US3681896A - Control of frost formation in heat exchangers by means of electrostatic fields - Google Patents

Abstract

Control of frost formation in heat exchangers by applying an electrostatic charge to the air-vapor stream and to water introduced into the stream. The charged water droplets induce coalescence of the water vapor in the air. An electrical potential is applied to repel the charged fluid and in the next region the surface is at a potential to attract the charged vapor thereby permitting the air fluid stream to pass to the heat exchanger vapor free.

Description

United States Patent m1 3,681,896 Velkoff [451 Aug. 8, 1972 [54] CONTROL OF FROST FORMATION IN 3,475,917 11/1969 HEAT EXCHANGERS BY MEANS OF 3.247.091 1966 n urm- ELECTROSTATIC FIELDS 3,221,475 12/1965 Wiemer....

. 3,365,858 1 19 [72] Invent: wmhmgw", 2,691,280 10/1933 2:22 062/27 5 [73] Assignee: The Ohio State University, Colurn- P imary inverter-Meyer Perlin bus, Ohio Assistant Examiner-Ronald C. Capossela [22] Filed: July 9 1970 Attorney-Cenrtamo, Dunbar & Kremblas [21] App1.No.: 53,450 [57] ABSTRACT Control of frost formation in heat exchangers by ap- [52] US. Cl. ..55/ 107, 55/121, 55/135, plying an electrostatic charge to the air-vapor stream 55/138, 55/149, 55/154, 55/269, 62/151, and to water introduced into the stream. The charged 62/71, 165/17 water droplets induce coalescence of the water vapor [51] Int. Cl ..B03e 5/00 in the air. An electrical potential is applied to repel [58] Field of Search ..55/4, 5, 10, 11, 107, 122, the charged fluid and in the next region the surface is 55/135-138; 165/1, 17; 34/1; 62/85, 151, at a potential to attract the charged vapor thereby per- 474, 475 mitting the air fluid stream to pass to the heat exchanger vapor free. 6 Reteren [5 1 l4Clalms,5DrawingFigures UNITED STATES PATENTS 2,992,177 7/ 1961 Morrisson ..165/1 X PATENTED M19 8 SHEET 10F 2 FIG?) CICI a b 2 l K 6 TA R FIG! INVENTOH HENRY R VELKOFF ATTORNEYS PATENTEDM BIB?! 3.681.896

swan 2a? 2 1 Ty-D l iplv INVENTOR HENRY RVELKOFF F G. 4

ATTORNEYS CONTROL OF FROST FORMATION IN HEAT EXCHANGERS BY MEANS OF ELFXITRMTATIC FIELDS BACKGROUND OF THE INVENTION Heat exchangers have been used extensively in commercial and industrial refrigeration systems. One such type of heat exchanger is the cross-flow type wherein two fluids flowing along the heat-transfer surface move at right angles to each other. in one of these types the fluid is unmixed as it passes through the exchanger and, therefore, the temperatures of the fluids leaving the heater section are not uniform, being hotter on one side than on the other. A flat-plate type heater, a design used for turbine regenerators to reclaim the energy of the exhaust gases, or an automobile radiator approximates this type of exchanger. In the other, one of the fluids is unmixed and the other is perfectly mixed as it flows through the exchanger. The temperature of the mixed fluid will be uniform across any section and will vary only in the direction of flow. The air flowing over the bank of tubes is mixed, while the hot gases inside the tubes are confined and therefore do not mix. In a third case, both of the fluids are mixed as they flow through the exchanger; that is, the temperature of both fluids will be uniform across the section and will vary only in the direction of flow.

In order to increase the effective heat-transfer surface area per unit volume, most commercial heat exchangers provide for more than a single pass through the tubes, and the fluid flowing outside the tubes in the shell is routed back and forth by means of baffles. In a baffled exchanger, the flow pattern on the shell side is complex; part of the time the flow is perpendicular, and part of the time parallel to the tube.

