GB2052719A

GB2052719A - Method and Apparatus for Cleaning and Increasing the Efficiency of a Heat Exchanger

Info

Publication number: GB2052719A
Application number: GB7921031A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-06-16
Filing date: 1979-06-16
Publication date: 1981-01-28
Also published as: GB2052719B

Abstract

For improving the heat transfer efficiency of a heat exchanger through which is flowing a heat delivering fluid, and for subjecting the heat exchange surfaces 2 contacted by said fluid to a cleaning action during operation, a compressible fluid is injected under pressure into the heat- delivering fluid so that it expands in and accelerates the latter fluid. The compressible fluid is preferably supplied by a perforate tube 10 into a preliminary chamber 9 which communicates with the interior of the heat exchanger housing 1 via openings 11. The heat delivering fluid flows in the housing 1 and may be water, and the compressible fluid may be steam or compressed air, which may be injected continuously or intermittently. The heat transfer surfaces 2 may be provided by a finned tube coil through which passes a heat receiving medium. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Increasing the Efficiency of a Heat Exchanger This invention relates to a method and an apparatus for increasing the net efficiency of a heat exchanger and for cleaning it as well so that the efficiency achieved in practice is improved to a value substantially equal to the theoretically highest possible.

When designing a heat exchanger there are many differenty factors having an influence on the efficiency achieved, e.g. the size and shape of the heat transfer surfaces, the choice of material in these surfaces, their surface structure and the flow velocity past them.

A factor contributing to an increased efficiency of a heat exchanger is a high flow velocity of the media flowing through the heat exchanger relatively to the heat transfer surfaces thereof.

However, if the geometrical shape of the heat exchanger and the volume flow therethrough are given there are, when considering heat exchangers of conventional types, no possibilities of changing the flow velocity as this quantity is determined by the volume flow and the cross sectional area available.

Another factor having an influence on the efficiency obtainable in practice is the tendency of a heat exchanger to get dirty. Thus, if the heat transfer surfaces are covered by materials transported by or precipitated from the media flowing through the heat exchanger the result will be that these materials will have an insulating effect that can drastically decrease the efficiency.

Heat exchangers of conventional types can be very difficult to clean which implies that the efficiency achieved in practice often is much lower than that efficiency for which the heat exchanger theoretically is designed.

This invention has for its object to provide a method of the kind indicated above and further a method that eliminates the problems encountered when using heat exchangers of conventional types.

According to the invention this object is achieved in that a compressible fluid is injected under pressure into the fluid flowing through the heat exchanger, that the mixture so obtained is allowed to expand so that the flow velocity thereof, at least in a part of the heat exchanger, is increased which results in an increase of the heat transfer proper and furthermore in a cleaning of the heat transfer surfaces so that a further increase of the efficiency is achieved.

If the fluid flowing through the heat exchanger is a liquid mainly consisting of water there is, according to the invention foreseen that the liquid is pressurized that the compressible fluid is mixed with the pressurized liquid and allowed to expand from a pressure higher than the pressure of the liquid to the pressure there of so that a high flow velocity is achieved. When mixing-the compressible fluid and the liquid also the mixture gets a high flow velocity. In one embodiment the compressible fluid is steam. In this embodiment there is achieved, by means of the heat input from the steam, a higher temperature and a further improved heat transfer during the condensing phase. In another embodiment there is injected pressurized air and the pressure of the mixture so obtained is lowered so that expansion occurs at least in a portion of the heat exchanger.

The invention also has for its object to provide and apparatus for carrying the method indicated above into effect. According to the invention this object is achieved if the apparatus comprising a flow chamber for the heat delivering fluid said flow chamber at least partially being defined by heat transfer surfaces, is characterized in that a mixing chamber is provided in or in the flow direction up-stream the flow chamber, that the mixing chamber has one inlet for heat delivering fluid and one inlet for compressible fluid said mixing chamber having flow connection with the flow chamber via a pressure reducing means.

In a preferred embodiment of the apparatus there is, according to the invention, foreseen that the mixing chamber is provided as a pre-chamber exposeable to pressure and provided at the inlet portion of the flow chamber, that the pressure decreasing means comprises a partition wall between the pre-chamber and the flow chamber said pressure decreasing means having at least one opening of small cross sectional area relatively to the cross sectional area of the flow chamber.

