MICRO-CHANNEL EVAPORATOR WITH FROST DETECTION AND CONTROL

Abstract

A refrigerant vapor compression system includes an evaporator having 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 defining a plurality of discrete, longitudinally extending refrigerant flow passages. One or more frost detection sensor(s) is/are installed in operative association with the evaporator for detecting a presence of frost/ice formation on at one of the flattened heat exchange tubes and associated heat transfer fins. A defrost system is provided and operatively associated with the evaporator heat exchanger A controller, operatively coupled to the frost detection sensor(s) 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 formation on the flattened heat exchange tubes and heat transfer fins.

Description

FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

FIG. 1 is a schematic diagram of a refrigerant vapor compression system incorporating a multi-channel heat exchanger as an evaporator;

FIG. 2 is a perspective view of an exemplary embodiment of an evaporator heat exchanger equipped with a defrost sensor;

FIG. 3 is a schematic diagram of a refrigerant vapor compression system incorporating a multi-channel heat exchanger evaporator with a defrost sensor and an associated electric defrost heater; and

FIG. 4 is a schematic diagram of a refrigerant vapor compression system incorporating a multi-channel heat exchanger evaporator with a defrost sensor and an associated hot gas defrost system.

DETAILED DESCRIPTION OF THE INVENTION

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 FIG. 1. Although the exemplary refrigerant vapor compression cycle illustrated in FIG. 1 is a simplified air conditioning cycle, it is to be understood that the heat exchanger of the invention may be employed in refrigerant vapor compression systems of various designs, including, without limitation, heat pump cycles, economized cycles, cycles with tandem components such as compressors and heat exchangers, chiller cycles, cycles with reheat and many other cycles including various options and features.

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 FIG. 2, each of the heat exchange tubes 40 extends in a generally horizontal direction along its longitudinal axis between a generally vertically extending first header 20 and a generally vertically extending second header 30, thereby providing a plurality of parallel refrigerant flow paths between the two headers. Although the refrigerant headers 20 and 30 are shown of a cylindrical configuration, the may be of a rectangular, half of a cylinder or any other shape, as well as have a single chamber or multi-chamber design, depending on the refrigerant path arrangement. Each heat exchange tube 40 has a first end mounted to the first header 20, a second end mounted to the second header 30, and at least one flow channel 42 extending longitudinally, i.e. parallel to the longitudinal axis of the tube for the entire length of the tube, thereby providing a flow path in refrigerant flow communication between the first header 20 and the second header 30. The heat exchanger refrigerant pass arrangement may be of a multi-pass configuration, such as depicted in FIG. 2, or of a single-pass configuration, depending on particular application requirements.

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 FIG. 2, the heat transfer fins 50 constitute segments of a fin plate formed as a serpentine series of generally V-shaped or generally U-shaped segments and are disposed in heat transfer contact with the both lower external surface of the lower wall 48 of one heat exchange tube 40 and the upper external surface of the upper wall 46 of the adjacent heat exchange tube 40 next therebelow. Alternatively, the fins may constitute a plurality of plates disposed in parallel, spaced relationship and extending generally vertically between the heat transfer tubes 40. It is to be understood that other fin configurations, such as, for example, generally corrugated, wavy, louvered or offset fins forming triangular, rectangular, or trapezoidal airflow passages may be used in the heat exchanger of the invention.

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 FIG. 2, a frost detection sensor 60 is mounted to the exterior surface of one of the heat exchange tubes 40. However, it is to be understood that a frost detection sensor 60 could instead be mounted on the surface of one of the heat transfer fins 50. Additionally, it is to be understood that a plurality of frost detection sensors 60 may be installed on the heat exchanger 10, including the locations on the heat exchange tubes and/or the heat transfer fins, with each defrost detection sensor 60 mounted at a different location within the heat exchanger 10. For example, a frost detection sensor may be installed in that region of the heat exchanger 10 where frost/ice tends to accumulate first and most excessively or a frost detection sensor 60 may be installed at each of a number of different locations throughout the heat exchanger 10 whereat frost/ice tends to accumulate. The precise location or locations at which frost detection sensors should be installed in a particular heat exchanger is a matter of choice within the skill of the ordinary practitioner in the art. The selection of the type of frost detection sensor 60 to be used is also within the skill of the ordinary practitioner in the art, and not limiting of the invention. The frost detection sensor 60 may be a heat flux sensor, a strain gauge sensor or any other type of sensor capable of detecting the formation of frost on the exterior surface of the heat exchange tubes 40.

