GB2402465A

GB2402465A - Split flow heat exchanger

Info

Publication number: GB2402465A
Application number: GB0311963A
Authority: GB
Inventors: Edwin Robinson
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-05-27
Filing date: 2003-05-27
Publication date: 2004-12-08
Anticipated expiration: 2023-05-27
Also published as: GB0311963D0; GB2402465B

Abstract

A split flow heat exchanger comprises a cylindrical chamber 5 having an inlet communicating combustion products 8 or an alternative heating fluid to a header and an additional heat transfer device comprising an enclosure 2 with a plurality of tubes 14 and exits through a discharge conduit 9. Additional heat transfer device is partitioned from the chamber 5 by a dividing wall 12 which splits a flow of process fluid into two streams 1, 10 and also forms one passage 11 communicating a first stream 10 of the process fluid with an annular passage 6a, 6b concentric with the chamber 5 whereby the process fluid passes over the chamber 5 in a clockwise, anticlockwise or both directions, and a second passage communicating a second stream 1 of process fluid passing over the tubes 14 before discharging from both passages into a common or separate exit duct 7. Tubes 14 may be bent or straight with or without fins. A manual or powered damper 15 may be used to adjust the flow proportions through each passage. A perforated plate 16 may be included to distribute the process fluid evenly over the chamber 5. The inner wall of the annular passage 6a, 6b may be coated with another metal to improve conductivity and the outer wall may be coated with ceramic or paint to reduce heat loss. Outer walls of chamber 5 may convoluted or have attachments and baffles may be added in passage 11.

Description

1 2402465

A SPLIT-FLOW HEAT EXCHANGER

This invention relates to heat exchangers and in particular but not limited to, those of the fired "gas to gas" type, comprising a hot gas or combustion chamber with co u eating tube array and box headas through which the heating medium flows before exiting via an exhaust conduit, and ova which the process gas to be heated is usually passed. In this type of heat exchange the heat transfer process is invariably a combination of "cross flow" and "counta flow" in which the two gas streams substantially flow in opposite directions separated by the thin walls of the heat exchange surfaces. By this method, a significant factor in heat transfer equations, known as the temperahc diffacnce (delta t), is at its highest value throughout the process.

"Gas to gas" heat exchanges are used extensively for process heating, e.g. in drying processes and in indirect fired forced convection balking ovens. Typically the heat excharga is arranged Bobbin an enclosure and utilizes a burna flung into a combustion chamber producing hot combustion products that flow through several passes of straight tubes arranged between box headers positioned to one side. The heat of combustion is transferred to the process by using a fan to deliva and re-circulate the process gas to and from the drying or baking chamber and passing it ova the heat transfer surfaces. The cooled combustion products exit the heat exchange and are vanted to atmosphere via a separate conduit, which Assures that the product cannot be coated thereby. À:-

Heat exchangers of this type have relatively high metal temperatures that vary significantly, particularly surrounding the combustion charnba and in the first pass of tubing. The resultir4; differential expansion causes stress between adjacent elements, leading to premabue failure at their interfaces. Additionally the pressure drop (delta p) through and ova the heat exchanger, which is relatively high to maintain good distribution and heat transfer between the gasses and surfaces, requires significant fan powa to overcome frictional losses.

In known systems with multi-pass straight tube box headas, the delta p on the combustion products side arises mainly from the number of entry and exit losses as the gas changes direction in the box headas, rather than that incurred whan it flows through the tubing. To create ideal conditions for heat transfer the velocity through the tubing is the more important factor, however due to the cumulative losses caused by the plurality of antry/exits and changes in direction, the design is open compromised to reduce backpressure at the burna. Therefore the internal longitudinal heat transfer coefficient via the tubing is relatively poor and a greata surface area is required to achieve a reasonable overall efficiency.

The process gas side pressure drop arises from the combination of the cross flow passes ova the tubes followed by high velocity circulation around the combustion chamber. Because the absolute viscosity of the process gas increases with rising temperature, the final gas pass velocity needs to be high to create sufficient turbulence to break down the boundary laya next to the combustion chamber wall that inhibits heat transfer and increases metal temperatures. Therefore the cumulative delta p is relatively high and the combustion chamber usually requires an extended surface area to reduce metal temperatures to an acceptable leveL The purpose of the following invention is to provide a heat exchanger which overcomes the foregoing disadvantages by splitting the flow of the process fluid to optimise pressure drop, improving distribution and heat transfer, particularly whale the metal temperatures tend to be high, thereby providing better cooling and limiting the thermal stress due to differential expansion.

