The refrigerator is a common item in many households around the world today. Its invention arose out of the need to preserve and store for later use what we cannot eat now. Lacking one means the food bought has to be eaten within two to three days of purchase otherwise the food will decompose. Refrigerators also have other uses like preservation of caught fish. This is employed extensively by large fishing trawlers that are out at sea for months or even big trucks that transport frozen fish. Other uses include preservation of bodies in the morgue and storage of some volatile chemicals just to name a few. There are the big industrial freezers that are of bigger volume and can maintain much lower temperatures than the regular refrigerators in households and also the small mini refrigerators that have a wide variety of uses because of their portability (Cowan: p. 10).
Regardless of the size and volume, all of them employ more or less the same basic principle of preservation whereby heat is drawn from the food or sample being preserved and dissipated somewhere else so as to maintain temperatures close to zero degrees in the compartment. If you are looking to maintain sub-zero temperatures, a freezer is much more handy though most (rather all) of the refrigerators manufactured nowadays also contain a freezer.
History
Before the invention of the refrigerator, most families would rely on ice boxes to preserve various foods like raw meat but their limited usage due to melting of ice was its major disadvantage. Furthermore, the ice that was used for preservation had to be mechanically cut from winter ponds and stored at a central place until needed. This entire process made the ice box to be quite expensive for most families. For the families that could not afford the icebox, the next best option was the root cellar which is really burying your food in the basement of the house which was a bit cooler. Obviously this had some limitations since attaining temperatures close to zero degrees would have been difficult when the winter season has passed.(Shepard: p. 103).
The first practical refrigerator was constructed by Jacob Perkins in 1834 and it used ether as a preservative whereby the ether will vaporize as it draws heat and it will be compressed back into liquid phase again so that the cycle continues. Later on, refrigerators used toxic gases like ammonia, methyl chloride and sulfur dioxides as refrigerants. These gases were very toxic and several deadly accidents were reported in the 1920s when methyl chloride occasionally leaked on food samples. An alternative refrigerant had to be discovered if the new invention was going to succeed because the prices alone were discouraging consumers from purchasing them.
In 1928, Thomas Midgely Jr together with Charles Franklin Kettering invented something of a miracle compound called Freon whose basic structure was made up several chlorofluorocarbons. Freon had a high volatility which is an important characteristic for a refrigerant and it was also non-toxic, which eliminated the danger of poisoning due to leakages. It is only decades later that people are realizing how hazardous Freon is to the ozone layer. Later on, improved innovations like automatic defrosting and automatic ice making increased the appeal of the refrigerator. There is a distinct difference between refrigerators and mini refrigerators.
The Mini-refrigerator
There is a common misconception that mini refrigerators are only made to be portable as compared to the standard refrigerator. A mini refrigerator does look and perform similar functions as your regular refrigerator but it is much smaller in height and volume so as to save on space. They are mostly used by students in dorms and hostels, people going camping but this requires you find an alternative power supply and also some cars are fitted with this apparatus usually on request from the owner. They are usually used in the preservation of basic foods like fruits and beverages which only require standard refrigeration temperatures. Others are more sophisticated and can contain ice trays which are used for freezing ice and storing limited amounts of frozen food like fresh meat.
Advantages of a Mini-refrigerator
There are several reasons why a person may decide to purchase a mini refrigerator. They are cheaper than your standard refrigerator and this is convenient for someone living alone or starting out on life. Since they are smaller, they are obviously, lighter and much easier to carry if you are going camping or down to the beach to enjoy a few cold drinks. The limited amount of items means they consume less power than your standard refrigerator hence you save on your electricity bill. The only major disadvantage is the limited volume it has inside and some foods that require complete refrigeration cannot be stored inside because of the temperature limitations. The ones that are capable of cooling to more extreme temperatures are usually more expensive.
As mentioned earlier, the mechanisms used in the preservation of foods in most refrigerators are more or less similar, the only difference being mini refrigerators use lighter and much smaller materials so as to save on weight.
How does it Work?