In a shell-and-tube heat exchanger with segmental bafl'les with two tube passes, in one shell pass the tube plates are fixed at each end and the tubes are welded or expanded into the plates. This type of construction has the lowest initial cost but can only be used for small temperature differences between the hot and the cold fluid. No provision is made to prevent thermal stresses due to the differential expansion between the tubes and the shell. Another arrangement one tube plate is fixed but the other is bolted to a floating-head cover which permits the tube bundle to move relative to the shell. The floating tube sheet is clamped between the floating head and a flange so that it is possible to remove the tube bundle for cleaning. For certain special applications such as regenerators for aircraft or automobile gas turbines, the rate of heat transfer per unit weight and unit volume is the prime consideration. Compact, lightweight heat exchangers for this type of service are included in the prior art.

An air-to-oil cooler was designed for cooling an aircraft electronics system operable at high altitudes. This type of chamber utilizes aluminum plate-fin construction. The in thermal bypass valve controls the temperature of the oil, which is returned to the system. Control is maintained despite wide fluctuations of the system heat load, and without the necessity of separate temperature control apparatus. The bleed-port facilitates system drainage.

in operation of each of the heat exchangers in cooling frost will form on the heat exchanger surface whenever the surface temperature is below the freezing point of water.

The forrmtion of frost in heat exchangers in commercial or industrial types of refrigeration continues to be a serious and aggravating problem. The effects of the problem created by frost formation are very much appreciated and are typified by the general trends ob served during the course of a test run conducted at a fixed mass flow rate of air where it was shown that there was a decrease in heat transfer rate with frost formation.

The test data revealed that the variation in heat transfer ooeflicient with time was much less ordered or regular than the variation in pressure drop. This rather random time variation, coupled with the relatively small change in value, dictated the correlation of the time averaged heat transfer coefficient rather than the instantaneous heat transfer coefficient.

The variation in heat transfer coefficient with time results partially from the dependence of the heat transfer coefficient on the frost thermal properties. The

0 mechanism of heat flow through the frost layer is transient in nature and is dependent on the frost thermal conductivity, specific heat, and density, which are dependent on the air velocity, humidity ratio, etc. The changes in frost physical properties will be reflected in the computed heat transfer coefficients and could, to some extent, contribute to the random nature of the time variation. The other parameter affecting the heat transfer coefficient is the physical deposition of the frost depending on its formation as dendritic, crystalline, or ice-like structures.

The increase in pressure loss which occurs with frost formation is primarily a function of the reduction in free flow area resulting from frost formation and as such is independent of the frost physical properties. The occurrence of different frost formations has little influence on the pressure loss, regardless of the type of structure, since the flow blockage will be approximately the same.

The normal approach to overcoming the frost formation is similar to many other commercial or industrial problems. Specifically, the approach to overcoming the effects of frost is to attack the effect rather than the cause. In the gaseous type of chamber, the most common way to overcome frost is to include heating elements that are periodically activated. In more recent systems wherein liquid coolants are used, a reverse cycle of a hot liquid is used to eliminate the frost that has formed.

The utilization of electrostatics to control the direction of droplets comprising a liquid fluid stream is known in the art. Specific reference is made to the systems referred to generally as ink drop printers or ink spitters. Many patents covering these systems are found in Cl. 346-75.

In the prior art ink drop printers, it is essential that the fluid stream generally under pressure be converted to droplets. This may be accomplished generally with a nozzle or other spray means. One sophisticated way of converting the liquid stream to droplets is to apply a sonic field to the fluid stream being emitted from the noule. The sonic field accomplishes two things; first, the stream is broken into droplets with the number of droplets being exactly equal to the frequency of the applied sonic force; and secondly, each droplet is converted into a perfectly round shape of equal size. The fluid droplets are then fed through an electrical arrangement wherein an electrostatic charge is applied to each droplet. The electrostatic charge is a function of the instantaneous value of an intelligence signal, or a video signal, to be recorded. The charged droplets are then passed between a pair of electrostatic deflection plates. The droplets as they pass through the electric field are deflected by an amount which is a function of the magnitude of their charge and in a direction which is a function of the polarity of the charge to a recording medium. The unaffected droplets are caught and removed. Variations of the prior art ink spot printers are also known. t

It can be appreciated that the ink spot printer is basically identical to the cathode ray tube system with the substitution of liquid droplets for electrons.

