US20060096308A1

US20060096308A1 - Vapor compression system with defrost system

Info

Publication number: US20060096308A1
Application number: US10/984,284
Authority: US
Inventors: Dan Manole
Original assignee: Individual
Current assignee: Tecumseh Products Co
Priority date: 2004-11-09
Filing date: 2004-11-09
Publication date: 2006-05-11
Also published as: CA2525360A1

Abstract

A vapor compression system including a circuit having operably coupled thereto, in serial order, a compressor, an interior heat exchanger, a second heat exchanger, an expansion device, an exterior heat exchanger, and an accumulator. A first bypass line extends from circuit between interior exchanger and second exchanger to between expansion device and third exchanger. A second bypass line extends from circuit between the exterior exchanger and accumulator to accumulator, and is operably coupled to second exchanger. A bypass expansion device is operably coupled to second bypass line. A first valve is coupled to first bypass line, and a second valve is coupled to second bypass line. During a defrost cycle first valve is in a first position wherein refrigerant flowing from interior exchanger flows to exterior exchanger through first bypass line, and second valve is in a second position wherein refrigerant flowing from exterior exchanger flows through second bypass line.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to vapor compression systems, particularly, vapor compression systems having a defrost system.
2. Description of the Related Art
Vapor compression systems, such as heat pumps, typically include a refrigerant circuit through which a compressible refrigerant flows and which fluidly connects, in serial order, a compressor, an indoor heat exchange coil, a sub-cooler, an expansion valve, and an outdoor heat exchange coil. When the heat pump is in the heating mode, the indoor heat exchange coil acts as a condenser transferring thermal energy from the compressed refrigerant flowing therein to the ambient air indoors to warm the air and condense the refrigerant. In the meantime, the outdoor heat exchange coil acts as an evaporator transferring the thermal energy from the ambient air outdoors to the refrigerant flowing through the coil. However, if the temperature of the outdoor heat exchange coil falls below the dew point, condensation may form on the coil. Under certain conditions, this condensation may freeze thus causing frost to build-up on the outdoor heat exchange coil. The build-up of ice and frost on the outdoor coil may impair the ability of the outdoor coil to transfer thermal energy, thus resulting in reduced efficiency.
In order to melt the ice on the outdoor coil, conventional heat pumps are often configured to switch to the cooling mode when ice is detected on the outdoor coil. In the cooling mode, the flow of the refrigerant is reversed and the indoor coil acts as an evaporator, while the outdoor coil acts as a condenser. As a result, hot refrigerant discharged from the compressor flows directly to the outdoor coil thereby heating the outdoor coil and melting the ice. Once the ice is melted, the heat pump switches back to the heating mode. Unfortunately, when the heat pump is in the cooling mode the indoor coil acts as an evaporator transferring thermal energy from the ambient air indoors to the refrigerant within the coil thereby cooling the air indoors. This phenomenon is commonly referred to as “cold blow.”
In order to alleviate the effects of cold blow, heat pump systems often include supplemental electric or gas heaters to heat the air that circulates over the indoor coil. However, these supplemental heaters often increase overall power consumption, can reduce the efficiency and reliability of the system, and can often cause temperature fluctuations. Accordingly, a need remains for a vapor compression system having an effective and efficient defrost system for defrosting the outdoor coil.

