START-UP PROCEDURE FOR REFRIGERANT SYSTEMS HAVING MICROCHANNEL CONDENSER AND REHEAT CYCLE

RELATED APPLICATIONS

[0001] This application claims priority to United States Provisional Patent Application Number 61/061142, which was filed June 13, 2008.

BACKGROUND OF THE INVENTION

[0002] Refrigerant systems utilize a refrigerant to condition a secondary fluid, such as air, delivered to a climate-controlled space. In a basic refrigerant system, the refrigerant is compressed in a compressor, and flows downstream to a condenser in a subcritical refrigerant cycle or to a gas cooler in a transcritical refrigerant cycle, where heat is typically rejected from the refrigerant to ambient environment, during heat transfer interaction with this ambient environment. Then refrigerant flows through an expansion device, where it is expanded to a lower pressure and temperature, and to an evaporator, where during heat transfer interaction with a secondary fluid (e.g., indoor air), the refrigerant is evaporated and typically superheated, while cooling and often dehumidifying this secondary fluid.

[0003] In recent years, much interest and design effort has been focused on the efficient operation of the heat exchangers (condenser and evaporator) of the refrigerant systems. One relatively recent advancement in heat exchanger technology is the development and application of parallel flow, or so-called microchannel or minichannel, heat exchangers (these two terms will be used interchangeably throughout the text), as the condensers and evaporators.

[0004] These heat exchangers are provided with a plurality of parallel heat exchange tubes, typically of a non-round shape, among which refrigerant is distributed and flown in a parallel manner. The heat exchange tubes are orientated generally substantially perpendicular to a refrigerant flow direction in the inlet, intermediate and outlet manifolds that are in flow communication with the heat exchange tubes. The heat exchange tubes typically have a multi-channel construction, with refrigerant distributed and flowing within these multiple channels in a parallel manner. Heat transfer fins are inter-disposed in between and rigidly attached to heat exchange tubes. The primary reasons for the employment of the

parallel flow heat exchangers, which usually have aluminum furnace-brazed construction, are related to their superior performance, high degree of compactness, structural rigidity, lower weight, lower refrigerant charge and enhanced resistance to corrosion.

[0005] One concern with utilizing microchannel heat exchangers, also related to their advantage, is their low internal volume. Due to low internal volume, microchannel heat exchangers are more susceptible to refrigerant pressure variations due to instantaneous changes in refrigerant flow throughout the refrigerant circuit. MicroChannel heat exchangers are also very sensitive to refrigerant charge amounts, with even a small amount of extra refrigerant charge in the system leading to higher than desirable discharge operating pressures and instantaneous pressure spikes. These problems are especially pronounced during startups. Nuisance interruptions of the refrigerant system operation can be a result of emergency shutdown by control software on a high pressure alarm or by mechanical safety, such as a high pressure switch, leading to complete inability to operate the refrigerant system, if a discharge pressure spike exceeded predetermined allowable safe limit (typically for a preset number of times). This consequently would results in a failure to keep a climate-controlled environment within desirable temperature and humidity ranges, leading to occupant discomfort and liability claims. Under certain circumstances, repeated starts and shutdowns in short periods of time can potentially lead to a compressor failure.

[0006] Another refrigerant cycle component is a reheat cycle utilizing primary refrigerant circulating throughout the main refrigerant circuit. In the reheat cycle, at least a portion of refrigerant passes through a reheat heat exchanger which is positioned in the path of air flowing across the evaporator. The reheat heat exchanger is typically positioned in the path of the air downstream of the evaporator. With a reheat cycle actuated, air can be cooled in the evaporator below normally desirable temperature, allowing for a greater amount of moisture removal from the air stream. The air then passes over the reheat heat exchanger and is heated back toward the target temperature. Typically, reheat cycles are provided with a refrigerant flow control device, such a three-way valve, that can selectively route at least a portion of refrigerant through the reheat heat exchanger when reheat is desired. The reheat cycle has not been operated at system start-up, in order to prevent high pressure spikes and nuisance refrigerant system shutdowns.

SUMMARY OF THE INVENTION

[0007] A refrigerant system has a compressor delivering a compressed refrigerant to a condenser. Refrigerant from the condenser passes through an expansion device and an evaporator. From the evaporator it is returned to the compressor. The condenser is a microchannel heat exchanger. A reheat cycle includes a refrigerant flow control device for selectively routing at least a portion of refrigerant through a reheat heat exchanger. The reheat heat exchanger is positioned in a path of air that has passed over the evaporator. A control for the refrigerant system selectively actuates a switch to route refrigerant through a reheat heat exchanger at system start-up.