The requirements for the prior art ink drop printers are extremely critical one pulse or one droplet error, a minor misalignment, etc. results in the ceiling being sprayed with ink.

SUMMARY OF THE INVENTION The present invention utilizes the electrostatically charged liquid droplet principle of the prior art; however, its utilization in a heat exchanger field has caused extreme modifications to the system. Also, the criticality of the system from that of the prior art has been reduced enormously. In essence, liquid droplets are utilized to prevent frost formation in a heat exchanger. The droplets do not need to be uniform in spacing or in configuration.

As the air containing the water vapor moves towards the heat exchanger surface, an electrostatic charge is induced in the air-vapor stream. Additional water is injected at the corona point and hence introduced into the stream as charged droplets. The charged water droplets will tend to induce coalescence of the water vapor in the air. The stream of charged droplets passes by the first stage of a heat exchanger. An electrical potential is applied by way of the heat exchanger to repel the charged fluid. As the fluid moves downstream, it enters a region where an auxiliary heat exchanger surface is at an opposite potential to attract the charged vapor while at the same time the remainder of the fluid stream containing a greatly reduced vapor content passes through the primary heat transfer surface.

OBJECTS It is accordingly the principal object of the present invention to reduce greatly or prevent the formation of frost in heat exchangers through the utilization of electrostatic fields.

Another object is to prevent the formation of frost with an electrostatic field in a manner that is not critical and is simple in principle and operation.

Other objects of the invention, together with certain exemplary features will become apparent from a detailed description of the preferred embodiment when taken in conjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 2 is a side view of a heat exchanger with an improved embodiment of the present invention including a rotary frost removal;

FIG. 3 is a view in perspective of a heat exchanger with an improved embodiment of the present invention including a rotary frost removal;

FIG. 4 is another side view of another embodiment of the present invention as applied to a heat exchanger with frost collection and removal sections; and

FIG. 5 is a view in perspective of an embodiment of the present invention as applied to a commercial heat exchanger with frost collection and removal sections.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring specifically to the schematic illustration of FIG. 1, air enters into the heat exchanger at the input channel 3. Also entering the chamber adjacent channel 3 is the liquid nonle 11. A water reservoir 15 feeds under pressure a source of water to the inlet or the nozzle 11. The purpose of the water source is to interject liquid droplets into the input channel 3.

The liquid emitted by the nozzle 1 l is conventionally droplets as the nozzle 1 l is a modified fuel spray nozzle of conventional design. In accordance with the primary concepts of the present invention, each droplet emitted from the nozzle 11 into the input channel 3 is electrically charged.

In one embodiment of electrically charging the liquid droplets the metallic nozzle 11 is connected to a source of high electrical potential, such as at point 2. The water droplets acquire an electrical charge as the liquid water comes into contact with the charged metal nozzle.

In a first alternative embodiment, the droplets are charged with a corona point, i.e., sharp needle type point 16. The needle point 16 is positioned midst the spray 5. It may be more expedient to utilize a plurality of points positioned around the outside of the fluid stream. The corona points 16, in a conventional manner, are connected to a suitable high voltage source such as the positive field shaping electrode 4. In turn, the points 16 emit an electrical charge, i.e., electrostatic charge, electrons or ions which subsequently attached to the water droplets S.

In a more sophisticated embodiment the liquid 15 in the nozzle 11 is subjected to a vibratory force 13. In this way the liquid stream is broken into liquid droplets of uniform size, shape and number.

The embodiment illustrated in FIG. 1 utilizes the corona point charge at the spray 5. The air containing water vapor enters the chamber at l in channel 3 and interrnixes with the liquid spray 5. Simultaneously then, at the corona point, the liquid droplets from the spray and the water vapor receive the electrical charge. Significantly the charged water droplets induce coalescence of the charged water vapor in the air stream. This results in a combined charged water vapor droplet.