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 FIG. 3, in the refrigerant vapor compression system 100 therein depicted, an electric defrost system is operatively associated with the evaporator heat exchanger 10. The electric defrost system comprises an electric defrost heater that includes at least one electric heating element 65 disposed at or slightly upstream, with respect to air flow, of the air inlet to the evaporator heat exchanger 10. In the exemplary embodiment depicted in FIG. 3, a plurality of electric heating elements 65 are provided, one electric heating element 65 associated with each heat exchange 40. When the controller 80 determines that a defrost cycle is to be initiated, the controller 80 energizes the electric heating elements 65. The electric heating elements 65 operate as in conventional practice to heat the air entering the evaporator heat exchanger 10 sufficiently above 0° C. to cause the frost formed within the heat exchanger to melt as the heated air flows through the heat exchanger. Also, the electric heating elements 65 would heat the external surfaces of the heat exchanger 10, which would assist in melting the ice as well. Alternatively, if the safety and isolation means are installed in place, at least some of the heat exchange tubes 40 or heat transfer fins 50 can be used as electric heating elements 65. After a pre-selected time interval, the controller 80 will de-energize the electric heating elements 65 thereby ending the defrost cycle.

In the exemplary embodiment of the refrigerant vapor compression system 100 depicted in FIG. 4, a hot gas defrost system is operatively associated with the evaporator heat exchanger 10. The hot gas defrost system includes a hot gas defrost line 70 and a flow control valve 90 operatively disposed in the hot gas defrost line 70. The hot gas defrost line 70 has an inlet opening in refrigerant flow communication with an intermediate pressure stage of the compressor 105 and an outlet opening in refrigerant flow communication with refrigerant line 104 at a location upstream, with respect to refrigerant flow, of the evaporator heat exchanger 10 and downstream, with respect to refrigerant flow, of the expansion device 120. Thus, the hot gas defrost line 70 provides a refrigerant flow path from an intermediate pressure stage of the compressor 105 to the refrigerant inlet line to the evaporator heat exchanger 10. The flow control valve 90 may be selectively positioned between a closed position whereat the flow control valve 90 closes the hot gas defrost line 70 to refrigerant flow therethrough and an open position whereat the flow control valve 90 opens the gas defrost line 70 to refrigerant flow therethrough. In an embodiment, the flow control valve 90 may be a solenoid electrically operated flow control valve. Further, the flow control valve 90 can be of a modulating or pulsating type, respectively modulating or cycling between the closed and open positions. Also, if the compressor 105 is not equipped with the intermediate pressure port, a discharge refrigerant vapor can be utilized instead for the defrost purposes, which is obviously within the scope of the invention.

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.

Claims

1. A refrigerant vapor compression system including a refrigerant flow circuit comprising:

a refrigerant compressor, a heat rejection heat exchanger, an expansion device and an evaporator connected serially in the refrigerant flow circuit in refrigerant flow communication, said evaporator having a plurality of longitudinally extending, flattened heat exchange tubes disposed in parallel, spaced relationship, each of said heat exchange tubes having flattened cross-section;

at least one frost detection sensor installed in operative association with said evaporator for detecting a presence of frost or ice formation on the evaporator external surfaces and generating a signal indicative of the presence of frost or ice formation at least at one location within the evaporator;

a defrost system operatively associated with said evaporator; and

a controller operatively coupled to said at least one frost detection sensor and to said defrost system, said controller selectively activating said defrost system to initiate a defrost cycle of said evaporator in response to said signal indicative of the presence of frost or ice formation at least at one location within the evaporator.

2. A refrigerant vapor compression system as recited in claim 1 wherein each of said flattened heat exchange tubes of said evaporator defines a plurality of internal discrete, longitudinally extending refrigerant flow passages.

3. A refrigerant vapor compression system as recited in claim 1 wherein said defrost system comprises an electric heating system operatively associated with said evaporator.

4. A refrigerant vapor compression system as recited in claim 1 wherein said defrost system comprises a hot gas defrost system for selectively passing at least a portion of refrigerant vapor from said compressor through said heat exchange tubes of said evaporator.

5. A refrigerant vapor compression system as recited in claim 4 wherein said hot gas defrost system comprises:

a hot gas defrost line having an inlet opening in refrigerant flow communication with an intermediate pressure stage of said compressor and an outlet opening in refrigerant flow communication with the refrigeration cycle circuit at a location upstream, with respect to refrigerant flow, of said evaporator and downstream, with respect to refrigerant flow, of said expansion device; and

a refrigerant flow control device disposed in said hot gas defrost line and operatively coupled to said controller, said refrigerant flow control device being selectively positionable between a closed position and an open position.

6. A refrigerant vapor compression system as recited in claim 5 wherein said refrigerant flow control device is of a modulation or pulsation type.

