MULTI-CIRCUIT REFRIGERANT SYSTEM USING DISTINCT REFRIGERANTS

BACKGROUND QF THE INVENTION This application relates to multi-circuit refrigerant systems, wherein distinct refrigerants are used in the multiple circuits to provide the ability to tailor operation to environmental conditions and load requirements.

Refrigerant systems are utilized in many applications to condition an environment. In particular, air conditioners and heat pumps are employed to cool and/or heat air entering the environment. The cooling or heating load of the environment may vary with ambient conditions, occupancy level, other changes in sensible and latent load demands, and as the temperature and/or humidity set points are adjusted by an occupant of the environment.

Multi-circuit refrigerant systems are also applied in the industry, wherein several independent circuits operate under a single control to provide various levels of sensible and latent capacity in response to the external load demands and wherein each circuit can independently function in one of several operational regimes.

One option available to a designer of refrigerant systems relates to the selection of available refrigerants. Various refrigerants are known, and each has individual properties and characteristics. Thus, the different refrigerants can provide different capacities, efficiencies, dehumidification capabilities as well as safety and toxicity levels, various degrees of compatibility with the environment, etc.

For instance, some refrigerants, such as R134a, may be best utilized in an air conditioning mode where the ambient temperatures are relatively high, and other refrigerants, such as R410A, may be better employed when ambient temperatures are typically lower. Similarly, for heat pump applications, some refrigerants, such as R744, might be best suited for the heating operations, while other refrigerants, such as R245fa, may be better fitting for the cooling operations.

In at least one proposed application, a composition of the refrigerant circulating throughout the refrigerant system has been selectively adjusted based upon the particular operation mode using a rectification tower concept (see United States patents 6,070,420 and 5,848,537). However, the circuitry, system schematic,

operation control, etc. for altering and optimizing the refrigerant composition is unduly complex and expensive.

SUMMARY OF THE INVENTION In a disclosed embodiment of this invention, a refrigerant system is provided with multiple circuits operating in parallel. At least two of the circuits are provided with distinct refrigerants. In one example, the refrigerant system is an air conditioning system, and the two refrigerants may be selected to be best utilized at distinct ambient temperatures. As an example, each refrigerant might provide efficiency benefits at the indicated temperature range. One circuit may be charged with a refrigerant best utilized at higher ambient temperatures, while the other circuit may have a refrigerant best utilized at lower ambient temperatures. The control for the system monitors the ambient temperature, and utilizes the two circuits in a sequence based upon a sensed ambient temperature. As is known, controls for multiple circuits operate the circuits in combination with each other to best address a particular environmental conditioning situation. The control in this invention is operable to sense ambient temperature, and initially use the circuit charged with the refrigerant that is best suited for an existing ambient temperature range. Generally, the other circuit is engaged only if an additional boost in capacity is required. Thus, the circuit that is most efficient for the particular environmental situation provides the bulk of the space conditioning. The other circuit is utilized less frequently and as a "trimming" circuit. That is, it is utilized to supplement the main circuit. In case the environmental conditions change significantly enough for the refrigerant circulating through another independent circuit of a multi-circuit refrigerant system to be more efficient, then that circuit becomes primary and is brought online first to address space conditioning demands.

In another embodiment, similar strategy can be exercised in relation to the temperature ranges for the environment to be conditioned. For example, one refrigerant can be mostly thermodynamically advantageous for a higher temperature range and the other refrigerant may have benefits at the lower temperature range. The system control determines and optimizes a sequence of operation for the multi-circuit

refrigerant system in accordance to the sensed conditioned space temperature. In a logical extension of this control strategy, both indoor and outdoor temperatures could be utilized for determination of an appropriate (for instance, most efficient) sequence of operation based on a two-dimensional map with indoor and outdoor temperatures as independent variables.

In still another embodiment, the refrigerant system is a heat pump having multiple circuits. One of the multiple heat pump circuits is provided with a refrigerant that is most efficient for cooling situations, and another circuit is provided with the refrigerant which is most efficient for heating situations. Again, the control is operable to initially utilize the circuit which is charged with the best refrigerant as a "main" circuit for the particular operation. Further, when it is desired to simultaneously operate some independent circuits of a multi-circuit system in the air conditioning mode and some circuits in the heat pump mode, for instance, to control humidity, the best refrigerant is matched to a particular mode of operation by the system controls.

In yet another embodiment, since the circuits charged with different refrigerants are inherently unbalanced (or provide different capacities at identical environmental conditions), the system controls utilize an operational sequence logic to optimize overall unit performance and reliability based on minimizing a number of start-stop cycles (and associated losses) to match particular capacity demands as well as taking into account continuous operational efficiency of a particular circuit charged with a particular refrigerant.

Furthermore, the some circuits could be provided with a single component refrigerant ("pure substance" refrigerant) while the other circuits could be provided with a mixture of multiple constituent refrigerants or the circuits can each be provided with a refrigerant mixture consisting of different components. In one embodiment, this mixture can consist of the same distinct refrigerant constituents mixed in different compositions (proportions) between the two circuits.

While the embodiments disclosed include only two circuits, it should be understood that additional circuits could be utilized within this invention. A worker

of ordinary skill in the art would recognize which refrigerants are best suited for the particular challenges.

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

Figure 1 shows a first schematic. Figure 2 shows a second schematic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a refrigerant system that may be utilized as an air conditioning system, and which includes a plurality of circuits 50 arranged in a parallel fashion. Each circuit 50 includes a compressor 12 delivering refrigerant to a condenser 20, to an expansion device 22, and to an evaporator 24. For cooling applications, the condenser 20 in an air conditioning system is positioned outside of the environment to be conditioned while the evaporator 24 is positioned as an inside heat exchanger.