SUMMARY OF THE INVENTION

The present invention provides a vapor compression system with defrost system for use with a refrigerant to heat and/or cool an interior space defined by a structure. The system, in one form, includes a fluid circuit having operably coupled thereto, in serial order, a compressor, a first heat exchanger located in the interior space, a second heat exchanger, an expansion device, a third heat exchanger located exterior to the structure, and an accumulator. A first bypass line extends from a first point in the fluid circuit between the first heat exchanger and the second heat exchanger to a second point in the fluid circuit between the expansion device and the third heat exchanger. A second bypass line extends from a third point in the fluid circuit between the third heat exchanger and the accumulator to a fourth point in the fluid circuit between the third point and the accumulator, and is operably coupled to the second heat exchanger. A bypass expansion device is operably coupled to the second bypass line between the third point and the second heat exchanger. A first valve is disposed in the fluid circuit between the first heat exchanger and the second heat exchanger and is in communication with the first bypass line. The first valve has a first position wherein at least a substantial amount of the refrigerant flowing from the first heat exchanger flows to the third heat exchanger through the first bypass line without passing through the second heat exchanger and the expansion device thereby defrosting the third heat exchanger, and a second position wherein the refrigerant flowing from the first heat exchanger flows to the second heat exchanger though the fluid circuit without passing through the first bypass line. A second valve is disposed between the third heat exchanger and the accumulator, and has a first position restricting the flow of refrigerant from the third heat exchanger to the accumulator through the fluid circuit without flowing through the second bypass line, and a second position wherein the refrigerant flowing from the third heat exchanger flows through the second bypass line and thereby passes through the bypass expansion device and the second heat exchanger before entering the accumulator. During an operating cycle the first valve is in the second position and the second valve is in the first position, and during a defrost cycle the first valve is in the first position and the second valve is in the second position.
The present invention also provides a method for defrosting a heat exchanger of a vapor compression system. The method, in one form, includes the step of circulating a refrigerant during an operational cycle through, in serial order, a compressor, a first heat exchanger located in an interior space defined by a structure, a second heat exchanger, an expansion device, a third heat exchanger located exterior to the structure, and an accumulator. The method also includes the step of circulating the refrigerant during a defrost cycle through, in serial order, the compressor, the first heat exchanger, the third heat exchanger, a bypass expansion device, the second heat exchanger, and the accumulator. During the defrost cycle at least a substantial amount of the refrigerant flowing from the first heat exchanger flows through a first bypass line to the third heat exchanger without passing through the second heat exchanger to thereby defrost the third heat exchanger. During the operational cycle the refrigerant flowing from the first heat exchanger bypasses the first bypass line and flows to the second heat exchanger without passing through the first bypass line.
The vapor compression system, in another form, includes a fluid circuit having operably coupled thereto, in serial order, a compressor, a first heat exchanger located in the interior space, a second heat exchanger, an expansion device, a third heat exchanger located exterior to the structure, and an accumulator. A first bypass line is fluidly coupled to the fluid circuit and provides fluid communication between the first heat exchanger and the third heat exchanger without passing through the second heat exchanger and the expansion device. A second bypass line is fluidly coupled to the fluid circuit and is in thermal communication with the second heat exchanger. The second bypass line provides fluid communication between the third heat exchanger and the accumulator. A bypass expansion device is operably coupled to the second bypass line between the third heat exchanger and the second heat exchanger. A first valve is operably coupled to the first bypass line, and has a first position restricting the flow of refrigerant to the second heat exchanger and communicating the refrigerant to the first bypass line, and a second position restricting the flow of the refrigerant through the first bypass line and communicating the refrigerant toward the second heat exchanger. A second valve is operably coupled to the fluid circuit between the third heat exchanger and the accumulator, and has a first position and a second position. In the first position, second valve restricts the flow of the refrigerant through the second bypass line and the refrigerant flows to the accumulator without flowing through the bypass expansion device and the second heat exchanger. In the second position, the refrigerant flowing from the third heat exchanger flows through the second bypass line and thereby passes through the bypass expansion device and the second heat exchanger before entering the accumulator. During an operating cycle the first valve is in the second position and the second valve is in the first position. During a defrost cycle the first valve is in the first position and the second valve is in the second position.
One advantage of the present invention is that the defrost cycle melts the ice on the exterior heat exchanger without converting the system to cooling mode. As a result, the interior heat exchanger does not act as an evaporator during the defrost cycle and, therefore, does not produce cool air or a “cold blow” effect.
Another advantage of the present invention is that it does not require the use of supplemental heaters to eliminate the effect of cold blow and, thus, efficiency is maintained.
Additional advantages of the present invention will become apparent when referencing the descriptions below.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is a schematic view of a vapor compression system according to one embodiment of the present invention wherein the vapor compression system is performing an operating cycle;
FIG. 2 is a schematic view of the vapor compression system of FIG. 1 wherein the vapor compression system is performing a defrost cycle; and
FIG. 3 is a schematic view of the vapor compression system of FIG. 1 wherein the vapor compression system is performing a start-up cycle.
Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed.

