US3269457A - Method and apparatus for preventing deposit formation on a heat exchange surface - Google Patents

Description

Aug. 30, 1966 R. SHINNAR 3,269,457

METHOD AND APPARATUS FOR PREVENTING DEPOSIT FORMATION ON A HEAT EXCHANGE SURFACE Original Filed Aug. 1, 1962 United States Patent 3 269,457 METHUD AND APPARATUS FOR PREVEN'HNG DEPOSET FURMATEON ON A HEAT EXCHANGE SURFACE Reuel Shinnar, Princeton, NJ., assignor to The Technion Research and Development Foundation, Haifa, Israel, a corporation of lsrael Continuation of application Ser. No. 213,970, Aug. 1, 1962. This application June 21, 1965, Ser. No. 473,883 7 (Jlaims. (Cl. 165--1) The present application is a continuation of application Serial No. 213,970, filed August 1, 1962, and now abandoned.

The present invention concerns a method and means for effecting heat exchange between two fluids of which at least one is a liquid.

Where of two fluids exchanging heat in a heat ex changer one is a solution from which solids are liable to separate in the course of operation and form a deposit on the heat-transfer surfaces, the heat-transfer capacity of the heat exchanger is seriously impaired. Therefore, unless this deposit is removed continuously during operation or even better, is prevented from forming on the heattransfer surfaces, the operation of a heat exchanger has to be interrupted periodically for cleaning.

Accordingly, various means for inhibiting the formation of deposits on the heat-transfer surfaces have been devised. In one known arrangement the vessel or vessels through which the liquid passes, for example the tube of a shell-andtube type heat exchanger, is provided with continuously operating mechanical scrapers capable of preventing the formation of deposits on the inner walls of the tube and of removing any rdeposit already formed. While this arrangement is considered as quite eflicient, it increases considerably the costs of the heat exchanger and is therefore too expensive for many purposes.

It is also known to incorporate in the deposit-forming liquid chemicals capable of inhibiting the deposition. This approach is, however, limited in scope as on the one hand it is confined to selected systems while on the other hand it imposes severe temperature limitations.

Finally, it is also known to pre-treat the deposit forming liquid prior to its introduction into the heat exchanger so as to remove therefrom all or part of the dissolved matter, e.g., softening in the case of hard water. This procedure is, however, again expensive.

It is the object of the present invention to provide a new method and means for preventing the formation of deposits on the heat-transfer surfaces of a heat exchanger.

It is also among the objects of the invention to provide a method for the above purpose which is particularly adapted to the treatment of liquids which have relatively large amounts of substances which tend to deposit, such as saline waters.

The invention consists in a method of effecting heat transfer in a heat exchanger from or to a deposit producing liquid, comprising charging any duct of the heat exchanger that is designated for the passage of said liquid with a bed of solid granular material inert towards the liquid, positioning the heat exchanger substantially vertically and flowing the liquid through said bed or beds ascendingly at such a rate as to bring and maintain said bed or beds in a fluidized state.

The invention also consists in a heat exchanger for use with deposit producing liquids in which ducts designated for an ascending passage of the liquid contain a bed of solid granular material inert toward the designated liquid and capable of being brought by the passing liquid into a fluidized state.

In the system the granular material acts as a focus or nucleus on which substances in the liquid are deposited. Such deposits are relatively soft, so that they may be readily removed from the granules by washing, and the granules may then be recirculated thru the system. Because of this action, most of the substances deposit on the granules and the deposition on the walls of the apparatus is minimized, thus obtaining good heat conduction thru the walls for a longer time and avoiding frequent scraping or cleaning operations.

The choice of the inert, bed-forming solid depends to a large extent on the nature of the designated liquid. Generally, high-density particles permit higher liquid ve locities and a high intensity of the agitation in the fluidized bed since they cause a larger difference between the local velocities of the solid and liquid, thereby permitting the particles to penetrate through the boundary layer and to increase the scraping action.

On the other hand, too small particles and/ or particles of the same density as the liquid will tend to move with the latter. Accordingly lead particles, e.g., lead shot or particles of other heavy metals are preferable where a strong scraping action is desired th ough any inert particles may be used and even particles consisting of the deposit-forming salt or salts themselves may sometimes be of advantage.

By a suitable choice of the bed the ensuing prevention of deposit formation is very pronounced. This is so because in a dense fluidized bed the solid particles impinging on the heat-transfer surface have a strong scraping effect by which any scale already formed is scraped off.

At the same time temperature gradients inside the liquid are reduced owing to the mixing action of the fluidized bed.

