What kind of fluid do air conditioners have
Lexicon> Letter K> Refrigerants
Definition: a medium that can extract heat from a cold medium in a cooling machine or heat pump
Categories: Basic Terms, Warmth and Cold
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 01.11.2012; last change: 17.03.2020
A cooling machine (e.g. as part of an air conditioning system) or a heat pump plays a role Refrigerant (or Working gas) plays an essential role. It arrives in liquid form in an evaporator, where it evaporates at low pressure (usually by pumping out with a compressor) and thereby absorbs latent heat. The evaporator is designed as a heat exchanger so that heat can be absorbed from the environment (e.g. a liquid or air). The refrigerant, which is now gaseous, is then compressed, making it liquid and releasing heat again. It is somewhat different in an absorption refrigeration machine or absorption heat pump: Here the refrigerant is absorbed by a solvent at moderate pressure and thus becomes part of this liquid.A coolant is not the same as a refrigerant!
Occasionally, refrigerants are mistakenly called Coolant designated. This is not the same: a coolant can only transport cold (or dissipate heat), while a refrigerant can generation of cold serves. A system often requires both a refrigerant and a coolant. For example, in a split air conditioner, a refrigerant such as R-410A is used as the refrigerant, while the unit uses air as the refrigerant for the condenser.
Important properties of refrigerants
Refrigerants must have a number of properties. In relation to their function, their thermodynamic properties are important:
- At all temperatures occurring in the evaporator during operation, the refrigerant should be able to evaporate sufficiently, if possible without the pressure having to be reduced too much. At the same time, the condensation in the condenser (liquefier) should be possible at the intended temperatures with not too high a pressure in order to limit the technical effort.
- A high volumetric heating output is favorable, so that the required volume flow is low and the device can often also be built more compactly.
- Under no circumstances should the refrigerant freeze.
- The enthalpy of evaporation (heat of evaporation) should be as high as possible so that a given cooling capacity can be achieved with the smallest possible volume flow of the refrigerant.
- The critical temperature must be high enough because the desired phase transition no longer occurs above it.
- The temperature glide should be absent or slight, i.e. H. the temperature remains almost constant during evaporation. This is often not the case with mixtures of substances with different boiling points; But there are also “azeotropic” mixtures that also avoid segregation during boiling and thus behave in a similar way to single-component refrigerants.
In addition, there are a number of other properties that can be important for practical use:
- The viscosity (viscosity) of the liquid should be as low as possible so that the liquid can flow easily through refrigerant lines.
- Good compatibility with typically used materials for lines, compressors, lubricating oils etc. is also desirable. For example, it is unfavorable if a refrigerant causes corrosion in metals.
- Ideally, refrigerants are non-flammable or even explosive so that such accidents can be avoided.
- Refrigerants are ideally not harmful to health (e.g. poisonous or corrosive) and ecologically harmless. So they have little or no greenhouse effect and no ozone depletion potential.
- If a product is at least easily perceptible through its odor before dangerously high concentrations are created in the breath, the dangers of explosion or poisoning are at least significantly reduced.
Unfortunately, it is difficult to achieve all of these properties at the same time. Therefore, compromises are often necessary in practice. For example, in some applications it is accepted that a refrigerant such as R-717 = ammonia is toxic and highly corrosive if the necessary safety precautions are feasible and significant advantages are achieved through good properties of a different kind (e.g. high heat of vaporization). In other cases, the flammability of a substance is accepted (e.g. R-290 = propane) if this eliminates the harmful effects on the climate that other agents (such as fluorocarbons) would have; however, the risk of explosion, for example for compact room air conditioners, must be avoided, so that propane cannot be used here.
Because of such problems, it is not always easy to replace climate-damaging refrigerants. In some cases it fails for the time being because major adjustments to the devices (e.g. with regard to the pressure generated by the compressor) are necessary; So you would have to specifically for environmentally friendly refrigerants (e.g. CO2) use constructed devices.
The severity of certain disadvantages or how important certain advantages of a refrigerant are can depend to a large extent on the respective application; This is one of the reasons why a wide range of very different refrigerants (see the next section) are in use.
