Method of action of the plastic heat exchanger and its constructions

Abstract

The within invention makes use of plastic tubing to create an effective two-phase heat exchanger whereby the vapor phase is collected in bubbles in the tubes separated by the areas of liquid phase. The vaporization of the liquid removes the heat, the surrounding medium and the vapor is condensed when cooled. The movement of the vapor and liquid phases is aided by the plastic walls.

Description

RELATED APPLICATIONS

[0001] The applicant claims priority of Provisional patent application Serial No. 60/315,378, filed Aug. 27, 2001, entitled “THE METHOD OF ACTION OF THE PLASTIC HEAT EXCHANGER AND ITS CONSTRUCTIONS”, inventor, Genrikh Smyrnov.

FIELD ON THE ART

[0002] The present invention relates to the method and use of a plastic heat exchanger (PHE) and its construction using a transitional two-phase heat carrier. The utility of this invention and practical applications are for power engineering, chemical industry, heat recovery, refrigerant and air conditioning systems and also ecological systems, etc.

BACKGROUND OF THE INVENTION

[0003] The practical application of plastic heat exchangers has expanded over the last few years. The plastic heat exchangers have two big advantages; they are economical, and have anticorrosion properties. Known plastic heat exchangers have been used as air heaters (see U.S. Pat. No. 4,069,807), passing the combustion products of a flame through water. The combustion products and whatever moisture added by evaporation is passed through a heat exchanger to heat air such as for a house. The construction of this heat exchanger in this patent doesn't contain essential design consideration as set forth herein. The polymer or plastic materials application is common with waste gases and high temperature, which can produce the high corrosion action for metallic surface of a heat exchanger. U.S. Pat. No. 4,213,498 discloses the low-cost flexible plastic heat exchanger, which “ . . . includes at least one heat transfer element having parallel outer and inner plastic sheets separated by a plurality of parallel ribs . . . ” etc. It does not include new information about the process of heat transfer in this plastic heat exchanger and it uses a liquid one-phase heat carrier.

[0004] The invention of U.S. Pat. No. 5,582,238 “ . . . provides a heat exchanger mat, comprising a twin-duct manifold having a duct member comprised of all inlet duct and outlet duct . . . ” etc. The mat also includes a plurality of tubes for circulation of the liquid, one phase heat carrier. All components of this heat exchanger are made of a thermoplastic material such as UV-resistant polypropylene. This invention makes use of one of the known forms of tube heat exchanger with some peculiarities connected with polymer materials application.

[0005] The invention (set out in WO 00/39517) relates to the traditional tube gas-liquid heat exchanger where instead of metallic tubes with small diameters, plastic capillary tubes are applied. The tubes, for example made of a plastic material, are designed to transport the liquid, one phase heat carrier. The authors of this invention suggest: “The tubes are designed to be generally shaped like substantially sinusoidal lines. The sinusoidal of two contacting tubes of two consecutive rows are phase offset relatively to each other such that the two tubes are maintained in two contact zones per sinusoid interval, thereby leaving interstices between the tubes to enhance the penetration of the flux.” These and other patents or plastic heat exchangers do not address the use of a transitional two-phase heat carrier fluid that compliments the plastic materials of the heat exchanger to increase the heat transfer rate efficiency.

[0006] The present invention uses a transitional two-phase heat carrier, with the plastic heat exchangers, to simplify their construction, reduce volume, mass and cost and to improve efficiency.

[0007] The present invention uses the plastic heat exchanger and its construction to incorporate a transitional two-phase heat carrier and thus, to reduce the level of wet ability between the heat carrier and the plastic material of the heat exchanger. Properties of the plastic materials, such as their flexibility and wet ability in connection with the special design of the plastic heat exchanger, yields new plastic heat exchangers with high efficiencies.

SUMMARY OF THE INVENTION

[0008] Existing plastic heat exchangers have been used for their non-corrosive properties and its practical applications with the different special working conditions such as heating or cooling corrosion active substances (for example, acids, alkalis, active solutions etc.). In addition their low cost provides economical feasibility. Generally, application of the plastic heat exchangers is rejected for other than non-corrosive working conditions, for example for gas-gas heat exchanger, due to inefficiencies due to the plastic materials having low heat conductivity. This was especially true for plastic heat exchangers with transitional two-phase heat carriers.

