The challenge in refrigeration and air conditioning is to remove heat from a low temperature source and dump it at a higher temperature sink. Compression refrigeration cycles in general take advantage of the idea that highly compressed fluids at one temperature will tend to get colder when they are allowed to expand. If the pressure change is high enough, then the compressed gas will be hotter than our source of cooling (outside air, for instance) and the expanded gas will be cooler than our desired cold temperature. In this case, we can use it to cool at a low temperature and reject the heat to a high temperature.
Vapour-compression refrigeration cycles specifically have two additional advantages. First, they exploit the large thermal energy required to change a liquid to a vapour so we can remove lots of heat out of our air-conditioned space. Second, the isothermal nature of the vaporization allows extraction of heat without raising the temperature of the working fluid to the temperature of whatever is being cooled. This is a benefit because the closer the working fluid temperature approaches that of the surroundings, the lower the rate of heat transfer. The isothermal process allows the fastest rate of heat transfer
Vapour compression refrigeration is the primary method to provide mechanical cooling. All vapor compression systems consist of the following four basic components alongwith the interconnecting piping. These are the evaporator, condenser, compressor and the expansion valve.
The evaporator and the condenser are heat exchangers that evaporate and condense the refrigerant while absorbing and rejecting the heat. The compressor takes the refrigerant from the evaporator and raises the pressure sufficiently for the vapor to condense in the condenser. The expansion device controls the flow of condensed refrigerant at this higher pressure back into the evaporator. Some typical expansion devices are throttle valves, capillary tubes and thermostatic expansion valves in case of large refrigeration systems.
Here, the dry saturated working medium at state 1 is compressed isentropically to state 2. Constant pressure heat transfer occurs from state 2 until the compressed vapor becomes saturated liquid or condensate at state 4. The compressed vapor is next throttled from the high pressure region in the condenser (state 4) to the low pressure region in the evaporator (state 5). Since throttling is an irreversible process, it is represented by a broken line. After throttling to evaporator pressure, the heat transfer in the evaporator causes vaporization of the working medium until state 1 is reached, thus completing the cycle. The process 4-5 is assumed to be adiabatic during throttling, an isenthalpic process.
CARNOT VAPOR COMPRESSION SYSTEMS
Here, the compression is imagined to take place in two stages: isentropic compression up to state 2 and isothermal compression from state 2 to 3 as shown in Figure
Schematic Representation of a Carnot Vapour Compression System and T-S Diagram
The working medium is condensed in a heat exchanger giving saturated liquid at state 4. The isentropic expansion from state 4 to state 5 gives the refrigeration effect, the area under line5- 1.
Comparing figs, we can see that the Carnot vapor compression cycle gives a greater refrigeration effect than the vapor compression cycle. It can be seen that the refrigeration system working on the Carnot vapor compression cycle has the highest COP.
LIMITATIONS OF CARNOT VAPOR COMPRESSION SYSTEMS WITH VAPOR AS REFRIGERANT
Although in theory, the Carnot vapor compression cycle has the highest COP; it is not suited for use in practical refrigeration systems. This is because it is virtually impossible to compress the refrigerant isothermally from state 2 to state 3 in a finite time interval. To offset this difficulty, we can follow the alternate path 1′-3- 4-5. However, this results in other difficulties which are mentioned in detail below:
Dry vs. Wet Compression
If the Carnot vapour cycle follows the path 1-2-3-4, then there is dry compression of the refrigeration vapor since the refrigerant is dry saturated at state 1. This type of compression is desirable in the compressor. But, in this case we see that the refrigerant now has to be compressed isothermally from state 2 to state 3, which is impossible to achieve in practice. The alternate path 1′-3-4-5 involves a wet compression of the vapor from state 1′ to state 3. Wet compression is highly undesirable as the compressor now has to deal with two different fluid phases. Besides, the liquid droplets present in the vapor would now react with the lubricant in the compressor which is highly undesirable. Thus, we see that both the paths of the Carnot vapor cycle are not suitable for use in practical refrigeration systems.
Throttling vs. Isentropic Compression
In the Carnot vapour compression cycle, there is isentropic expansion from state 4 to state 5. This is achieved by the use of a turbine. However, in actual cycles, the expansion from saturated liquid at state 4 to liquid-vapor mixture at state 5 produces very little work. A turbine working under such conditions would have very low efficiency which would not justify the cost involved in using a turbine. Also, the refrigeration system would become very bulky and not suitable for domestic use.
In actual practice, an expansion valve is used to achieve the desired expansion from state 4 to state 5. The refrigerant gets throttled in the expansion valve from saturated liquid to liquid- vapor mixture. The expansion no longer remains isentropic. The expansion now becomes an isenthalpic process.
Thus, we see that the Carnot vapour refrigeration cycle is not suitable for use in refrigeration systems. A better ideal cycle is the vapor compression refrigeration cycle.