DETAILED DESCRIPTION

The embodiments hereinafter disclosed are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following description. Rather the embodiments are chosen and described so that others skilled in the art may utilize its teachings.
Referring first to FIG. 1, a vapor compression system 10 in accordance with the present invention is illustrated. Vapor compression system 10 includes refrigerant fluid circuit 12 (represented by bold flow lines in FIG. 1) through which flows a compressible refrigerant, such as carbon dioxide. Operably coupled to fluid circuit 12, in serial order, is compressor 14, first heat exchanger 16, second heat exchanger or sub-cooler 18, expansion device 20, third heat exchanger 22 and accumulator 24. Vapor compression system 10 is depicted in FIGS. 1-3 as a heat pump system for heating and/or cooling an interior space defined by building or other structure. As such, first heat exchanger 16 is positioned in the interior space, while third heat exchanger 22 is positioned exterior to the structure. A blower or fan 26 is positioned adjacent interior heat exchanger 16 and is adapted to circulate the ambient air of the interior space over interior heat exchanger 16. It should be understood that although the present invention is illustrated in FIGS. 1-3 as a heat pump system, the present invention may be similarly adapted for use in other heating and cooling systems, water heating systems, and other heating and cooling applications.
Compressor 14 may be any known single-stage or multi-stage compressor suitable for compressing a refrigerant fluid, such as carbon dioxide. Such suitable compressors may include one or more compressor mechanisms, including rotary vane mechanisms, reciprocating piston mechanisms, orbiting scroll mechanisms and centrifugal impeller mechanisms. Interior and exterior heat exchangers 16, 18 may be of any conventional condenser/evaporator design and may include a series of evaporator/condenser coils. The structure and design of second heat exchanger 18 is discussed in further detail below. Expansion device 20 may be any conventional expansion device or valve suitable for use in heating and/or cooling systems.
Turning now to FIGS. 1-3, vapor compression system 10 also includes first bypass line 28 and second bypass line 34. First bypass line 28 extends from a first point 30 in fluid circuit 12 between interior heat exchanger 16 and second heat exchanger 18 to a second point 32 in fluid circuit 12 between expansion device 20 and third heat exchanger 22. As illustrated in FIGS. 2 and 3, first bypass line 28 is adapted to communicate fluid from first point 30 in fluid circuit 12 directly to second point 32 in fluid circuit 12, the fluid thereby bypassing second heat exchanger 18 and expansion device 20.
As shown in FIGS. 1-3, a first valve 42 is operably coupled to first bypass line 28 and has a first position and a second position. In its first position, depicted in FIG. 2, first valve 42 permits the refrigerant flowing from interior heat exchanger 16 to flow from first point 30 through bypass line 28 to second point 32, thereby flowing from interior heat exchanger 16 directly to exterior heat exchanger 22 without passing through second heat exchanger 18 and expansion device 20. In its second position, depicted in FIG. 1, first valve 42 prevents the flow of refrigerant through first bypass line 28. As a result, the refrigerant is forced to flow through second heat exchanger 18 and expansion device 20 before flowing to exterior heat exchanger 22.
As illustrated in FIG. 2, first valve 42 may be disposed in first bypass line 28. In this configuration, when first valve 42 is in the first position, first valve 42 is open and permits fluid to flow through bypass line 28 but does not positively prohibit fluid from also flowing through second heat exchanger 18 and expansion device 20. Thus, at first point 30 refrigerant flowing from interior heat exchanger 16 may flow to exterior heat exchanger 22 either through first bypass line 28 or through fluid circuit 12 including second heat exchanger 18 and expansion device 20. However, due to the resistance created by second heat exchanger 18 and expansion device 20, the natural fluid dynamics of system 10 causes at least a substantial amount of the refrigerant flowing from interior heat exchanger 16 to flow to exterior heat exchanger 22 through bypass line 28 when first valve 42 is in the first position. First valve 42 may be any conventional valve capable of controlling the flow of high pressure refrigerant fluid. In one embodiment, for example, valve 42 is a solenoid valve.
In an alternative embodiment, first valve may be positioned at first point 30. Furthermore, first valve may be a three way valve. In this configuration, when in the first position, the three way valve permits refrigerant to flow from first point 30 through first bypass line 28, while positively prohibiting refrigerant from flowing to second heat exchanger 18 and expansion device 20. In the second position, the three way valve directs the flow of refrigerant through second heat exchanger 18 and expansion device.