The water content of typical saturated air is as follows:

SOF 0.00764 64: 1 0.01 105 l30F 0.1 l 140 Based upon these data, approximately 0.001 to 0.1 pounds of water would be introduced per pound of air flowing (or other gas) in the system. The most likely rate would fall in the range of 0.001 to 0.01 pounds of water added per pound of air flowing.

In certain installations charged droplet formation and controlled deposition may be had without any added water spray. That is, the corona point (or points) could act as nucleating sites for the water vapor in the air without any addition of water. Thus it is possible for the device to reduce frost formation even with water addition rates approaching or equal to zero.

Referring again to P16. 1 the liquid droplets from the spray 5 together with the water vapor from the air stream passes from the first stage 3 of the heat exchanger into a second stage 6 wherein the surface of the chamber is cooled to cool the air stream. In the cooling region 6 the liquid droplets in the stream are repelled by the field on the electrode 19. The field is created by the negative charge applied from source 12. The fluid stream 5 then progresses into an auxiliary cooling chamber 7. The surface of this chamber has a positive charge applied thereto by source 14 and hence comprises an electrode 21 of opposite polarity to that of the prior cooling chamber. The surface of the auxiliary cooing chamber 7 attracts the fluid droplets 5 at the inner wall 9.

The primary heat exchanger 8 also has positioned adjacent thereto electrode 23 negatively charged from source 116 for applying a repelling charge to the air stream. In this way the repelling charge applied to the cooling chamber 6 and primary heat exchanger 8, together with the attracting charge applied to the stream in auxiliary camber 7, causes the liquid droplets to be attracted to the surface of the wall of the auxiliary chamber 7.

With the water vapor removed from the air stream, the air is caused to pass out of the chamber at outlet with greatly reduced frost formation on the primary heat exchanger 8.

1n the auxiliary cooler 7 the water vapor and droplets adhere to the surface. At this station the water will cause ice to be deposited thereon. In a preferred embodiment the frost deposition is removed by mechanical means such as a rotating surface that is moved into and out of the stream at periodic intervals. It must be pointed out, however, that since the water vapor is removed prior to entry into the main heat exchanger that the amount of ice accretion is at a minimum. Empirical design of the surface in the auxiliary chamber can be made to meet the necessary demands. For instance, extended lengths or increased surface, or increased potentials may be chosen to meet the demand.

With reference to FIG. 2 there is shown a side view, and FIG. 3, a view in perspective, of another embodiment of the present invention as applied to a heat exchanger. Specifically exemplified is the rotary frost removal 47 comprising a series of discs 41.

In operation of this embodiment the air intake 30 into the channel 31 passes beyond the area of the nozzle 33 having a negative coronal discharge point 34 applied through source 37 for charging the water droplets 35 much in the same manner as that shown and described relative to FIG. 1. The air entering the channel 32 and formed in negatively charged droplets strikes the precooler heat exchanger 36. The surface of the precooler 36 reduces the temperature of the moist air to approximately 32 F or just above the freezing point of water. The precooler 36 has also placed thereon a negative charge by source 51 the same as the coronal discharge.

Adjacent the precooler 36 is on auxiliary rotating cooling surface 47 comprising a series of parallel rotating discs 41. The discs are of a diameter approximating twice that of the precooler heat exchanger 36. The discs 41 extend down into a lower cooling chamber 45 where they are cooled. The discs are electrically charged positively opposite to that of the corona discharge. lt is to be appreciated that the electrical charge is applied conventionally by source 51 and when charged the surfaces of the discs become electrodes. With the positive charge the discs attract the water droplets and thereby cause the frost to collect on the discs. As the discs rotate they carry the frost from the main (upper) air stream chamber 32 to the lower chamber 45. In the lower chamber 45 frost scrapers 50 remove the frost from the discs 41 at region 42.

in the meantime the cooled air in the upper chamber 32, passing beyond the rotating discs 41, enters the primary heat exchanger 48. There is applied to the primary heat exchanger a negative charge by source 53 similar to that of the corona discharge point. As pointed out above this repelling charge further causes the liquid droplets to be attracted to the discs 41 of the auxiliary rotating surface 47.

With the water removed from the air stream the air is caused to pass out of the chamber at outlet 52 with greatly reduced frost formation on the primary heat exchanger 48.