According to the present invention there is provided a split-flow heat exchanger, comprising a cylindrical chamber with an inlet at one end for combustion products or an alternative heating fluid and a header at the other communicating internalb with additional heat transfer surface positioned to one side and connected to a common discharge conduit, said additional heat transfer surface is externally partitioned from the cylindrical chamber by a dividing wall forming a separate passage or bypass and extended to create an annular passage concentric with the chamber, which is interrupted to provide a means of exit, thereby the heat transfer surface is divided in two with individual inlet and outlet passages, and the process fluid to be heated when presented to the heat exchanger is split into two streams, one of which passes over the additional heat transfer surface then directly to an exit duct, whilst the other passes around the cylindrical chamber in a clockwise or counter clockwise direction or combination of both, before discharging to a common or separate exit duct. À e

A significant feature of the proposed invention applied to gas to gas heat exchangers, is that the process gas to be heated is split into two streams each passing over a different area of the heat exchanger and therefore the cumulative pressure drop in known systems is eliminated. A damper or dampers can be used to vale the delta p and hence the volume of each stream, using the available heat transfer to trim the final temperature. The additional heat transfer surface is also protected from exposure to the hot cylindrical chamber by a partition wall, therefore thermal expansion is more uniform and predictable under conditions of variable load or temperature, mimmising thermal stress. The process gas stream, which is channelled to the hot cylindrical chamber, is a fraction of the normal volume and therefore the pressure drop and width of the annular gas passage can be relatively small In one embodiment of the invention the flow is split yet again so that half the volume passes around the annular passage clockwise and the other half counter-clockwise. In the latter case the radiation heat transfer component from the incandescent combustion chamber contacts the smaller concave area of the enclosing walls and the greater heat flux will increase their temperature, thus adding to the hot surfaces in intimate contact with the process gas. Therefore the delta t between the cooler process gas at its initial temperature and the hot combustion chamber will cool the metal walls more effectively and the heat transfer coefficient resulting from convection and radiation will improve the overall heat transfer.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figures 1.ú 2 are cross sections of a typical heat exchanger of known design.

Figure 3 illustrates a split-flow heat exchanger with tubular heat transfer surfaces.

Figure 4 is a sectional plan view of Fig 3 showing a bent tube arrangement.

Figures 5 6 illustrate a further embodiment of a split-flow heat exchanger.

Referring to Figures 1 2. The process gas I flows between walls of enclosure 2 and over several rows of tubes 3 welded between box headers 4, then around cylindrical chamber 5 constrained by curved was 6 finally exiting into duct 7. The hot combustion products or heating fluid 8 flows through chawher 5 and tubes 3 to the outlet 9 and transfers heat to the process gas 1 as it passes over the surfaces defined hereinbefore.

Referring to Figures 3 4. The process fluid or gas is split into two streams whereby stream 1 flows over tubes 14, constrained by the lower wall of enclosure 2 and the underside of dividing wall 12, to outlet 7. Stream 10 can be varied by damper 15 and flows along passage 11, constrained by the upper wall of enclosure 2, removable wall 13 and the topside of dividing wall 12, to distributor 16 followed by an annular passage formed between chamber 5 and circumferential wads 6a and 6b, whereupon the fluid divides into two further streams flowing clockwise and counter clockwise around the cylindrical chamber to outlet duct 7. The hot combustion products 8 flow through chamber 5 and tubes 14 to the outlet 9 and transfer heat to the process fluid 1 as it passes over the surfaces defined hereinbefore.

Referring to Figures 5 6. In this embodiment ofthe invention the process fluid is split into two streams whereby stream 1 flows over tubes 3 constrained by the lower was of enclosure 2 and the underside of dividing wall 12 to outlet duct 7. Stream 10 Bows along passage 11 constrained by the upper wall of enclosure 2, removable wall 13 and the topside of was 12, to an annular passage formed between the chamber 5 and circumferential wall 6 and thereby to outlet duct 7.

The hot combustion products 8 flow through chamber 5, tubes 3 and box headers 4 to the outlet 9 and transfer heat to the process fluid 1 as it passes over the subraces defined hereinbefore.

Additionally in Fig 3 when specifically applied to 'gas to gas' heat exchangers, wall 6b of the annular passage is heated from both sides, by radiation from the combustion chamber and by convection from the process gas stream 1 as it exits the tube array 14 at a burgher temperature.

The tube array 14 of Fig 4 comprises bent tubes of which the effective cross flow heat transfer surface is greater than that in systems of comparable rating with multiple passes of short straight tubes e.g. tubes 3 welded between box headers 4 of Figs 1 2. The presence ofthe box headers reduces the space available for cross flow area and increases the amount of less efficient longitudinal heat transfer surface. By replacement of the straight tubes 3 and intermediate box headers 4 with individual bent tubes 14 of Fig 4, the entry and exit losses and flow reversals within the heat exchanger are Agonized and the pressure drop through the tubing may be increased to optimize the internal longitudinal heat transfer process, rather than being compromised by considerations of overall fan power verses the pressure drop e.g. in typical 'gas to gas' heat exchangers of multi-pass tube and box header designs.

Each element of the bent tube array 14 of Fig 4, is of course free to expand independently and overall the net effect of the longitudinal expansion that occurs between each pass, results in very little displacement between the inlet register of chamber 5 and exhaust discharge 9, therefore the stress at the tube to tube plate interfaces are _.

