US3926586A

US3926586A - Method and apparatus for cooling a gas with water droplets

Info

Publication number: US3926586A
Application number: US427060A
Authority: US
Inventors: Sigvard Matts
Original assignee: Svenska Flaktfabriken AB
Current assignee: Svenska Flaktfabriken AB
Priority date: 1973-12-21
Filing date: 1973-12-21
Publication date: 1975-12-16
Anticipated expiration: 1992-12-16

Abstract

Electrically charged water droplets are injected into a stream of hot gas to cool the gas before it enters an electrical precipitator, and an alternating electric field is applied to increase the speed of motion of the droplets with respect to the gas, thereby to accelerate the cooling and permit use of a smaller cooling chamber. The electric field may be sinusoidal or square wave, may also have a DC component, and may be singlephase or multi-phase. The electrodes may be spaced-apart parallel planes, or concentric tubes, or planar arrays of generally cylindrical members, for example.

Description

Matts States atet [191 Dec. 16, 1975 [75] Inventor: Sigvard Matts, Vaxjo, Sweden [73] Assignee: Aktiebolaget Svenska Flaktfabriken,

Nacka, Sweden [22] Filed: Dec. 21, 1973 [21] Appl. No.: 427,060

[52] US. Cl. 55/10; 55/122; 55/123;

[51] Int. Cl. 803C 3/16 [58] Field of Search 55/2, 7, 8, 10, 101, 107, 55/117, 122,123,139

[56] References Cited UNITED STATES PATENTS 1,472,231 10/1923 Schmidt 55/138 1,865,907 7/1932 Heinrich 55/123 1,937,265 11/1933 Crowder..... 55/122 2,086,063 7/1937 Brion et al.. 55/123 2,525,347 lO/195O Gilman 55/107 3,656,440 4/1972 Grey 55/138 3,739,552 6/1973 Webster et al 55/138 3,807,137 4/1974 Rommell 55/10 FOREIGN PATENTS OR APPLICATIONS 25,158 8/1930 Australia .1 55/122 556,939 l0/l943 United Kingdom .1 55/107 Primary Examiner-Bernard Nozick Attorney, Agent, or Firm1-1owson and Howson [5 7] ABSTRACT Electrically charged water droplets are injected into a stream of hot gas to cool the gas before it enters an electrical precipitator, and an alternating electric field is applied to increase the speed of motion of the droplets with respect to the gas, thereby to accelerate the cooling and permit use of a smaller cooling chamber. The electric field may be sinusoidal or square wave, may also have a DC component, and may be single phase or multi-phase. The electrodes may be spacedapart parallel planes, or concentric tubes, or planar arrays of generally cylindrical members, for example.

9 Claims, 3 Drawing Figures US. Patent Dec. 16,1975 Sheet1of3 3,926,586

Fig.7

U.S. Patent Dec. 16, 1975 Sheet20f3 3,926,586

METHOD AND APPARATUS FOR COOLING A GAS WITH WATER DROPLETS This invention relates to a method and apparatus for causing solid or liquid particles finely distributed in a gas to cool said gas by oscillating therein.

A great number of industrial processes include mixing operations by which solid or liquid particles in the most effective way are to be brought into close contact with gases. These processes often are carried out in recipients or continuous vessels, such as cooling towers or the like, which usually are to be given large dimensions and thereby become expensive. Examples of such cooling towers are those applied in connection with electrostatic precipitators for separating dust from hot gases and cooling the gas prior to its entering the precipitator.

The cooling often is effected by injection of water into the gas, whereby part of the heat content of the gas is consumed in heating the water droplets, and a substantially greater amount is used to vaporize said droplets. During their heating the water droplets are surrounded by a cooled gas layer, which during the subsequent vaporization of the droplets is completed by a layer of vapour. These layers are heat-insulating and delay the vaporization, which thereby, for being carried out, requires relatively much time and a relatively long passage way through the cooling towers. The cooling towers, consequently, must be large and expensive. It is, therefore, desired to speed up the cooling process in order thereby to reduce the dimensions and costs of the towers.

