Which is gas produced in refrigerators



 

A fridge is an electrical or gas-powered device that is integrated in a cabinet. The interior is cooled down from the outside temperature (room temperature) by means of a cooling unit, which is usually located on the rear and is regulated by a thermostat. The typical operating temperature inside is between 2 ° C and 8 ° C. Refrigerators are among the most common household appliances and thus the most prominent electricity consumers in the world. The housing of the refrigerator interior is thermally insulated in order to keep the amount of energy required to maintain the temperature difference between the interior and the ambient temperature low.

It is used for the storage of food, medicines or chemicals, etc. The purpose of a refrigerator is to keep it cool below ambient temperature. In the domestic sector, food can be kept longer than when stored at ambient temperature (bacteria proliferation is restricted). Furthermore, the cooling of beverages is used for refreshment.

History and development of the refrigerator

Cooling was already achieved in ancient times by transporting large amounts of ice from the mountains to the cities and using it to store food in deep cellars (so-called ice cellars). In 1748 William Cullen showed the first artificial cooling at the University of Glasgow. The modern chemical refrigerator was marketed commercially as early as 1834 by Alexander Twinning; his refrigerators cooled with air compression. An important further development was the use of ammonia by Ferdinand Carré in 1859.[1]

The refrigerator was further developed in 1876 by the German engineer and scientist Carl von Linde, the developer of the Linde process, which is fundamental to science and technology. Even its first development was still operated with ammonia at that time. The substance is corrosive, and not only caused leaks, but also a bad smell, so that refrigerators were only suitable for domestic use in the 1920s with the development of various substitute chemicals. In the 1930s it became standard equipment in private households in the USA and Cuba; By 1937, every second US household had a refrigerator.

The first European refrigerator was produced in 1929 by the Zschopauer Motorenwerke J.S., founded by Jørgen Skafte Rasmussen. Rasmussen developed. From the brand DKW cooling went in 1931 German cooling and power machines GmbH in Scharfenstein.

The first refrigerators were operated with methyl chloride, ammonia or sulfur dioxide, which, however, caused problems for the storage of moving parts in the compressor and, in the event of leaks, the escape of toxic gases or deflagrations. With the discovery and use of chlorofluorocarbons, whose ozone-depleting and climate-changing effects were not yet known at the time, these problems could be resolved.

The first CFC-free refrigerator in the world in recent times was developed and produced in 1992 by the Saxon company "dkk Scharfenstein" (later under the name "Foron"). The company was supported by Greenpeace and the Hygiene Institute Dortmund under the direction of Harry Rosin. At the time, other refrigerator manufacturers had no interest in introducing this technology. This device cools with propane and butane, which neither enlarge the ozone hole nor increase the greenhouse effect, but are flammable.

The invention of the refrigeration machine by Carl von Linde made it possible to produce ice industrially all year round, so that natural ice could be dispensed with.

With the spread of electricity and the refrigerator, its predecessor, the currentless refrigerator, became less attractive. From history, the term refrigerator is incorrectly used colloquially for the refrigerator that is in use today.

functionality

 

All refrigerator types are based on the following operating principle: Heat is extracted from the inside of the refrigerator and released to the outside (see refrigeration machine and heat pump). Both are done with heat exchangers

Depending on the way in which this is done, a distinction is made between three types of refrigerators: absorption refrigerators, compressor refrigerators, refrigerators with a Peltier element.

Compressor refrigerator

In a compressor refrigerator, a compressor compresses a gaseous refrigerant, which heats up in the process. In the condenser, which consists of black cooling coils and is attached to the rear of the device, the heat is released into the environment and the medium condenses. The working medium then flows through a throttle, e.g. B. an expansion valve or a capillary tube, then further into the evaporator inside the refrigerator. Here, the evaporating refrigerant takes the necessary heat of evaporation from the interior of the refrigerator and flows on as gas to the external compressor. The function of a compressor refrigerator is almost the same as that of a heat pump, they only differ in the use of the heat exchanger. The new refrigerators work with ammonia using the Linde method.

A detailed functional description can be found in the article of Compression refrigeration machine.

Absorption refrigerator

 

The absorption refrigerator works with a water-ammonia mixture. in the Stove ammonia and water are separated by adding heat (gas flame, electrical heating, solar heat ...). Then the liquid water and the gaseous ammonia are passed on through various pipe systems. The ammonia is in capacitor liquefied. This is where the refrigerator gives off heat. A Evaporator makes it gaseous again. This is where the refrigerator cools. Then the ammonia is im absorber merged with the water. A detailed functional description can be found in the article on the diffusion absorption chiller. Absorption refrigerators are z. B. used in motor vehicles or camping needs. At least when operated electrically, they are less efficient than compressor refrigerators. If they are operated directly with gas, they are almost on a par with the compressor devices thanks to the direct use of primary energy. Since they have no moving parts, they are practically silent. This property gives them a wide range of applications, e.g. B. as a minibar in hotels.