[0008] In one embodiment, the reheat cycle may also be actuated at certain environmental and operating conditions, when high pressure spikes are expected to occur. Such conditions may include, for instance, high ambient temperatures, higher operating speeds of variable speed compressors and a higher number of active tandem compressors. These environmental and operating conditions may be pre-programmed and stored in the memory of the refrigerant system controller.

[0009] These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure IA shows a first embodiment schematic.

[0011] Figure IB shows an alternative embodiment.

[0012] Figure 2A shows an exemplary microchannel heat exchanger.

[0013] Figure 2B is a cross-section through a portion of Figure 2A.

[0014] Figure 3 is a graph showing start-up utilizing the disclosed method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0015] A refrigerant system 20 is illustrated in Figure IA and includes a compressor 22 delivering refrigerant into a discharge line heading to a condenser 24. The condenser 24 is a parallel flow heat exchanger, and in one disclosed embodiment is a microchannel or minichannel heat exchanger. As mentioned above, these terms are used interchangeably here.

[0016] Heat is transferred in the condenser 24 from the refrigerant to a secondary fluid, such as ambient air. The high pressure, desuperheated, condensed and typically subcooled, refrigerant passes from the condenser 24 into an expansion device 38, where it is expanded to a lower pressure and temperature. Downstream of the expansion device 38, refrigerant flows through an evaporator 36 and back to the compressor 22. As known, the heat exchanger 24 operates as a condenser in subcritical applications and as a gas cooler in transcritical applications. Nevertheless, although both applications are within the scope of the invention, the heat exchanger 24 will be referred throughout the text as a condenser.

[0017] A reheat cycle is incorporated into the refrigerant system 20. As known, a refrigerant flow control device such as a three-way valve 30 selectively routes at least a portion of refrigerant downstream of the condenser 24 and through a reheat heat exchanger 32. An air-moving device such as a fan 34 blows air over an evaporator 36, and over the reheat heat exchanger 32. That is, the reheat heat exchanger 32 is positioned indoors, along with the evaporator 36, and downstream, with respect to the air flow, of the evaporator 36. As mentioned above, essentially, the reheat cycle is selectively actuated by opening (fully or partially) the three-way valve 30 to direct at least a portion of refrigerant through the reheat heat exchanger 32 when dehumidification in a climate-controlled environment X is desired. Under such circumstances, the refrigerant system is controlled such that the evaporator 36 cools the air to a temperature below that is desired in the environment to be conditioned X, which allows removing an additional amount of moisture from the air to be delivered to the conditioned environment X. As the air passes over the reheat heat exchanger 32, it is reheated toward the target temperature. As a result, temperature and humidity control are achieved in the climate-controlled environment X.

[0018] Downstream of the reheat heat exchanger 32, there is an optional check valve 40. Further, as shown, a condenser bypass line 26 selectively bypasses at least a portion of refrigerant around the condenser 24 and includes a refrigerant flow control device such as a valve 28. This allows for achieving variable dehumidification capability, or variable sensible heat ratios. The valve 28 can be adjustable (through modulation or pulsation) or of an on/off type.

[0019] Figure IB shows an alternative embodiment wherein the reheat cycle three-way valve 42 is positioned upstream of the condenser 24 and delivers at least a portion

of refrigerant through a reheat refrigerant line 44 to a reheat heat exchanger (not shown). For purposes of this application, the exact location of the three-way valve 42 and the reheat heat exchanger 32 is not critical, provided they are both located on the high pressure side of the refrigerant system 20. Also, as known, the three-way valves 30 and 42 can be replaced by a pair of conventional valves performing identical refrigerant routing function.

