Patent Application
20060288716
The present invention relates generally to heating, ventilation and air conditioner HVAC systems. In particular, the present invention is related to methods and/or systems that control HVAC system refrigerant pressure.
An HVAC system generally includes a closed loop refrigeration system with at least one evaporator, at least one condenser and at least one compressor. As the refrigerant travels through the evaporator, it absorbs heat from a heat transfer fluid to be cooled and changes from a liquid to a vapor phase. After exiting the evaporator, the refrigerant proceeds to a compressor, then a condenser, then an expansion valve, and back to the evaporator, repeating the refrigeration cycle. The fluid to be cooled (e.g. air) passes through the evaporator in a separate fluid channel and is cooled by the evaporation of the refrigerant. The cooled fluid can then be sent to a distribution system for cooling the spaces to be conditioned, or it can be used for other refrigeration purposes.
One type of air conditioner system is a split system where there is an indoor unit or heat exchanger, which is generally the evaporator, and an outdoor unit or heat exchanger, which is generally the condenser. Often, the outdoor unit is placed outdoors and is subject to outdoor ambient conditions, particularly temperature. When the outdoor ambient temperature falls, the amount of heat being removed from the refrigerant in the condenser increases. The increased heat removal in the condenser can result in a decrease in the refrigerant pressure at the suction line to the compressor, commonly referred to as head pressure. The decrease in head pressure results in a lowering of the temperature of the refrigerant at the evaporator. When the temperature of the refrigerant at the evaporator becomes too low, icing of the system can occur. Icing is a condition when the temperature at the exterior of the evaporator is sufficiently low to freeze water present in the atmosphere. The ice formed by the water frozen on the surface reduces the available heat transfer surface and eventually prevents the proper operation of the HVAC system by inhibiting heat transfer and/or damaging system components.
Some attempts to address the problem of icing have utilized the control of system pressure. In one approach, a variable speed condenser fan or a plurality of condenser fans having independent controls are used to control airflow over the condenser coil. As the amount of air passing over the coil decreases, the amount of heat transfer taking place at the coil decreases. Therefore, the temperature of the refrigerant in the condenser and the pressure of the system increase to allow the indoor coil to cool the air without icing problems. The use of the variable speed condenser fan or a plurality of condenser fans having independent controls has the drawback that it is expensive and requires complicated wiring and controls.
An alternate approach for the problem of low system pressure or icing is a parallel set of condensers in the refrigerant cycle, as described in U.S. Pat. No. 3,631,686. The parallel set of refrigerant condensers allows for two modes of operation. One mode of operation allows refrigerant to flow from only one of the refrigerant condensers. During this mode of operation, the condenser that does not permit the flow of refrigerant fills with liquid refrigerant. Because of this flooding, there is a reduction in the effective surface area of the condenser. The reduced surface area thereby reduces the ability of the condenser to remove heat from the refrigerant. Therefore, the temperature of the refrigerant in the condenser and the head pressure of the system increase allowing the indoor coil to cool the air without icing. The use of parallel refrigerant condensers has the drawback that it requires an additional condenser coil and additional piping, thereby increasing the space and cost required for installation. Another drawback associated with refrigerant flooding of the condenser coil is the resultant decrease in system capacity. Refrigerant normally available in a properly operating system is trapped in the condenser coil and not available to the compressor, thereby decreasing system capacity.
An additional alternate approach for the problem of low system pressure is the use of a valve that controls the discharge or flow of liquid refrigerant from the condenser to a receiver vessel downstream of the condenser to maintain control of the amount of condensing surface exposed to the outside temperature, as described in U.S. Pat. No. 2,874,550. The discharge of refrigerant from the condenser is controlled by a pressure-response valve that mechanically opens to allow the flow of liquid refrigerant from the condenser to the receiver vessel reducing the level of liquid inside the condenser, thereby lowering the system pressure. Alternatively, the valve is closed to stop the flow until the level of refrigerant rises in the condenser in an amount that reduces the effective cooling surface of the condenser. The reduced surface area thereby reduces the ability of the condenser to remove heat from the refrigerant, thereby raising the pressure of the system. The use of a pressure-response valve and a vessel downstream of the condenser to maintain control of the amount of condensing surface has the drawback that it includes a specially designed valve and additional piping, thereby increasing the required space and cost. As discussed above, another one of the drawbacks with refrigerant flooding the condenser coil is decreased system capacity. Refrigerant normally available in a properly operating system is trapped in the condenser coil and not available to the compressor, thereby decreasing system capacity.
