DE69208076T2 - Stack evaporator - Google Patents

Abstract

A stack type evaporator comprises tubular elements (1) each having a plurality of inwardly protruding recessed ribs (7) which extend from an upper header portion (1a) of the element to a lower header portion (1b), with the ribs serving as straight drain canals (7a). A hydrophilic resin coating of a specific composition covers the outer surfaces of the tubular elements (1) and fins (2) each interposed between two adjacent tubular elements. The combination of straight drain canals with the specific hydrophilic resin coating is effective to facilitate the drainage of condensed water so that the waterdrops are perfectly prevented from flying out of the evaporator and that any stinking mold or mildew is not permitted to grow within a reduced amount of remaining adherent water. Also, the hydrophilic coating itself does not emit any unpleasant smell. <IMAGE>

Description

Background of the invention 1. Technical field of the invention:

The invention relates to an evaporator for use in motor vehicle air conditioning systems and, in particular, relates to a stack evaporator which is improved to prevent a condensed water drop from flying away and avoid the problem of an unpleasant odor. 2. Description of the prior art:

Stack evaporators of this type (see, for example, Japanese Utility Model Publication Sho. 53-32375) usually have plate-like tubular members each consisting of a pair of plate-like support plates 60 as shown in Figure 11. These support plates are adjacent to each other and connected along their peripheries 60a. Such tubular members are stacked side by side in the thickness direction with fin members provided between adjacent tubular members. At the end of each tubular member, a header 60b as an inlet and a header 60c as an outlet are provided to form a refrigerant passage. A refrigerant flowing through the inlet header 60b will flow and turn along the flow path of the tubular member to the other end of the tubular member before flowing into the outlet header 60c. These tubular elements thus represent the stack evaporator "with one-sided collector" which is often used in this area.

The "single-sided header" structure is somewhat disadvantageous because, due to the turn the coolant makes, it cannot flow evenly through the tubular elements but in a staggered manner. This inevitably reduces the effective heat transfer area of each tubular element.

Therefore, another evaporator, which is of the "both-sided header" type structure, has been proposed and used in certain cases. In this evaporator, each tubular element has an inlet header at one end and an outlet header at the other end.

The prior art stack evaporators of the single-sided and double-sided types use recessed fins 70 distributed on each support plate 60 as shown in Figure 11. These fins are intended to impart turbulence to the flow of refrigerant within the tubular elements so that heat transfer is improved. In more detail, numerous recessed fins 70 project inwardly from two plate-like fins connected to each other along their circumferences to form the connected support plates 60 of each tubular element (see, for example, Japanese Utility Model Publication Sho. 56-6847 and ibid. 63-33100).

However, when using these stack evaporators, water condensed on the surfaces of the tubular elements and the fin members remains within the recessed fins 70. An angle of contact (hereinafter referred to as "contact angle" for convenience) between each water droplet and the surface of the tubular member or the fin member to which the water drop adheres is so large that it is difficult to drain the condensed water smoothly. As a result, the water inside the air vents which has accumulated between the tubular members and the fin members arranged therebetween is sprayed to fly into a vehicle interior, thereby impairing the air-conditioned comfort therein. Furthermore, the air vents become moldy due to the adhering condensed water, and an unpleasant moldy or musty smell impairs the air flow which inadvertently enters the interior.

A prior art invention disclosed in Japanese Patent Publication Sho. 60-45776 has been made to address this problem. According to this teaching, the tubular members and the fin members are coated with a hydrophilic surface. The hydrophilic coating reduces the contact angle between the surface of the tubular member or the fin member and the water drop. As a result, the condensed water forms a thin layer on the surfaces which reduces the airflow resistance along the surface and which does not adhere to it but flows off evenly, thus addressing the problem of water droplets flying away.

As described in Patent Publication Sho. 60-45776, a water glass based coating has been preferred as a hydrophilic coating. However, an inherent odor of this water glass coating is unpleasant and affects the air-conditioned automobile interior into which the outside air flow inevitably Therefore, such a coating known from the prior art still has a significant problem.

On the other hand, another Japanese patent publication Sho. 61-39589 (corresponding to EP-A 0 409 130) or Patent Laid-Open Gazette Hei. 3-49944 make another proposal that uses a polyamide resin instead of water glass as a component of the hydrophilic coating. Although a polyamide resin coating does not cause as bad an odor as water glass, the water adhering to it cannot form a sufficiently thin layer. Therefore, the air flow resistance through this evaporator is comparatively large, it is difficult to drain the condensed water, and the problem of flying water drops is not solved.

Furthermore, it should be considered that the recessed fins exert a certain retaining force. Even though the adhesion and flowability of the condensed water have been improved by means of the hydrophilic coating on the outer surface of the evaporator, condensed water remains in the outer recesses of the fins 70 provided individually over the outer surface of each tubular member. The drainage of condensed water has not been improved to a satisfactory degree and the flying away of water drops and other secondary problems remain unsolved. Furthermore, due to the remaining condensed water, there is a risk that the tubular members become moldy. Mold or mildew emits a bad smell into the vehicle interior and makes it uncomfortable.

Therefore, in its Japanese Patent Application Hei. 1-223685 (see Patent Laid-Open Gazette Hei. 3-87595) corresponding to EP-A 0 415 584, this applicant has proposed not to rely on any hydrophilic coatings but to give the tubular members an improved shape so that the discharge of condensed water is improved, to prevent water drops from flying off and to prevent bad smell from occurring.

