing (i.e. the openings of the inlet/outlet pipes 27 and 28 for the shell-side fluid, fluid drains, vents, inspection holes, etc.) and due to their rounded ends will not fold back or suffer damage even if these openings are quite sharp. In the event that the tyres 48 do become damaged or suffer deterioration, they can easily be replaced.As mentioned above, the manner in which the inlet/outlet pipes 27 and 28 are produced may give rise to localised distortion of the casing walls: however, by providing the baffles with flexible rims as described above, such distortion can be accepted without the need to machine the interior of the casing.

In the heat exchanger described above, the casing 26 can be made at low cost from relatively inexpensive tubing, while the provision of flexible rims on the baffles ensures high thermal performance by substantially preventing by-pass of the shell-side fluid, and at the same time permits removal of the tubestack 29 in service. A further advantage of the flexible rim baffle is its ability to minimise the transmission of externally induced vibrations to the tube elements 30 via the casing and baffles.

Figure 5 illustrates an alternative form of heat exchanger which is generally similar to the construction described above, with reference to Figures 2 to 5, similar parts being accorded the same reference numerals. In this embodiment, however, the portion 37 of the casing 26 is deformed inwardly rather than outwardly, and its internal diameter is accurately sized to a value smaller than the minimum possible diameter of the tubing bore, preferably the same as the internal diameter of the casing portion 36. In this latter case, the tubeplates 31 and 32 of the tubestack 29 have the same external dimensions, and the baffles 34 are sized so that they can pass easily through the portion 37 during insertion of the tubestack within the casing.

Claims (4)

1. A heat exchanger comprising a hollow casing through which a first fluid is passed, and a tubestack received within the casing and including a plurality of tube elements through which a second fluid is passed for heat exchange with said first fluid, the tubestack also including at least one baffle extending transversely to the tube elements and a flexible annular member provided on the baffle or on each of at least one of the baffles, the flexible annular member having a base portion which engages an outer periphery of the baffle and a pair of arm portions which extend in opposite axial directions from the base portion and which engage an internal surface of the hollow casing, a free end of each arm portion being curved radially inwardly of the baffle.

2. A heat exchanger as claimed in Claim 1, wherein the free end of each arm portion has the form of a hook or a bead.

3. A heat exchanger as claimed in Claim 1 or 2, wherein the arm portions extend at an included angle of between 30" and 90".

4. A heat exchanger substantially as hereinbefcre described with reference to Figures 2 to 5 of the accompanying drawings.