[0009] With the present invention, it is possible to create an effective plastic heat exchanger (PHE) using the transitional two-phase heat carrier.

[0010] The methods of action of the plastic heat exchangers with a transitional two-phase heat carrier will differ for small and big apparatuses. The small apparatuses with total transferred power about 0.1-10 kW will have moderate sizes and use the plastic pipes of the serpentine (coil) forms with diameters lower than 5-10 mm. The big apparatuses with total transferred power of about from 10 kW to 100 megawatt use plastic pipes of the serpentine (coil) forms with diameters on the order of 10-mm. The plastic materials of these pipes have more strength limitations in comparison with conventional metallic materials in areas of working temperatures and pressure levels. Pipe technology for the transitional two-phase heat carrier allows selection of the corresponding heat carrier fluid for any working temperature level with small pressures of saturation, to decrease the corresponding tension in the plastic material of the PHE. Using plastic materials in the PHE opens new possibilities for improvement of the PHE characteristics and consequently characteristics of different apparatuses. Selection of the pair of the transitional two-phase heat carrier fluid and the plastic material for the heat exchanger, will improve the heat and mass processes in the plastic heat exchangers in comparison to ordinary plastic heat exchangers with a one phase liquid heat carrier.

[0011] It is possible to obtain different levels of wet ability for the plastic-heat carrier pair. The low level of wet ability allows control inside the PHE of the process of drop wise condensation in the cooling zone. It corresponds with significant increase of the heat transfer coefficient and considerable initial temperature drop for the boiling point in the heating zone. This feature is important for improvement of the two-phase flow movement and heat transfer characteristics of the heat carrier in the small and big serpentine plastic pipes and consequently in the PHE. In addition decreasing the wet ability will enforce instability of liquid microfilms in the vapor slugs of the heat carrier. Thus it will help the corresponding thermal incongruities in the plastic pipe with small internal diameters and thus lead to enhancement of the two-phase flow movement of the heat carrier and resulting heat and mass transfer. The PHE design can be similar in physical construction to ordinary metallic heat exchangers, metallic serpentine pipes, and baffle, with attention to required spacing and pitches between different branches of the serpentine pipes.

[0012] The practical applications of the suggested new plastic heat exchanger (PHE) and its construction based on the heat pipe technology can be very wide. With two- phase flow, with boiling and condensing, a significant enhancement of the heat transfer rates in plastic heat exchangers is available. In some instances it may be better than metal, due to low wet ability in, for example, ‘Teflon’. The small PHE can be expedient for cooling systems of electronic equipment. The middle size PHE will be useful for air conditioning and refrigeration units. The large PHE have applications in the different heat recovery and ecological systems, in power engineering and chemical industry as heat exchangers for waste gases, air cooling and heating systems and so on.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 illustrates the conventional picture of the interactions of the elastic properties of the plastic (polymer) wall of the plastic heat exchanger (PHE) with a transitional two-phase heat carrier and the pressure imbalance in the adjacent branches of the serpentine (coil) pipe.

[0014] Here: 1—heating zone, 2—cooling zone, 3—transport zone, 4—plastic (polymer) wall in constricted state, 5—plastic (polymer) wall in expanded state, 6—liquid slug of the heat carrier, 7—vapor slugs of the heat carrier, 8—microfilm in the vapor slug, 9—the instantaneous pressure P1 and P2 in the vapor slugs 7 with the microfilm 8 and without it correspondingly.

[0015] FIG. 2 illustrates the peculiarities of the process in the plastic heat exchanger with a two-phase carrier with big internal diameters (more than 10 mm). Here: 10—vapor bubbles in the heating zone 1, 11—condensate drops in the cooling zone 2, 12—liquid streams in the transport zone 3.

[0016] FIG. 3 illustrates the way to obtain the optimal content and concentration of the transitional two-phase heat carrier fluid (may be one or mixture of many components) and optimal determination of filling of the internal volume of the plastic pipe exchanger by the heat carrier. Here: 13—valves, 14—vacuum pump, 15—vessel for determination of filling of volume of the pipe exchanger by the heat carrier, 16—temperature measurement system for determination of the average temperature of surface of the cooling zone 2, 17—temperature measurement system for determination of the average temperature of surface of the heating zone 1, 18—baffles, 19—shell of the PHE.

