This invention relates generally to evaporator heat exchangers and, more particularly, to providing for improved control of frost accumulation on the external surfaces of evaporator heat exchangers having a plurality of parallel, flattened heat exchange tubes.
Air conditioners and heat pumps employing refrigerant vapor compression cycles are commonly used for cooling or cooling/heating air supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used for cooling air, or other secondary media such as water or glycol solution, to provide a refrigerated environment for food items and beverage products within display cases, bottle coolers or other similar equipment in supermarkets, convenience stores, groceries, cafeterias, restaurants and other food service establishments.
Conventionally, these refrigerant vapor compression systems include a compressor, a condenser, an expansion device, and an evaporator serially connected in refrigerant flow communication. The aforementioned basic refrigerant vapor compression system components are interconnected by refrigerant lines in a closed refrigerant circuit and arranged in accord with the vapor compression cycle employed. The expansion device, commonly an expansion valve or a fixed-bore metering device, such as an orifice or a capillary tube, is disposed in the refrigerant line at a location in the refrigerant circuit upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. The expansion device operates to expand the liquid refrigerant passing through the refrigerant line connecting the condenser to the evaporator to a lower pressure and temperature. The refrigerant vapor compression system may be charged with any of a variety of refrigerants, including, for example, R-12, R-22, R-134a, R-404A, R-410A, R-407C, R717, R744 or other compressible fluid.
In some refrigerant vapor compression systems, the evaporator is a parallel tube heat exchanger having a plurality of flattened, typically rectangular or oval in cross-section, multi-channel heat exchange tubes extending longitudinally in parallel, spaced relationship between a first generally vertically extending header or manifold and a second generally vertically extending header or manifold, one of which serves as an inlet header/manifold. The inlet header receives the refrigerant flow from the refrigerant circuit and distributes the refrigerant flow amongst the plurality of parallel flow paths through the heat exchanger. The other header serves to collect the refrigerant flow as it leaves the respective flow paths and to direct the collected flow back to the refrigerant line for return to the compressor, in a single pass heat exchanger, or to a downstream bank of parallel heat exchange tubes, in a multi-pass heat exchanger. In the latter case, this header is an intermediate manifold or a manifold chamber and serves as an inlet header to the next downstream bank of parallel heat transfer tubes.
Each heat exchange tube generally has a plurality of flow channels extending longitudinally in parallel relationship the entire length of the tube, each channel providing a relatively small cross-sectional area refrigerant flow path. Thus, a heat exchanger with multi-channel tubes extending in parallel relationship between the inlet and outlet headers of the heat exchanger will have a relatively large number of small cross-sectional area refrigerant flow paths extending between the two headers. Sometimes, such multi-channel heat exchanger constructions are referred to as microchannel or minichannel heat exchangers as well. Commonly, for evaporator applications, the heat exchanger generally includes heat transfer fins positioned between heat transfer tubes for heat transfer enhancement, structural rigidity and heat exchanger design compactness. The heat transfer tubes and fins are permanently attached to each other (as well as to the manifolds) during a furnace braze operation. The fins may have flat, wavy, corrugated or louvered design and typically form triangular, rectangular, offset or trapezoidal airflow passages.
When a heat exchanger is used as an evaporator in a refrigerant vapor compression system, moisture in the air flowing through the evaporator and over the external surfaces of the refrigerant conveying heat exchange tubes and associated heat transfer fins of the heat exchanger condenses out of the air and collects on the external surfaces of those heat exchange tubes and heat transfer fins. Depending upon operating conditions, the moisture condensing out of the air may accumulate on the exterior surfaces of the heat exchange tubes and heat transfer fins of the evaporator and form frost or ice. As the accumulation of frost or ice on the heat exchange tubes and heat transfer fins increases and builds up closing the airflow passages between the fins and the tubes, particularly in the regions where the fins contact the tube, heat transfer between the refrigerant within the tubes and the air passing over the tubes decreases, as a result of the increase in thermal conduction resistance caused by the frost or ice layer. Additionally, if the frost build-up between the fins becomes excessive, the air-side pressure drop through the evaporator increases, resulting in a decrease in airflow delivered by an air-moving device, thereby further deteriorating the overall performance of the evaporator heat exchanger.