Thermoelectric refrigerator

For years, cool boxes based on the thermoelectric principle (Peltier effect) have been widely available for mobile use. These work directly with 12V DC voltage and are therefore ideal for use in the car. In principle, they also work completely silently, but are usually supported by fans, which generate a certain level of noise. The advantages, however, come at the price of extremely poor efficiency: while a compressor refrigerator consumes around 0.5 watts to transmit one watt of "cooling power", a Peltier element needs over 2 watts for the same energy transport. Using these devices in the household does not make sense in terms of energy.

construction

 

A typical table-top refrigerator (standing model) has a capacity of approx. 150 liters and weighs approx. 40 kg. A fridge / freezer combination has a capacity of around 250 liters and weighs around 65 kg.

There are different room layouts for refrigerators. The best known and most common is the variant with a large outer door and an inner flap to the freezer compartment in the upper area. The freezer compartment usually has a maximum capacity of 20 liters, the refrigerator compartment can take up to 250 liters (possibly larger).

Other variations have separate doors for the fridge and freezer. These are known as fridge / freezer combinations. The compartments can be on top of each other or next to each other; the latter version is particularly popular in the USA and usually has an integrated ice cube maker and, optionally, an additional drinks flap in the large door of the cooling segment. Such a "side-by-side" refrigerator can hold more than 500 liters (refrigerator compartment approx. 350 l, freezer compartment approx. 150 l).

Larger versions, for example, have a double door to the refrigerator area above and a very wide drawer for the freezer segment below.

business

Temperature zones

 

There are different temperature zones in a modern household refrigerator:

  • It is warmest upstairs, where cooked dishes and jams can be stored well.
  • In the middle area, dairy products (yoghurt, cheese) are in good hands.
  • It is coldest at around 2 ° C in the compartment above the vegetable compartments. Perishable things like meat and sausage belong here.
  • The drawers at the bottom are convenient for fruit and vegetables at around 8 ° C. Under the cover, the temperature and humidity are suitable for preserving vitamins and the appearance of the goods.
  • It's relatively warm in the door compartments, but cool enough for butter and eggs. Milk should not be kept in the doorway.

Ambient temperature

For refrigerators with freezer compartments, especially with freezer compartments, the ambient temperature of the installation location specified by the manufacturer, specified as the climate class, must be taken into account:

  • Climate class SN (subnormal): ambient temperatures from +10 ° C to +32 ° C
  • Climate class N (normal): ambient temperatures from +16 ° C to +32 ° C
  • Climate class ST (subtropics): ambient temperatures from +18 ° C to +38 ° C
  • Climate class T (tropics): ambient temperatures from +18 ° C to +43 ° C

While a low ambient temperature at the installation site initially helps to save energy, apparently paradoxically, falling below the minimum temperature leads to thawing in the ice / freezer compartment. This is due to the fact that the freezer compartments are only passively regulated by a thermostat that measures the temperature in the refrigerator compartment. When the outside temperature is low, e.g. B. 8 ° C, the compressor rarely needs to run to z. B. to ensure a temperature of 6 ° C in the refrigerator. This activity of the compressor is not enough to counter the significantly larger difference (heat flow) between the outside temperature and the temperature in the freezer, e.g. B. -18 ° C to work. Apart from that, lubricants in the compressor can become too viscous below the minimum operating temperature.

Refrigerators of climate class SN therefore often have a heater in the vicinity of the thermostat in the cooling room of approx. 8 watts. Sometimes the light bulb is simply not switched off in order to extend the operating temperature from climate class N to SN.

If you want to save energy and set up a refrigerator in an unheated room where permanent temperatures below 10 ° C are to be expected, it is better to opt for fridge / freezer combinations and separate refrigerators and freezers, or even better for a freezer. Recently, more and more small cold rooms and deep-freeze cells have become more and more popular in private households. For the commercial user there are some other cooling devices such as B. Wall cooling shelves (for the presentation of mainly packaged food), free cooling counters, beer cooling, etc.

In 1983, the Rocky Mountain Institute (RMI) built a Sun Frost refrigerator with only 0.19 kWh / l per year, the heat exchanger of which is attached to the outside of the building and generates half of the cooling energy required passively. The RMI considers a development of devices with even lower consumption values, e.g. B. by vacuum insulation layers, for possible. The most economical fridge / freezer combinations achieve consumption values ​​of 0.48 kWh / l per year (at 25 ° ambient temperature) like the Blomberg CT 1300A (no longer available) or 0.34 kWh / l per year (at 21 °) the Sun Frost RF16. A comparable device in energy efficiency class A requires 1.26 kWh / l per year (as of 2006). Pure refrigerators without a freezer compartment are much more economical. These are recommended if a separate freezer is already available. Pure cooling devices of the A ++ class have consumption values ​​below 0.27 kWh / l per year.

Defrost

Modern refrigerators and freezers usually have automatic defrosting. In models from the middle price range, automatic defrosting has been standard for the refrigerator compartment since around the 1980s, while automatic defrosting in the freezer compartment is only common on expensive models. Fridges of earlier years of construction have to be defrosted manually by switching them off for a few hours and letting the ice layer, which has formed on the inside rear wall of frozen condensation water, drain into an extra container or remove it from the interior.