[0020] As shown in Figure 2A, an inlet line 146 downstream of the compressor 22 delivers refrigerant into a first bank of parallel heat exchange tubes 148, and then across the condenser core to a first chamber of an intermediate manifold structure 133. From the intermediate manifold structure 133, the refrigerant passes back through a second bank of parallel heat exchange tubes 150 to an intermediate chamber in the manifold 147. Refrigerant then passes through yet another bank of parallel heat exchange tubes 152, returning to the intermediate manifold 133. From the intermediate manifold 133, the refrigerant passes through another bank of heat exchange tubes 154 back to the manifold 147, and an outlet refrigerant line. Of course, this is simply one illustrated embodiment. It should be noted that, in practice, there may be more or less refrigerant passes than the four illustrated passes 148, 150, 152, and 154. Further, it should be understood that, although for simplicity purposes, each refrigerant pass is represented by a single heat exchange tube, typically there are many heat exchange tubes within each pass amongst which refrigerant is distributed while flowing within the pass. In condenser applications, a number of the heat exchange tubes within each bank may decrease in a downstream direction, with respect to refrigerant flow. For instance, there could be 12 heat exchange tubes in the first bank, 8 heat exchange tubes in the second bank, 5 heat exchange tubes in a third bank and only 2 heat exchange tubes in the last forth bank. Separator plates 143 are placed within the manifolds 133 and 147 to separate the chambers positioned within the same manifold structure.

[0021] As shown in Figure 2B, the heat exchange tubes within the tube banks 148, 150, 152, and 154 may consist of a plurality of parallel channels 100 separated by walls 101. The Figure 2B is a cross-sectional view of the heat exchange tubes shown in Figure 2A. The channels 100 allow for enhanced heat transfer characteristics and assist in improved structural rigidity of the heat exchanger. The cross-section of the channels 100 may take different forms, and although illustrated as a rectangular in Figure 2B, may be, for instance, of triangular, trapezoidal, oval or circular configurations. The size of the channels 100 in a

microchannel heat exchanger is quite small. As disclosed, the channels could have a hydraulic diameter of less than or equal to 5 mm, and more narrowly, less than or equal to 3 mm. Notably, the use of "hydraulic diameter" does not imply the channels are circular.

[0022] As mentioned above, when microchannel heat exchangers are utilized as condensers, pressure spikes which can be particularly observed at the refrigerant system startup, can provide a challenge to a refrigerant system designer. One concern with microchannel heat exchangers is that their internal volume is relatively small, and thus they are particularly susceptible to pressure spikes and extremely sensitive to refrigerant charge amounts. Although pressure spikes are particularly pronounced at refrigerant system start-up, they can be also observed at changes of operating conditions such as, for instance, a sharp increase of the compressor speed or activating a larger number of tandem compressors, in order to satisfy thermal load demands in the conditioned space X.

[0023] In this invention, the reheat circuit is actuated at refrigerant system startup. Now, when the refrigerant is passing through both the condenser 24, and through the reheat heat exchanger 32, there is a larger combined internal volume on a high pressure side of the refrigerant system, and the amplitude of the pressure spike is thus reduced. In some instances, all of the refrigerant could pass through the reheat heat exchanger 32.

[0024] As shown in Figure 3, with a conventional start-up S, and without the reheat circuit being actuated, a pressure spike can be relatively high, and may exceed the safety limit Y. With the present application, and as shown at Z in Figure 3, the amplitude of the pressure spike is greatly reduced, due to the combined internal volume of the heat exchangers 24 and 32. In this manner, the pressure spike may well be below the safety limit Y, and nuisance shutdowns, caused by control software operating on a high pressure alarm or by mechanical safety, such as a high pressure switch, can be avoided. This provides uninterrupted control of temperature and humidity within the desired ranges and occupant comfort in the climate-controlled environment. Furthermore, repeated starts and shutdowns of the refrigerant system in short periods of time will be avoided, leading to improved compressor reliability and temperature/humidity variation reduction in the conditioned space.

[0025] The reheat heat exchanger 32 can be any type of a heat exchanger, including standard heat exchangers or a microchannel heat exchanger.

[0026] A control 110 for the refrigerant system may be of any appropriate electronic control type, as is known in the art. The control would typically control all system components, and not only the three-way valve 30 that can be adjustable (through modulation or pulsation) or of an on/off type. The control 110 can actuate the reheat cycle at certain environmental and operating conditions, when high pressure spikes are likely to occur. Such conditions may include, for instance, high ambient temperatures, higher operating speeds of variable speed compressors and a higher number of active tandem compressors. These environmental and operating conditions may be pre-programmed and stored in the memory of the refrigerant system control 110. Further, under some environment and operating conditions, it may be that there is less likelihood of a pressure spike at system start-up. Thus, the control may be programmed to not actuate the reheat cycle in these instances.

[0027] In addition, after some period of time following refrigerant system startup, the three-way valve 30 is deactivated to block flow of refrigerant through the reheat heat exchanger 32, unless dehumidification mode of operation is desired. This period of time can be on the order of 15 seconds to 3 minutes.

[0028] Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.