The present invention relates to a cross-fin type heat exchanger in which a plurality of heat transfer fins are arranged in an array around straight pipe portions of a serpentine heat transfer tube with a plurality of bends, and a refrigeration cycle apparatus including the cross-fin type heat exchanger.
In a typical cross-fin type heat exchanger in which a plurality of heat transfer fins are arranged in an array around straight pipe portions of a serpentine heat transfer tube with a plurality of bends, when heat transfer surfaces are cooled so that the surface temperature is at or below the air dew point temperature, condensation of water vapor in the air occurs on the heat transfer surfaces and water droplets are generated on the surfaces. In particular, when the temperature of the fins is at or below 0° C., a frosting phenomenon occurs such that water vapor in the air forms frost on the heat transfer surfaces. As the frost on the heat transfer surfaces grows, air paths through which the air passes are clogged. Disadvantageously, airflow resistance increases, so that the performance of an apparatus markedly decreases.
To avoid the performance decrease due to frost, a defrosting operation for removing frost formed on the surfaces of the heat exchanger has to be periodically performed. For the defrosting operation, for example, a hot gas system in which the heat exchanger, serving as a target, is heated from the inside by switching of flow directions of a refrigerant in a refrigeration cycle or a heater system in which the heat exchanger is heated from the outside by a heater disposed near the heat exchanger is used. During the defrosting operation, a role of the apparatus, for example, comfort of air conditioning, is reduced. Furthermore, the efficiency of such a device is also reduced. It is therefore necessary to shorten the time of the defrosting operation as much as possible.
As regards the frost problem, according to a related-art, the surface of each fin is coated with a hydrophilic coating layer, the hydrophilic coating layer is exposed to plasma to form fine asperities thereon so that the area of the hydrophilic coating layer on the surface of the fin is increased, thus enhancing the effect of the coating layer, namely, providing superhydrophilicity. Accordingly, adhesion water, which will cause frost, becomes to have affinity with the surface of the fin, thus facilitating gravitational flow discharge. Alternatively, the surface of each fin is coated with a water-repellent or hydrophobic coating layer, the hydrophobic coating layer is exposed to plasma to form fine asperities so that the area of the hydrophobic coating layer on the surface of the fin is increased, thus enhancing the effect of the coating layer, namely, providing superhydrophilicity. Accordingly, adhesion water, which will cause frost, tends to be shaped into a sphere, thus facilitating gravitational flow discharge from the surface of the fin. Consequently, forming of frost is delayed (refer to Patent Literature 1, for example).
As described above, in the cross-fin type heat exchanger of the related art, gravitational drainage is enhanced using the effect of the hydrophilic or hydrophobic coating layer on the surface of each fin, thus achieving the effect of delaying frost formation.
In a cross-fin type heat exchanger including, for example, flat heat transfer tubes through which a refrigerant flows, however, the flat heat transfer tubes are often arranged such that the longitudinal direction of each tube is horizontal. It is difficult to expect the effect of gravitational drainage in the horizontally arranged portions. For the same reason, it is also difficult to expect the effect of shortening defrosting time.
A technical challenge that the present invention addresses is to obtain a draining effect without relying on gravity in order to enable improvement of the drainage, extension of time until the spaces (air paths) between fins becomes clogged, and shortening of defrosting time.
A retainer for a cross-fin type heat exchanger according to the present invention has the following structure. That is, the cross-fin type heat exchanger in which a plurality of heat transfer fins are arranged in an array around straight pipe portions of a serpentine heat transfer tube with a plurality of bends includes holes being provided on heat transfer surfaces of the heat transfer tubes and the heat transfer fins for transferring heat between air, in which the holes each having a radius smaller than a critical radius of a nucleus that is generated upon phase change from water vapor to condensed water droplets.