Referring to FIGS. 1-3, second bypass line 34 extends from a third point 36 in fluid circuit 12 between exterior heat exchanger 22 and accumulator 24 to a fourth point 38 in fluid circuit 12 between third point 36 and accumulator 24. Second bypass line 34 is operably coupled to, and is in thermal heat exchange with, second heat exchanger 18. Bypass expansion device 40 is operably coupled to second bypass line 34 and reduces the pressure of the refrigerant flowing to second heat exchanger 18.
A second valve 44 is disposed in refrigerant circuit 12 between third and fourth points 36, 38, and has a first position and a second position. In the first position, depicted in FIG. 1, second valve 44 is open and refrigerant is permitted to flow from exterior heat exchanger 22 directly to accumulator 24 through fluid circuit 12 without flowing through second bypass line 34. In this position second valve 44 does not positively prohibit fluid from flowing to accumulator 24 through second bypass line 34. Thus, at third point 36 refrigerant flowing from exterior heat exchanger 22 may flow to accumulator 24 either through second bypass line 34 including bypass expansion device 40 and second heat exchanger 18 or through fluid circuit 12. However, due to the resistance created by bypass expansion device 40 and second heat exchanger 18, when second valve is in the first position, the fluid dynamics of system 10 causes at least a substantial amount of the refrigerant to flow directly to accumulator 24 via refrigerant fluid circuit 12 without flowing through second bypass line 34. In the second position, depicted in FIG. 2, second valve 44 is closed thereby positively prohibiting refrigerant from flowing directly to accumulator 24 via fluid circuit 12. As a result, the refrigerant flowing from exterior heat exchanger 22 is forced to flow through second bypass line 34. Second valve 44 may be any conventional valve capable of controlling the flow of high pressure refrigerant. In one embodiment, for example, second valve 44 is a solenoid valve.
Alternatively, second valve 44 may be positioned at third point 36 and may be a three way valve. In this embodiment the second valve has a first position positively directing the flow of refrigerant through second bypass line 34 and a second position positively directing the flow of refrigerant through second bypass line 34.
Vapor compression system 10 also includes sensor 48. Sensor 48 is operably coupled to either exterior heat exchanger 22, or fluid circuit 12 near the outlet of exterior heat exchanger 22. Sensor 48 is adapted to sense the temperature of the refrigerant in, or flowing from, exterior heat exchanger 22. Alternatively, sensor 48 may be adapted to sense the pressure of the refrigerant flowing from exterior heat exchanger 22. A controller 46 is electronically coupled to sensor 48 and is adapted to receive the sensed temperature from sensor 48. Controller 46 is also operably coupled to first and second valves 42, 44 and is adapted to affect the movement of valves 42, 44 between their first and second positions.
During the heating mode, vapor compression system 10 performs an operating cycle, illustrated by the bold flow lines in FIG. 1. During the operating cycle, first valve 42 is in the second position while second valve 44 is in the first position. The refrigerant fluid is compressed in compressor 14 to a high pressure and high temperature. The resulting hot compressed fluid discharged from compressor 14 flows through fluid circuit 12 to interior heat exchanger 16. Interior heat exchanger 16 acts as a condenser extracting heat from the hot compressed fluid and transferring it to the ambient air forced over interior heat exchanger 16 by blower 26. As a result, the compressed refrigerant is cooled and the ambient air within the interior space of the structure is heated. Although cooled in interior heat exchanger 16, the refrigerant exiting interior heat exchanger 16 is still quite hot and retains a significant amount of thermal energy. The hot refrigerant fluid then flows to second heat exchanger 18 where additional thermal energy is extracted to thereby further cool the refrigerant. The second heat exchanger 18 stores the extracted thermal energy and, ultimately, transfers the thermal energy to refrigerant flowing in another area of system 10 during a defrost cycle, which is discussed further below. The cooled refrigerant flows from second heat exchanger 18 to expansion device 20 which reduces the pressure of the compressed refrigerant and meters the refrigerant to exterior heat exchanger 22. Exterior heat exchanger 22 acts as an evaporator wherein thermal energy is transferred from the ambient air outside of the structure to the refrigerant, thereby cooling the air outside of the structure and evaporating the compressed refrigerant fluid. The refrigerant then flows through fluid circuit 12 to accumulator 24 which stores any liquid refrigerant remaining in the fluid exiting exterior heat exchanger 22 and meters the liquid refrigerant to compressor 14 or to another location in refrigerant circuit 12. The evaporated refrigerant flows through accumulator 24 and back to compressor 14 and the operational cycle is repeated.