According to the construction of the evaporator of this prior art, each tubular member is formed with a plurality of recessed fins. These fins extend longitudinally and parallel to each other from an upper header to a lower header. Since such fins serve as drainage channels, the condensed water flows downward to be discharged from the lower header. Therefore, the condensed water adhering to the surface of the tubular member can be continuously discharged through the projecting fins, thereby increasing the "water-repellent" property.

In the condenser of this type, corrugated fins are arranged between two adjacent tubular elements, and these are designed to be water-conducting so that the condensed water cannot easily flow from the fins onto the projecting fins of the tubular elements. Although the condensed water in the edge or end region of the fins adjacent to the tubular elements will flow onto the tubular elements and into their recessed fins, the condensed water present in the recesses of the corrugated fins will remain there due to a large surface tension. In other words, certain portions of the condensed water present on the fins will not necessarily discharged through the recessed ribs of the tubular elements.

As a result, the drainage of condensate is not brought to a satisfactory level, and the problems of flying water drops and the bad smell resulting, for example, from the mold that remains in the water and adheres to it are not solved.

It may seem obvious to apply the hydrophilic coating made of water glass or synthetic resin also to tubular elements having the longitudinally recessed ribs serving as improved drainage channels.

However, it is not possible to avoid the problems already described, namely that on the one hand the hydrophilic coating consisting of water glass is undesirable due to its stench and that on the other hand the previously known resin coating does not improve the adhesiveness and flowability of the condensed water. The condensed water does not move evenly towards these recessed ribs, although they have the shape of straight drainage channels, so that the drainage is not satisfactory and the problem of water drops flying away remains unsolved. Object and summary of the invention

It is an object of the present invention to solve the problems inherent in previously known evaporators and therefore to create a novel stack evaporator in which neither water drops fly away nor bad smells so that it can be used advantageously in air conditioning systems.

The present inventors have made research and investigations to improve the stack evaporator already proposed by them and having tubular members with vertical recessed fins as described above in such a manner that the recessed fins can serve as more effective longitudinal channels for discharging the condensed water. As a result of these investigations, the inventors have found that a hydrophilic resin coating on the surface of the tubular members must have a specific resin composition in order to make its contact angle fall within a specific range.

There is therefore proposed a stack evaporator comprising tubular members having a plurality of recessed projections formed therein serving as longitudinal drainage channels extending between an upper and a lower header of each tubular member so that the condensed water is guided along the fins toward the lower header so that it can be drained away from the tubular members. Furthermore, the evaporator according to this invention has a specific hydrophilic resin coating which does not emit a bad smell and covers both the surfaces of each fin and each tubular member so that the condensed water is carried from the fins onto the tubular member and moves smoothly from a flat surface thereon to the outer recesses of each fin.

The object of the present invention is achieved in one embodiment by a stack evaporator comprising: a plurality of plate-like tubular elements, each consisting of a pair of adjacent plate-like support plates connected to each other along their circumferences so as to form a refrigerant passage therebetween; a plurality of radiating fins each arranged between two adjacent tubular elements stacked side by side in the direction of their thickness; upper and lower headers arranged at an upper and a lower end of each tubular element, respectively, the headers being connected to the other corresponding headers so that the tubular elements are connected to form an evaporator; a plurality of recessed ribs projecting inwardly from each support plate and extending vertically and parallel to each other from the upper header to the lower header, an inner end of each rib of one support plate being adjacent to and welded to a flat portion between the ribs of the other support plate, and wherein the coolant passage extending through each tubular member is divided by the ribs into a plurality of individual passages extending between the upper and lower headers, each tubular member having on its surface a plurality of longitudinal drainage recesses also extending between the upper and lower headers, and a hydrophilic resin coating covering the outer surface of each tubular member and each radiating rib and composed of a main component and a hydrophilic agent, the main component being a polyvinyl alcohol resin and the hydrophilic agent being a polyamide and/or polyvinylpyrrolidone resin. The object of the present invention is achieved in a further embodiment by a stack evaporator comprising: a plurality of plate-like tubular members, each consisting of a pair of adjacent plate-like support plates connected to each other along their circumferences so as to form a refrigerant passage therebetween; a plurality of radiating fins each arranged between two adjacent tubular members stacked side by side in the direction of their thickness; upper and lower headers arranged at an upper and a lower end of each tubular member, respectively, the headers being connected to the other corresponding headers so that the tubular members are connected to form an evaporator; a plurality of recessed ribs projecting inwardly from each support plate and extending vertically and parallel to each other from the upper header to the lower header, an inner end of each rib of one support plate being adjacent to and welded to a flat portion between the ribs of the other support plate, and wherein the coolant passage extending through each tubular member is divided by the ribs into a plurality of individual passages extending between the upper and lower headers, each tubular member having on its surface a plurality of longitudinal drainage recesses also extending between the upper and lower headers, and a hydrophilic resin coating covering the outer surface of each tubular member and each radiating rib and having a contact angle θ ranging between 5° and 20°.

In addition to the polyvinyl alcohol resin as the main component blended with the hydrophilic agent, the resin coating may contain other components which may include a hardener, a surface active agent (hereinafter referred to as "surfactant") and a microbicide such as an antibacterial agent, a bactericide or an anti-mold agent for preventing malodorous mold formation on the surfaces of the evaporator.