According to the invention there is further foreseen that the inlet of the pre-chamber for compressible fluid comprises a substantially closed channel means extending into the prechamber and having connection to a source of pressurized compressible fluid the walls of said channel means being provided with a number of small openings in the pre-chamber.

The invention is now to be described more in detail below reference is being made to the accompanying drawings in which fig 1 shows the principle of an apparatus for carrying the inventive method into effect. Fig 2 shows examples of the temperature distribution curves of the media through the heat exchanger. Fig 3 shows a preferred embodiment of a heat exchanger designed for carrying the inventive method into effect. Fig 4 shows another embodiment of the heat exchanger.

ln fig 1 there is shown a much simplified apparatus for carrying the inventive method into effect. The apparatus comprises an outer housing 1 in which the heat transfer surfaces 2 of the heat exchanger are provided said heat transfer surfaces can, for example, be constituted by a coil of tubes. The heat transfer surfaces 2 shown in fig 1 can be designed in any conventional manner and it is preferred that they almost completely are occupying the space defined by the outer housing 1 and a partition wall 3 provided therein. This implies that the partion of the apparatus defined by the housing 1 and the partition wall 3 constitutes the heat exchanger proper. The apparatus shown in fig 1 is further provided with an inlet A for supplying a heat delivering medium and an outlet B for this medium.The apparatus further has an inlet C for heat receiving medium and an outlet D for this medium. Finally, the apparatus has an inlet E for supplying a compressible fluid. The heat delivering medium is in most cases a liquid mainly consisting of water.

The left hand portion of the apparatus according to fig 1 is provided as a pre-chamber 9 which is defined by the partition wall 3 and the left hand portion of the housing 1. The inlet A for heat delivering fluid is exhausting into this prechamber 9. Also the inlet E for the fluid having a lower density (the compressible fluid) than the heat delivering fluid is exhausting into this prechamber. The inlet E is connected to an injection device 10 provided in the pre-chamber. In a practical embodiment this injection device 10 can be provided as a pipe closed in one end and having its envelope surface perforated by a number of small openings. Further, the partition wall 3 separating the pre-chamber 9 from the heat exchanger proper is provided with openings 11 the number and sizes of which are carefully calculated.

To carry the inventive method into effect the apparatus described above is working in the following manner: The heat delivering medium, e.g. water, is supplied at a temperature T5 (vide fig 2A) via the inlet A to the pre-chamber 9 which is completely filled and which is held under a certain pressure.

In a practical operating condition T5 may amount to 900C. Via the inlet E and the injection device 10 there is supplied stream having a pressure higher than the pressure existing in the prechamber 9, and due to this high pressure, having a temperature T6 considerably higher than the temperature of the heat delivering fluid present in the pre-chamber. This implies that the temperature of the fluid in the pre-chamber will increase due to the energy supply caused by the steam injection. Also boiling can occur. However, the pressure of the steam will decrease due to expansion through the openings 11 so that a lower saturation pressure corresponding to the temperature T4 will result. The steam-water mixture delivers heat in the heat exchanger so that the steam is condensed.At the same time, however, the pressure drop through the heat exchanger results in the decrease of the saturation pressure and the saturation temperature. When all steam has been condensed the temperature of the liquid will decrease in a linear ratio to the decrease of enthalpy until the exit temperature T3 is reached.

Injecting a large amount of steam results in a decrease of the average density of the fluid flowing through the heat exchanger which results in an increase of the flow velocity of the fluid along the heat transfer surfaces 2 which in turn results in an increase of the heat transfer coefficient K as KNV" where K=heat transfer coefficient V=flow velocity a=a numeral (ofter 0.7) Except the increase of the flow velocity along the heat transfer surfaces 2 mentioned above there is achieved, by means of the described arrangement, a possibility to carefully control the exit temperature at D. This is achieved by regulating the flow of steam supplied at E.

Further, there is created a very advantageous possibility to clean the heat exchanger when in operation by means of the very high flow velocity that can be achieved if the injection of fluid via the inlet E is carried out with an appropriate volume flow. Apart from the pure mechanical cleaning effect that is obtained by means of the high flow velocity in the heat exchanger there is also achieved a possibility of increasing the surface temperature of the heat transfer surfaces 2 and particularly those heat transfer surfaces provided in vicinity of the partition wall 3. This possibility of increasing the temperature is of great importance since on these surfaces there are often deposited great quantities of impurities dissolved in the heat delivering fluid and these deposited impurities can be dissolved again and carried out from the heat exchanger when the temperature is raised.