Without going into much detail of the electrical circuits and various components that are crucial for the functioning of refrigerator, we will focus on the major components whose sole purpose is to maintain minimum temperatures essential for the preservation of food. These components are the compressor pump, a fan which gives the distinct humming sound when the refrigerator is powered, the condenser coils or tubes which are seen on the back of the refrigerator and the cabinet which is made from heat resistant materials so as to prevent the cool air inside the refrigerator from escaping outside. With new models being introduced into the market, these basic components have been slightly improved but their core functions still remain the same.
Most household refrigerators and mini refrigerators work on the principle of a vapor compression cycle, which in Layman’s language is a cycle of condensation and vaporization. Just think of it as a reverse heat engine whereby a work input is converted into a heat output. For the case of a refrigerator, the work input is provided by electricity which condenses the refrigerant, which is Freon vapor in this case, and pushes it into the heat exchanger coils or the condenser tubes. In the coils the temperatures of the Freon vapor drops as it liquefies, releasing the heat to the environment. Looking at this cycle in much detail, the refrigerant being used (Freon now in vapor form) enters the compressor pump at a lower vapor pressure than the standard atmospheric pressure and its temperature is also slightly above room temperature. The increase in temperature is due to the heat drawn from the food being preserved. The vapor is further compressed by the compressor pump whereby it leaves at a higher pressure than before and the vapor is superheated because of the increase in pressure. This can be attributed to the Gas Laws whereby an increase in pressure results in an increase in temperature because the two are directly proportional. (Marcus: p. 29).
Now the superheated vapor which is also under intense pressure travels through the winding coils and tubes which constitute the condenser where it is passively cooled by exposure to the room air from which the superheated vapor dissipates as heat. The condenser coils or tubes are painted black so as to dissipate heat faster because according to Kirchoff’s Radiation Law, objects that are good emitters are good absorbers. A blackened surface is an excellent emitter and will in turn act as an excellent absorber. For the case of coils, you want the heat contained in the vapor to be expelled as fast as possible and also the slightly cooler air from the surrounding room to enter the coils and cool the vapor. The main reason the tubes are also coiled is to increase their surface area and contact with the circulating air in the room. The heat is dissipated under the principle of conduction. For this to work, the surrounding environment (the air in the room) must be cooler than the temperature inside the condenser tubes. A cooler environment provides less strain to the functions of the refrigerator and less power is consumed from the source.
The removal of superheat from the Freon Vapor takes place in a series of steps whereby with each passing distance, the vapor is cooled to a much lower temperature than the previous step. It is worth noting that the vapor is still under intense pressure and the continued loss of heat with the effect of high pressure causes the vapor to undergo condensation and change phase into a hot liquid. As the refrigerant exits the condenser, the entire heat content has not been removed and it is slightly warmer than room temperature. The warm liquid refrigerant is being pushed through the system via its pressure through a device known as an expansion valve, beyond which its pressure abruptly decreases. The drop in pressure is attributed to increase in volume which according to Dalton’s Law of Gases, should decrease when the volume of a vessel increases. (Dyson: p. 52).
When the condensed warm liquid passes through the expansion valve, it undergoes an abrupt reduction of pressure which results in flash evaporation of the refrigerant. Flash evaporation is a phenomenon which occurs when heat energy is drawn from the warm liquid and this energy is used to explosively vaporize a section of the liquid (approximately half of the total volume). The end result of this process is the temperature of the partially evaporated liquid refrigerant abruptly drops and it is lower than the enclosed compartments of the refrigerator. This whole process is referred to as auto-refrigeration. The cold and partially vaporized Freon continues flowing through the coils of the evaporator unit. A fan now blows air from the freezer compartment or box air of the refrigerator over the coils or tubes. This air now represents the heat contained in the various food substances being preserved in the refrigerator.
The extraction of heat occurs slowly and the process is more efficient if warmer air from outside is not allowed to flow into the compartments The refrigerant draws in these cool air (rather slightly warm air) being blown by the fan and it further changes state from a partial mixture of liquid and vapor to a complete vapor phase. While most liquids like water need to be supplied with heat energy up to a temperature of a hundred degrees for it change state to vapor, the refrigerant has a characteristic of being able to vaporize at low temperatures due to its extremely low boiling point. This shows how the discovery of Freon and other refrigerants was such a breakthrough in the above process because without their specific characteristics, the extraction of heat from the preserved foods would have to occur at much higher temperatures and that is impossible.