[0017] FIG. 4 illustrates the scheme of the PHE on the bases of the transitional two-phase heat carrier with different serpentines in the heating 1 and cooling 2 zones, which are separated from each other and joined by at least two transport lines: vapor transport line 25 and condensate transport line 26 with two hot flows 111, 112 in the heating zone 1 and three cold flows 221, 222 and 223 in the cooling zone 2. Here: a—front view scheme and b—side view scheme; 19—shell of the PHE, 20 and 21—top and bottom collectors in the cooling zone 2, 23 and 24—top and bottom collectors in the heating zone 1 correspondingly.

[0018] FIG. 5 shows different variants of the PHE on base of the use of the transitional two-phase heat carrier with some baffles 18 and some hot and cold flows (see FIG. 5a) and single baffle 18 with only one hot and cold flows (see FIG. 5b and FIG. 5c). a—the PHE with two hot flows 111, 112 and two cold flows 221, 222 on the common serpentine pipe base (vertical position); b—the PHE with only one hot 11 and cold 22 flows and with the single baffle 18 (vertical position); c—the PHE as in b but in horizontal position.

[0019] FIG. 6 illustrates the PHE on the base on the use of a transitional two-phase heat carrier with the serpentine plastic pipe where all serpentine-pipes located in the same plane as the base for the PHE with frames, inserts and other accommodations to fix the pitches between branches. Here: 27—serpentine pipe, 28—the moving element for fixing a plastic serpentine, 29 and 30—the top and bottom strips of the frame, 31—the screw-nut for control of tension of the serpentine plastic pipe, 32—connecting rods.

[0020] FIG. 7 shows the PHE with a transitional two-phase heat carrier with the serpentine plastic pipe 27, where all serpentine-pipes 27 located in the multiple parallel planes. Here: 33—spacer for fixing a distance between branches, 34—the tube, which formed curvature of the pipe, 35—the fastener, 36—cross-arm, 37—the fastener for control of tension of the plastic serpentine pipe, 38—the dummy of the serpentine 27 and 39—the filling nipple.

[0021] FIG. 8 gives the scheme of the PHE on base of a transitional two-phase heat carrier with the serpentine plastic pipe 27, which contains the additional porous insert 40 and also the heat flux concentrator or additional heater 41. Here: 18—baffle, 19—shell of the PHE, 40—the porous insert with azimuth and axial channels near the wall (shell) of the insert, 42—the condensate accumulator.

DETAIL DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0022] The present invention provides the method of action of the plastic heat exchanger (PHE) and its construction based on the use of a transitional two-phase heat carrier.

[0023] The action of the PHE will be described with reference to FIG. 1 and FIG. 2. It is necessary to distinguish the hydrodynamic and heat and mass processes in the serpentine plastic pipes based on the use of a transitional two-phase heat carrier with small internal diameters (lower 5-10 mm) and considerable bigger one (more than 10 mm). The peculiarities of the hydrodynamic and heat and mass processes in the serpentine plastic pipes with the small and big diameters are considered separately.

[0024] As it is shown on FIG. 1 in the serpentine plastic pipes with the small diameters the transitional two-phase heat carrier will exist in the form of the alternating liquid 6 and vapor 7 slugs.

[0025] The area of vapor slugs 7 contain the liquid microfilms 8, which cover the plastic walls 5 of the pipe. Their occurrence is as a result of the movement of the liquid slugs 6, leaving the microfilm.