The two circuits 50 are provided with distinct refrigerants. In one embodiment, a control 100 is operable to sense ambient temperature. One circuit 50 is charged with a refrigerant that is best utilized at higher ambient temperatures, and the other circuit is provided with a refrigerant that is best utilized at lower ambient temperatures. The control 100 controls both circuits 50. Based upon a sensed ambient temperature, the control 100 selects which circuit 50 to be utilized initially to meet cooling demands. As an example, should the ambient temperature be somewhat low, the circuit 50, that is charged with the refrigerant that is best utilized at lower ambient temperatures, is initially brought online to meet a cooling load. If additional cooling is necessary to satisfy the cooling requirements, then the other circuit, which is "less-suited" for lower ambient temperatures, is actuated and used as a trimming circuit to fully meet the cooling demand. Conversely, should a higher ambient temperature be sensed by the control

100, then the circuits are used in a reverse order.

The present invention thus provides the ability to utilize distinct refrigerants to most efficiently and accurately meet a cooling challenge. It has to be noted that refrigerant systems typically operate at part-load conditions most of the time, so a supplementary circuit with "less-suited" refrigerant will be utilized very seldom and the attained benefits of matching refrigerant to particular environmental conditions will not be compromised.

Further, similar strategy can be exercised in relation to the temperature ranges for the environment to be conditioned. One refrigerant can be mostly thermodynamically advantageous for a higher temperature range and the other refrigerant may have benefits at the lower temperature range. The control 100 will determine and optimize a sequence of operation for the multi-circuit refrigerant system in accordance to the sensed conditioned space temperature. Also, as a logical extension of this control strategy, both indoor and outdoor temperatures could be utilized for determination of an appropriate (for instance, most efficient) sequence of operation based on a two-dimensional map with indoor and outdoor temperatures as independent variables. Obviously, this invention extends to the refrigerants operating in trans-critical and super-critical regions as well.

In the heating applications, the condensers 20 would be located indoors and the evaporators 24 positioned outdoors, but the abovementioned logic for the control 100 with respect to indoor and outdoor temperatures would be preserved (although may be referenced to a different performance map).

Also, more than two circuits and more than two refrigerants can be utilized within multi-circuit system configurations.

Figure 2 shows another embodiment wherein heat pumps are utilized in a multi-circuit system. As shown, there is a pair of circuits 10. Of course, a different number of circuits and more than two distinct refrigerants would be within the scope of this invention. Each circuit 10 includes a compressor 12 delivering refrigerant to a discharge line 14. A suction line 16 returns refrigerant to the compressor 12. A four- way valve 18 selectively routes refrigerant from the discharge line 14 to either an outdoor heat exchanger 20 in the cooling (or air conditioning) mode of operation, or to an indoor heat exchanger 24 in the heating (or heat pump) mode of operation. In

the cooling mode, the four-way valve 18 routes the refrigerant to the outdoor heat exchanger 20, then to an expansion device 22, and then to the indoor heat exchanger 24, from where it is returned, through the four-way valve 18 and suction line 16, to the compressor 12. In the heating mode, a direction of the refrigerant flow through the system is essentially reversed, and the refrigerant flows from the compressor 12, through the four-way valve 18, through the indoor heat exchanger 24, through the expansion device 22, through the outdoor heat exchanger 20, and then again through the four-way valve 18 and through the suction line 16 back to the compressor 12. This general operation is as known in the art. The four-way valve 18 is controlled to either achieve cooling or heating mode of operation. Furthermore, if the expansion device cannot handle the reversing flow, then, as one of the potential solutions, a pair of unidirectional expansion devices, with corresponding check valves, may be employed instead.

One of the multiple heat pump circuits 10 is provided with a refrigerant that is most efficient for cooling situations, and the other circuit is provided with a refrigerant, which is most efficient for heating situations. Again, the control 100 is operable to initially utilize the circuit which includes the best refrigerant for the particular operation. Further, a similar strategy could be employed for providing humidity comfort as well, while simultaneously operating one of the heat pump circuits in the cooling mode and the other circuit in the heating mode and while the best refrigerant is matched to a particular mode of operation by the control 100.

Also, since the circuits containing different refrigerants will be inherently unbalanced (typically delivering different capacity levels at identical environmental conditions), an operating sequence can be developed to optimize the unit performance by reducing the cycling losses while selecting the circuit to minimize, for example, an overall number of starts and stops. Further, an optimization strategy can be developed where efficiency of operation of each circuit at particular conditions and the cycling losses are evaluated at the same time to determine the most efficient and reliable sequence/control logic of unit operation. In all cases considered above, a transducer communication feedback of system operating and/or environmental conditions to the

control 100 will be a deciding factor of switching between the circuits on a primary- secondary basis.

A main aspect of the invention is that each circuit in the multi-circuit refrigerant system may have a different refrigerant, which provides enhanced capability in system operation and control in satisfying a wide spectrum of external sensible and latent load demands. The teachings of this invention are not limited to a specific system configuration or refrigerant, and the benefits of the invention can be easily extended to other design arrangements by a person ordinarily skilled in the art.

Also, it has to be noted, that a refrigerant can be a mixture of various components ("pure" substances), and changing a composition of such a mixture would constitute a different refrigerant as well.

Providing an appropriate control for operation of all of the refrigerant system components and devices would also be within the skill of a worker in this art.

Although a preferred 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.