The re-cooled air from the evaporator now returns back to the refrigerator compartments and envelopes the foods as slightly warmer air is blown by the fan back to the evaporator for the process to continue. It is worth noting that the cool air in the refrigerator or freezer is really “hot” when you compare it to the temperature of the refrigerant inside the evaporator. Otherwise, the complete vaporization of the Freon will not occur. The refrigerant exiting the evaporator is now fully vaporized and contains the heat absorbed from the food samples. It returns to the compressor pump where it is compressed again to a higher pressure and superheated vapor; and the cycle begins again.
The purpose of a thermostat in a refrigerator is to set the temperature at which the compartments will be maintained. It works by altering the temperature at which the refrigerant will be completely converted to vapor. If say the temperature is set at -4 degrees Celsius the cool air being blown by the fan will be used to completely vaporize the refrigerant in the evaporator up to a temperature of -4 degrees Celsius. Once this temperature is attained, no further vaporization takes place and the refrigerant might still be in the partial liquid-vapor phase. The fan no longer blows cool air and the state of equilibrium is maintained. This is observed when you open the door of the refrigerator and the fan immediately starts humming since warm air has entered from outside and has raised the temperature.
The refrigerator discussed above is a compressor refrigerator because of the use of a compressor pump to generate the high pressure and superheated vapor. Other types like the absorption refrigerator employ a source of heat for example combustion of LPG or an electric heating element to generate the compressor-like conditions. Due to the absence of a compressor pump, refrigerators using heat sources are much quieter; with the fan being the only moving part. Absorption refrigerators are usually used in office buildings where they are used to chill a concentrated solution of Nacl, or Brine that is circulated throughout the building. (Marcus: p. 30).
Power Sources of Mini-refrigerators
Some mini refrigerators operate on alternating current or AC which is the power source you get from a socket at home. Others use direct current or DC. The DC power sources are usually obtained from rechargeable battery packs like the lead acid accumulator or the regular car battery. Most of them are charged using the AC power supply but their output is only DC. For the mini refrigerator, the only setting that can be changed is whether the AC power source is 120V like in the US, or 240V like in Great Britain. This is visible at the back of the device where a switch with the two power settings is visible. However, if you bought it at your local dealer, the settings have already been made and there is no need to alter anything; just plug in and use it. The AC mini-refrigerator characterizes your average refrigerator whereby usage is dependent on the availability of power in the house. As long it is plugged in, it can be continuously used throughout the day and food preservation is assured. If it is connected to a DC power source it won’t function since its electrical circuits only work with alternating current. (Dyson: p. 59).
The DC mini refrigerator usually obtains its power from a battery source or a generator with a rectifying circuit (to convert AC to DC) since there aren’t any households around the world being supplied with DC power by their power companies. The DC mini-refrigerator has limited usage which is dependent on the duration of time power is being provided by a battery or generator. Power conservation techniques are necessary like avoiding preserving non-perishable foods like sodas so as to conserve energy. Furthermore, they also have limited features like absence of a freezer or automatic defrosting so as to conserve energy. Most of the portable mini refrigerators fall in this category and they are smaller than their AC counterparts. The ones found in automobiles are also DC powered since they are using power from the car batteries. Some of them are as small as a microwave and are quite easy to carry if you are going to the beach or camping out.
The mini refrigerators running on both DC and AC are the more recent ones and are slightly more expensive. They are quite convenient since you can just plug in at home and use them or connect it to a battery pack when out camping. Since they are an improvement of the previous AC and DC exclusive mini refrigerators, they have much more advanced features, are more power efficient through use of better insulation materials and are also lighter because of the extensive usage of plastic composite materials. The way their power input ports are designed, they can automatically detect if the source is AC or DC. Most household appliances that are connected to AC power supply have a rectifying circuit consisting of diodes and choke stabilizers that is used to convert AC to DC. This same principle applies to the mini-refrigerator when it is connected to an AC source.