Finally, owing to the presence of the fluidized bed much of the deposit that would have formed normally on the heat-transfer surfaces now forms on the surfaces of the bed-forming solid particles and can thus easily be removed in the course of operation by replacing the fluidized bed without interrupting the operation as known per se. Moreover, some of the precipitated solids are not deposited at all; they form around nucleation centers provided by solid particles scraped oif the heat-transfer surfaces and are entrained with these particles by the flowing liquid.

All the above effects act in a cumulative manner with the net result that a heat exchanger used for heat transfer from or to a deposit-producing liquid and operated in accordance with the invention has a considerably higher heat-transfer capacity than a similar exchanger not provided with fluidizable beds but otherwise operating under the same conditions and with the same fluids. Therefore, when proceeding in accordance with the invention, smaller heat-transfer surfaces are required than would otherwise be the case. Thus, for example, the invention can advantageously be embodied in any water distillation arrangement operating by forced circulation evaporation, e.g., a water desalination plant.

The invention is applicable both where the deposit producing liquid is the heat acceptor or the heat donor. Likewise, the application of the invention is not confined to a particular type of heat exchanger, provided that the ducts designated for the passage of the deposit-producing liquid enable the creation therein of a fluidized bed. For example, a multi-tube shell-and-tube type heat exchanger may be used and either the shell or the tube may be fitted with fluidized beds depending on where the deposit-producing liquid is designated to pass.

The invention is further illustrated by the following example described with reference to the annexed drawing which is a diagrammatic illustration of a heat-exchange arrangement according to the invention.

The heat-exchange arrangement illustrated in the drawing comprises a vertical shell-and-tube heat exchanger 1 comprising a shell 2 and one single coaxial tube 3 fitted near its bottom with a liquid-permeable packing 4 serving as a retainer for a bed 5 of solid granular particles. In its fixed state bed 5 occupies only the lower portion of tube 3 up to a level 6, indicated by crossing lines, whereas in the fully fluidized state the bed expands up to level 7.

The top section of a vessel 8 is linked with the bottom of column 3 by means of pipes 9 and 10 via a pump 11 and a valve 12. The top of tube 3 is linked by means of a double bent pipe 13 with the bottom section of container 8. Container 8 is further fitted with a double bent pipe 14 comprising a valve 15 and serving for overflow discharge. At its bottom container 8 is fitted with a discharge pipe 16 comprising a valve 17 and serving for the withdrawal of samples.

A valve controlled pipe 19 serves for the introduction of fresh liquid into the system, and the pressure drop between the bottom and top of tube 3 can be measured by means of a manometer 20.

In operation, liquid to be cooled passing thru shell 2 transfers heat to the liquid in tube 3. The latter liquid containing granular material passes upwardly, by action of pump 11 forming a fluidized bed on which the scaleforming material is deposited as a soft coating. The flow detaches the coating which forms a sludge with the liquid, the sludge passing into separator 8 where the scale-forming material is deposited and passes out thru exit 16. The liquid freed from the scale-forming material is then returned thru pipe 9 to the fluidized bed for recirculation.

The invention is of particular advantage for use in desalination of liquids containing at least 200 ppm. of

salts. It is also of advantage in evaporators where the liquids contain several grams per liter of material which is deposited from solution. In evaporators operating at about 140 C. or higher, the present invention is capable of preventing undue deposition of scale, whereas known methods 'have failed.

A practical experiment was conducted under the following conditions: Tube 3 measured 2" in diameter and 1.20 m. in height while the diameter of shell 2 was 4" and its height 70 cm. The bed consisted of a charge of lead shots of 0.5 mm. average diameter. The fixed bed was 35 cm. high.

Hard water containing 335 ppm. of 'CaHCO was circulated through tube 3 at a rate of approximately 1./minute as a result of which the bed was fluidized and expanded up to a height of l m. Fresh water was introduced via pipe 19 at a rate of 1.0 l./minute. Steam was condensed in the outer tube at 130 C. The circulating water was heated inside tube 3 up to 95-105 C. and the soheated water was discharged into vessel 8. The hot Water accumulating in the vessel was partly recycled and partly discharged through overflow pipe 14 at a rate of 1.0 l./minute. The discharged water contained 250 ppm. of CaHCO The apparatus was operated in this manner for two weeks continuously after which it was found to be completely free of scale.

The experiment was repeated with the same apparaus under the same conditions but without a bed in tube 3 and with the further difference that the rate of fresh water addition and hot water withdrawal was reduced to 0.5 l./minute since the heat-transfer coeflicient was only half as large as when a fluidized bed was employed. The heat exchanger showed heavy scaling after one week.