A preferred composition of such a resin coating includes: 30 - 65 parts by weight polyvinyl alcohol resin as the main component; 20 - 65 parts by weight polyamide and/or polyvinylpyrrolidone resin as the hydrophilic agent; 1 - 15 parts by weight of the hardener; 0.1 - 2.0 parts by weight of the surfactant; and 3 - 30 parts by weight of the microbicide.

Although either the polyamide or the polyvinylpyrrolidone resin can be used alone as hydrophilic agents, a mixture of both is advantageous.

The hardener can be either a phenolic resin or a polyurea resin, although the former is less odorous and therefore more preferred.

A preferred surfactant is an anionic surface active agent.

The appropriate microbicide includes: bis-(2-pyridylthio)- zinc 1,1'-diphoxide; methylbenzimidazole carbamate; and 2-(4-thiazoly )-1H-benzimidazole.

The contact angle of the hydrophilic resin coating is an essential feature of the present invention. With an angle of less than 5º, the condensate adhere too much to the surface of the tubular elements and the ribs, whereas at an angle above 20º the condensate adheres less but is less mobile with respect to the longitudinal drainage channels. In these cases the longitudinal channels do not fully fulfil their drainage function. Therefore the contact angle θ should be between 5º and 20º, in particular between 7º and 13º.

The inwardly directed projecting recessed ribs constitute the longitudinal drainage channels, which shall be such that their channel width "W", channel depth "D" and channel pitch "P" are within the ranges given below to ensure better drainage. Also, a proportion ratio (%) of the channels to the total surface area of each support plate, from which the area of the enlarged portions located on both sides of the said support plate is deducted, should be within a range given below.

The channel width "W" is between 1 and 3 mm, preferably between 1.3 and 2.4 mm.

The channel depth "D" is between 1 and 2.5 mm, preferably between 1.5 and 2.1 mm.

The channel spacing "P" is between 7 and 14 mm, preferably between 8 and 11 mm.

The channel share ratio (%) is between 5 and 40%, preferably between 15 and 25%.

It will be understood that the recessed ribs directed inward from each tubular element and extending from its upper header to its lower header are advantageous in that all the condensate water on the tubular elements and the fin members can flow outwards evenly along the fins. The condensate water will then quickly leave the evaporator without causing problems due to the flying water droplets.

The hydrophilic resin coating covering the outer surfaces of the tubular members and the fin members and comprising a polyvinyl alcohol resin as a main component and polyamide and/or polyvinylpyrrolidone resins as hydrophilic agents does not cause a bad smell which was unavoidable in the water glass coating. Furthermore, the resin coating in the particular composition of the present invention advantageously cooperates with the inwardly projecting recessed fins, such as the longitudinal drainage channels, to impart a water-repellent property to the evaporator for facilitating the drainage of the condensed water. Therefore, the problem of flying water drops has been more completely dealt with by the evaporator according to the invention. Brief Description of the Figures

The present invention will be more easily understood from the described embodiments with reference to the accompanying drawings, in which:

Fig. 1 is a plan view of a support plate forming an evaporator in an embodiment in a lateral view of the uniform coolant passages;

Fig. 2 is an enlarged cross-sectional view taken along line 2-2 of Figure 1;

Fig. 5; 3 is a further enlarged cross-sectional view taken along line 3-3 of Figure 1;

Fig. 4a is another enlarged cross-sectional view taken along line 4-4 of Figure 1;

Fig. 4b is yet another enlarged cross-section taken along line 5-5 of Figure 1;

Fig. 5 is an enlarged cross-section of a portion of a tubular member with an adjacent collector;

Fig. 6 is a perspective view showing components of the evaporator shown without holding means;

Fig. 7 is a front view showing the completed evaporator;

Fig. 8 shows the coolant flow;

Fig. 9 shows a plan view of a support plate forming a partition provided in the collector;

Fig. 10 is an enlarged cross-sectional view taken along line 10-10 of Fig. 9;

Fig. 11 is a plan view of the prior art support plate from a side angle looking toward a uniform coolant passage;

Fig. 12 is a diagram showing the relationship between an "area" ratio (%) and a "proportion" ratio (%) of water retained on the support plates 6, wherein the "area" ratio is a ratio of the areas of the longitudinal drainage channels with respect to the total surface area of the support plate, from which areas expanded on both sides have been subtracted; and the "proportion" ratio is a ratio of the amount of water retained with respect to an outer surface of the support plate in contact with air, which is given in %, taking as a standard (e.g. 100) a value for a case where no channels are formed on the support plate;

Fig. 13 is a graph showing a relationship between a cooling capacity and a refrigerant pressure at an outlet;

Fig. 14 is a graph showing a relationship between a coolant flow resistance and a coolant flow rate; and

Fig. 15 is a graph showing a relationship between an air flow resistance and an air flow rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention used for stack evaporators made of aluminum or aluminum alloys for use in automotive air conditioning systems are described in detail below:

As shown in Figure 7 in its entirety, the evaporator comprises a plurality of plate-like tubular members 1 stacked upright and arranged side by side. The evaporator also comprises corrugated fin members 2, most of which are arranged between two adjacent tubular members 1 and the others outside the outer tubular members 1. The corrugated fin members 2 are brazed to the tubular members.