[0026] This microfilm 8 will be evaporated in the heating zone 1 under action of the heat flux, and this evaporative process will be unstable. Its instability will be increased by growth of the heat flux. The slug's structure or slugs 6 and 7 alternate throughout the adjacent branches. The microfilm 8 existence in the vapor slug 7 in the heating zone 1 will be accompanied with a high level of evaporation of the microfilm. This will lead to rapid growth of the saturation pressure P1 in the vapor slugs 7. This growth will be the same in the adjacent branches where surface of the vapor slugs 7 will be in similar heating zones 1 where the microfilms 8 exist in all vapors slugs 7 at the same time. Usually such coincidence is impossible. As imbalance in the pressure growth P1 and P2 in the branches appears (with the driving force), it will start the two-phase flow movement of the heat carrier, (from P1 to P2). Such a situation will exist in plastic serpentine pipe or metallic pipe. However the plastic material has much better elastic property in comparison with the metallic. During the pressure increase the cross-section of the plastic pipe begins to grow in elastic expansion. It will be accompanied by storing of mechanical energy of the pressure growth. It will be in the form of the potential mechanical energy by the extended elastic plastic walls 4 of the pipe. When two-phase flow of the heat carrier (or alternating liquid 6 and vapor 7 slugs) begins to move under action of the driving force, it will allow restoration of the microfilm 8 in one of the zones where, due to evaporation, it had become dry and disappearance in the adjacent one where evaporation eliminates it. The accumulated mechanical energy of the plastic wall 4 will aid the two-phase flow movement of the heat carrier and therefore it will enhance the process of the heat and mass transfer. The surface of most plastic materials will promote lower wet ability on the internal surfaces of the plastic pipes. The low wet ability of the surface increases instability of the liquid microfilm 8 on this surface. This enforces the pressure imbalance in the adjacent branches of the serpentine pipe and, as a result, enforces the two-phase periodical movement of the heat carrier and also improves the process of heat and mass transfer. Some of above mentioned interactions between the transitional two-phase heat carrier and the plastic wall are used as the foundation for thermal-hydraulic processes for the large diameters (more 5-10 mm) of the serpentine pipes (see FIG. 2).

[0027] Decreasing of the wet ability helps to start the boiling or evaporation process at lower temperatures (see FIG. 2, bubbles 10 in the heating zone 1) and realize the drop wise condensation with resultant increasing of the heat transfer coefficients in the cooling zone 2 (see FIG. 2, condensed drops 11 in the cooling zone 2). Therefore, the peculiarities of the plastic materials and their interaction with the selected heat carrier can improve the action of the plastic serpentine pipes and consequently the PHE. For these reasons, this thermo hydraulics process with the transitional two-phase heat carrier in the plastic pipes is much better than in the metallic pipes.

[0028] The application of the one component heat carrier fluid limits the best results compared to the interaction between two-phase flow and the plastic wall of the serpentine pipe.

[0029] FIG. 3 illustrates the method of filling PHE with a heat carrier two-phase fluid and optimize the concentration of the heat carrier. This scheme contains (see FIG. 3) the heating 1, cooling 2 and transport 3 zones, which are divided by two baffles 18. The internal volume from one side of the pipe over the valve 13 is connected with the vacuum pump 14, and from another side it is connected with the liquid supply system for filling the sample of this pipe. This supply system contains some vessels 15, which are joined by the valves 13 with the internal volume of the tested sample and a source of the liquid heat carrier (or sources of the different components if the heat carrier is a mixture). Here there are also temperature measurement systems of sensors 16 and 17 (for example, thermocouples), which are located on the external surfaces of the heating 1 and cooling 2 zones. The transitional two-phase heat carrier is placed in the pipe system by first removing of all non-condensable gases by vacuum pump 14 from the internal volumes of the serpentine pipe (SP). Then liquid flow is directed from a source of the liquid heat carrier via flow meter to the internal volume of the SP. With the flow rate of the external heat carrier and average temperatures of the heating 1 and cooling 2 zones the total thermal resistance of the SP of PHE can be determined.

[0030] FIG. 4 illustrates the PHE using a transitional two-phase heat carrier with different serpentines in the heating 1 and cooling 2 zones, which are separated from each other and joined by at least two transport lines 25 (vapor line) and 26 (condense line). In FIG. 4 the views a and b are from different sides of the PHE. The heat input into the heating zone 1 is created by two cross flows 111, 112 and the heat removal from the cooling zone 2 is created by three cross flows 221, 222 and 223. The baffles 18 divide the flow from each is other. Using the top 20 and bottom 21 collectors in the cooling zone 2 and top 23 and bottom 24 collectors in the heating zone 1 combines the different serpentines. There can be multiple collectors to combine the serpentines with the advantage of creating smaller differences in the temperatures between the cooling zone 2 and the heating zone 1.