An additional alternate approach for the problem of low system pressure is the use of a refrigerant bypass around the condenser, as described in U.S. Pat. No. 3,060,699 and U.S. Reissued Pat. No. Re. 27,522. If the temperature and pressure of the refrigerant in the condenser are sufficiently high, a valve will close on a condenser bypass and the flow of refrigerant will be directed to the condenser. If the temperature and pressure of the condenser are not sufficiently high, the valve will open on a condenser bypass and at least some of the flow of refrigerant will be directed away from the condenser. The result of the bypass is an increase in pressure through the pipe leading to the evaporator downstream of the compressor. The use of a bypass has the drawback that it includes a specially designed valve and additional piping, thereby increasing the required space and cost.
What is needed is a method and system for controlling the system refrigerant pressure without the drawbacks discussed above.
The present invention includes a method for controlling refrigerant pressure in an HVAC system. The method includes providing a compressor, a condenser and an evaporator connected in a closed refrigerant loop. The condenser has a header arrangement capable of distributing refrigerant to a plurality of refrigerant circuits within the condenser. The header arrangement also is capable of selectively isolating at least one of the refrigerant circuits from refrigerant flow. Refrigerant pressure is sensed at a predetermined location in the refrigeration system. At least one of the refrigerant circuits is isolated when the refrigerant pressure is less than or equal to a predetermined pressure.
The present invention also includes a method for controlling refrigerant pressure in an HVAC system. The method includes providing a closed loop refrigerant system comprising a compressor, a condenser and an evaporator. The condenser has a header arrangement capable of distributing refrigerant to a plurality of circuits within the condenser. The header arrangement is also capable of selectively isolating at least one of the circuits from refrigerant flow. Refrigerant pressure is measured at a predetermined location in the refrigeration system. At least one of the circuits is isolated from refrigerant flow when the measured pressure is equal to or less than a predetermined pressure. The number of circuits isolated within the condenser varies with the measured pressure with respect to the predetermined pressure. The isolation of the refrigerant circuits continues until the measured pressure is greater than the predetermined pressure.
The present invention also includes a heating, ventilation and air conditioning system. The HVAC system includes a refrigerant system having a compressor, an evaporator, and a condenser connected in a closed refrigerant loop. The HVAC system also includes a refrigerant pressure measuring device for sensing refrigerant pressure disposed at a predetermined location within the refrigerant system. The condenser includes a plurality of refrigerant circuits, a first valve arrangement and a second valve arrangement. The first valve arrangement is arranged and disposed to isolate one or more of the refrigerant circuits from flow of refrigerant when the refrigerant pressure is below a predetermined pressure. The second valve arrangement is arranged and disposed to draw refrigerant into or out of the isolated circuits of the condenser in response to the refrigerant pressure sensed by the refrigerant pressure measuring device.
The present invention provides an inexpensive method and system to control head pressure. The method and system requires little or no additional piping in order to implement the method and system. The system requires less in materials and therefore costs less. Additionally, the method and system of the present invention does not require the use of variable speed or multiple stage fans to control air flow across the heat exchangers of the HVAC system.
The lack of additional piping also allows retrofitting of the system into existing HVAC systems. Because, little or no additional piping is required, the system occupies approximately the same volume as existing HVAC systems. Therefore, the method and system of the present invention may be used in existing systems whose piping has been arranged according to the present invention or as a new system.
Another advantage of the present invention is that the air conditioning or heat pump unit can operate at lower ambient temperatures. The method and system of the present invention provides an increase in system pressure, thereby allowing the system to operate at lower ambient temperatures without icing of the system components.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
In addition to the various ratios of the first condenser portion 220 to the second condenser portion 230, the locations along the face of the condenser, perpendicular to the air, of the first and second condenser portions 220 and 230 may be selected to provide a greater efficiency in heat transfer when a condenser portion is isolated. In one embodiment, the first condenser portion 220 is arranged and disposed to isolate heat transfer circuits 210 that are positioned along the face of the condenser 120 in locations having a decreased overall heat transfer efficiency. Suitable locations for the isolated first condenser portion 220 in this embodiment include the heat transfer circuits 210 at the edges of the condenser, where the flow of heat transfer fluid is lower. The heat transfer circuits 210 on the outer edges of the condenser 120 typically receive less heat transfer fluid flow and have a lower heat transfer efficiency. Isolating the heat transfer circuits 210 having a lower efficiency and allowing the flow of refrigerant in heat transfer circuits 210 having a higher efficiency, such as the heat transfer circuits 210 near the center of the condenser 210, permits the condenser 120 to operate at a higher overall efficiency, while controlling the head pressure of the system. The isolation of the heat transfer circuits 210 may take place with each of the condenser portions in a single continuous area along the face of the condenser, or may be discontinuous, such that the heat transfer circuits of a single condenser portion may be split into two or more sections to provide increased heat transfer efficiency for the condenser 120. In this embodiment, the first condenser portion 220 may be arranged and disposed along the face of the condenser such that the less efficient heat transferring edge portions may be isolated in discontinuous portions of the face of the condenser, leaving a continuous second condenser portion in the more efficient heat transferring center portion of the condenser 120.