Each tubular element 1 is, as shown in Figures 1 to 7, provided with an upper header 1a and a lower header 1b which are voluminous and arranged longitudinally at opposite ends of the element. Uniform coolant passages 1c, extending longitudinally of the element 1, are provided between the headers 1a and 1b and in fluid communication therewith, the uniform coolant passages 1c forming a flat coolant passage as a whole. The adjacent tubular elements 1 are firmly connected to one another in the region of their headers 1a and 1b, these regions being in fluid communication with one another via coolant passages 1d.

Each tubular element 1 is formed by arranging two plate-like support plates, with the respective inner sides facing each other, which are then brazed together along their peripheries 6a to be one piece. The support plates 6 are formed by pressing a brazing sheet with a core sheet, whose front and back are covered with a solder layer. The solder layer is applied by weld plating so that the support plates 6 can be easily soldered to one another and also to the adjacent corrugated fins 2.

The end portions of each support plate 6, except for the outer support plate 6, which form the outer tubular members 1, protrude outward to form flared portions. Each outer support plate 6 has flat-shaped ends as shown in Figure 6 and has three coolant passage openings ld arranged in the transverse direction.

Three further coolant passage openings ld penetrate a curvature of each expanded region in the transverse direction of the support plate so that the collectors of adjacent tubular elements 1 are connected to one another. However, as shown in Figure 7 using a complete evaporator, there are no openings ld in the adjacent side walls of the expanded regions 9 which belong to the lower collector 1b of the fifth and sixth tubular elements 1, counting starting from the right. Likewise, there are no such openings 1d in the adjacent side walls of the expanded regions of the upper collector 1a of the tenth and eleventh tubular elements 1. Similarly, there are no openings 1d in the adjacent side walls of the expanded regions 9 of the lower collector 1b between the fifteenth and sixteenth tubular elements 1. Those side walls of the expanded regions that are not pierced by openings serve as dividing members provided between adjacent expanded regions.

As shown in Figure 7, each corrugated fin is arranged between two adjacent tubular members 1, which in this arrangement are brazed together by virtue of the above-mentioned solder layer. A coolant inlet pipe 3 is connected to the lower header 1b on the rightmost tubular member 1 in fluid communication therewith. A coolant outlet pipe 4 is connected in a corresponding manner to the lower header 1b of the leftmost tubular member 1 also in fluid communication therewith.

Due to the above-mentioned dividing members, the refrigerant entering the evaporator through the inlet pipe 3 is guided in a zigzag shape as shown in Figure 8, with its flow direction being changed at each boundary between adjacent groups of tubular members before the refrigerant leaves the evaporator through the outlet pipe 4. Heat transfer is thus effective between the refrigerant thus flowing and the air flow passing through air passages, each air passage being a gap between the adjacent tubular members and the fin members 2 arranged therebetween. The reference numeral 5 in Figures 6 and 7 denotes a side plate which is arranged on the outside of the outer corrugated fin member.

In this embodiment, the "groups" include the same number of tubular elements 1, whereby the evaporator ensures excellent heat transfer. However, the total number of tubular elements may occasionally make it impossible to divide them into equal groups of tubular elements. In a case where the inlet pipe 3 and the outlet pipe 4 are connected at the lower part of the outermost tubular elements, it may be possible to divide the tubular elements into groups of equal numbers despite an odd number of tubular elements. number of said elements in the groups, it may be desirable to allocate a larger number of said elements to a group connected to the inlet than to the other groups. Other cases may arise where it is desirable to increase the number of tubular elements progressively from the inlet group to the outlet group. In such cases, details will be determined with a view to gradually increasing the essential cross-sectional area of the refrigerant passage towards the outlet, taking into account the total number of said elements, the number of turns around which the refrigerant must flow, the positions at which the inlet pipe 3 and the outlet pipe 4 are connected to the evaporator body or other conditions. In other words, the preferred grouping of the tubular elements should be selected having regard to all relevant conditions.

As shown in Figures 1, 3 and 6, recessed ribs 7 are arranged on an inner side of each support plate 6 between its two widened portions 9 at regular intervals in the longitudinal direction of the support plate. The arrangement of the recessed projections is offset with respect to the longitudinal side of the support plate. The inwardly directed recessed ribs 7, which extend longitudinally from one widened portion 9 to the other, serve as longitudinal drainage channels, as will be described in more detail below. A pair of support plates 6 with ribs 7 are brought into close contact with each other and brazed together along their peripheries 6a so that they are integral. As can be seen in Figures 1 and 3, the ribs 7 of one support plate 6 are shown as solid lines and those of the other support plate are shown as dot-dash lines, arranged alternately with each other. Inner end surfaces of the ribs 7 of one Support plate 6 are closely arranged and brazed to the flat portions 8 between two adjacent ribs 7 of the other support plate so as to form a plurality of uniform coolant passages le extending longitudinally from the input header 1b to the output header 1a within the coolant passage 1c of each tubular member 1. The longitudinal drainage channels 7a are formed by the inwardly projecting recessed ribs 7. For the purpose of improving the drainage of condensate, in consideration of other requirements, it is desirable to select the parameters or dimensions of the drainage channels such as their width "W", their depth "D" and their pitch "P" as shown in Figure 3, as well as the area ratio (%) of the channels to the surface of the support plate 6, from which the expanded portions 9 are excluded.

The channel width is preferably within a range of 1 to 3 mm. If the channels are narrower than 1 mm, the condensate cannot flow into them evenly, so that the longitudinal channels cannot serve effectively as drainage recesses. If the channels are wider than 3 mm, the uniform passages le become too narrow to keep the coolant pressure loss below a certain limit. The preferred range of this width is therefore between 1.3 and 2.4 mm.