Meanwhile, sensor 48 senses the temperature and/or pressure of the refrigerant in, or flowing from, exterior heat exchanger 22 and communicates the sensed temperature and/or pressure to controller 46. A sensed temperature below a certain level could be an indication of frost build-up on exterior heat exchanger 22. Similarly, a sensed pressure below a certain level may also indicate inefficient/ineffective evaporation due to frost build-up on exterior heat exchanger. Therefore, when the sensed temperature and/or pressure falls below a pre-determined value, controller 46 initiates a defrost cycle by switching first valve 42 to the first position and second valve 44 to the second position.
During the defrost cycle the refrigerant circulates through system 10 along the flow path illustrated in bold in FIG. 2. More particularly, hot refrigerant flowing from interior heat exchanger 16 flows to first point 30 at which point a majority of the hot refrigerant flows through first bypass line 28 directly to exterior heat exchanger 22 bypassing second heat exchanger 18 and expansion device 20. As a result, the hot fluid exiting interior heat exchanger 16 flows directly to exterior heat exchanger 22, wherein the hot refrigerant flows through exterior heat exchanger 22 thawing any frost that has formed on exterior heat exchanger 22 and cooling the refrigerant. The refrigerant then flows from exterior heat exchanger 22 to third point 36 at which point the refrigerant is forced to flow through second bypass line 34. The refrigerant is expanded in expansion device 40 and is metered to second heat exchanger 18. In second heat exchanger 18 the cool refrigerant absorbs the sensible heat of second heat exchanger 18 (e.g. the thermal energy stored in second heat exchanger 18 during the operational cycle), thereby warming the refrigerant and cooling second heat exchanger 18. The warm refrigerant then flows to accumulator 24 and then to compressor 14 and the defrost cycle continues.
During the defrost cycle, sensor 48 continues to sense the temperature and/or pressure of the refrigerant in, or flowing from, exterior heat exchanger 22. When the sensed temperature and/or pressure of the refrigerant reaches a pre-determined value, controller 46 ceases the defrost cycle and initiates the operating cycle.
Second heat exchanger or sub-cooler 18 may be any conventional heat exchanger capable of exchanging thermal energy between the refrigerant flowing in fluid circuit 12 and the refrigerant flowing in second bypass line 34. Because second heat exchanger 18 extracts and stores thermal energy during the operational cycle, second heat exchanger 18 is preferably constructed of a material having significant thermal storage potential. Such materials include metals, such as steel and copper. In one embodiment, a mass of material capable of storing heat may be added onto the body of second heat exchanger 18 in order to increase the thermal storage potential of the heat exchanger. Alternatively, or additionally, second heat exchanger 18 may incorporate a layer or section of phase change material, such as water, paraffin wax, or salt hydrates including, for example, NaOH, CaCl₂, Na₂SO₄, Na₂HPO₄, Ca(NO₃)₂or Na₂S₂O₃. Second heat exchanger 18 may alternatively include adsorption/desorption pairs capable of storing and releasing heat. Examples of such pairs are ammonia/strontium chloride, carbon/water, activated carbon/ammonia, zeolites/water and methenol/metal hydrides. Chemicals capable of undergoing a reversible exothermic process may also be used to increase their heat storage potential.
In addition to the operational and defrost cycles, the vapor compression system 10 may be adapted to perform a start-up cycle, during which the refrigerant circulates through system 10 along flow lines illustrated in bold in FIG. 3. During initial start-up of vapor compression system 10, controller 46 initiates the start-up cycle by switching first valve 42 to the first position and maintaining second valve 44 in the first position. The refrigerant flows from compressor 14 to interior heat exchanger 16. From interior heat exchanger 16, the refrigerant flows through first bypass line 28 to exterior heat exchanger 12, thereby bypassing second heat exchanger 18 and expansion device 20. The refrigerant then flows from exterior heat exchanger 12 to accumulator 24, bypassing second bypass line 34, expansion valve 40 and second heat exchanger 18. From accumulator 24, the refrigerant flows to compressor 14 and the cycle is repeated until the system is warmed up. By directing the refrigerant to bypass expansion device 20, the torque load placed on compressor 14 by the refrigerant during start-up is reduced. As a result, the start-up cycle reduces the stress on compressor 14 and the power spike caused by the compressor during start-up, thereby promoting the life of compressor 14. Once vapor compression system 10 is fully operating, controller 46 switches system 10 to the operational cycle, illustrated in FIG. 1, by moving first valve 42 from the first position to the second position.
While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.