One method of accelerating the cooling process is to cause the water droplets to move rapidly relative to the gas. Thereby the droplets are moved out of the heatinsulating gas layer and caused to intimately contact the hot gas. Such an agitation of the droplets in the gas can now be effected by a method in which, according to the invention the droplets are charged electrically, preferably in connection with their injection into the gas, and thereafter together with the gas are subjected to the action of force from an electric field. In order to restrict the vaporization to taking place in a relatively small activity range, the droplets preferably are caused to move in a zigzag path and, therefore, the electric field has to alternate in direction, preferably periodically. To subject the water particles to the greatest possible action or force, the field strength must change rapidly from a fully positive to a fully negative value. The curve shape, thus, is preferably square, but a sinusoidal field strength variation may be used. In the latter case, however, a somewhat greater vaporization zone is required for the same cooling capacity of the cooling tower.

The zigzag movement relative to the gas is brought about by coaction of a horizontally oriented alternating field with the force of gravity. Hereby, thus, the water droplets move down faster than the gas. The height of the cooling tower which is determined by the require' ment that all water must have been vaporized before the gas leaves the tower, can now be reduced compared with the height required if the particles were subjected only to the gravity effect.

The tower height, however, can be reduced still more according to one preferred feature of the invention, if the water droplets are given a motion component directed against the gas stream, thereby causing the drop lets to remain longer in the vaporization zone. This is effected by an electric field produced by three-phase or multiphase charged electrodes. The water droplets, being subjected to attracting and repelling forces from all electrodes nearby, move to and fro in the gas in a way they would if they were influenced by the force of an imaginary resultant electrode located somewhere between the real electrodes and moving relative to these electrodes responsive to the phase succession of the electrode potentials. If the electrodes are located in parallel with each other in one plane, the imaginary resultant electrode migrates in the plane, and its action of force on the water droplets tries to cause the droplets to move in a continuous zigzag path in a direction determined by the phase succession.

By a suitable choice of phase-succession potential, electrode gas and frequency, the water droplets can be caused to move in a counterflow to the gas with a speed such that they substantially remain in the vaporization zone until they are completely vaporized.

For obtaining a field at all, there is always required an antipole (though at times this may be infinitely far away). One may cause negatively charged particles to move from a negative electrode, but then other negative particles near the necessary positive electrode will be attracted thereto. Only a field is capable of producing a force. An electrode at one end of a field line always has more positive (or negative) potential than the electrode at the other end. Otherwise there would be no field, and no force would be developed.

The invention is described in greater detail in the following, with reference to the accompanying drawings showing the invention applied by way of example, viz. in an arrangement for cooling hot dust-loaded gas prior to its entering an electrostatic precipitator for separation of the dust.

FIG. 1 shows in a schematic way a typical general arrangement of cooling tower and electrostatic precipitator,

FIG. 2 shows in a schematic way the location of sprinklers and one-phase charged electrodes in the cooling tower,

FIG. 3 shows in a schematic way the location of three-phase charged electrodes in the cooling tower.

In FIG. 1, the designation 1 refers to the tower inlet for gas which via the distribution means 2 is distributed in the tower space 8. The tower outlet 3 extends to an electrostatic precipitator 5. Dust possibly precipitated in the tower is discharged at 4.

In connection to the tower inlet water is sprayed into the gas by sprinklers 7 supplied with water from a supply line 6 common to the sprinklers. The water is vapoiized successively in the tower space 8. The tower size is chosen such that all water is vaporized before it arrives at the outlet 3.

FIG. 2 shows, for better clarity, only the range 2-8 in FIG. 1. The sprinklers 7 and their associated annular counterelectrodes 9 are connected to a source for high direct potential 10. When the counterelectrodes 9 are given positive polarity in relation to the sprinklers 7, the water droplets leaving the sprinklers are charged highly negatively in a known manner. The droplets follow the gas downwardly in the tower and enter the space between the plane electrodes 12. These elec trodes are located in parallel with the gas stream in a number and with a spaced relationship determined by the cooling tower capacity. The electrodes, according to the Figure, are connected altematingly to the poles of a high one-phase alternating potential source.

In a certain moment when the electrode 12 farthest to the left is presupposed to be positive, each negatively charged droplet moves to the left, designated by 14. In the next moment the polarisation has been reversed and the droplet moves to the right, designated by 15. By action of the force of the electrodes as well as of the gravity and flowing gas, each water droplet performs a zigzag movement 16 with a speed relative to the gas which by some tenth power exceeds the speed of free fall in the gas.