When connected to a DC source the already existing circuit is designed to use DC power and it simply bypasses the rectification stage. It is important to note that though conversion from AC to DC is possible through an inverter circuit, it is not preferred because of the loss of power through excessive heating. The use of DC power is preferred because of its stability and lack of fluctuations. Meanwhile, power is transported in AC to minimize power losses over long distances and conversion of AC to DC back at the power station is a cumbersome affair.
Materials Used
Most mini refrigerators have their outside cabinets made from aluminum alloys which are lighter than steel and provides a similar amount of protection. It is also very resistant to corrosion. The newer models hitting the market are mostly made of plastic composites which are much lighter than aluminum alloys and are also easily recycled hence conserving the environment. The inside of the cabinet is usually made from vacuum formed plastic to prevent the heat from outside (normal room temperature) from reaching the refrigerator compartments. The foam also acts as a shock absorber when the device is knocked. This prevents damage by absorbing the shock waves that would otherwise reverberate to the fan and the electrical connections.
On the inside of the vacuum formed plastic, there is a layer of fiberglass or polyfoam Polyfoam is usually preferred because of its thermal properties with regards to minimizing heat exchange between the refrigerator compartments and the environment. Also, it acts as a shock absorber and improves stability of the entire structure. However, the major reason for using polyfoam and vacuum formed plastic on the inside of the cabinet is to prevent the loss of cool air from the mini refrigerator. The two materials create a partial vacuum between the outside and the inside of the refrigerator. This vacuum prevents any movement of heat or cold air in either direction through minimization of conduction and convection means of heat transmission. Just think of it working with on the same principle as a thermos flask which keeps your coffee warm for a limited amount of time. It is not a perfect design like the ones found in morgues but it achieves its goal.
The compressor pump and condenser coils are mostly made of copper and its alloys. Copper is the most practical conductor around for both heat and electricity. Silver is the best conductor known but you can imagine how mush a refrigerator will cost if it is made using silver. Copper improves the dissipation of heat by the coils and painting them black further improves their performance. It is also useful in the functioning of the compressor because it is light and can be used to make movable parts. Its ability to withstand high temperatures makes it durable and reduces the instances of wear and tear. If the various components do break down, soldering copper is not a tedious process and due to the availability of the metal, finding spare parts is an easy task.
Freon is the most common refrigerant. With its low boiling point and less toxic tendencies when compared to ammonia and sulfur dioxide, it revolutionized the cooling process in the earlier refrigerators and made this device common in every household. However, it has been discovered that even though it is harmless to humans, it is a very toxic substance when it comes into contact with the ozone layer. The continued depletion of the ozone layer has been blamed on CFCs and Freon contains some CFC elements. Tetraflouroethane is another refrigerant that has gained popularity and it is preferred over Freon because it is not harmful to the ozone layer. It is a chemically inert gas with a boiling point of -23 degrees Celsius. However, even tetrafluoroethane is facing similar environmental challenges as Freon due to its contribution to climate change. (Wilson: p. 15).
References
Barbara Krasner-Khiat, (2003). “The Impact of Refrigeration,” History Magazine. Web.
Cowan, Ruth Schwartz, 1983, “More Work for Mother: The Ironies of Household Technology From the Open Hearth to the Microwave.” New York: Basic Books, pp. 10-15.
Dyson, James, 2001, “A History of Great Inventions.” New York: Carol & Graf Publishers101. Pp. 52-63.
Marcus, Alan I., and Howard P. Segal, 1999, “Technology in America”. 2nd Ed. Fort Worth: Harcart Brace & Company, pp. 29-32.
Shepard, Sue, 2001 “Pickled, Potted, and Canned: How the Art and Science of Food Preserving Changed the World” New York: Simon & Schuster, pp. 101-110.
Wilson, C. Anne, 1999, “Waste Not Want: Food Preservation From Early Times to Present Day” Edinburgh: Edinburgh University Press, pp. 12-20.