7. A refrigerant vapor compression system as recited in claim 4 wherein said hot gas defrost system comprises:

a hot gas defrost line having an inlet opening in refrigerant flow communication with a discharge pressure side of said compressor and an outlet opening in refrigerant flow communication with the refrigeration cycle circuit at a location upstream, with respect to refrigerant flow, of said evaporator and downstream, with respect to refrigerant flow, of said expansion device; and

a refrigerant flow control device disposed in said hot gas defrost line and operatively coupled to said controller, said refrigerant flow control device being selectively positionable between a closed position and an open position.

8. A refrigerant vapor compression system as recited in claim 7 wherein said refrigerant flow control device is of a modulation or pulsation type.

9. A refrigerant vapor compression system as recited in claim 1 wherein said refrigerant system is a heat pump and said defrost system comprises switching between heating and cooling modes of operation.

10. A refrigerant vapor compression system as recited in claim 1 wherein said at least one frost detection sensor comprises a frost detection sensor mounted on an exterior surface of one of said flattened heat exchange tubes.

11. A refrigerant vapor compression system as recited in claim 1 wherein said at least one frost detection sensor comprises a frost detection sensor mounted on a surface of one of the heat transfer fins positioned between said heat exchange tubes and permanently attached to these heat exchange tubes.

12. A refrigerant vapor compression system as recited in claim 1 wherein said at least one frost detection sensor comprises a plurality of frost detection sensors, each of said frost detection sensors installed at a different location within said evaporator.

13. A refrigerant vapor compression system as recited in claim 1 wherein said at least one frost detection sensor comprises a plurality of frost detection sensors, each of said frost detection sensors mounted on an exterior surface of a different one of said flattened heat exchange tubes.

14. A refrigerant vapor compression system as recited in claim 1 wherein said at least one frost detection sensor comprises a plurality of frost detection sensors, each of said defrost detection sensors mounted on a surface of a different one of the heat transfer fins positioned between said heat exchange tubes and permanently attached to these heat exchange tubes.

15. A refrigerant vapor compression system as recited in claim 1 wherein said flattened heat exchange tubes have a flattened rectangular or flattened oval cross-section.

16. A refrigerant vapor compression system as recited in claim 2 wherein said plurality of internal discrete, longitudinally extending fluid flow passages have a circular cross-sectional flow area.

17. A refrigerant vapor compression system as recited in claim 2 wherein said plurality of internal discrete, longitudinally extending fluid flow passages have a non-circular cross-sectional flow area.

18. A refrigerant vapor compression system as recited in claim 1 further comprising a plurality of heat transfer fins extending between adjacent flattened heat exchange tubes of said evaporator.

19. A refrigerant vapor compression system as recited in claim 18 wherein said plurality of heat transfer fins comprises a plurality of generally vertical fins extending between adjacent heat exchange tubes.

20. A refrigerant vapor compression system as recited in claim 18 wherein said plurality of heat transfer fins comprises serpentine-like fins extending between adjacent heat exchange tubes.

21. A refrigerant vapor compression system as recited in claim 20 wherein said a serpentine-like heat transfer fins extending between adjacent heat exchange tubes form one of generally triangular, rectangular or trapezoidal airflow passages.

22. A refrigerant vapor compression system as recited in claim 18 wherein said plurality of heat transfer fins are at least one of louvered, wavy, offset strip or flat plate configurations.

23. A refrigerant vapor compression system as recited in claim 1 wherein said evaporator has a first manifold and a second manifold and said flattened heat exchange tubes extend longitudinally between sad first and second manifolds in a single-pass configuration.

24. A refrigerant vapor compression system as recited in claim 1 wherein said evaporator has a first manifold and a second manifold and said flattened heat exchange tubes extend longitudinally between sad first and second manifolds a multi-pass configuration.

25. A refrigerant vapor compression system as recited in claim 1 wherein said flattened heat exchange tubes of said evaporator have a generally horizontal orientation.

26. A refrigerant vapor compression system as recited in claim 1 wherein said flattened heat exchange tubes of said evaporator have a generally vertical orientation.

27. A refrigerant vapor compression system as recited in claim 1 wherein said flattened heat exchange tubes of said evaporator have an inclination angle between 0 and 90 degrees, with respect to the horizontal axis.

28. A refrigerant vapor compression system as recited in claim 1 wherein said evaporator has a generally vertical orientation.

29. A refrigerant vapor compression system as recited in claim 1 wherein said evaporator has an inclination angle between 0 and 90 degrees, with respect to the horizontal axis.

30. A refrigerant vapor compression system as recited in claim 1 wherein said flattened heat exchange tubes of said evaporator have a generally straight configuration.