In the prior art, air conditioners are used that can switch between indoor heating and cooling. An example of such an air conditioner is a so-called split-type air conditioner in which a refrigerant circuit is configured by connecting a heat source unit having a compressor, a four-way switching valve, and a heat source-side heat exchanger, and a utilization unit having an expansion valve and a utilization-side heat exchanger, the connection being established via two refrigerant communication pipes.
In a split-type air conditioner capable of switching between indoor heating and cooling, the use of carbon dioxide, which has little effect on the environment, is considered desirable as the refrigerant sealed within the refrigerant circuit instead of a CFC refrigerant, an HCFC refrigerant, or an HFC refrigerant, which have been heretofore used.
However, in the air conditioner described above, when carbon dioxide is used as the refrigerant, the carbon dioxide as the refrigerant is compressed by the compressor until critical pressure or greater is reached. During the cooling operation, a refrigeration cycle operation is performed in which the refrigerant compressed to critical pressure or greater in the compressor flows to the heat source-side heat exchanger via the four-way switching valve, the refrigerant is cooled in the heat source-side heat exchanger, the refrigerant is sent to the expansion valve via first refrigerant communication pipe, the refrigerant is reduced in pressure until a low pressure is reached, the refrigerant flows to the utilization-side heat exchanger, and the refrigerant is heated in the utilization-side heat exchanger, after which the refrigerant returns to the compressor via the second refrigerant communication pipe and the four-way switching valve. During the heating operation, a refrigeration cycle is performed in which the refrigerant compressed to critical pressure or greater in the compressor flows to the utilization-side heat exchanger via the four-way switching valve and the second refrigerant communication pipe, the refrigerant is cooled in the utilization-side heat exchanger, the refrigerant is sent to the expansion valve, the refrigerant is reduced in pressure until a low pressure is reached, the refrigerant flows to the heat source-side heat exchanger via the first refrigerant communication pipe, and the refrigerant is heated in the heat source-side heat exchanger, after which the refrigerant returns to the compressor via the four-way switching valve. Specifically, during the cooling operation, carbon dioxide compressed to critical pressure or greater passes through a portion leading from the compressor up to the expansion valve via the four-way switching valve, the heat source-side heat exchanger, and the first refrigerant communication pipe; and during the heating operation, the carbon dioxide compressed to critical pressure or greater passes through a portion leading from the compressor up to the expansion valve via the four-way switching valve, the second refrigerant communication pipe, and the utilization-side heat exchanger.
Thus, when an air conditioner is configured that can use a refrigeration cycle in which the refrigerant is compressed to critical pressure or greater and that can switch between indoor heating and cooling, substantially all of the components constituting the refrigerant circuit, including the refrigerant communication pipes, must be designed at a maximum pressure determined based on the pressure to which the refrigerant is compressed by the compressor, bringing about the problems of increased costs of materials due to an increase in the thickness of the refrigerant communication pipes, reduced workability due to increased thickness, and further increased costs due to reduced workability.
In Japanese Laid-open Patent Application No. 2003-139422, a procedure is disclosed in which the expansion valve is connected to the heat source unit side, the refrigerant cooled in the heat source-side heat exchanger is reduced in pressure by the expansion valve, and the refrigerant is then sent to the utilization-side heat exchanger via a first refrigerant communication pipe, thereby suppressing increases in the thickness of the refrigerant communication pipes.