The channel depth is preferably in the range between 1 and 2.5 mm. If the channels are shallower than 1 mm, the uniform passages le become too narrow to keep the coolant pressure loss below a certain limit and the condensate cannot flow through them to a sufficient extent, so that the longitudinal channels no longer function. as drainage channels. If the channels are designed to be deeper than 3 mm, the hydraulic diameter of the uniform passages becomes too large to maintain a very good heat transfer property. The preferred range of depth is therefore between 1.5 and 2.1 mm.

A preferred range of channel spacing is between 5 and 14 mm. If the spacing is less than 7 mm, the uniform passages le become too small to keep the coolant pressure loss below a certain limit. If the channel spacing is greater than 14 mm, the condensate cannot flow evenly into the channels. The preferred spacing is therefore between 8 and 11 mm.

The area ratio (%) between the longitudinal drainage channels and the support plate surface excluding the widened portions 9 should be in the range of 5 to 40%. If the ratio is not within this range, the longitudinal channels will no longer be good drainage recesses because an excessive amount of condensed water will be retained by the support plate. Figure 12 is a diagram showing a state in which the area ratio (%) of the longitudinal channel portion to the amount of water retained with respect to a unit surface in contact with the air is given as a reference value (e.g. 100%) in the case where no channels are provided in the support plate. If the drainage channels have an area ratio of more than 40%, not only will they lose their drainage property, but the passage 1c will also become too narrow to keep the refrigerant pressure loss below an allowable limit. Therefore, the preferred ratio is between 15 and 25%. The amount of water retained in the graph refers to the amount of water retained that remained on evaporators after they were immersed in a water container and weighed 30 minutes after withdrawal.

The cross-section of the inwardly directed, projecting, recessed ribs 7 need not necessarily have the rectangular shape shown in Figure 3, but may also have a trapezoidal shape with a width that decreases progressively towards their bottom or any other shape. However, the shape shown is desirable in this embodiment in order to ensure a good drainage effect of the longitudinal channels.

The side plates 5, which are arranged outside the outer corrugated fins 2, have a plurality of groove-shaped projections 5a arranged on their inner sides. The projections 5a extend vertically and parallel to each other, thereby providing a further plurality of additional vertical drainage channels between the outermost fins and the side plates attached to their outer surfaces. The water condensing in the gaps between the outermost tubular elements and the side plates flows downwards through these additional channels, thereby providing drainage also for these air passages formed by said gaps. In this embodiment, as shown in Figure 5, a coating "S" of a hydrophilic resin covers the surfaces of the tubular elements 1, the corrugated fins 2 and the side plates 5.

The hydrophilic resin must contain a polyvinyl alcohol resin as the main component and must be mixed with polyamide and/or a polyvinylpyrrolidone resin for the following reasons. First, this resin composition does not have an unpleasant odor caused by the previously known water glass coating, which has deteriorated the environment of an automobile interior. In other words, the air-conditioned space of an automobile remains pleasant when the resin coating according to the present invention is used as the coating of the evaporator. Second, the resin coating can also protect an underlying oxide layer from odor emission. Therefore, the problem of evaporator odor is much better dealt with. Third, together with the longitudinal drainage channels of the present invention, the proposed resin coating is advantageous because the channels serve more effectively as drainage recesses. The condensed water adheres to the water glass coating to such an extent that the longitudinal drainage channels cannot fully perform their drainage function. On the other hand, the previously known resin coating results in a lower water adhesion capacity, but this was less mobile, so that the longitudinal drainage channels could not fulfill their intended function, so that the problem of flying water drops was not addressed. The novel resin coating according to the present invention is of such a nature that it can be classified between a water glass coating and the previously known resin coating, whereby the longitudinal drainage channels can fulfill their drainage function to a satisfactory extent.

The polyvinyl alcohol resin as the main component of said hydrophilic resin can be mixed with either polyamide resin or a polyvinylpyrrolidone resin. However, It is advantageous if both the polyamide and the polyvinylpyrrolidone resin are added to the polyvinyl alcohol resin so that the resin coating has an improved initial hydrophilic property as well as a higher durability.

In addition to the polyamide and/or polyvinylpyrrolidone resins as hydrophilic agents mixed with the polyvinyl alcohol resin as a main ingredient to prepare a mixture, a hardener such as a phenolic resin or a polyurea resin and a surfactant such as an anionic surfactant and a microbicide such as bis-(2-pyridylthio)zinc 1,1'-diphoxide; methylbenzimidazole carbamate; or 2-(4-thiazolyl)-1H-benzimidazole may be mixed with the mixture.

The phenolic resin as a hardener has less odor and is therefore better suited than the polycarbamite resin.

The preferred contents of the polyvinyl alcohol resin, the hydrophilic agent, the hardener, the surfactant and the microbicide are 30 - 65 parts, 20 - 65 parts, 1 - 15 parts, 0.1 - 2.0 parts and 3 - 30 parts, respectively, all of which are by weight. Reasons for this are described below.

If the content of polyvinyl alcohol resin contained as a main ingredient to serve as a raw material of the hydrophilic resin coating is below 30 parts by weight, the coating is not sufficiently hydrophilic and also becomes too thin to disperse the microbicides therein. However, a content of more than 65 parts by weight increases the manufacturing cost of the hydrophilic coating and at the same time impairs its hydrophilic properties. A particularly preferred content of the polyvinyl alcohol resin is therefore between 40 and 60 parts by weight.