One technique for avoiding this problem is to use a circulating air system inside the refrigerator to ensure that the air is fed to an evaporator - outside the actual refrigerator - where ice then forms. This evaporator in turn defrosts itself regularly and the resulting liquid is caught outside the device in a collecting tray and can evaporate there supported by the compressor waste heat. This means that the air in the refrigerator is dry and hardly any ice can form.

Star designation for freezer compartments

* -6 ° C suitable for short-term storage of frozen food (approx. 1 week)
** -12 ° C suitable for medium-term storage of frozen food (approx. 2 weeks)
*** -18 ° C suitable for storing frozen food
**** below -18 ° C suitable for freezing and long-term storage of food

Related refrigeration equipment

Freezer / chest

Freezers and chest freezers work on the same principle as a refrigerator, but cool with an internal temperature of at least -18 ° C, which enables long-term storage of frozen food. With 4-star freezers, you can also freeze food yourself. These are usually equipped with separate quick freezers (these are usually on top and have a separate flap equipped, while the other freezer compartments mostly just like one drawer are built).

Cooling shelf

In supermarkets, special open cooling devices are used to keep food that needs permanent cooling easily accessible for customers. Since a constant exchange with the warmer ambient air is possible here, unlike with closed refrigeration units, the energy consumption of refrigerated shelves is significantly higher.

Alternative cooling methods

The Nigerian cool pot

The principle of evaporative cooling was refined around the turn of the millennium by the Nigerian teacher Mohammed Bah Abba as follows: Take two similarly shaped clay pots of different sizes and place them inside one another. Fill the gap between the two pots with wet sand. Fill the inner pot with fruit or vegetables. Then put a large wet cloth over the fruit. At high temperatures, water vapor forms on the outer clay pot, which acts as an insulating layer and thus keeps the contents of the inner pot cool. Provided that the sand filling and the cover are well moistened, fruits or vegetables stay fresh for several weeks with this method without any electricity.

Mohammed Bah Abba won the “Rolex Award for Enterprise” with this simple device and was awarded US $ 75,000 by the jury.

Problems

environmental issues

Refrigerators used in households work on the compressor principle. The CFC-containing coolants used there as coolants for a long time are ecologically very questionable, as they have a strong ozone-depleting effect. However, since the CFCs are only released when the refrigerator is scrapped, the refrigerators in question should not be replaced prematurely for this reason alone. Since the mid-1990s, other coolants, such as butane or R134a, have mainly been used in newer cooling devices. Refilling refrigerators or air conditioning systems with coolants originally containing CFC is prohibited, or only permitted with suitable CFC-free replacement coolants. Older, already installed industrial cooling systems are exempt from this regulation.

In winter, the operation of refrigerators is particularly uneconomical in most areas, since the device is often located in heated rooms (e.g. kitchen) and because it has to cool down from this heated ambient temperature. In the past, this was avoided by keeping the goods to be stored cool in a cupboard with a connection to the outside world during the relevant season.

Health risks

After an investigation by Jean-Pierre Hugot from the Parisian Hôpital Robert Debré it could be that the climatic conditions inside a refrigerator favor the spread of certain cold-loving microbes such as Yersinia and Listeria. These microorganisms may cause Crohn's disease (a disease of the digestive system) [2]. However, the consumption of spoiled food because of not using refrigeration is likely to be associated with greater disease risks.

A disadvantageous consequence of using antibacterial silver coatings in refrigerators is the transfer of silver particles into food.

Repairs

Components such as the compressor, the starter of the compressor and the thermostat are exposed to higher stresses. While the replacement of a thermostat can be carried out by almost any electrical company, if the compressor is damaged, you usually have to contact the repair service of the device manufacturer or a specialist company for refrigeration technology. Replacing a compressor can be so expensive that it can make more sense to replace the entire cooling device, since not only the compressor but also the entire filling with refrigerant has to be replaced. The starter is exposed to high currents and high temperature differences. In the case of refrigerators and freezers designed to be customer-friendly and environmentally friendly, the starter can be replaced separately from the compressor.

turn on

If a compressor refrigerator is switched off, the compressor cannot restart immediately against the pressure that is still present in the condenser. Only after some time (1–2 minutes) does the pressure equalize itself again through the throttle and the condenser and restart is possible again. The control built into the refrigerator automatically observes this waiting time. However, if the plug is pulled during operation, it should only be plugged in again after a few minutes in order not to unnecessarily overload the compressor drive. If the refrigerator is plugged in again immediately, a waiting time is caused by a (self-resetting) motor protection switch after unsuccessful start attempts.

transport

If a compressor refrigerator designed for standing operation has been transported horizontally for a long time, lubricant may have shifted from the compressor into the cooling circuit. In this case, the refrigerator should be left in its normal position for several hours before it is used again. This gives the lubricant enough time to flow back into the compressor.

See also

  • thermodynamics
  • Conduction
  • Heat pump
  • Chiller
  • Refrigerator poetry

Individual evidence

  1. Britannica 2004
  2. Jean-Pierre Hugot, Corinne Alberti, Dominique Berrebi, Edouard Bingen, Jean-Pierre Cézard. Crohn's disease: the cold chain hypothesis. Lancet 2003; 362: 2012-15

Category: Cooling technology