Further, unlike the larger diameter round heat exchange tubes with relatively large spaces between the tubes, commonly used in conventional refrigerant evaporators, flattened, multi-channel tubes defining a plurality of small cross-sectional area flow passages are subject and more susceptible to damage from the accumulation of frost or ice on the external surfaces of the heat exchange tubes and associated heat transfer fins. For the conventional round tube and plate fin heat exchanger constructions condensing water tends to more readily drain off the heat exchange tubes and along heat transfer fins. However, on flattened tubes, the condensing water tends to accumulate rather than drain off the tubes. Consequently, the accumulating water, unless removed from the tube, will alternately freeze, at certain operating conditions, forming frost or ice and then melt (fully or partially) during a defrost cycle. Since water expands upon freezing, repeated freezing and thawing of the accumulated condensate, particularly in the confined spaces between the heat transfer fins and the flattened heat exchange tubes (e.g., in the region where the fins contact the flattened tubes), can damage the heat exchanger by deforming or cracking the tube and causing separation of the fins from the tubes. Furthermore, during sequential defrost cycles, more ice may accumulate on external surfaces of the heat exchange tubes and heat transfer fins of the evaporator heat exchanger and may even completely block airflow passages, forcing the evaporator to run outside of a specified operational envelope (in terms of suction pressure) and compromising refrigerant system reliability or causing nuisance shutdowns, both of which are obviously highly undesirable events.
In refrigerant vapor compression systems having conventional finned round tube and plate fin evaporators, it is common practice to defrost the evaporator either periodically for a timed interval or on demand as the need to defrost is sensed. For example, U.S. Pat. No. 6,205,800 discloses a demand defrost method for defrosting the evaporator of a refrigerated display case, wherein a defrost cycle is initiated when the difference between the sensed air temperature within the case and the sensed refrigerant temperature equals or exceeds a defrost threshold. The refrigerant temperature sensor is mounted externally on the refrigerant inlet tube to the evaporator or other location in the evaporator coil or internally within the refrigerant inlet tube. Examples of frost sensors disclosed in the art for use in connection with evaporator defrost on demand control systems include thermistors, such as disclosed in U.S. Pat. No. 4,305,259; capacitive sensor plates, such as disclosed in U.S. Pat. No. 4,347,709; air velocity sensors, such as disclosed in U.S. Pat. No. 4,831,833; fiber optic sensors, such as disclosed in U.S. Pat. No. 4,860,551; and heat flow sensors, such as U.S. Pat. No. 6,467,282.
A refrigerant vapor compression system includes a refrigerant flow circuit having a refrigerant compressor, a condenser, an expansion devise and an evaporator connected serially in refrigerant flow communication. The evaporator has a plurality of longitudinally extending, flattened heat exchange tubes disposed in parallel, spaced relationship. Each of the heat exchange tubes has a flattened cross-section and may define a plurality of discrete, longitudinally extending refrigerant flow passages. At least one frost detection sensor is installed in operative association with the evaporator for detecting a presence of frost or ice formation on at least one of the flattened heat exchange tubes or heat transfer fins and generates a signal indicative of the presence of frost or ice formation on that flattened heat exchange tubes and heat transfer fins. A defrost system is operatively associated with the evaporator. A controller, operatively coupled to the frost/ice detection sensor and to the defrost system, selectively activates the defrost system to initiate a defrost cycle of the evaporator in response to the signal indicative of the presence of frost or ice formation on at least one of the flattened heat exchange tubes and heat transfer fins. The frost/ice detection sensor may be a single sensor installed at a single location on the heat exchanger or a plurality of frost detection sensors installed at different locations on the heat exchanger.
In an embodiment, the frost detection sensor may be a sensor mounted on an exterior surface of one of the flattened heat exchange tubes or heat transfer fins. In an embodiment, a plurality of frost detection sensors may be mounted on the exterior surfaces of a number of different flattened heat exchange tubes, heat transfer fins or a combination of thereof. In an embodiment, the defrost system may be an electric defrost heater system. In an embodiment, the defrost system may be a hot gas defrost system for selectively passing at least a portion of refrigerant vapor from the compressor through the heat exchange tubes of the evaporator.
The heat exchanger may have flattened heat exchange tubes having a flattened generally rectangular or oval cross-section, each of which may define multiple internal fluid flow passages having a flow area of a circular cross-section or a non-circular cross-section. The heat exchanger may also include a plurality of fins extending between adjacent flattened heat exchange tubes. The fins may be a plurality of generally vertical fins extending between adjacent heat exchange tubes or a plurality of fins may comprise serpentine-like fins extending between adjacent heat exchange tubes and may be of a louvered, wavy, offset strip or flat plate configuration.
In the following detailed description of the invention, reference will be made to and is to be read in connection with the accompanying drawing, where:
The heat exchanger of the invention will be described herein in use as an evaporator in connection with a simplified air conditioning cycle refrigerant vapor compression system 100 as depicted schematically in
The refrigerant vapor compression system 100 includes a compressor 105, a condenser 110, an expansion device 120, and the heat exchanger 10, functioning as an evaporator, connected in a closed loop refrigerant circuit by refrigerant lines 102, 104 and 106. The compressor 105 circulates hot, high pressure refrigerant vapor through discharge refrigerant line 102 into the inlet header of the condenser 110, and thence through the heat exchange tubes of the condenser 110 wherein the hot refrigerant vapor is desuperheated, condensed to a liquid and typically subcooled as it passes in heat exchange relationship with a cooling fluid, such as ambient air, which is passed over the heat exchange tubes by the condenser fan 115. Although the heat exchanger 110 is referred to as a condenser throughout the text, as known to a person ordinarily skilled in the art, a predominantly two-phase subcritical condenser heat exchanger becomes a single-phase gas cooler, in transcritical applications. Both subcritical and transcritical applications of the heat exchanger 10 can equally benefit from the invention described herein.