Refering now to FIG. 4 there is shown in a side view schematic the application of the preferred embodiment of the present invention into a heat exchanger. in this embodiment the heat exchanger includes frost collection and removal sections. HQ 5 is a similar embodiment to FIG. 4 and is shown in perspective to illustrate the applicability of the present invention to a standard and commercial heat exchanger.

With reference to FIG. 4 the air entering through intake into the channel 62 passes beyond the area of the nozzle 63 having a negative potential applied through source 70 to corona discharge point 64 for charging the water droplets 65 much in the same manner as the embodiments shown in FIGS. 1 and 2.

1n the embodiment of FIGS. 4 and 5 a series of frost collector heat exchangers 71, 73, 75, 77 are interspersed with the primary heat exchangers 72, 74, 76. The frost collectors 71, 73, 75, 77 are similar in design and are of a configuration to provide very large passage ways and to provide very little heat transfer surface. The primary heat exchangers 72, 74, 76 are of conventional design and configuration and may be of the fintube or plate-fin type.

The frost collectors 71, 73, 75, 77 are electrically charged with a positive potential from sources 85, 87, 89 and 91 respectively. The charge is opposite in polarity to that of the corona point 65. In contrast to create attracting and repelling fields, the primary heat exchangers 72, 74, 76 are charged with a negative potential from sources 86, 88, and 90. In operation then, the water droplets 65 formed at the corona point or discharge of the nozzle 64 are given a negative charge. The frost collectors 71, 73, 75, and 79 having a positive charge applied thereto attract the negatively charged water droplets. To enhance the attraction of the negatively charged water droplets to the frost collectors 71, 73, 75, and 79, the primary heat exchangers are also negatively charged and thereby create a repelling force to the negatively charged water droplets.

The passage area of the frost collectors 71, 73, and 75 are chosen in design to be relatively large. In this way there can be a large frost buildup without a significant reduction of the cross section flow area. In turn, operation of the heat exchangers occur for relatively long periods of time before impairment of flow or severe pressure losses occur.

The removal of the frost from the frost collectors 71, 73, 75, 77 may be accomplished by the conventional reverse cycle of a hot liquid through the frost collectors. The water that is accumulated is expelled from drains 81, 82, 83 and 84.

Since the preferred embodiment of FIGS. 3 and 4 includes a plurality of frost collectors, it would be expeditious to defrost on a cycle basis, i.e., one collector at a time. In this way at no time would there be a shut-down time for the over-all heat exchanger it being appreciated that the primary heat exchanger units 72, 74, and 76 are essentially frost free at all times and would not require defrosting.

Although certain and specific embodiments have been shown, it is to be understood that modifications may be made without departing from the true spirit and scope of the invention.

What is claimed is:

I. In a heat exchanger having a plurality of serially positioned chambers defining regions of sequential cooling the improvement comprising:

a first chamber, means for converting the moisture of the air taken into the first of said chambers into liquid droplets,

means for applying an electrical potential to each of said droplets,

means for applying an electrical potential of opposite polarity to the chamber defining region adjoining said first chamber,

means for applying an electrical potential of opposite polarity to said last-named electrical potential and of same polarity as said polarity of said liquid droplets to a third chamber defining region,

whereby said second chamber attracts said liquid droplets and said last-named region repels said droplets thereby causing said droplets to accumulate in the intermediate region.

2. The heat exchanger improvement of claim 1 wherein said exchanger includes at least a fourth chamber defining region and means for applying an electrical potential to said chamber of a opposite polarity to that of said liquid droplets.

3. The heat exchanger improvement of claim 1 wherein said chamber defining region having an opposite polarity to that of said liquid droplets accumulated therein further includes means for removing said if fifi liif fiiir i f lggiovemenr of claim 3 wherein said rrreans for removing frost from said chamber includes scraper means.

5. The heat exchanger improvement of claim 1 wherein said chamber defining region having an opposite polarity to that of said liquid droplets accumulated therein further comprises a plurality disc-like plates for the accumulation therein of said liquid droplets as frost therefrom.