Applicant is aware that fluidized beds have been proposed in heat transfer apparatus. But they were for the purpose of improving the transfer of the heat. The medium in which the fluidized bed was used was gaseous. The present invention is directed to the prevention of scale formation on the walls, and the formation of the scale in an easily removable form, which allows for continuous removal of the scale (or any other solids formed). The invention is in a method of operating a heat exchanger under conditions of heavy scale and crust formation at the walls.

Such heavy scaling conditions occur in the evaporation of sea water at high temperatures, and certain types of crystallizers, where the crystals tend to adhere to the wall and form an insulating hard layer. In this case the fluidized bed, if correctly operated, allows:

(a) Continuous operation for long times without clean- (b) Continuous high heat transfer coeflicients due to the prevention of scale; and

(0) Continuous efficient removal of the scale forming deposit from the recirculating liquid.

The prevention of scale deposit is not only due to the scouring of the walls. Applicant found in his experiments that the concentration of scale forming ions was considerably lower in the fluidized bed. This is due to the fact that the fluidized bed provides alternative nucleation sites for the formation of the scale inside the body of the hot liquid.

Another advantage of the fluidized bed is the fact that the scale can easily be removed from the recirculating liquid by standard separation methods. This would not be the case for a recirculating slurry, such as used in the prior art. In that case, the slurry contains both the scale and the scouring material, and separation would be diflicult.

What is claimed is:

1. A method of heating a liquid containing a substantial amount of scale-forming material comprising heating said liquid by heat exchange introducing into said liquid a bed of solid granular material which is inert to said liquid, passing said liquid upwardly thru a substantially vertical duct at a suflicient velocity to fluidized said bed, causing scale-forming material to deposit on said granular material as a soft coating, removing said liquid containing said material as a sludge from the upper end of said duct, thereby separating said granular material from said liquid, said granular material remaining in said duct as a fluidized body, and thereafter removing said deposit from said liquid.

2. A method according to claim 1 characterized in that said granular material is metallic.

3. A method according to claim 1 characterized in that said granular material is lead shot.

4. A method according to claim 1 characterized in that said freed liquid is returned to the lower end of said duct.

5. A heat exchanger comprising at least one substantially vertical duct, an inlet for liquid at the lower end thereof and an exit at the upper end thereof, said liquid containing scale-forming material, a bed of granular material in said duct, means at said inlet for flowing liquid upwardly thru said duct at a suflicient pressure to fluidize said bed, said granular material being adapted to have said scale-forming material deposited thereon as a soft coating, and said flow of liquid being adapted to form a sludge with said scale-forming material, a separator for removing said scale-forming material, said separator being adjacent to said duct, a connection from said exit to said separator, means at the lower end of said separator for removal of deposited scale-forming material, and means at the upper part of said separator for the outflow of said liquid.

6. A heat exchanger according to claim 5 characterized in that means are provided for in the upper part of said separator for return of said liquid to said flowing means.

7. A heat exchanger according to claim 6 characterized in that said separated liquid is returned to the lower 15 end of said duct.

References Cited by the Examiner UNITED STATES PATENTS 1,795,348 3/1931 Schmidt 165-419 X 1,992,472 2/1935 Craig 165-l 19 X FOREIGN PATENTS 590,907 1/1960 Canada.

OTHER REFERENCES Othmer, Donald R; Fluidization, Reinhold Publishing Corp, N.Y., 1956 (page 4 relied on).

ROBERT A. OLEARY, Primary Examiner.

FREDERICK L. MATTESON, J 11., Examiner. N. R. WILSON, Assistant Examiner.

Claims (1)

1. A METHOD OF HEATING A LIQUID CONTAINING A SUBSTANTIAL AMOUNT OF SCALE-FORMING MATERIAL COMPRISING HEATING SAID LIQUID BY HEAT EXCHANGE INTRODUCING INTO SAID LIQUID A BED OF SOLID GRANULAR MATERIAL WHICH IS INSERT TO SAID LIQUID, PASSING SAID LIQUID UPWARDLY THRU A SUBSTANTIALLY VERTICAL DUCT AT A SUFFICIENT VELOCITY TO FLUIDIZED SAID BED, CAUSING SCALE-FORMING MATERIAL TO DEPOSIT ON SAID GRANULAR MATERIAL AS A SOFT COATING, REMOVING SAID LIQUID CONTAINING SAID MATERIAL AS A SLUDGE FROM THE UPPER END OF SAID DUCT, THEREBY SEPARATING SAID GRANULAR MATERIAL FROM SAID LIQUID, SAID GRANULAR MATERIAL REMAINING IN SAID DUCT AS A FLUIDIZED BODY, AND THEREAFTER REMOVING SAID DEPOSIT FROM SAID LIQUIDS.

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