If the content of the hydrophilic agent added to improve the hydrophilic properties of the resin coating is less than 20 parts by weight, the coating will be insufficiently hydrophilic. A content exceeding 65 parts by weight of said hydrophilic agent will cause excessive solubility of the resin coating. If a microbicide is included, it will be lost when the coating dissolves, so that prevention of mold growth is no longer ensured. A particularly preferred content of said hydrophilic agent is between 35 and 45 parts by weight.

If the hardener used to adjust the hardness of the coating to a certain level is contained in less than 1 part by weight, resulting in an uncured coating, or if it is contained in more than 15 parts by weight, this will trigger a reaction with the hydrophilic atom groups in the hydrophilic resin molecules, resulting in the hydrophilic properties not being achieved. A particularly preferred content of the hardener is therefore between 5 and 10 parts by weight.

The surfactant is added to stabilize the resin solution in which the evaporator or its parts are immersed to form the hydrophilic coating so that it does not become blistered during use. A content of less than 0.1 part by weight is therefore too low to prevent blistering in the solution. Such a low content is also insufficient to distribute the microbicide homogeneously in the resin coating, but an excess content of more than 2.0 parts by weight will also cause numerous bubbles in the resin solution, resulting in unevenness of the cured resin coating. A particularly preferred content of the surfactant is therefore between 0.5 and 1.5 parts by weight.

The microbicide in this specification includes an antibacterial agent, a bactericide, an anti-mold agent or the like. The hydrophilic resin coating containing such a microbicide protects the evaporator from mold formation when condensed water adheres to prevent odor generation due to the mold. The above-mentioned surfactant is also effective to disperse such a microbicide in the resin solution.

3 to 30 parts by weight of the microbicide may be added to the resin. Although a content below 3 parts by weight is not as effective in completely preventing mold growth, a content above 30 parts by weight results in a white powder of the microbicide forming on the surface of the evaporator. Such a powder will fly off and enter the air-conditioned vehicle interior, thereby impairing its comfort. Therefore, a preferred content is between 5 and 15 parts by weight.

The thickness of the above-described hydrophilic resin coating "S" is preferably between 0.2 and 1.5 µm. A resin coating thinner than 0.2 µm fails to fulfill the intended function of a hydrophilic coating, and a thickness greater than 1.5 µm causes an odor inherent in the resin to be conspicuous. A preferred range of thickness is between 0.5 and 1.3 µm.

As previously noted, the inventors have found that, with respect to the longitudinal drainage channels 7a, the hydrophilic resin coating must have a certain contact angle between 5° and 20° in order for these channels to fully function as drainage recesses.

According to the concept before this invention was made, the contact angle should be as small as possible. However, the present inventors have found that at a contact angle smaller than 5°, the condensed water adheres disproportionately to the evaporator surfaces and that the high drainage capacity exhibited by the longitudinal channels 7 is therefore impaired. A contact angle above 20° has also been found to be unsuitable since a low adhesion of the condensed water has been observed to similarly impair the drainage of the channels 7a. Therefore, a preferred contact angle is in the range of 5° to 20° and in particular between 7° and 13°.

The hydrophilic resin coating "S" can be prepared, for example, in the following manner.

After assembly to produce the described structure, the stack evaporator is subjected to a pretreatment of an acid wash and a rinse in that order and under usual conditions. Then a chromate primer is applied to the thus treated surface by means of a suitable process using a mixed solution of phosphate and chromate components or a solution of a suitable chromate component. The primer imparts to the surface a higher corrosion resistance and enables the resin to adhere closely to the surface.

Following these treatments, the stack evaporator is washed and then immersed in a hydrophilic resin solution of the composition described above. An uncured resin coating is thus applied to the surface and finally the evaporator is subjected to a baking process to cure and finish the coating.

A surprising effect provided by the invention was confirmed in the following tests. First, 6 samples of the stack evaporator were prepared, which were substantially the same as those described above in the embodiment. These samples were either of the single-sided or double-sided collector type and had different types of projections on their tubular elements and different types of or no hydrophilic coating, as shown in Table 1.

Their drainage properties and odors were tested using the procedures described below to obtain the result shown in Table 2. In addition, a content of retained water was measured as a proportion of the heat transfer surface in the area of contact with the air. The values obtained by the latter test are also shown in Table 2 and expressed as a % of the value for sample No. 1.

In the drainage property test, the samples were immersed in water, pulled out to stand for 30 minutes and then weighed for the water retained at that time (corresponding to a process status in actual use). The reference symbols "O", "Δ", "X" and "XX" in Table 2 indicate: A small amount of water retained without flying water drops; Table 1

Notes on Table 1:

W = Width L = Length H = Height D = Depth

ger. = just vorbek. = previously known

* = Composition of the coating

G% = weight percent #= of the coating

etc.** = 18 wt% polyamide + 18 wt% polyvinyl pyrrolidone

+ 9 wt% phenolic resin + 1 wt% anionic surfactant

+ 9 wt% bis-(2-pyridylthio)-zinc 1,1'-diphoxide

etc.*** = 65 wt% polyamide

etc.**** = 2 wt% hardener

PVA = polyvinyl alcohol resin PA = polyamide resin Table 2

a a slightly higher proportion of retained water with only occasional water droplets flying away; an increased proportion of locally retained water from which water droplets can fly away; and a significant amount of absorbed water at which water droplets will definitely fly away.