FIG. 3 shows another electrode arrangement causing the water droplets to move against the gas stream. The droplets thereby remain for a longer period in the tower space, which thereby is given a greater capacity. The electrodes in this arrangement are of rod or tube shape and located in-one or several planes. depending on the tower capacity. Each plane includes a number of mutually parallel electrodes 12, located perpendicu larly to the gas stream and consecutively connected to the poles of a source of high three-phase potential 17.

The succession of phases being R, S, T, as indicated in the Figures, the electric fields 20 between the electrodes together cause the water droplets to perform an oscillating movement with an upwardly directed component as shown at 19. In the immediate vicinity of each electrode element 12 the force field is highly curved, and in this field every partial movement of the oscillation of the droplets is given a curved shape. The centrifugal force in this movement has the tendency of moving the droplet in the average away from the electrodes 12 and thereby reduces the risk of water coming into contact with the electrodes.

In FIG. 2 the counterelectrodes shown are of annular shape with positive polarity. They may have many other shapes, and their polarity may be negative. The Figure further shows plane electrodes, which, however, may be shaped in various different ways, for example comprise concentric cylinders placed on the outside of each other. The charging of the droplets may also be effected by other known methods, for example by a corona discharge as in an electrostatic precipitator.

The principle of agitating particles in a gas may also be applied to rendering other industrial processes more effective.

At absorption processes, for example, it may at times be desirable rapidly-to remove the absorbent (particles, droplets) from the gas stream after its saturation. This may be carried out, for example in a final step, by superimposing an alternating potential with a direct potential, as indicated at 130 in FIG. 2. The effectof the alternating potential brings about a more rapid absorption, while the direct potential results in a faster removal of the absorbent, which precipitates on the electrodes with opposite polarity.

EXPLANATION OF REFERENCE NUMERALS 1- Cooling tower inlet (FIG. 1)

2 Gas distribution means 3 Cooling tower outlet 4 Discharge from tower 5 Electrostatic precipitator, shown schematically 6 Water supply line Y 7 Sprinklers 8 Tower space proper 9 counterelectrodes (FIG. 2)

v 10 High direct potential 11 Charged water droplets 12 Plane'electrodes l3- Alternating potential source 132 Superimposed direct potential 14 Movement to the left 15 Movement to the right 16 Zigzag movement 7 17 High three-phase potential (FIG. 3).

19 Upwardly directed component 20 Electric fields I claim:

1. In a method for cooling a hot gas as it flows along a path extending froma gas inlet of a generally-vertical cooling chamber to a gas outlet thereof spaced downstream of said gas inlet, by injectingdroplets of coolant water into the gas in said chamber at a position adjacent said gas inlet, the gas being cooled by the resultant heating and vaporization of the water droplets in said chamber, the improvement comprising the steps of electrically charging said droplets with a single common polarity of electrical charge;.and inducing a repetitive back-and-forth component of motion of said charged droplets in said chamber without producing substantial precipitation of them, by providing first and second spaced-apart electrode means extending along respective substantially-parallel planes on opposite sides of said gas-flow path in said chamber downstream of said droplet-injecting position and applying an alternating voltage between said first and second electrode means, alternately to attract andrepel said charged droplets, the frequency of said alternating voltage being sufficiently high that for most of said droplets said component of motion is repetitively reversed and said droplets vaporized before they are deflected sufficiently to be precipitated by striking any of saidelectrode means; whereby the speed of motion of said droplets with respect to said gas is increased and the rate of cooling of said gas by said charged droplets thereby accelerated. 2. The method of claim 1, wherein said gas is flowed continuously along said path adjacent said electrodes and said water droplets are injected into said gas prior to its passage adjacent said electrode means.

3. The method of claim 1, comprising applying a unidirectional component of voltage between said first and second electrode means in addition to said alternating voltage.

4. The method of claim 1, in which said voltage varies substantially sinusoidally with time.

5. The method of claim 1, in which said voltage alternations are of substantially rectangular waveform as a function of time.

6. The methodof claim 1, in which said alternating voltage has different phases at different positions within the region occupied by said gas and droplets.