[0031] The reliable action of the PHE is aided if the cooling zone 2 is located above the heating zone 1. But it is also possible for reliable action of the PHE, when both heating 1 and cooling 2 zones are located on the same level in gravity field. The PHE (see FIG. 4) works as follows. Under action of the hot flows 111, 112, the heat carrier begins to boil in the heating zone 1 and vapor moves up, collecting in the top collector 23 and lifting via vapor transport line 25 to the top collector 20 of the cooling zone 2. Hereafter the vapor distributes from the top collector 20 to all serpentine of the cooling zone 2, where this vapor is condensing under action of the cold flows 221, 222 and 223. The condensate under vapor pressure comes to the bottom collector 21 and then it comes via the condensate transport line 26 to the bottom collector 24 of the heating zone 1. Here the condensate distributes from collector 24 to all serpentines of the heating zone 1, where it is boiling and so on. Serpentines of this PHE can be horizontal or vertical, however the horizontal position will give steadier flow and greater stability.

[0032] FIG. 5 illustrates another embodiment of the PHE using the transitional two-phase heat carrier, where the serpentines are not limited to only the heating 1 and cooling 2 zones. They are spread across the entire cross section of the PHE. It is preferable for this design to exclude the transport zone 3 and to use only baffles 18 to divide the different zones (as it is shown in FIG. 5). This PHE can be successively used for small units with only two flows (one hot and one cold) and also for multiple flows (see FIG. 5a). It is possible to use this design for vertical (FIG. 5a and FIG. 5b) and horizontal (FIG. 5c) locations of the serpentines.

[0033] Experimental tests have shown that the optimal size for some positions the PHE are the same as the determination of the corresponding optimal parameters for metal thermal siphon heat exchangers. Actually the working lengths of the branches in the cooling LC and heating Lh zones correspondingly are related to each other as inverse proportional values of the average total heat transfer coefficients Kc and Kh in said zones, correspondingly: LC/Lh={square root}Kh/KC. This optimal ratio corresponds to the minimum of the external heat exchange surface for the plastic heat exchanger (PHE).

[0034] Using for the PHE of the new plastic materials with high level of thermal conductivity, it allows very efficient finned plastic pipes. If the plastic material of the plastic pipes has high thermal conductivity, for example plastic “CoolPoly”, the outer surface of the heating 1 and cooling 2 zones of the PHE can embody the plastic fins. The height of these plastic fins has to satisfy the condition:

h∂2&agr;/&lgr;f&dgr;f≦1,

[0035] where: h—height of the plastic fin, &agr;—coefficient of heat transfer, &lgr;f and &dgr;f—thermal conductivity and thickness of the plastic fin, correspondingly. Therefore applying the proposed invention together with the plastic materials with high thermal conductivity gives unique ability to eliminate the limitations for using plastic materials for heat transfer processes.

[0036] The advantage of applying of the plastic material for the PHE is connected with elasticity. Elasticity helps to aid the periodical movement of two-phase flow of the heat carrier and correspondingly to enhance the process of heat and mass transfer. Simultaneously this elasticity decreases rigidity for plastic elements of the PHE.

[0037] The PHE require frames, inserts and other accommodations, to fix spacing and pitches between different branches of pipes and other associated apparatus.

[0038] The main embodiments of design of the bundle of the serpentine pipes (SP) and consequently PHE on its base can be presented in the forms shown in FIG. 6 and FIG. 7. FIG. 6 illustrates the PHE using a transitional two-phase heat carrier, where all plastic serpentines 27 are located in the same plane. The serpentines 27 of the pipe are located on the plane of the frame. The top and bottom strips 29 and 30 and moving elements 28 for fixing plastic serpentines 27 are located on the frame. They have to fix the sizes of the serpentines 27 and its curvature. The connecting rods 32 with help of the screw-nut 31 ensure control of tension of the plastic serpentines 27. Simple packaging of the frames with the serpentine pipe 27 with baffle 18 and shell 19 can assemble the PHE on it.

[0039] FIG. 7 illustrates the PHE using a transitional two-phase heat carrier, where all plastic serpentines 27 are located in the multiple parallel planes. Any section consists of the plastic serpentine 27, spacer 33 for fixing distance between branches, tube 34, which support the right curvature of the pipe, screw-nut 35 for pressing the system of the serpentines 27 and spacers 33. The cross-arm 36 commonly with the dummy 38 for the serpentine 27 places of a role of any frame. The screw-nut 37 controls the tension of the SP. The filling nipple 39 is using for finding the optimal content and quantity of the transitional two-phase heat carrier to be placed in the pipes.