In the cross-fin type heat exchanger according to the invention, since the holes arranged on the heat transfer surfaces, used for transferring heat between air, of the heat transfer tubes and the heat transfer fins each have a radius smaller than the critical radius of each nucleus that occurs upon phase change from water vapor to condensed water droplets, condensed water droplets are not formed in the holes. The holes are filled with air at all times. Furthermore, each heat transfer surface includes air parts and metal part at all times. As the surface energy of an object is higher, the object is more likely to be wet with water. Accordingly, water moves to the metal part having high surface energy rather than to the air having low surface energy. The movement of water from the holes filled with the air to the metal part causes driving force that facilitates drainage, thus improving drainage. Advantageously, frost formation can be delayed due to removal of condensed water droplets, serving as nuclei for frost growth, and the defrosting time can be shortened by improvement of the drainage during defrosting. Furthermore, a highly efficient operation of a refrigeration cycle apparatus including the cross-fin type heat exchanger can be achieved.
In the case where the four-way valve 2 is in a switching position as illustrated in
For example, in an air-conditioning apparatus, in the case where an outdoor heat exchanger functions as the evaporator 5 in a heating operation and the temperature of air flowing into the evaporator 5 is 2° C., an evaporating temperature of the refrigerant in the evaporator 5 is approximately −5° C. The temperature of the heat transfer surfaces is at or below 0° C. and frost occurs on the heat transfer surfaces by water vapor in the flowing air. Due to frost formation, each space (air path) between the heat transfer fins 8 is clogged with a frost layer 11 as illustrated in
When there is frosting on the heat exchanger, it is important to delay clogging of the spaces (air paths) between the heat transfer fins 8 by reducing the amount of frost generated on the heat transfer surfaces or even with the same amount of frost, generating frost with higher density.
To remove the frost layer 11 generated on the heat transfer surfaces, the apparatus performs a defrosting operation. In the air-conditioning apparatus, for example, the four-way valve 2 performs switching as illustrated in
During a defrosting operation, since the heating operation is stopped, room temperature decreases. The decrease of the room temperature impairs comfort. In addition, heating load increases in accordance with the decreased room temperature when the operation is returned to the heating operation, thus degrading efficiency. As defrosting time becomes longer, a reduction in room temperature becomes larger. Accordingly, the shorter the defrosting time, both comfort and energy saving are improved. However, if the heating operation is resumed while the melt water 12 still remains on the heat transfer surfaces, frost occurs such that the remaining melt water 12 on the heat transfer surfaces serves as the starting points of frost. It is therefore important to surely remove the melt water 12 from the heat transfer surface.
In particular, in the cross-fin heat exchanger, illustrated in
A method of improving the drainage to delay clogging of the spaces (air paths) between the heat transfer fins will be described in detail below. First, the critical radius of a nucleus that occurs upon phase change from water vapor to condensed water droplets will be described. Phase change is a phenomenon in which nuclei occur in a stable environmental phase and the growth of the nuclei causes a different phase. For the growth of the nuclei, the free energy, dG, of the entire phase has to be reduced thermodynamically. The free energy upon the occurrence of a nucleus having a radius r is given by the following Equation (1).
In this equation, v denotes the volume of a single molecule, dμ denotes a variation in chemical potential per molecule, and y denotes the surface energy density. A reduction in dG by the growth of the nuclei means that an increase in y may lead to reduced dG. The r dependence of Equation (1) is illustrated as a graph in
Next, control of phase change from water vapor to condensed water droplets will be described. It is assumed that the above-described generation process corresponds to phase change from water vapor to condensed water droplets. In considering the change of vapor, dμ in Equation (2), namely, a variation in chemical potential per molecule is given using a pressure in each phase by the following Equation (3).
In this equation, k denotes the Boltzmann constant, T denotes the temperature, p denotes the vapor pressure, and pe denotes the equilibrium vapor pressure of condensed water droplets.
Substitution of Equation (3) into Equation (2) yields the following Equation (4).