Claims

1. A vapor compression system for use with a refrigerant to heat or cool an interior space defined by a structure, the system comprising:

a fluid circuit having operably coupled thereto, in serial order, a compressor, a first heat exchanger located in the interior space, a second heat exchanger, an expansion device, a third heat exchanger located exterior to the structure, and an accumulator;

a first bypass line extending from a first point in said fluid circuit between said first heat exchanger and said second heat exchanger to a second point in said fluid circuit between said expansion device and said third heat exchanger;

a second bypass line extending from a third point in said fluid circuit between said third heat exchanger and said accumulator to a fourth point in said fluid circuit between said third point and said accumulator, said second bypass line being operably coupled to said second heat exchanger;

a bypass expansion device operably coupled to said second bypass line between said third point and said second heat exchanger;

a first valve disposed in said fluid circuit between said first heat exchanger and said second heat exchanger and in communication with said first bypass line, said first valve having a first position wherein at least a substantial amount of the refrigerant flowing from said first heat exchanger flows to said third heat exchanger through said first bypass line without passing through said second heat exchanger and said expansion device thereby defrosting said third heat exchanger, and a second position wherein the refrigerant flowing from said first heat exchanger flows to said second heat exchanger though said fluid circuit without passing through said first bypass line; and

a second valve disposed between said third heat exchanger and said accumulator, said second valve having a first position restricting the flow of refrigerant from said third heat exchanger to said accumulator through said fluid circuit without flowing through said second bypass line, and a second position wherein the refrigerant flowing from said third heat exchanger flows through said second bypass line and thereby passes through said bypass expansion device and said second heat exchanger before entering said accumulator, and

wherein during an operating cycle said first valve is in the second position and said second valve is in the first position; and wherein during a defrost cycle said first valve is in the first position and said second valve is in the second position.

2. The vapor compression system of claim 1 wherein the refrigerant is carbon dioxide.

3. The vapor compression system of claim 1 wherein said first valve is a three-way valve and is disposed at said first point.

4. The vapor compression system of claim 1 wherein said second valve is a three-way valve and is disposed at said third point.

5. The vapor compression system of claim 1 wherein said first and second valves are electronically controlled valves.

6. The vapor compression system of claim 5 further comprising a controller operably coupled to said first and second valves and a sensor operably coupled to said controller, said sensor sensing the temperature of the refrigerant at a location between said third heat exchanger and said compressor in said fluid circuit and communicating said temperature to said controller, said controller switching said system from said operating cycle to said defrost cycle when said temperature falls below a pre-determined value.

7. The vapor compression system of claim 6 wherein said controller switches said first valve from the second position to the first position at initial start-up of the system.

8. The vapor compression system of claim 1 further comprising a controller operably coupled to said first and second valves and a sensor operably coupled to said controller, said sensor sensing the pressure of the refrigerant flowing from said third heat exchanger and communicating said pressure to said controller, said controller switching said system from said operating cycle to said defrost cycle when said pressure falls below a pre-determined value.