According to the invention it is possible tb operate the apparatus described above in several ways. In the first place the steam injection can be carried out continuously which corresponds to a continuous expansion of the fluid in the heat exchanger and an increase of the flow velocity therethrough. Such a way of operation results in an increased heat transfer coefficient due to the increased flow velocity but offers also, as mentioned above, a means for regulating the exit temperature. In the second place the projection of steam can be carried out in an intermittent or pulsating way. At the intermittent operation the steam injection is of such power that the prechamber is emptied from liquid. At first this results in a very rapid exhausting and expanding of the fluid present in the pre-chamber through the heat exchanger and then in a steam blowing through the heat exchanger. This way of operation is mainly intended for cleaning the heat exchanger. When injecting steam in a pulsating way the purpose is to achieve an increase of the heat transfer coefficient and to avoid deposition of impurities in the heat exchanger as well. In such a way of operation the pulse period can be approximately of the same length as the time of flow of the water through the heat exchanger.

According to the invention it is also possible to use compressible media other than steam. In practice pressurized air can be used and this pressurized air is injected via the inlet E in the same manner as the steam. When using pressurized air no temperature rise occurs in the pre-chamber and thus no boiling can occur in the heat exchanger unless the temperature of the fluid supplied via the inlet A is above the boiling point at the pressure existing in the heat exchanger. In most cases, however, there is achieved by the expansion of the pressurized air at the openings 1 'a significant decrease of the average density of the medium flowing through the heat exchanger proper whereby an increase of the flow velocity is achieved. The corresponding temperature distribution curve is shown in fig.

2 B.

As was the case when using steam the pressurized air can be injected continuously and intermittently or.pulsatingly as well. When injecting continuousfy the aim is the above mentioned increase of the flow velocity through the heat exchanger while, when injecting intermittently the aim is only to clean the-heat exchanger. When injecting in the pulsating manner the object is to achieve both cleaning effect and the increase of the heat transfer coefficient.

In fig,,3 there is shown, one embodiment of the .invention, having many practical advantage$. In this embodiment the pre-chamber9 is provided as a vertical central tube or housing the envelope surface3 of which corresponds to the partition wall 3 described above. Provided around the pre chamber sphere are a number of plane helixes of finned tubes piled on each other but separated by means of baffle plates 14. The inner ends of the helixes are connected to a vertical- exhaust pipe 1 2 the upper end of which constitutes the outlet D of the heat exchanger. In a similar manner the outer ends of the helixes are connected to a vertical inlet tube, 1. and the free end thereof constitutes the inlet C of the heat exchanger.The baffle plates 14 have a snug (sealing) fit to the outer surface of the pre-chamber 9 and are provided in such distances above each other that the finned tubes 2 just can be received therebetween. In this way there is provided radially directed flow passages about the finned tubes from the wall 3 of the pre-chamber radially outwardly to the outer periphery of the heat exchanger where arrows B are representing the outlet for heat delivering fluid. As was the case in the embodiment of fig 1 the pre-chamber is perforated by a number of openings 11 which have carefully selected number and sizes. In a modified embodiment the number of the openings 11 or possibly their sizes can be adjusted to values which are appropriate as regards the flow through the inlet A at a certain moment.As was the case in the embodiment of fig 1 there is provided inside the pre-chamber 9 an injecting device 10 having the shape of a tube closed in one end and provided with a great number of small openings through the envelope surface thereof. The end of the tube 10 not closed constitutes the inlet E, e.g. for injecting steam.

The apparatus shown in fig 3 has the same principle of operation as the apparatus of fig 1.

Thus, it is possible to continuously supply via the inlet E steam and pressurized air as well. It is also possible to use an intermittent or pulsating supply in order to-achieve the advantages mentioned above. Finally, i4is also possible to reverse the flow direction through the helixes of finned tubes 2. In this case the directions of arrows C and D are also reversed and further the' heat exchanger is working adcording the parallel flow principle instead of the counter flow principle.

In fig 4 there is shown another embodiment ot the apparatus for carrying the inventive method into effect. This apparatus comprises an outer housing 1 in which there -is centrally provided a column having provided around it coils of tubing coupled in parållel and possibly in the form of finned tubings 2. These coils of tubing 2 are connected to an inlet pipe at arrow C at an upper portion of the housing 1 and to an outlet pipe at arrow D at a lower portion of the housing 1.

