Engineering in the Kitchen: Refrigeration and Thermal Management
In the 11th century a Persian physicist, Ibn Sina, invented a coil which condensed aromatic vapors so that he could produce essential oils desired at the time, thus creating the first form of managed refrigeration. The first refrigerator to utilize this technology was not designed until 1748 by William Cullen at the University of Glasgow. However, in 1805, Oliver Evans, the “father of refrigeration” invented a vapor-compression system which removed heat from the system by recycling vaporized refrigerant. While many different designs circulated over the next hundred years, it wasn’t until the mid-1920s that Freon replaced ammonia as the refrigerant and made refrigerators a more effective product. Freon allowed refrigerators incredibly efficient, but it was banned due to numerous environmental issues in the 1980s. To replace Freon, which is at Chloro-Flouro-Carbon (CFC), modern refrigerators now use tetrafluoroethane (HFC). Current refrigerators are divided into four main food storage zones, the freezer at 0oF, the meats zone at 32oF, the main refrigeration zone at 41oF and the vegetables zone at 50oF. The process which keeps a refrigerator cold is known simply as the vapor-compression refrigeration cycle.
The Vapor-Compression Refrigeration Cycle
There are four main phases within the vapor-compression cycle that the refrigerant substance must undergo. The first phase begins when a slightly superheated vapor enters the compressor at a low pressure. As the name would suggest, the compressor increases the pressure that the vapor is under. In most home refrigerators the compressor is at the rear of the unit and is hermetically sealed. This means that a motor and the compressor are mounted in a sealed housing and the electric leads pass through this housing. This is mainly done to prevent refrigerant leakage. The second phase in the vapor-compression refrigeration cycle is known as the condenser. During this phase the vapor refrigerant is condensed through heat transfer to cool water in the surrounding system. The condenser is also located at the back of the refrigerator and is placed so that the ambient air flows pass it via free convection. From there the refrigerant, which is now a high pressure liquid, enters the third phase of the cycle known as the expansion valve. As the liquid enters the expansion valve the pressure is decreased causing some of the liquid to immediately flash to vapor. If you have ever looked at the back of a refrigerator the long winding tube is where you will find the expansion valve. The final phase takes place in the evaporator where the low pressure liquid that remains from the previous phase is vaporized as a result of the heat transfer from the refrigerated space. The evaporator is normally found in the walls of the freezing compartment in the refrigerator. This process works in the same way that sweating cools down a human. As a liquid changes phases to a gas and evaporates it takes a certain amount of energy with it, hence the cooling affect.
Alternatives to Vapor-Compression Refrigeration
There are currently 5 alternatives to the standard vapor-compression system that range from prototypes to commercial development. These alternatives are Magnetic, Thermionic, Thermoacoustic, Thermoelectric and Thermotunneling. Magnetic cooling utilizes an alternating magnetic field to take advantage of the magnetocaloric effect. Applying a magnetic field to a material will heat it under adiabatic conditions. When the magnetic field is removed, the material is cooled below its initial temperature. Thermodynamic modeling suggests that this cooling cycle could have up to a 25% efficiency advantage over vapor-compression systems. However the design for this system is still in the prototyping phase. Thermionics emission was first discovered by Edison during experiments with the light bulb, but it was not used for cooling until 1994. Thermionic cooling works by applying a voltage across two materials with a gap thickness less than an electron mean free path. High energy electrons will leave one surface and be replaced by average energy electrons which lower the temperature. Thermodynamics models have revealed that the potential efficiency is fairly high but not higher than current vapor-compression systems. However theoretical and experimental studies have revealed that thermionic cooling has the capability of higher heat fluxes than other methods so they may be useful in microelectronics. Thermoacoustic cooling utilizes pressure waves to expand and compress a gas inside of a resonator. This in turn also causes displacement of the gas. If the timing between the pressure and the displacement phases line up correctly then the gas can serve as a heat pump. While thermoacoustic cooling is low cost, reliable, environmentally friendly and easily controllable it is less efficient than current vapor compression models and lacks the power density seen in other alternatives. The fourth alternative to vapor-compression cooling is known as thermoelectric cooling. Applying a DC current to a charge carrier provides the necessary work that allows heat to flow from one junction of two dissimilar conductors to the other junction. Of these alternatives, thermoelectric cooling is the only model currently commercially available. However, due to its inefficiency and high cost it is only suitable for certain niche markets. The final alternative to vapor-compression cooling is known as thermotunneling cooling. Thermotunneling is a process where the transfer of relatively high energy (meaning warm) electrons is made possible by gaps that span just nanometers across. Applying an external voltage to one surface causes all of the electrons to move in the same direction, resulting in a cooled surface. While this concept is estimated to have about the same efficiency as vapor-compression there are many challenges involved in design.
While refrigerators have evolved greatly since the 11th century, the same basic concept exists: using vapor compression to cool a space.