Today we are going to discuss refrigeration cycles!
VIDEO COMING SOON
What is the refrigeration cycle?
The term refrigerate means to cool something down (to remove heat from something). By the second law of thermodynamics (which states that total entropy, or disorder, of an isolated system can never decrease over time), heat travels from areas of high tempreature to low temperature to even out an uneven temperature gradient, and this process happens naturally. But what if we want to do the reverse – move heat from a cold area to a warm one? In that case, we need to use something called a refrigerator (or a heat pump, which we talk about in a different lesson). Yes, this is like the refrigerator in your house! There are other items that fall under this category of “refrigerators,” such as air conditioners too.
We use a refrigeration cycle to move unwanted hot air from a refrigerator and it exits the refrigeration cycle at a different point, in order to keep it cold inside the fridge, using work input into the system. In refrigeration, there is usually a gas or liquid used for cooling (called a refrigerant), such as CFCs, ammonia, and more. It usually has 4 parts to it: Compressor, condenser, expansion device, and evaporator.
When is it used?
Refrigerators have different cycles that are used within them with different liquids inside to transfer the heat from the hot area to the cold area and then loop back into a circle – these liquids or gases running through the refrigerators are called refrigerants. Common examples of refrigerants are the following:
(a) Halocarbons refrigerants: eg. Chlorofluorocarbons (CFCs) such as R11, R12, R113, R114, R115
(b) Azeotropic refrigerants: These are mixtures of two or more refrigerants whose vapour and liquid phases retain identical compositions over a wide range of temperatures. Eg. R-502 (made up of 8.8% R22 and 51.2% R115)
(c) Zeotropic refrigerants: These are mixtures whose composition in liquid phase differs to that in vapour phase/ Zeotropic refrigerants do not boil at constant temperatures unlike azeotropic ones. Eg. R404a (made up of 44% R125, 52% 143a and 4% 134a)
(d) Inorganic refrigerants: eg. Carbon dioxide, ammonia, water, air
(e) Hydrocarbon refrigerants: eg. Ethane (R170), propane (R290), butane (R600), isobutane (R600a)
The objective of a refrigerator is to remove
heat (QL) from the cold medium
Equations
COP: The performance of refrigerators is expressed in terms of the Coefficient Of Performance
Different Types of Refrigeration
There are different types of refrigeration cycles, and all of them work against a temperature gradient by transferring heat from one area to another, in order to keep a designated area cold (by removing the hot air from there). However, there are differences between refrigeration types based on the processes that occur within the cycles – different processes in different cycles can be isothermal (no temperature change over the process), isobaric (no pressure change over the process), isentropic (no entropy change over the process), and more.
(a) Cyclic: Vapor Cycles such as Gas Refrigeration Cycle, Reversed Carnot Cycle, and Gas Cycles such as Vapor Compression Refrigeration, Vapor Absorption Refrigeration Cycle
(b) Non-Cyclic: In this method, refrigeration is accomplished by melting ice or with sublimation of dry ice. Usually these methods are used on a small-scale such as in the lab or in a portable cooler (like when you put ice in a cooler to keep cold drinks!
(c) Thermoelectric Refrigeration: This method electronically pumps out heat using the Peltier effect to create a heat flux between two materials by applying a voltage to the thermoelectric module. One module face ends up heated when the voltage is applied. Also usually used in small scale operations such as in a cooler.
(d) Magnetic Refrigeration: This method works using the principle of the magnetocaloric effect (MCE), where magnetic fields cause metals to heat up. A substance (Eg. helium) is applied to a metal that’s under a magnetic field. The substance carries away the unwanted heat, the metal cools down, and the magnetic field is taken away to make the metal cold.