Very different natural and synthetic, organic and inorganic substances are used as refrigerants:
- In the past, chlorofluorocarbons (CFCs) were mainly used for cooling units and heat pumps. However, these were later banned because of their ozone-depleting properties. Initially, under the Montreal Protocol of 1987, this only affected fully halogenated CFCs such as R-12 and R-114; From 1994 onwards, partially halogenated CFCs such as R-22 and fluorocarbons (HFCs, e.g. R-134a = tetrafluoroethane), which have a significantly lower ozone depletion potential, were also restricted in their use. Today in Germany only refrigerants without ozone destruction potential are allowed for new systems. Unfortunately some of them (especially fluorinated hydrocarbons = PFCs) are because of their compared to CO2 by far greater greenhouse effect, significantly damaging the climate. For example, R-410A, which is now used in stationary air conditioning systems, is not ozone-damaging, but very damaging to the climate with a GWP value of 1725; Great care should therefore be taken to ensure that this gas never escapes into the environment. (R-12 and R-23 with GWP values above 10,000 are even worse.) The climate-damaging R-134a (with GWP = 1430) was banned for car air-conditioning systems from 2011 because it is more likely to leak there than for stationary ones Investments. R-1234yf (tetrafluoropropene, GWP only approx. 4) was initially favored as a substitute. However, this substance came under criticism because it is flammable, can also self-ignite on hot parts in the engine compartment and, when burned, releases very toxic and caustic substances, in particular hydrogen fluoride (HF) and the likewise very toxic gas carbonyl fluoride. In the event of an accident, the vehicle occupants are therefore at considerable risk. In addition, the Swiss Empa has found that R-1234yf breaks down in the atmosphere to form trifluoroacetic acid, which is a plant poison and is only broken down very slowly.
- Non-halogenated (chlorinated or fluorinated) hydrocarbons such as propane, propene and butane (→Liquefied petroleum gas) avoid the problems mentioned: They are not ozone-damaging, hardly harmful to the climate and also non-toxic. However, they are flammable, and if they get into the air in significant concentrations, there is a risk of explosion. Accordingly, more or less complex technical measures are required in order to use such substances safely. In some cases, however, isobutane (R-600a) can be used as a climate-friendly alternative e.g. B. apply to R-134a.
- Carbon dioxide (CO2, R-744) is in principle harmful to the climate, but far less than the above-mentioned CFCs and PFCs, and the quantities required in refrigeration units and heat pumps are relatively small. (More problematic are the CO2Emissions from power generation for operation.) In addition, such CO2 used, which would otherwise get into the atmosphere anyway. CO2 is also relatively safe in terms of safety, as it is non-flammable, and is available very inexpensively. A disadvantage, however, is that it can only be liquefied at a fairly high pressure, so that CO2-based aggregates are correspondingly more complex. Otherwise, the refrigeration properties are quite favorable; In particular, you only need the circulation of relatively small amounts of this refrigerant, since its volumetric refrigeration capacity is very high, and the devices can be built very compact.
- Ammonia (R-717) has long been used as a refrigerant both in industrial systems and in small systems (including absorption heat pumps). It is not harmful to the climate, but quite toxic, so problematic z. B. in the event of leaks. In connection with water it also attacks various metals such as B. copper. For these reasons, ammonia has largely been replaced by halogenated hydrocarbons, especially in small systems, despite their harmful effects on the climate.
- Water (R-718) can also be used as a refrigerant, but only for temperatures above 0 ° C (also in the evaporator). Since it has a very low pressure in gaseous form, correspondingly large volumes have to be conveyed. For these reasons, water is rarely used as a refrigerant.
Mixtures of refrigerants are often used instead of single-component refrigerants. For example, the R-410A commonly used in air conditioning systems today is a mixture of CH2F.2 and CHF2CF3.
Desired ban on fluorocarbons
There are efforts to reduce the use of climate-damaging fluorocarbons (HFC) z. B. in air conditioners and refrigerators to be banned as soon as possible. Although these refrigerants could theoretically be prevented from entering the atmosphere, unfortunately this often does not succeed (e.g. due to improper disposal or unwanted refrigerant leaks) - especially in developing countries.
For these reasons, an extension of the Montreal Protocol was decided in 2016, which initially only concerned chlorofluorocarbons (CFCs), but now also affected fluorocarbons (PFCs). This progress should ensure that the global warming that will occur in the coming decades will not increase by roughly half a degree.Discharge of refrigerant from a refrigeration machine into the environment is punishable today with high penalties!
In principle, refrigerants may only be handled by appropriately trained persons (with detailed knowledge of handling regulations, occupational safety, etc.) and they may never be released into the environment without an emergency. In the past it happened relatively often that when repairing a refrigeration machine, the refrigerant was simply released into the air and then new refrigerant was added again. Today, refrigerants must be extracted as much as possible with suitable devices and, if necessary, safely disposed of; otherwise there is a risk of high fines or even imprisonment. Corresponding measures must be precisely documented. Nevertheless, according to the Federal Environment Agency, several hundred tons of climate-damaging refrigerants are released into the atmosphere every year through the use of stationary air conditioning systems, which is CO2-Equivalents of several hundred thousand tons per year. In terms of magnitude, however, this corresponds to only one per thousand of the total CO2Emissions. Without the strict legal regulations (with severe threats of punishment) this would probably be significantly different.
The EU Ecodesign Directive defines, among other things. Minimum requirements for the Energy Efficiency Ratio (EER) of air conditioning units. In some cases, somewhat less strict limit values apply here for devices with refrigerants whose climate damage is relatively low (GWP below 150).
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See also: chiller, heat pump, compression heat pump, absorption heat pump, fluorocarbons, ammonia, cold, latent heat
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