In a further embodiment ofthe invention the process stream 10 of Fig 5 bypasses the tube array 3 and is channelled around the combustion chamber 5 in a single annular passage. Because the process stream is split, the passage can be relativeb narrow to accommodate the smaller volume of stream 10 and as it is at the initial temperature and in more intimate contact with the heat exchange surface, it will cool the metal surfaces more efficiently extracting the heat. Furthermore in 'gas to gas' heat exchangers ofthis type, the final row oftubes 3, next to chamber 5, which are shielded from radiated heat by the wall 12, passage 11 and the annulus formed by wall 6, will expand more evenly and the variation between the adjacent tubes/rows, together with the associated stress will be reduced.

Because the outa wall 6 of the annular passage is at a Louisa temperature than that of the combustion chamber, selection of the material for its construction can be made on the basis of conductive and re-radiation properties. These may be enhanced by adding e.g. deposits of a suitable metal to the inner surface to improve conductivity and/or a layer of ceramic/paint to the outer surface to reduce heat loss.

The combustion chamber 5 or the walls 6, 6a and 6b of Figs 3 and 5, may be convoluted or have attachments added to increase their surface area and make use of the improved heat transfer Alternatively baffles may be added to form channels in passage 11 to divert more volume of stream 10 to the hotter surfaces of the cylindrical chamber 5 and reduce the flow to the cooler surfaces.

Downstream mixing baffles may correct any variations in the process fluid temperature, due to uneven flow or heat transfer across the unit. Alternatively it is possible to arrange the two process streams from the split-flow heat exchanger to have different temperatures when required by adding a partition wall to duct 7.

In the split-flow bent tube variant of the invention, the amount of welding to make the tube to header connections for a given heat transfer area, is less than 50% of that required in known straight tube, box header systems. Indeed the total welding throughout the heat exchanger is substantially less due to the absence of intermediate box headers; therefore production costs are significantly reduced.

In a further embodiment of the invention applied to the heating of liquids, the split-flow heat exchanger can be used to provide e.g. hot water at two different temperatures using one heating source. In such a system, some of the water to be heated would pass over the additional heat transfer surface leaving at the required temperature, whilst that passing over the cylindrical chamber would exit by a separate conduit. A communicating conduit would be provided with a control valve between the exit conduits, to allow mixing between the streams and balance irregularities in the flow and temperature under variable load or demand.

The split-flow heat exchanger is suitable for waste heat recovery applications wherein hot flue gas from a gas turbine or similar high tanpee process passes through the cylindrical chamber and additional heat transfer surface and the process fluid to be heated passes over the heat transfer surfaces as descried hereinbefore. Alternatively the exhaust gas from a low-pressure process may be drawn through the heat exchanger using and induced draft fan.

Claims

1. A split-flow heat exchanger, comprising a cylindrical chamber with an inlet at one end for combustion products or an alternative heating fluid and a header at the other communicating internally with additional heat transfer surface positioned to one side and connected to a common discharge conduit, said additional heat transfer surface is externally partitioned from the cylindrical chamber by a dividing wall fomung a separate passage or bypass and extended to create an annular passage concentric with the chamber, which is interrupted to provide a means of exit, thereby the heat transfer surface is divided in two with individual inlet and outlet passages, and the process fluid to be heated when presented to the heat exchanger is split into two streams, one of which passes over the additional heat transfer surface then directly to an exit duct, whilst the other passes around the cylindrical chamber in a clockwise or counter clockwise direction or combination of both, before discharging to a common or separate exit duct.

2. A heat exchanger according to Claim 1 where the additional heat transfer surfaces comprises bent or straight tubes, finned tubes or the combination of other suitable surfaces or materials. : : .

3. A heat exchanger according to Claims 1 or 2 wherein the process fluid flows in one I'..

direction only around the annular passage surrounding the cylindrical chamber.

4. A heat exchanger according to Claims 1, 2 or 3 wherein a manual or powered damper is....

included to adjust the proportions ofthe process fluid flowing over each area. À

5. A heat exchanger according to Claims 1, 2, or 4 wherein a perforated plate or similar À ' device is included to distribute the process fluid evenly over the hot gas chamber.

6. A heat exchanger according to Clains 1, 2, 3 or 4 wherein the wall ofthe annular passage surrounding the cylindrical chamber is manufactured from a material or composite to improve its absorption, radiation and heat transfer whilst reducing heat loss from the external surface.

7. A heat exchange as claimed in any preceding claim wherein the cylindrical chamber is convoluted and/or has extended surface added.

8. A heat exchanger as claimed in any preceding claim wherein down stream mixing devices are included to ensure the process fluid stream is isothermal.

9. A heat exchanger as claimed in any preceding claim wherein a partition wall is included at the exit ofthe heat exchanger to maintain separation ofthe process fluid streams such that fiercer use may be made of any tine ddoe when required.

10. A heat exchanger as claimed in any preceding claim, wherein baffles are included in the bypass duct to direct more process fluid to the hotter surfaces of the cylindrical chamber.

11. A heat exchanger substandalb as herein described and illustrated in the accompanying drawings.