For example, when the air condition is 7° C. and the relative humidity is 85%, the vapor pressure in the air is 854 [Pa]. Furthermore, when the temperature of the heat transfer surfaces is −10° C., the temperature of condensed water droplets may be equal to the surface temperature, −10° C. Accordingly, the equilibrium vapor pressure in the condensed water droplets at −10° C. is pe=286 Pa. In other words, p is three times higher than pe. As regards the critical radius r* under such conditions, r*=1 nm as illustrated in
As regards a heat exchanger, if holes 21 each having a radius smaller than the critical radius determined by air conditions and cooled surface conditions are arranged on each heat transfer surface of the evaporator 5 as illustrated in
In the defrosting operation, the movement of water from each hole 21 filled with air to the metal part causes driving force which facilitates the drainage. Such an effect achieves smooth drainage of water from the heat transfer tubes 9 in the cross-fin type heat exchanger employing the flat tubes functioning as the heat transfer tubes 9. Upon frost formation, subcooled water droplets are removed before freezing, thus reducing the amount of frost. Advantageously, clogging of the spaces (air paths) between the heat transfer fins 8 is delayed.
As described above, by providing the holes 21 having a radius smaller than the critical radius of a nucleus, in which the critical radius is determined by use conditions (the air conditions and the cooled surface conditions) of the apparatus, on each heat transfer surface, drainage is improved, thus defrosting time is shortened. In addition, clogging of the spaces (air paths) between the heat transfer fins is delayed, thus reducing the number of defrosting operations.
Each of the arranged holes has a nanosize diameter that is sufficiently smaller than the diameter of foreign matter or dust typically expected to exist in an indoor space and an outdoor location. Accordingly, the hole is not clogged with foreign matter or dust. The performance can be maintained over time.
In consideration of the strength of each actual fin and that of each actual heat transfer tube, the depth of each hole is preferably a depth that does not penetrate therethrough. Examples of methods of forming holes in, for example, aluminum fins and aluminum heat transfer tubes include anodizing illustrated in
The oxide film 54, formed by anodizing, has high corrosion resistance. Advantageously, reliability is increased. In the case where the heat transfer fins 8 and the heat transfer tubes 9 are made of metal, such as aluminum, which can be treated by anodizing, the heat transfer fins and the heat transfer tubes assembled into the heat exchanger, as illustrated in
The technique described in Embodiment 1 is to improve the drainage and delay clogging of the spaces (air paths) between the heat transfer fins. It is needless to say that this technique can be applied to a cross-fin type heat exchanger including heat transfer tubes with other shapes, for example, rounded heat transfer tubes as well as the cross-fin type heat exchanger including the flat heat transfer tubes 9.
By using the cross-fin type heat exchanger according to Embodiment 2 in the refrigeration cycle apparatus as described above, the time until clogging of the spaces (air paths) between the heat transfer fins can be extended and the defrosting time can be shortened, such that a highly efficient operation can be achieved. This results in energy saving. Application of this refrigeration cycle apparatus to, for example, an air-conditioning apparatus or a refrigerator enables the air conditioning apparatus or refrigerator to perform a highly efficient operation. In the application to, for example, an air-conditioning apparatus, the technique can be applied to a heat exchanger in which the fin pitch (fin interval) ranges from 1.0 mm to 2.5 mm and the outside diameter of each rounded heat transfer tube ranges from approximately 4 mm to approximately 13 mm. In the application to an apparatus used as a unit cooler, a display case, or a refrigerator, the technique can be applied to a heat exchanger in which the fin pitch (fin interval) ranges from 4.0 mm to 10 mm and the outside diameter of each rounded heat transfer tube ranges from approximately 6 mm to approximately 16 mm.
The structure of the cross-fin type heat exchanger according to Embodiment 2 of the invention will now be described with reference to
In the cross-fin type heat exchanger according to Embodiment 2, heat transfer fins 8 and heat transfer tubes 9, which constitute an evaporator 5, have surfaces for transferring heat between air. As illustrated in
Specifically, although attention has been paid to the phase change from water vapor to condensed water droplets in Embodiment 1 described above, attention will be paid to phase change from condensed water droplets to ice droplets in Embodiment 2. As regards a change in melt phase, a variation dμ in chemical potential per molecule is given using a temperature T in liquid phase by the following Equation (5).
In this equation, L denotes the latent heat of melting and Tm denotes the freezing temperature.
Substitution of Equation (5) into Equation (2) yields the following Equation (6).
The left side of Equation (6) denotes the difference between the freezing temperature and the temperature in the liquid phase. Since the right side of Equation (6) is non-negative, Tm<T which expresses a depression of the freezing temperature in the liquid phase.