9. A method for defrosting a heat exchanger of a vapor compression system, the method comprising the steps of:

circulating a refrigerant during an operational cycle through, in serial order, a compressor, a first heat exchanger located in an interior space defined by a structure, a second heat exchanger, an expansion device, a third heat exchanger located exterior to the structure, and an accumulator; and

circulating the refrigerant during a defrost cycle through, in serial order, the compressor, the first heat exchanger, the third heat exchanger, a bypass expansion device, the second heat exchanger, and the accumulator,

wherein during the defrost cycle at least a substantial amount of the refrigerant flowing from the first heat exchanger flows through a first bypass line to the third heat exchanger without passing through the second heat exchanger to thereby defrost the third heat exchanger, and wherein during the operational cycle the refrigerant flowing from the first heat exchanger bypasses the first bypass line and flows to the second heat exchanger without passing through the first bypass line.

10. The method of claim 9 further comprising the step of sensing the temperature of the refrigerant exiting said third heat exchanger and switching the system to the defrost cycle when the temperature of the refrigerant falls below a pre-determined value.

11. The method of claim 9 wherein during the defrost cycle the refrigerant flowing from said third heat exchanger flows through a second bypass line to the bypass expansion device and the second heat exchanger before entering the accumulator.

12. The method of claim 9 wherein the refrigerant is carbon dioxide.

13. The method of claim 9 further comprising the step of sensing the pressure of the refrigerant in said third heat exchanger and switching the system to the defrost cycle when the pressure falls below a pre-determined value.

14. The method of claim 9 further includes the step of circulating the refrigerant during a start-up cycle through, in serial order, the compressor, the first heat exchanger, the third heat exchanger, and the accumulator, wherein during the start-up at least a substantial amount of the refrigerant flowing from the first heat exchanger flows through a first bypass line to the third heat exchanger without passing through the expansion device.

15. A vapor compression system for use with a refrigerant to heat or cool an interior space defined by a structure, the system comprising:

a first bypass line fluidly coupled to said fluid circuit, said first bypass line providing fluid communication between said first heat exchanger and said third heat exchanger without passing through said second heat exchanger and said expansion device;

a second bypass line fluidly coupled to said fluid circuit, said second bypass line being in thermal communication with said second heat exchanger, said second bypass line providing fluid communication between said third heat exchanger and said accumulator;

a bypass expansion device operably coupled to said second bypass line between said third heat exchanger and said second heat exchanger;

a first valve operably coupled to said first bypass line, said first valve having a first position restricting the flow of refrigerant to said second heat exchanger and communicating the refrigerant to said first bypass line, and a second position restricting the flow of the refrigerant through said first bypass line and communicating the refrigerant toward said second heat exchanger;

a second valve operably coupled to said fluid circuit between said third heat exchanger and said accumulator, said second valve having a first position restricting the flow of the refrigerant through said second bypass line and wherein the refrigerant flows to said accumulator without flowing through said bypass expansion device and said second heat exchanger, and a second position wherein the refrigerant flowing from said third heat exchanger flows through said second bypass line and thereby passes through said bypass expansion device and said second heat exchanger before entering said accumulator, and

16. The vapor compression system of claim 15 wherein the refrigerant is carbon dioxide.

17. The vapor compression system of claim 15 further comprising a controller operably coupled to said first and second valves and a sensor operably coupled to said controller, said sensor sensing the temperature of said third heat exchanger and communicating said temperature to said controller, said controller switching said first valve to said first position and said second valve to said second position when said temperature falls below a pre-set level.

18. The vapor compression system of claim 15 wherein said controller switches said first valve from said second position to said first position at initial start-up of the system.

19. The vapor compression system of claim 15 further comprising a controller operably coupled to said first and second valves and a sensor operably coupled to said controller, said sensor sensing the pressure of the refrigerant in said third heat exchanger and communicating said pressure to said controller, said controller switching first valve to said first position and said second valve to said second position when said pressure falls below a pre-set level.