GB2066937A

GB2066937A - Improvements in or relating to heat exchangers

Info

Publication number: GB2066937A
Application number: GB8000582A
Authority: GB
Original assignee: Watt Joule & Therm Co Ltd
Current assignee: Watt Joule & Therm Co Ltd
Priority date: 1980-01-08
Filing date: 1980-01-08
Publication date: 1981-07-15
Also published as: GB2066937B; JPS56142397A

Abstract

A heat exchanger 10 has an outer tube 11 and an inner tube 12 within the outer tube, said inner tube being provided with external, axially-extending fins 27. Within at least some of the axially-extending spaces defined by adjacent fins and the outer tube, are provided devices 28 for increasing the angular momentum of a fluid, e.g. engine exhaust gas, flowing along said spaces, thereby promoting turbulence in the boundary layer and so increasing heat transference. Each such device 28 may comprise a twisted strip (e.g. Figure 7). In use, a fluid such as water flows through the inner tube 12 in counterflow direction to that of the exhaust gas flowing in the outer tube 11. <IMAGE>

Description

SPECIFICATION Improvements in or relating to heat exchangers This invention relates to heat exchangers and particularly to heat exchangers using the fluid counterf low principle.

It is well known that, with two fluids in counterflow separated by a heat-conducting wall and with one at a higher temperature than the other, the main impediment to the most effective transfer of heat from the higher temperature fluid to the other is the fluid boundary layer, particularly the stationary sub-layer, contiguous with the wall.

It is an object of the present invention to provide such a heat exchanger having a provision for disturbing the boundary layer thereby giving increased heat transference.

According to the present invention there is provided a heat exchanger comprising an outer tube, an inner tube within the outer tube and provided with external, axially-extending fins, and, within at least some of the axially-extending spaces defined by adjacent fins and the outer tube, devices for increasing the angular momentum of the fluid flowing along said spaces, thereby promoting turbulence in the boundary layer and so increasing heat transference.

Preferably, the turbulence-promoting device is a rod of strip metal twisted at intervals along its length and supported stationarily between a pair of adjacent fins. Merely by way of example, in a 12 foot rod length a convenient number of twists is 30. For convenience, such a device is hereinafter called "a turbulator".

Each turbulator is preferably supported at one end by an annular mounting plate within the outer tube and surrounding a non-finned part of the othertube and at its other end by an integral foot resting on the inner tube.

Fluid inlet and outlet to the outer tube is preferably normal to fluid flow along the other tube and at non-finned parts of the inner tube. Fluid ingress is preferably by way of a tube or pipe which extends slightly inboard of the outer tube to prevent a smooth change of direction of the incoming fluid and thus promote turbulence in the region of fluid entry to the outer tube.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa longitudinal section of a heat exchanger according to the invention; Figure 2 is an enlarged sectional detail of the heat exchanger at the fluid entry to the outer tube; Figure 3 is an enlarged sectional detail of the heat exchanger in the region of the turbulator mounting plate; Figure 4, Sand 6 are respectively a front elevation, a rear elevation and a sectional view on the line VI-VI of Figure 4 of the turbulator mounting plate; Figure 7 is a fragmentary side elevation of a turbulator; Figure 8 is an end elevation of a turbulator; Figure 9 is a sectional view on the line IX-IX of Figure 4; Figure 10 is a detail view showing the mounting of a turbulator on the plate;; Figure 11 is an end elevation of a framework for supporting two heat exchangers connected in series; and Figure 12 is a detail view of Figure 11.

The heat exchanger according to the present invention is of modular construction permitting two or more heat exchangers to be connected in series and/or parallel depending on the particular heat exchange requirement called for.

The module of the present invention consists of two heat exchangers 10 connected in series. The particular heat exchange application is the transfer of heat from the exhaust gases of internal combustion engines under test to water to be heated. The heat exchanger can, of course, be employed for any convenient heat transfer application. It is considered that there is special advantage in using as the higher temperature fluid various forms of exhaust gases which tend to contain particulate material due to the ease of cleaning and heat exccanger as a result of the axial finning as will be described later.

The heat exchanger 10 comprises an outer tube or case 11 surrounding an inner and concentric tube 12.

The outer tube 11 has an exhaust pipe or stack 13 and an entry pipe 14 spaced axially along the outer tube 11 and disposed at right angles thereto.

To connect two such heat exchangers in series it is merely necessary to bolt the flanged end of the exhaust pipe 13Ato the flanged entry pipe 14 of another with appropriate intervening sealing and to connect the inner tubes together by a pipe 15. The two heat exchangers are supported vertically spaced on axially-spaced upright frameworks 16 being tied to the frameworks 16 by U-bolts 17.

The outer tube 11 has welded to each end an external flange 18 each of which is bolted, as indicated at 19, to a flange 20 external of innertube 12.

