What is refrigerant
How chillers and heat pumps work
Regardless of whether at home in your own refrigerator or in a complex, industrial system: In many cases, the generation of cold is always subject to the same basic principle, in which a thermodynamic cycle process extracts cold from a room or environment.
The evaporation of a liquid refrigerant such as water or liquefied under pressure such as ammonia, propane and fluorinated refrigerants (HFCs) requires the supply of heat from the environment. This cools the environment down. Chillers use this effect. They extract heat from a cooling medium, i.e. cool it down. This means that no “cold” is added to the medium to be cooled, but heat is withdrawn.
So that the evaporating substance (the refrigerant) does not have to be continuously replaced, a small amount of refrigerant is repeatedly evaporated in a circulation system (pipeline) of a refrigeration machine and then liquefied:
- A compressor driven by an electric motor sucks in refrigerant vapor from an evaporator through which a medium to be cooled flows and compresses it, which significantly increases the pressure and temperature.
- The refrigerant then flows through a condenser, through which cold air or cold water flows on the outside, thereby cooling the refrigerant, which is still in vapor form, and finally liquefies it.
- The liquid refrigerant then passes through an expansion valve into the evaporator, where it evaporates while absorbing heat from the medium to be cooled and is sucked in again by the compressor.
- Direct systems are used when the evaporator is at the place to be cooled, e.g. B. in a cold room, a refrigerated shelf in a supermarket or a room to be air-conditioned; the coolant that supplies heat to the evaporator is usually air that is transported by means of a fan. In direct systems, the refrigerant lines are usually long and therefore have a large refrigerant content, have a small pipe cross-section, but do not have to be thermally insulated. Non-flammable refrigerants are used as refrigerants.
- Indirect systems are used when the evaporator absorbs heat from a liquid coolant (or cooling brine), which is pumped to a cooling point (and from there back again) by means of a circulation pump. The cooling point is then a heat exchanger through which the coolant flows and to which, in turn, heat is supplied with air. The coolant lines have a large diameter and must be thermally insulated. In indirect systems, the evaporator is located near the compressor, which means that the refrigerant lines are short and the refrigerant charges are low. The probability of leakage and the possible amount of leakage are therefore low. On the one hand, non-flammable and on the other hand flammable and toxic substances can be used as refrigerants; the latter then require additional safety technology.
- The condenser can also be operated directly and indirectly.
If the cold side (of the evaporator) is used technically, the machine is referred to as a refrigeration machine, if the warm side (of the condenser) is used as a heat pump. With simultaneous or staggered use (by means of hot water or ice storage) of heat and “cold”, such a “hybrid” machine works particularly energy-efficiently.
In many application cases, heat supply by burning fossil fuels can be dispensed with (e.g. for space heating or hot water preparation), or cooling operation with stored "cold" can be maintained without the refrigerant compressors having to run. So z. B. photovoltaic or wind power generated energy can be stored wear-free and used with a time delay.
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