6. The heat exchanger improvement of claim 5 wherein said discs are rotatable and further includes means for scraping the frost therefrom.

7. The heat exchanger improvement of claim 6 wherein said discs are of a diameter sufficiently greater than that of said chamber to cause said discs to extend into an adjoining region and wherein said means for removing said frost accumulated thereon is in said adjoining region.

8. The heat exchanger improvement of claim 1 wherein said means for converting the intake air moisture into liquid droplets further comprising means for introducing liquid droplets into said first chamber defining region.

9. The heat exchanger improvement of claim 8 wherein said means for applying an electrical potential to said liquid droplets further causes said air liquid droplets and said introduced liquid droplets coalesce.

10. The heat exchanger improvement of claim 1 wherein said means for applying said electrical potential comprises at least one corona point.

11. The heat exchanger improvement of claim 8 wherein said means for introducing liquid into said intake chamber further comprises means for pro-forming said liquid droplets.

12. The heat exchanger improvement of claim 1 wherein said chamber defining chambers regions includes primary heat exchangers interspersed with frost collector heat exchangers.

13. The heat exchanger improvement of claim 12 wherein said frost collector heat exchangers are of a configuration to provide relatively large air passageways and relatively little heat transfer surface.

14. The heat exchanger improvement of claim 13 further comprising means for removing said frost accumulated on said frost collector heat exchangers.

* a: a m m

Claims (14)

1. In a heat exchanger having a plurality of serially positioned chambers defining regions of sequential cooling the improvement comprising: a first chamber, means for converting the moisture of the air taken into the first of said chambers into liquid droplets, means for applying an electrical potential to each of said droplets, means for applying an electrical potential of opposite polarity to the chamber defining region adjoining said first chamber, means for applying an electrical potential of opposite polarity to said last-named electrical potential and of same polarity as said polarity of said liquid droplets to a third chamber defining region, whereby said second chamber attracts said liquid droplets and said last-named region repels said droplets thereby causing said droplets to accumulate in the intermediate region.

2. The heat exchanger improvement of claim 1 wherein said exchanger includes at least a fourth chamber defining region and means for applying an electrical potential to said chamber of a opposite polarity to that of said liquid droplets.

3. The heat exchanger improvement of claim 1 wherein said chamber defining region having an opposite polarity to that of said liquid droplets accumulated therein further includes means for removing said liquid droplets as frost therefrom.

4. The heat exchanger improvement of claim 3 wherein said means for removing frost from said chamber includes scraper means.

5. The heat exchanger improvement of claim 1 wherein said chamber defining region having an opposite polarity to that of said liquid droplets accumulated therein further comprises a plurality disc-like plates for the accumulation therein of said liquid droplets as frost therefrom.

6. The heat exchanger improvement of claim 5 wherein said discs are rotatable and further includes means for scraping the frost therefrom.

7. The heat exchanger improvement of claim 6 wherein said discs are of a diameter sufficiently greater than that of said chamber to cause said discs to extend into an adjoining region and wherein said means for removing said frost accumulated thereon is in said adjoining region.

8. The heat exchanger improvement of claim 1 wherein said means for converting the intake air moisture into liquid droplets further comprising means for introducing liquid droplets into said first chamber defining region.

9. The heat exchanger improvement of claim 8 wherein said means for applying an electrical potential to said liquid droplets further causes said air liquid droplets and said introduced liquid droplets coalesce.

10. The heat exchanger improvement of claim 1 wherein said means for applying said electrical potential comprises at least one corona point.

11. The heat exchanger improvement of claim 8 wherein said means for introducing liquid into said intake chamber further comprises means for pre-forming said liquid droplets.

12. The heat exchanger improvement of claim 1 wherein said chamber defining chambers regions includes primary heat exchangers interspersed with frost collector heat exchangers.

13. The heat exchanger improvement of claim 12 wherein said frost collector heat exchangers are of a configuration to provide relatively large air passageways and relatively little heat transfer surface.

14. The heat exchanger improvement of claim 13 further comprising means for removing said frost accumulated on said frost collector heat exchangers.

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