[0040] FIG. 8 illustrates the PHE using a transitional two-phase heat carrier, which contains the additional porous insert 40, heater 41 (it can be any kind of the heat flux concentrator) and also condensate accumulator (storage) 42. The porous insert 40 and the condensate accumulator 42 can be produced from plastic material. The condensate accumulator 42 is joined with one of the branches in the cooling zone 2 so, that it can flow of condensate into the condensate accumulator 42. The porous insert 40 is filled by condensate from the condensate accumulator 42, using capillary forces in the porous insert 40. The heat flux comes to the wetted porous insert 40 from its external surface and starts the evaporative process from porous body into the small azimuth and axial channels. Only vapor of the heat carrier (with pressure considerable more than pressure of condensate in the inlet of the condensate accumulator 42) comes into the outlet of the porous insert 40. The condensate accumulator 42 has to be protected from heating. Outlet vapor pressure can be more than inlet condensate pressure due to capillary pressure, a value which increases for smaller radius. Therefore in the plastic serpentine pipe (SP) additional driving force aids two-phase flow movement of the heat carrier, increase stability of this movement and improve the thermal and hydraulic characteristics of the PHE. It is possible (see FIG. 8) to install not only one but many such porous inserts (as the porous insert 40). Therefore it will be necessary to address both advantages and drawbacks of the numbers of the additional porous inserts 40.

[0041] All the proposed embodiments of the PHE using a transitional two-phase heat carrier can be developed for practical applications of the plastic heat exchangers. Plastic heat exchangers (PHE) constructed with pipes with small diameters (less 5-10 mm) can be for cooling electronic equipment, residential and commercial heaters and air conditioning systems, vehicles etc. PHE with small and big diameters can be for low-temperature heat and energy recovery systems, especially for using heat of waste gas. In these cases these systems have ecological and economical advantages.

Claims

1. A method of action of: a) a plastic tube heat exchanger, b) filled by a transitional two-phase heat carrier, c) circulating in heating and cooling zones, d) where the evaporation and condensation of the heat carrier causes circulation.

2. The method of claim 1, where the channels are made with small diameters less than 5 mm, which are partly filled by the two-phase heat carrier.

3. The method of claim 1, where the channels are of a diameter less than 10 mm.

4. The method of claim 1, where the channels are of a diameter of approximately 10 mm.

5. The method of a plastic heat exchanger as defined in claim 1, wherein the plastic material is combined with a selection from the heat carrier group: ammonia-water, ethanol-water or water-refrigerant.

6. The method of action of the plastic heat exchanger as defined in claim 5, wherein the optimal filling by the heat carrier of the internal volume of the plastic heat exchanger is in the range 30%-70% of the total internal volume of the heating, transport and cooling zones of the plastic heat exchanger for diameters between approximately 5-10 mm.

7. The method of claim 5 wherein the volume of the heat exchanger is filled in the range of 80% to 100% for diameters of 10 mm or greater.

8. The plastic heat exchanger for effecting the method to claim 1, comprising the continuous elongate plastic tube to form a continuous flow of the alternating liquid and vapor slugs of the heat carrier between the heating and cooling zones.

9. The plastic heat exchanger according to claim 8, wherein, the cooling and heating zones are located in the same serpentines and they have at least one baffle, which divides these zones.

10. The plastic heat exchanger according to claim 8, wherein the working lengths of the branches in the cooling Lc and heating Lh zones correspondingly are related each to the other as proportional values of the average total heat transfer coefficients Kc and Kh in said zones, correspondingly: Lc/Lh={square root}Kh/Kc.

11. The plastic heat exchanger according to claim 8, wherein inside serpentines are installed frames or inserts and other accommodations to fix the pitches between the branches and exclude bypass of flow.

12. The plastic heat exchanger according to claim 8, wherein the branches contain at least one porous insert, which covers at least one cross-section area of the place of the branch, and said insert embodies the azimuth and axial channels which is located on the outer surface of the insert.

13. The plastic heat exchanger according to claim 1, where the plastic material of the tubes has high thermal conductivity, 1) the plastic heat exchanger of claim 1 wherein the outer surface of the cooling and heating zones embodies the plastic fins, height which satisfy the condition:

h&dgr;2&agr;/&lgr;f&dgr;f≦1,

where: h—height of the plastic fin, &agr;—coefficient of heat transfer, &lgr;f and &dgr;f—thermal conductivity and thickness of the plastic fin.