An evaluation of the unpleasant smell was carried out relying on the human sense of smell, but in an environment simulating the actual operation of the capacitor. The reference symbols "O", "Δ", "X" and "XX" indicate: Odorless at the beginning of the test and remaining odorless; not smelly at the beginning, but emitting odor after use for a long time; slightly smelly at the beginning, and emitting odor for a long time; and remarkably smelly from the beginning of use.

As can be seen from the above result, the evaporator having the tubular members having the inwardly and vertically directed recessed fins and the surfaces of which are coated with the specific hydrophilic resin coating according to the present invention is superior not only in its odor but also in its drainage properties compared with all other reference samples of the evaporators. Therefore, both the problem of flying off of water drops and the bad odor have been simultaneously addressed by the invention.

The data on water uptake per surface area in contact with air indicate that the particular resin coating in the invention works well with the longitudinal drainage channels to provide the best drainage. Although the resin composition in the invention (with a contact angle of less than 20º and between 7º and 13º in the embodiment) is not necessarily more hydrophilic than the water glass coating (most hydrophilic and with a contact angle of less than 5º). The previously known coating is less suitable for water absorption. This indicates an "organic" and effective combination of the particular resin coating with the longitudinal drainage channels.

Furthermore, the sample No. 1 (invention) was compared with the sample No. 2 which is the single-side header type stack evaporator and which is a typical and widely used one. Figures 13 to 15 show the results of comparison tests conducted on: their cooling capacity at different outlet refrigerant pressure; their refrigerant flow resistance at different refrigerant flow rates; and their air flow resistance at different air flow rates.

The cooling capacity of the sample No. 2 decreases significantly with increasing outlet refrigerant pressure, while the capacity of the sample No. 1 (invention) decreases only gradually. This means that the evaporator proposed by the invention is improved in its cooling capacity at different outlet refrigerant pressures. Regarding the refrigerant flow resistance, it is found that the sample No. 1 offered a lower resistance to the refrigerant flow in the order of 0.1 kg per cm² or more at different refrigerant flow rates than the sample No. 2. Furthermore, it was found that the sample No. 1 offered a lower resistance to the air flow of about 2 mmAq at different air flow rates. These data show that the evaporator is also excellent in its cooling capacity and operation.

In summary, the evaporator according to the present invention comprises tubular members having an inlet header at one end and an outlet header at the other end. Each tubular member consists of a pair of support plates, each having the recessed ribs projecting inwardly therefrom and extending vertically and parallel between the headers. The uniform flow passages are provided for refrigerants flowing through the tubular members in a manner that allows for offset or turbulent flow.

Consequently, a uniform and effective heat transfer is achieved throughout the evaporator, thereby improving its heat transfer capacity as a whole and reducing refrigerant pressure loss.

Furthermore, the fins projecting from one support plate are arranged alternately with respect to the fins of the other support plate in each pair of support plates. The inner end surfaces of these fins of one support plate abut against the flat portions of the other and are secured thereto. This feature is advantageous because any slight error in the arrangement between the opposing support plates can never result in a leaky connection of the two plates, although this was unavoidable in prior evaporators in which the fins were immediately adjacent to each other. Due to such a feature, a less precise assembly is feasible, whereby stable tubular elements are made with a pair of rigidly connected support plates. Moreover, such a structure has a larger area for heat-transferring coolant, which increases the heat transfer rate of the evaporator.

Another important feature is that the outer projections of the recessed fins, which project inwardly and extend vertically between the upper and lower headers, of each tubular member serve as longitudinal drainage recesses or channels for draining the condensate. The condensate on the surfaces of the tubular members and fin members continuously flows downward along the fins through the longitudinal channels and is quickly removed from the evaporator.

The unique combination of such longitudinal drainage channels (e.g. one feature) with the particular hydrophilic resin coating (e.g. the other feature) covering the surfaces of the tubular elements and the fin members acts in the invention as an unexpected synergistic effect of these two features. The synergistic effect is greater than the simple sum of the individual effects resulting from the features, so that the drainage or water-repellent properties of the evaporator have been improved in a surprising manner.

As a result, the flying away of water droplets is prevented to an almost perfect degree and, consequently, at the same time, the formation of mold in the adhering condensate is prevented, so that the air-conditioned environment in the vehicle cabin or in the vehicle interior is kept pleasant.

The resin composition in the invention includes polyvinyl alcohol as a main component and as a hydrophilic agent (e.g. polyamide and/or polyvinylpyrrolidone resins), which are mixed with it. This composition does not emit such a foul smell as is the case with water glass coating, so this also contributes to a better environment in the vehicle cabin.

In a preferred embodiment as claimed in claims 2 and 3, in which the hardener, the surfactant and the microbicide are mixed with the main component and the hydrophilic agent just mentioned, the "anti-mold effect" is significantly improved, while also ensuring good drainage through the longitudinal channels.

In a further preferred embodiment as claimed in claim 4, in which the hydrophilic resin coating has the contact angle θ between 5° and 20°, the water drop drainage effect of the longitudinal drainage channels is doubled, so that the stack evaporator has no flying water droplets and no bad smell, so that it can be advantageously used in air conditioning systems.