The high pressure, liquid refrigerant leaves the condenser 110 and thence passes through the liquid refrigerant line 104 to the evaporator heat exchanger 10, traversing the expansion device 120 wherein the refrigerant is expanded to a lower pressure and temperature to form a refrigerant liquid/vapor mixture. The now lower pressure and lower temperature, expanded refrigerant thence passes through the heat exchange tubes 40 of the evaporator heat exchanger 10 wherein the refrigerant is evaporated and typically superheated as it passes in heat exchange relationship with air to be cooled and, in many cases, dehumidified, which is passed over the heat exchange tubes 40 and associated heat transfer fins 50 by the evaporator fan 15. The refrigerant leaves the evaporator heat exchanger, predominantly in a vapor thermodynamic state, and passes through the suction refrigerant line 106 to return to the compressor 105 through the suction port.
As the airflow traversing the evaporator heat exchanger 10 passes over the heat exchange tubes 40 and heat transfer fins 50 in heat exchange relationship with the refrigerant flowing through the heat exchange tubes 40, the air is cooled and the moisture in the air flowing through the evaporator beat exchanger 10 and over the external surface of the refrigerant conveying tubes 40 and heat transfer fins 50 of the evaporator heat exchanger 10 condenses out of the air and collects on the external surfaces of the heat exchange tubes 40 and heat transfer fins 50. A drain pan 45 is provided beneath the evaporator heat exchanger 10 for collecting that condensate which drains from the external surface of the heat exchange tubes 40 and heat transfer fins 50.
The parallel flow heat exchanger 10 includes a plurality of heat exchange tubes 40 of generally flattened cross-section, which are arranged in parallel relationship in a generally vertical array. In the exemplary embodiment of the heat exchanger 10 depicted in
Each heat exchange tube 40 comprises an elongated tubular member extending along its longitudinal axis and having a generally flattened cross-section, for example, a rectangular cross-section or oval cross-section. The flattened tubular member has an upper wall 46 and a lower wall 48 and defines the at least one longitudinally extending internal fluid flow passage 42. The at least one internal fluid flow passage 42 may be subdivided into a plurality of parallel, independent internal fluid flow passages 42 which extend longitudinally parallel to the longitudinal axis of the heat exchange tube 40 in a side-by-side array, thereby providing a multi-channel heat exchange tube. Each flattened heat exchange tube 40 has a leading edge 41 which faces upstream, with respect to the airflow through the heat exchanger 10, and a trailing edge 43 which faces downstream, with respect to the airflow through the heat exchanger 10.
Each flattened multi-channel tube 40 may have a width as measured along a transverse axis extending from the leading edge 41 to the trailing edge 43 of, for example, fifty millimeters or less, typically from ten to thirty millimeters, and a height of about two millimeters or less, as compared to conventional prior art round heat exchange tubes having a diameter of ½ inch, ⅜ inch or 7 mm. The heat exchange tubes 40 are shown in the accompanying drawings, for ease and clarity of illustration, as having ten internal channels 42 defining flow paths having a rectangular cross-section. However, it is to be understood that in applications, each multi-channel heat exchange tube 40 may typically have from about ten to about twenty internal flow channels 42. Generally, each internal flow channel 42 will have a hydraulic diameter, defined as four times the cross-sectional flow area divided by the “wetted” perimeter, in the range generally from about 200 microns to about 3 millimeters. Although depicted as having a rectangular cross-section in the drawings, the internal flow channels 42 may have a circular, triangular, oval or trapezoidal cross-section, or any other desired non-circular cross-section. Also, heat transfer tubes 40 may have other internal heat transfer enhancement elements, such as mixers and boundary layer destructors.
As in conventional practice, to improve heat transfer between the air flowing through the heat exchanger 10 over the external surfaces of the flattened heat transfer tubes 40 and the refrigerant flowing through the internal parallel flow channels 42 of the heat transfer tubes 40, the heat exchanger 10 includes a plurality of external heat transfer fins 50 extending between each set of the parallel-arrayed tubes 40. The heat transfer fins are brazed or otherwise securely attached to the external surfaces of the upper and lower walls of the respective tubular members of adjacent heat exchange tubes 40 to establish heat transfer contact, by heat conduction, between the heat transfer fins 50 and the external surface of the flat heat exchange tubes 40. Thus, the external surfaces of the heat exchange tubes 40 and the surfaces of the heat transfer fins 50 together form the external heat transfer surface that participates in heat transfer interaction between the air flowing through the heat exchanger 10 and refrigerant flowing through the internal channels 42. The external heat transfer fins 50 also provide for structural rigidity of the heat exchanger 10 and quite often assist in air flow redirection to improve heat transfer characteristics.
