In recent years, a power recovery cycle has been proposed having an expander in place of an expansion valve in order to further enhance the efficiency of a refrigerating machine. In this power recovery cycle, the expander acts to recover, when a refrigerant expands, expansion energy in the form of electric power or mechanical power, thereby reducing the input of a compressor by the amount of energy being recovered (see, for example, patent document 1).
FIG. 10 depicts the conventional refrigerating machine as disclosed in the patent document 1. The compressor 1 is driven by a drive means (not shown) such as, for example, an electric motor or an automobile engine to suck and compress the refrigerant. A high-temperature and high-pressure refrigerant discharged from the compressor 1 is cooled by a radiator 2, which in turn discharges the refrigerant towards an expander 3 in which the refrigerant is reduced in pressure and expands accordingly. The expander 3 then converts expansion energy of the refrigerant into mechanical energy (rotational energy) so that the mechanical energy (rotational energy) so converted may be supplied to a generator 4 for generation of electric power. The refrigerant that has been reduced in pressure and has expanded in the expander 3 evaporates in an evaporator 5 before it is again sucked into the compressor 1.
In this refrigerating machine, because the expander 3 reduces the pressure of the refrigerant while doing expansion work by converting expansion energy into mechanical energy, the refrigerant discharged from the radiator 2 reduces enthalpy while undergoing a phase change along an isentropic curve (c→d), as shown in FIG. 11. Accordingly, as compared with a case in which during a pressure reduction, the refrigerant merely undergoes adiabatic expansion without doing any expansion work (an isenthalpic change), the phase change along the isentropic curve can increase a difference in specific enthalpy between an inlet side and an outlet side of the evaporator 5 by an amount corresponding to expansion work Δiexp, making it possible to increase the refrigerating capacity. Also, because mechanical energy (rotational energy) can be supplied to the generator 4 by the expansion work Δiexp, the generator 4 can generate electric power (Δiexp×power generation efficiency), which is in turn supplied to the compressor 1. As such, electric power required for driving the compressor 1 can be reduced and, hence, the coefficient of performance (COP) of the refrigerating cycle can be enhanced.
Patent document 1: Japanese Laid-Open Patent Publication No. 2000-329416
However, when the compressor 1 is held at a standstill, the refrigerant moves from the side of the radiator 2 towards the side of the evaporator 5 due to a pressure difference created in the refrigerating cycle during operation of the compressor 1. Accordingly, in the conventional construction referred to above, the refrigerant that has moved from the side of the radiator 2 flows into the expander 3 and is brought into contact with oil contained in an oil sump within the expander 3. When the expander 3 is held at a standstill, the oil sump contains a lot of oil and, in particular under low-temperature conditions, a lot of refrigerant dissolves in the oil. As such, when the refrigerating machine is started again, it runs short of the proper amount of flow of the refrigerant. Also, the viscosity of the oil in the expander 3 lowers due to the presence of a lot of refrigerant dissolving in the oil.
If the amount of flow of the refrigerant is insufficient, the refrigerant pressure in the evaporator 5 lowers and, hence, the temperature of pipes and fins disposed within the evaporator 5 also lowers. When this temperature becomes less than 0° C., frost may be formed on such pipes and fins within the evaporator 5. Accordingly, the resistance to flow in the evaporator 5 increases and, at worst, there is a possibility of the evaporator 5 clogging. If the evaporator 5 clogs, the amount of air flowing though the evaporator 5 greatly reduces, and the amount of heat-exchange is extremely reduced. As a result, the compressor 1 sucks and compresses a liquid refrigerant in the evaporator 5, and there arises a possibility of the compressor 1 being damaged. Further, the lowering of the viscosity of the oil in the expander 3 may cause damage to the sliding surfaces of the expander 3, which in turn causes a reduction in reliability of the expander 3.