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Buildings 2020+. EnErgy sourcEs chemical and thermodynamic fluids characteristics. The essential requirement of the combination of the absorbent/refrigerant is that they must be mixed in the liquid phase during the cycle temperature range. The following requirements apply to the mixture: • the difference in boiling temperature must be as high as possible; • the refrigerant should have high latent heat of vaporization and good absorption in absorbent material in order to keep low circulation range between the generator and the absorber; • favourable viscosity, thermal conductivity and diffusion coefficients; • refrigerants and absorbents must be cheap, non-corrosive, environmentally friendly. The most commonly used liquids are water/NH3 and LiBr/water. The water/NH3 fluid is widely used for both cooling and heating purposes. Both NH3, refrigerant and water, absorbent are very stable for a large range of temperature and pressure. NH3 has a high latency evaporation temperature, which is essential for the effective operation of the system. The freezing point of NH3 is -77°C, therefore it can be used for low temperature applications. There are several disadvantages such as high pressure, toxicity and corrosion of copper and copper alloys. The NH3 and water solution are volatile during the process, so it is necessary to reactivate the solution. Without the return of NH3, water will accumulate in the evaporator and degrade the system performance (Althouse, 2013). The other most commonly used liquid for the absorption cooling and heating purposes is LiBr/water. Two extraordinary LiBr/water properties are: LiBr absorbency of material, regeneration and a very high solution evaporation temperature. The use of water as a refrigerant limit the temperature of the sublimation to 0°C. Since water is the refrigerant, the system must operate under vacuum conditions to achieve lower evaporation temperature. In addition to the extensive use of LiBr/water and water/NH3 for many years, new functional fluids are being tested, such as fluorocarbon refrigerant-based working fluids, a mixture using inorganic salt absorbent NaOH/water and a mixture of LiBr+ZnBr2/water. 5.10.2. Types of absorption systems Single-effect absorption system It is the most often used scheme. This system uses a non-volatile absorbent such as LiBr/water. The high temperature heat QH supplied to the generator (Fig. 5.20) is used to evaporate the refrigerant from the solution. The refrigerant is supplied to a condenser (Fig. 5.20) 170PDF Image | Heat Pumps 978-83-65596-73-4
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