For example, assuming that the holes 31 each have a radius of 10 nm in
Furthermore, each hole 31 is filled with water at all times as illustrated in
As described above, by providing, on the heat transfer surfaces, holes having a radius smaller than the radius determined by use conditions of the apparatus and Equation (6), namely, the holes 31 having the radius at which the freezing temperature of water droplets in the holes is lower than the temperature of the heat transfer surfaces, advantageously, the drainage is improved, thus defrosting time is shortened. In addition, clogging of the spaces (air paths) between the heat transfer fins is delayed, thus reducing the number of defrosting operation times.
Each of the arranged holes in Embodiment 2 also has a nanosize diameter that is sufficiently smaller than the diameter of foreign matter or dust typically expected to exist in an indoor space and an outdoor location. Accordingly, the hole is not clogged with foreign matter or dust. The performance can be maintained over time.
In Embodiment 2 as well, when the strength of each actual fin and that of each actual heat transfer tube is taken into consideration, the depth of each hole is preferably a depth that does not penetrate therethrough. Examples of methods of forming holes in, for example, aluminum fins and aluminum heat transfer tubes include anodizing illustrated in
As described above, the oxide film, formed by anodizing, has high corrosion resistance. Advantageously, improved reliability is obtained. In the case where the heat transfer fins 8 and the heat transfer tubes 9 are made of metal, such as aluminum, which can be treated by anodizing, the heat transfer fins and the heat transfer tubes assembled into the heat exchanger, as illustrated in
The technique described in Embodiment 2 is also to improve the drainage and delay clogging of the spaces (air paths) between the heat transfer fins. It is needless to say that this technique can be applied to a cross-fin type heat exchanger including another shaped heat transfer tubes, for example, rounded heat transfer tubes as well as the cross-fin type heat exchanger including the flat heat transfer tubes 9.
By using the cross-fin type heat exchanger according to Embodiment 2 in the refrigeration cycle apparatus as described above, the time until clogging of the spaces (air paths) between the heat transfer fins can be extended and the defrosting time can be shortened, such that a highly efficient operation can be achieved. This results in energy saving. Application of this refrigeration cycle apparatus to, for example, an air-conditioning apparatus or a refrigerator enables the air conditioning apparatus or refrigerator to perform a highly efficient operation. In the application to, for example, an air-conditioning apparatus, the technique can be applied to a heat exchanger in which the fin pitch (fin interval) ranges from 1.0 mm to 2.5 mm and the outside diameter of each rounded heat transfer tube ranges from approximately 4 mm to approximately 13 mm. In the application to an apparatus used as a unit cooler, a display case, or a refrigerator, the technique can be applied to a heat exchanger in which the fin pitch (fin interval) ranges from 4.0 mm to 10 mm and the outside diameter of each rounded heat transfer tube ranges from approximately 6 mm to approximately 16 mm.
The structure of the cross-fin type heat exchanger according to Embodiment 3 of the invention will now be described with reference to
In the cross-fin type heat exchanger according to Embodiment 3 of the invention, heat transfer fins 8 and heat transfer tubes 9, which constitute an evaporator 5, have surfaces for transferring heat between air. As illustrated in
The holes 21 enable the density of frost layers to be increased, thus obtaining the effect of delaying clogging of the spaces (air paths) between the heat transfer fins. The holes 31 reduce the amount of frost, thus obtaining the effect of delaying clogging of the spaces (air paths) between the heat transfer fins. Advantageously, the synergy of these effects further delays clogging of the spaces (air paths) between the heat transfer fins. Furthermore, the mixed arrangement of the holes 21 and the holes 31, as illustrated in
As described above, the first holes 21 having a radius smaller than the critical radius of a nucleus that occurs upon phase change from water vapor to condensed water droplets and the second holes 31 having a radius at which the freezing temperature, determined by use conditions of an apparatus, of the water droplets is lower than the temperature of the heat transfer surfaces are arranged on each heat transfer surface. Advantageously, the drainage is improved, thus shortening the defrosting time. In addition, clogging of the spaces (air paths) between the heat transfer fins can be delayed, thus reducing the number of defrosting operation times.