In the exemplary embodiment of the heat exchanger 10 depicted in
As noted hereinbefore, a heat exchanger used as an evaporator in refrigerant vapor compression system, such as for example, but not limited to, an air conditioning or refrigeration system, are subject to water condensing out of the air flow passing through the evaporator and collecting on the external surfaces of the heat exchange tubes and heat transfer fins of the heat exchanger. Under certain operating conditions typically experienced over the course of normal operation, the condensate will freeze forming frost or ice on the upper and lower exterior surfaces 46, 48 of the flatted heat exchange tubes 40 and on the heat transfer fins 50, particularly in the region where the heat transfer fins 50 contact the upper and lower exterior surfaces of the heat exchange tubes 40.
To detect frost or ice formation on the heat exchanger 10, at least one frost detection sensor 60 is installed in operative association with the heat exchanger 10. In the exemplary embodiment of the heat exchanger 10 depicted in
The frost detection sensor 60 is operatively coupled to a controller 80 and provides a signal to the controller 80 indicative of the formation of frost on the exterior surface of the heat exchange tube 40 with which the sensor 60 is associated. In an embodiment, the frost detection sensor 60 provides a signal to the controller 80 indicative of the actual degree of frost formation on the exterior surface of the heat exchange tube 40 with which the sensor 60 is associated. The controller 80 processes the signal received from the frost detection sensor(s) 60 and determines whether or not the amount of frost formation indicated is excessive. If so, the controller 80 then initiates a defrost cycle to melt the frost formed on the evaporator heat exchanger 10.
Referring now to
In the exemplary embodiment of the refrigerant vapor compression system 100 depicted in
The flow control valve 90 is operatively coupled to the controller 80. As noted before, the controller 80 processes the signal received from the frost detection sensor(s) 60 and determines whether or not the amount of frost/ice formation indicated is excessive. If so, the controller 80 then initiates a defrost cycle to melt the frost/ice formed on the external surfaces of the evaporator heat exchanger 10 by sending a command signal to the flow control valve 90 causing the flow control valve 90 to partially or fully open. With the flow control valve 90 open, at least a portion of an intermediate pressure or discharge pressure refrigerant vapor passes from the compressor 105 through the hot gas defrost line 70 to enter the refrigerant line 104 and mix with the expanded refrigerant vapor passing from the expansion device 120, thereby raising the temperature of the refrigerant vapor passing through the heat exchange tubes 40 of the evaporator heat exchanger 10. This higher temperature refrigerant vapor raises the temperature of the tubular elements defining the heat exchange tubes 40 as it traverses the flow passages 42 therethrough to a temperature sufficiently above 0° C. to cause the frost/ice formed within the heat exchanger 10 to melt as the heated air flows through the evaporator heat exchanger. After a pre-selected period of time, the controller 80 commands the flow control valve 90 to close, thereby preventing refrigerant vapor flowing therethrough from the compressor 105 to the refrigerant line 104 and terminating the defrost cycle. Also, if desired, the refrigerant flow through the main refrigerant circuit could be completely blocked, when the defrost cycle is initiated. In this case, an additional flow control valve would be installed on the discharge line 102 and closed during the defrost cycle by the controller 80.
As mentioned previously, if intermediate pressure vapor is not available as a defrost medium, discharge pressure vapor may be used. In that case, the inlet of the hot gas defrost line 70 would be in refrigerant flow communication with the discharge pressure side of the compressor 105. The outlet of the hot gas defrost line 70 would again be in refrigerant flow communication with the refrigeration circuit at a location upstream, with respect to refrigerant flow, of the evaporator 10 and downstream, with respect to refrigerant flow, of the expansion device 120. Furthermore, if the refrigerant system 100 is a heat pump, switching between heating and cooling modes of operation can be employed as the defrost means.
Also, it has to be understood that although the embodiments of the invention are disclosed and would be most beneficial for application to evaporator heat exchangers with a horizontal orientation of the straight heat exchange tube array, the invention disclosed herein would also be beneficial in application to evaporator heat exchangers with other heat exchange tube orientations and configurations, for example vertically oriented heat exchange tubes or heat exchange tubes oriented at an inclination angle between 0 to 90 degrees with respect to the horizontal axis.
While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US07/05724 | 3/6/2007 | WO | 00 | 8/28/2009 |