1. A stacked evaporator comprising a plurality of plate-like tubular elements (1), each consisting of a pair of opposed plate-like support plates (6) connected to each other along their peripheries (6a) so as to form a refrigerant passage therebetween, a plurality of fins (2) each arranged between two adjacent tubular elements (1) stacked side by side in the direction of their thickness, upper headers (1a) and lower headers (1b) arranged at the upper and lower ends of each tubular element (1), respectively, the headers being connected to other corresponding headers so that the tubular elements are connected to form the evaporator, each tubular element (1) having on its surface a plurality of recessed fins (7) serving as longitudinal drainage channels (7a) and which extend from the upper header (1a) to the lower header (1b), characterized in that a hydrophilic resin coating covers the outer surfaces of the tubular elements and the fins, and that the hydrophilic resin coating consists of a polyvinyl alcohol resin as a main component and polyamide and/or polyvinylpyrrolidone resins as their hydrophilic agents mixed with the main component.

2. A stack evaporator according to claim 1, characterized in that the plurality of recessed fins (7) project inwardly from each support plate (6) and extend parallel to each other from the upper header (1a) to the lower header (1b), an inner end of each fin of one support plate abutting and connecting to a flat portion between the fins of the other support plate, the refrigerant passage (1e) being formed by each tubular member and divided by the fins into a plurality of individual unitary passages (1c) extending from the upper header (1a) to the lower header (1b).

3. A stack evaporator according to claim 1 or 2, characterized in that the hydrophilic resin coating contains a hardener, a surfactant and a microbicide in addition to the polyvinyl alcohol resin as the main component and polyamide and/or polyvinylpyrrolidone resins as their hydrophilic agent.

4. A stack evaporator according to claim 3, characterized in that the hydrophilic resin coating consists of 30 - 65 parts by weight of polyvinyl alcohol resin as the main component, 20 - 65 parts by weight of the hydrophilic agent, 1 - 15% by weight of the hardener, 0.1 - 2.0% by weight of the surfactant and 3 - 30% by weight of the microbicide.

5. A stack evaporator according to claim 1, characterized in that the longitudinal drainage channels (7a) for the condensate have a width "W" of 1 - 3 mm and a depth "D" of 1 - 2.5 mm and are arranged at a distance "P" of 7 - 14 mm.

6. Stack evaporator according to claim 1, 2 or 5, characterized in that a ratio between the surface area of the longitudinal drainage channels (7a) for condensed water to the surface area of the support plate (6) without its upper and lower widened regions (9) is between 5 - 40%.

7. Stack evaporator according to claim 1, characterized in that, with the exception of the collectors (1a, 1b) of the tubular elements (1) arranged at predetermined positions of the evaporator, the other tubular, adjacent elements are in fluid communication with one another via coolant flow openings which are introduced into the apex regions of the collectors, the walls of the collectors at the predetermined positions of the first-mentioned tubular elements serving as dividing members which divide the tubular elements to form groups, the dividing members causing the coolant flowing into the evaporator through a coolant inlet pipe (3) to turn in a meandering manner at each boundary between the groups before it leaves the evaporator through a coolant outlet pipe (4).

8. A stack evaporator comprising a plurality of plate-like tubular elements (1), each consisting of a pair of opposed plate-like support plates (6) connected to each other along their peripheries (6a) so as to form a refrigerant passage therebetween, a plurality of fins (2) each arranged between two adjacent tubular elements (1) stacked side by side in the direction of their thickness, upper collectors (1a) and lower headers (1b) arranged at the upper and lower ends of each tubular member (1), respectively, the headers being connected to other corresponding headers so that the tubular members are connected to form the evaporator, each tubular member (1) having on its surface a plurality of recessed ribs (7) serving as longitudinal drainage channels (7a) and extending from the upper header (1a) to the lower header (1b), characterized in that a hydrophilic resin coating covers the outer surfaces of the tubular members and the ribs, and that the hydrophilic resin coating has a contact angle in the range of 5° to 20°.

9. A stack evaporator according to claim 8, characterized in that the plurality of recessed fins (7) project inwardly from each support plate (6) and extend parallel to each other from the upper header (1a) to the lower header (1b), an inner end of each fin of one support plate abutting and connecting to a flat portion between the fins of the other support plate, the refrigerant passage (1e) being formed by each tubular member and divided by the fins into a plurality of individual unitary passages (1c) extending from the upper header (1a) to the lower header (1b).

10. A stack evaporator according to claim 8, characterized in that the longitudinal drainage channels (7a) for the condensate have a width "W" of 1 - 3 mm and a depth "D" of 1 - 2.5 mm and are arranged at a distance "P" of 7 - 14 mm.

11. Stack evaporator according to claim 8, 9 or 10, characterized in that a ratio between the surface area of the longitudinal drainage channels (7a) for condensed water to the surface area of the support plate (6) without its upper and lower widened regions (9) is between 5 - 40%.

12. Stack evaporator according to claim 8, characterized in that, with the exception of the collectors (1a, 1b) of the tubular elements (1) arranged at predetermined positions of the evaporator, the other tubular, adjacent elements are in fluid communication with one another via coolant flow openings introduced into the apex regions of the collectors, the walls of the collectors at the predetermined positions of the first-mentioned tubular elements serving as dividing members which divide the tubular elements to form groups, the dividing members causing the coolant flowing into the evaporator through a coolant inlet pipe (3) to turn in a meandering manner at each boundary between the groups before it leaves the evaporator through a coolant outlet pipe (4).