7. In apparatus comprisinga cooling chamber, means for flowing a hot gas through said chamber along a path extending from a gas inlet of said chamber to a gas out let of said chamber, and means for injecting droplets of coolant water into said gas-flow path in said chamber to cool said gas by heating and vaporization of said droplets, the improvement comprising;

means adjacent said inlet for electrically charging said droplets with a single common polarity of charge;

first and second spaced-apart electrode means positioned between said inlet and outlet and downstream of said droplet-injecting means and of said charging means, said first and second electrode means being responsive to an alternating voltage for applying an alternating electric field to said charged droplets adjacent thereto along said path thereby to produce repetitive alternate deflections 5 of said charged droplets in opposite directions; and

means for generating and applying between said first and second electrode means a voltage having a frequency sufficiently high that for most of said charged droplets said deflections are repetitively reversed and said charged droplets vaporized before they are deflected sufficiently to be precipitated by striking any of said electrode means, thereby to accelerate the heating and vaporization of said droplets by said hot gas.

8. The apparatus of claim 7, in which said electrode means comprises at least two substantially plane parallel electrodes spaced apart from each other and generally parallel to said flow path so as to receive said flow of said gas and charged droplets between them, and means for applying an alternating voltage to said electrodes so that each electrode is supplied with a polarity of voltage opposite to that of the nearest adjacent one of the others of said electrodes.

9. The apparatus of claim 7, in which said means for applying an alternating electric field comprises at least two spaced-apart arrays of generally cylindrical spaced-apart electrodes in said path of said gas flow, each of said electrodes extending transversely to the direction of said gas flow path, each of said arrays extending in a plane generally parallel to said gas flow path, a source of a multi-phase alternating potential, and means for supplying different successive phases of said alternating potential to consecutive ones of said electrodes in each of said arrays.

Claims

1. IN A METHOD FOR COOLING A HOT GAS AS IT FLOWS ALONG A PATH EXTENDING FROM A GAS INLET OF A GENERALLY-VERTICAL COOLING CHAMBER TO A GAS OUTLET THEREOF SPACED DOWNSTREAM OF SAID GAS INLET, BY INJECTING DROPLETS OF COOLANT WATER INTO THE GAS IN SAI CHAMBER AT A POSITION ADJACENT SAID GAS INLET, THE GAS BEING COOLED BY THE RESULTANT HEATING AND VAPORIZATION OF THE WATER DROPLETS IN SAID CHAMBER, THE IMPROVEMENT COMPRISING THE STEPS OF ELECTRICALLY CHARGING SAID DROPLETS WITH A SINGLE COMMON POLARITY OF ELECTRICAL CHARGE; AND INDUCING A REPETITIVE BACK-AND-FORTH COMPONENT OF MOTION OF SAID CHARGED DROPLETS IN SAID CHAMBER WITHOUT PRODUCING SUBSTANTIAL PRECIPITATION OF THEM, BY PROVIDING FIRST AND SECOND SPACED-APART ELECTRODE MEANS EXTENDING ALONG RESPECTIVE SUBSTANTIALLY-PARALLEL PLANES ON OPPOSTE SIDE OF SAID GAS-FLOW PATH IN SAID CHAMBER DOWNSTREAM OF

2. The method of claim 1, wherein said gas is flowed continuously along said path adjacent said electrodes and said water droplets are injected into said gas prior to its passage adjacent said electrode means.

6. The method of claim 1, in which said alternating voltage has different phases at different positions within the region occupied by said gas and droplets.

7. In apparatus comprising a cooling chamber, means for flowing a hot gas through said chamber along a path extending from a gas inlet of said chamber to a gas outlet of said chamber, and means for injecting droplets of coolant water into said gas-flow path in said chamber to cool said gas by heating and vaporization of said droplets, the improvement comprising; means adjacent said inlet for electrically charging said droplets with a single common polarity of charge; first and second spaced-apart electrode means positioned between said inlet and outlet and downstream of said droplet-injecting means and of said charging means, said first and second electrode means being responsive to an alternating voltage for applying an alternating electric field to said charged droplets adjacent thereto along said path thereby to produce repetitive alternate deflections of said charged droplets in opposite directions; and means for generating and applying between said first and second electrode means a voltage having a frequency sufficiently high that for most of said charged droplets said deflections are repetitively reversed and said charged droplets vaporized before they are deflected sufficiently to be precipitated by striking any of said electrode means, thereby to accelerate the heating and vaporization of said droplets by said hot gas.