What is a refrigeration cycle and how does it work?
The simple refrigeration cycle consists of four main processes: compression, condensation, expansion, and evaporation. These processes take place respectively in the compressor, condenser, expansion valve, and evaporator.
The following figure indicates the refrigeration cycle schematic with the above-mentioned processes, which can be also represented in the log(p)-h diagram, as shown in figure 1. These thermodynamic processes form a closed cycle called the theoretical Linde circuit, which is standard circuit for real compressor refrigeration systems.
As shown in fig. 1 the basic refrigeration cycle starts at point 1′. Here the compressor sucks the superheated refrigerant vapor through the suction pipe and starts to compress it. The pressure, temperature, and energy of compressed vapor rapidly increase. The specific volume, on the other hand, decreases significantly. Compression ends at point 2, which determines the parameters of vapor leaving the compressor. At this point, the temperature and energy of refrigerant vapor have the highest value in the whole cycle.
The distance between points 2 and 3 represents the process of cooling this superheated high-pressure vapor up to the point where it begins to condense. Its energy and specific volume decrease slightly. At point 3, the vapor is no longer superheated. Instead, it becomes 100% saturated (x = 1).
The condensation process is shown between points 3 and 4. It is a phase transition in which a saturated vapor gradually becomes a saturated liquid. During this process, a lot of energy is released in the form of heat, which must be rejected. Condensation occurs under constant pressure and temperature conditions and ends at point 4 where the quality “x” of refrigerant is equal to zero (x = 0).
It is usually recommended to make the liquid refrigerant slightly subcooled in practice. The subcooling process takes place from point 4 to 4’ and ensures that liquid refrigerant enters the expansion valve. With the subcooling, we also increase the specific cooling capacity (the segment between points 5 and 1 on the chart).
An expansion valve is used to decrease the pressure of the refrigerant, which enters it as subcooled liquid (point 4′) and flows out as a liquid-vapor mixture at point 5. The quality of this mixture has a value between zero and one (0 < x < 1). The expansion process is treated as an isenthalpic one – the energy (enthalpy, kJ/kg) of refrigerant is constant.
The line from points 5 to 1 indicates the evaporation process at constant temperature and pressure. The heat load from the cooled medium is transferred to the refrigerant via the evaporator heat exchange surface. Therefore, the energy of the refrigerant increases significantly. When all the liquid phase in the liquid-vapor mixture is evaporated, point 1 is reached, defining saturated refrigerant vapor (x = 1).
Finally, the saturated refrigerant vapor flows through the suction pipe. In this section, we control the superheating of the refrigerant, to make sure that the refrigerant entering the compressor will not contain the liquid phase.
The entire cycle ends at point 1′, where the specific volume of the superheated refrigerant is the highest, and where the compression process begins again.