The flange 20 at the outlet end of the innertube 12 is welded to the inner tube 12. The flange 20 near the inlet end of the inner tube has welded thereto a short tube 21 provided with an external flange 22 at its other end bolted, as indicated at 23 to an external flange 24 surrounding the inner tube 12; Asbestos rope packing 25 fills the space between the inner tube 12 and the tube 21.

Sealing material 26 is provided between flanges 18 and 20 and flanges 22 and 24.

It is to be noted that the entry pipe 14 extends slightly inboard of the outer tube 11 (see Figure 2) to ensure a non-smooth change of direction of the ingressing hot exhaust gases thereby promoting turbulence in this region.

The inner tube 12 between the entry pipe 14 and the exhaust stack is finned as indicated at 27 (Figure 1). The finning 27 is a number equiangularly-spaced, radially-extending axial fins extending outwardly of the innertube 12 and stopping just short of the outer tube 11. A preferred number of fins is ten but this number can be varied to suit particular heat exchange requirements as in the present instance where there are eight fins. Thus the fins and outer tube define a number of equiangularly-spaced channels and, in accordance with the invention, there is disposed along each of these a turbulator 28.

Each turbulator 28 is a rod 29 which is twisted along its length at regular intervals to provide a series of twists, say for example 2.5 twists per foot of rod 29. At one end the rod 29 has a foot 30 whereby it rests on inner tube 12, and at its other end an apertured plate 31 whereby it is secured to an annular mounting plate 32.

The annular mounting plate 32 has two diametrically spaced plain holes 33 whereby it is bolted as indicated at 34 to the flange 20 at the inlet end of the inner tube 12. The annular mounting plate 32 has eight equiangularly-spaced mounting holes 35, one for each turbulator 28, and which are recessed at theirturbulator-remote side to accommodate the heads 36 of turbulator securing bolts 37. Clamping washer is indicated at 38.

The provision of these turbulators 28, which terminate just short of exhaust stack 13, increases the angular momentum of the hot exhaust gases in their flow from entry pipe 14 to exhaust stack 13 thus promoting a turbulence which disturbs the boundary layer at the wall of the inner tube 12 thus increasing heat transfer from the gases to the water counterflowing along the inner tube 12.

It will be manifest that a variation in heat transference can be achieved if each passage is not provided with a turbulator 28.

Referring to Figure 3, it will be seen that access to the turbulators can be gained by unbolting flange 10 from flange 18 and sliding along the inner tube 12 and tube 21 which will carry with it the mounting plate 32 and turbulators 28. This permits mainte nance and replacement of the turbulators 28.

Referring to Figure 1 and the outlet end of the inner tube 12 it will be manifest if the linking pipe 15 is disengaged from the inner tube 12 at the flange connection 39 and flanges 18 and 20 are released the inner tube 12 can be exposed to permit mechanical or water or steam or solvent cleaning between the fins.

The outer tube 11 would normally be lagged or insulated externally.

It should be noted that with the above described design of heat exchanger there is a relatively low pressure drop through the passage defined by the inner tube 12 and the annular passage defined by the inner tube 12 and outer tube 11.

Claims

1. A heat exchanger comprising an outer tube, an inner tube within the outer tube provided with external, axially-extending fins, and, within at least some of the axially-extending spaces defined by adjacent fins and the outer tube, devices for increas ing the angular momentum or a fluid flowing along said spaces, thereby promoting turbulence in the boundary layer and so increasing heat transference.

2. A heat exchanger as claimed in claim 1, in each turbulence-promoting device is a rod of strip metal twisted at intervals along its length and supported stationarily between a pair of adjacent fins.

3. A heat exchanger as claimed in claim 1 or 2 in which each turbulence-promoting device is supported at one end by an annular mounting plate within the outer tube and surrounding a non-finned part of the other tube and at its other end by an integral foot resting on the inner tube.

4. A heat exchanger as claimed in any one of claims 1 to 3 in which the outer tube has an inlet and an outlet spaced axially along the tube and disposed normally to the outer tube for effecting fluid inlet to and outlet from the outer tube at non-finned parts of the inner tube.

5. A heat exchanger as claimed in claim 4 in which fluid ingress to the outer tube is by way of a tube or pipe which extends slightly inboard of the outer tube to prevent a smooth change of direction of the incoming fluid and thus promote turbulence in the region of fluid entry to the outer tube.

6. A heat exchanger as claimed in claim 4 or 5, in which fluid egress from the outer tube is by way of an exhaust pipe or stack.

7. A heat exchanger as claimed in any one of claims 3 to 6 in which the annular mounting plate is carried by a support tube releasably connected to the outer tube and in sealing and slidable engagement with the inner tube, which arrangement allows access to the turbulence-promoting devices.

8. A heat exchanger, substantially as hereinbefore described with reference to the accompanying drawings.

9. The combination of two or more heat exchangers as claimed in any one of claims 1 to 8, with the fluid exhaust from one heat exchanger communicating with the fluid inlet of another and with the inner tubes of these heat exchangers connected by a pipe.