Each of the arranged holes in Embodiment 3 has a nanosize diameter that is sufficiently smaller than the diameter of foreign matter or dust typically expected to exist in an indoor space and an outdoor location. Accordingly, the hole is not clogged and performance can be maintained over time.
In Embodiment 3 as well, when the strength of each actual fin and that of each actual heat transfer tube is taken into consideration, the depth of each hole is preferably a depth that does not penetrate therethrough. Examples of methods of forming holes in, for example, aluminum fins and aluminum heat transfer tubes include anodizing illustrated in
As described above, the oxide film 54, formed by anodizing, has high corrosion resistance. Advantageously, improved reliability is obtained. In the case where the heat transfer fins 8 and the heat transfer tubes 9 are made of metal, such as aluminum, which can be treated by anodizing, the heat transfer fins and the heat transfer tubes assembled into the heat exchanger, as illustrated in
In anodizing, the diameter of each hole depends on the current. In the case where the heat exchanger is to be the anode and an electrode 41 is connected to a heat transfer tube 9 as illustrated in
To improve drainage from the heat transfer tubes as in the case of the cross-fin type heat exchanger employing the flat heat transfer tubes, it is therefore preferable that the diameter of each hole in the heat transfer tubes 9 be increased in order to increase the area of water having high surface energy so that the drainage is improved.
Furthermore, in the case where the fin pitch is so narrow that a bridge of water droplets is formed between the heat transfer fins and the drainage from the heat transfer fins 8 accordingly deteriorates, it is preferable that the diameter of each hole in the heat transfer fins 8 be increased in order to improve the drainage.
The technique described in Embodiment 3 is also to improve the drainage and delay clogging of the spaces (air paths) between the heat transfer fins. It is needless to say that this technique can be applied to a cross-fin type heat exchanger including another shaped heat transfer tubes, for example, rounded heat transfer tubes as well as the cross-fin type heat exchanger including the flat heat transfer tubes 9.
By using the cross-fin type heat exchanger according to Embodiment 3 in a refrigeration cycle apparatus as described above, the time it takes for the spaces (air paths) between the heat transfer fins to be clogged can be extended and the defrosting time can be shortened, so that a highly efficient operation can be achieved. This results in energy saving. Application of this refrigeration cycle apparatus to, for example, an air-conditioning apparatus or a refrigerator enables the air conditioning apparatus or refrigerator to perform a highly efficient operation. In the application to, for example, an air-conditioning apparatus, the technique can be applied to a heat exchanger in which the fin pitch (fin interval) ranges from 1.0 mm to 2.5 mm and the outside diameter of each rounded heat transfer tube ranges from approximately 4 mm to approximately 13 mm. In the application to an apparatus used as a unit cooler, a display case, or a refrigerator, the technique can be applied to a heat exchanger in which the fin pitch (fin interval) ranges from 4.0 mm to 10 mm and the outside diameter of each rounded heat transfer tube ranges from approximately 6 mm to approximately 16 mm.
With application of the invention, problem of frost formation, at or below 0° C., on the surface of a heat exchanger that exchange heat with air can be solved. In an air-conditioning apparatus or a refrigerator including a refrigeration cycle apparatus, clogging of the spaces (air paths) between heat transfer fins or the defrosting operation has been causing reduction in efficiency. By using the refrigeration cycle apparatus including the cross-fin type heat exchanger of the invention to an air-conditioning apparatus or a refrigerator, time until the spaces (air paths) between the heat transfer fins becomes clogged can be extended and defrosting time can be shortened, such that a highly efficient operation of the air-conditioning apparatus or refrigerator can be achieved; hence, energy saving cab be achieved.
1. compressor; 3 condenser; 4 expansion valve (expansion means); 5 evaporator; 8 heat transfer fin; 9 heat transfer tube; 21 hole (hole having a radius equal to or smaller than the critical radius of a nucleus); 22 condensed water droplet; 31 hole (hole having a radius that offers the Gibbs-Thomson effect); 53 base metal; 54 oxide film.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2010/003216 | 5/12/2010 | WO | 00 | 10/10/2012 |