As shown in
In the HVAC system according to the present invention, when the pressure in the suction line 145 to the compressor 130 falls, the temperature of the refrigerant in the evaporator 110 likewise falls. When the pressure falls to a certain level, the evaporator 110 operates at temperatures that may result in icing of the evaporator 110. Icing is a condition when the temperature at the exterior of the evaporator is sufficiently low to freeze water present in the heat transfer fluid. In particular, in a residential system, the heat transfer fluid is typically air and the water that freezes is water present in the air in the form of humidity. The ice formed by the water frozen on the surface eventually prevents the proper operation of the HVAC system by inhibiting heat transfer and/or damaging system components. This icing generally begins at temperatures of from about 25° F. to about 32° F. In order to prevent the freezing of the evaporator, the pressure in the suction line 145 is preferably maintained above the temperature that corresponds to the freezing point of the evaporator 110.
The method and system for controlling the refrigerant pressure of an air conditioning or heat pump unit according to the present invention includes an HVAC unit that can operate at lower ambient temperatures. The present invention involves a piping arrangement that partitions the circuits within the condenser of a refrigeration system. The piping arrangement includes valves positioned so that one or more of the circuits within the condenser may be isolated from flow of refrigerant. The piping arrangement may be applied to a new system or may be applied an existing system. Applying the piping arrangement to the existing system has the advantage that it allows control of the refrigerant pressure without the addition of expensive piping, equipment and/or controls.
When the temperature around the condenser coil falls (e.g. when the outdoor temperature falls), the system refrigerant pressure falls proportionally. To help build head pressure, the present invention uses the valves connected to the circuits of the condenser to isolate a portion of the condenser from flow of refrigerant. The portion of the condenser that is not isolated remains in the active circuit and receives refrigerant. Because the refrigerant is only permitted to flow into a portion of the condenser 120, the heat transfer area and the corresponding amount of heat transfer is reduced. Therefore, less heat is removed from the refrigerant. Likewise, less heat is transferred to the first heat transfer fluid 150, thereby maintaining a higher refrigerant temperature. Additionally, because the temperature of the refrigerant is higher, the corresponding pressure of the refrigerant is also higher. Therefore, the refrigerant pressure of the system is increased.
In one method according to the invention, the pressure of the refrigerant is measured and compared to a predetermined pressure. The pressure measurement may be taken from any point in the system. However, the preferred point of measurement of refrigerant pressure is on the suction line 145 to the compressor. The suction line 145 to the compressor also corresponds to the outlet of the evaporator 110. The outlet of the evaporator 110 represents a low pressure point in the system, due the phase change of the refrigerant to a vapor resulting from the heat exchange relationship existing between the refrigerant and the second heat transfer fluid 155 in the evaporator 110. The lowest pressure point where liquid refrigerant is undergoing evaporation also corresponds to the lowest temperature in the system. The predetermined pressure is preferably a pressure that is greater than or equal to the pressure that corresponds to a temperature that results in icing at the evaporator 110.
The piping arrangement of the condenser 120 of the present invention includes piping sufficient to isolate the two or more heat transfer circuits 210 within the condenser. In one embodiment, the isolation valves 240 are positioned inside the vapor header 290 of the condenser 120. In an alternate embodiment, the isolation valves 240 are positioned on piping upstream from the vapor headers 290 of the condenser 120.
In an alternate embodiment according to the invention, refrigerant stored in the isolated portion of the condenser 120 after isolation valves 240 are closed may be drawn out of the isolated portion of the condenser 120 into the active system by suction pressure. Because the refrigerant from the isolated portion of the condenser adds to the amount of refrigerant per unit volume of the refrigeration system 100 not isolated, the pressure of the refrigerant is increased. Therefore, this addition of refrigerant into the system from the isolated portion of the condenser further assists in raising the system pressure. Alternatively, refrigerant may also be drawn out of the active portion of the refrigerant system 100 to reduce the pressure of the refrigerant, when a reduced refrigerant pressure is desirable. Drawing refrigerant out of the isolated portion of the coil provides additional control of the refrigerant pressure that provides a decrease in refrigerant pressure, particularly during times of unexpected, temporary or small refrigerant pressure increases. For example, the isolated condenser portion may not be opened during a particular pressure increase and the refrigerant may be drawn into the system. This operating condition may be desirable during times such as when the system is subject to gusting wind, changes in sunlight intensity or other temporary change in ambient conditions.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