Further, the housing 1 has an inlet A for heat delivering fluid and an outlet B provided at the periphery of a lower portion of the housing. By means of a partition wall 3 an upper portion 9 of the housing 1- is separated from the lower portion receiving the coils df tubing 2-.'The-partition wall 3 is provided with openings 11 as was the case in the embodiments of figs t and 3 and is, therefore, together with the upper portion of the housing 1 definingtthe pre-ehamber 9.Inside the pre chamber 9 the inlet E-is connected tb an injecting, device -10 e.g; constituted by a- pipe closed ~at one end. and having its envelopesiirface perforated.

The apparatus according to fig 4 can operate in the same manners as those according to figs 1 and 3..Thus; it is possible to inject via the inlet E steam and pressurized air as well, both continuously and pulsatingly. In these alternatives the advantages mentioned above are achieved.

In a modified embodiment the pre-chamber and the injecting device provided therein need not provided at the inlet of the heat exchanger proper. Thus, it is possible to divide a heat exchanger into several sections in which the pressure drop occurs step by step and where in the different sections there are provided pre chambers for injecting e.g. steam. As an alternative one section can constitute the pre chamber of a down-stream section.

It may be an advantage if the tubings 2 are provided with finns 1 5 as is shown in fig 3. Even if the compressible fluid can only clean the passages between the tubings leaving plugs 1 6 between the tubes the result is that most parts of the finns are exposed to the heat delivering fluid.

Claims

1. A method of increasing the efficiency of a heat exchanger through which is flowing a heat delivering fluid, characterized in that a compressible fluid under pressure is injected into the heat delivering fluid, that the mixture so obtained is allowed to expand for obtaining an increased flow velocity at least in a portion of the heat exchanger so that an increase of the heat transfer proper is achieved and the heat exchanger is cleaned when in operation.

2. A method of claim 1 and in which the heat delivering fluid is a liquid, e.g. mainly consisting of water, characterized in that the liquid is kept under pressure and steam is injected into the liquid, that the pressure of the mixture so obtained is lowered so that boiling and expansion occur at least in a portion of the heat exchanger and that a certain steam content is kept at least in a portion of the heat exchanger.

3. A method of claim 1 and in which the heat delivering fluid is a liquid, e.g. mainly consisting of water, characterized in that the liquid is kept under pressure and pressurized air is injected into the liquid, that the pressure of the mixture so obtained is lowered so that expansion occurs at least in a portion of the heat exchanger.

4. A method of either claim 1 or 3, characterized in that the steam or the pressurized air is supplied continuously.

5. A method of either claim 2 or 3, characterized in that the steam or the pressurized air is supplied intermittently or pulsatingly.

6. A method of claim 2, characterized in that the steam and the pressurized air are supplied at the same time.

7. An apparatus for carrying the method of claim 1 into effect and comprising a flow chamber for the heat delivering fluid said flow chamber being defined at least partially by heat transfer surfaces (2), characterized in that a mixing chamber (9) is provided in or up-stream of the flow chamber, that the mixing chamber has an inlet (A) for heat delivering fluid and an inlet (E) for compressible fluid and that the mixing chamber is provided in flow connection with the flow chamber via a pressure reducing means (3, 11).

8. An apparatus of claim 7, characterizediin that the mixing chamber is provided as a prechamber (9) exposable to pressure and situated at the inlet end of the flow chamber, that the pressure reducing means comprises one partition wall (3) provided with at least one opening (11) and separating the pre-chamber from the flow chamber said opening being of small cross sectional area relatively to the cross sectional area of the flow chamber.

9. An apparatus of claim 8, characterized in that the inlet (E) for supplying compressible fluid to the pre-chamber (9) comprises a substantially closed channel (10) extending into the prechamber and being connected to a source of pressurized compressible fluid said channel having its wall provided with a number of small openings inside the pre-chamber.

1 0. An apparatus of either of claims 7-9, characterized in that the mixing chamber (9) is provided centrally surrounded by the pressure reducing means (3, 11) and the flow chamber (2, 14).

11. An apparatus of claim 10, characterized in that the pressure reducing means is provided as a perforated substantially cylindrical housing (3) while the heat transfer surfaces (2) are provided as plane possibly finned helixes of tubing arranged coaxially around the housing and separated by baffle plates (14) preventing axially directed flow.

12. A method of increasing the efficiency of a heat exchanger substantially as hereinbefore described.

13. A heat exchanger apparatus for increasing the efficiency thereof substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.