Refrigerants.
Refrigerant, also known as refrigeration working fluid, is a working substance that is continuously circulated in the refrigeration system and changes through its own state to achieve refrigeration.
The refrigerant absorbs the heat of the cooling medium (water or air, etc.) in the evaporator and vaporizes, and in the condenser it transfers the heat to the surrounding air or water and condenses. Its properties are directly related to the refrigeration effect, economy, safety and operation management of refrigeration equipment, so the understanding of refrigerant property requirements cannot be ignored.
1.Requirements for refrigerants.
1) The critical temperature should be high and the solidification temperature should be low. This is a basic requirement for the properties of the refrigerant. The high critical temperature facilitates condensation with general cooling water or air: the low solidification temperature prevents it from solidifying at the evaporation temperature, which facilitates the refrigeration requirements at lower temperatures.
2) The evaporation temperature at atmospheric pressure should be low. This is a necessary condition for cryogenic refrigeration.
3) The pressure should be moderate. The evaporation pressure is best approached to and slightly higher than the atmospheric pressure to prevent air from seeping into the refrigeration system, thereby reducing the refrigeration capacity. The condensing pressure should not be too high to reduce the pressure on the refrigeration equipment, so as not to consume too much compression power and reduce the possibility of leakage in the high-pressure system.
4) The cooling capacity per unit volume Q should be large. In this way, when the refrigeration capacity is constant, the refrigerant circulation can be reduced and the size of the compressor can be reduced.
5) Thermal conductivity (thermal conductivity) should be high, and the viscosity and density should be small. This increases the heat transfer coefficient of each heat exchanger and reduces its loss of flow resistance in the system.
6) Adiabatic index x should be small. This reduces compression power consumption and lowers the exhaust temperature, which is conducive to improving power efficiency and simplifying system design.
7) Chemically stable. No combustion, no **, no decomposition at high temperature, no corrosion to metal, no chemical reaction with lubricating oil, no damage to human health.
8) Cheap and easy to buy.
9) It has a certain water absorption, so as not to produce "ice plug" and affect the normal operation when a very small amount of water is infiltrated into the refrigeration system.
2.Classification of refrigerants.
1. According to the chemical composition, refrigerants can be divided into inorganic compounds, freons, azeotropic mixtures and hydrocarbons.
Inorganic compounds mainly include ammonia, water, carbon dioxide, etc. Among them, ammonia and water are still the commonly used refrigerants.
There are many types of Freon refrigerants, with large differences in thermal properties, and can be used in different refrigeration systems. Freon is a general term for halogen derivatives of saturated hydrocarbons. At present, the main derivatives used as refrigerants are methane and ethane, which replace some or all of the hydrogen atoms in the original compound with atoms of fluorine, chlorine and bromine. The advantages of freon are: non-toxic, non-combustible, non-corrosive to metal; The main disadvantage is that it is easy to leak and it is not easy to detect.
Azeotropic mixture is to mix more than two refrigerants in a certain proportion, their boiling points are the same, and the composition ratio of gas and liquid phases remains unchanged. Azeotropic mixtures are one of the future development directions of refrigerants. The first digit after the code R of the azeotropic mixture is 5, and the commonly used ones are R502, R500, etc.
Hydrocarbon refrigerants include methane, ethane, propane, n-butane, ethylene, propylene, etc., which are mainly used in the petrochemical industry.
2. Classified according to the condensing pressure and evaporation temperature during normal operation.
According to the saturation pressure PK and normal evaporation temperature t0 of the refrigerant condensed in the condenser at room temperature, it can be divided into the following three categories.
Low-pressure, high-temperature refrigerant.
Condensing pressure pk<3kg C (absolute), t0>0.
For example, R1 (CFC13) with t0 = 237℃。These refrigerants are suitable for use in centrifugal refrigeration compressors in air conditioning systems. Usually at 30, pk<306kg/c㎡。
Medium-pressure medium-temperature refrigerant.
Condensing pressure PK<20kg C (absolute), 0 > T0, >-60. For example, R717, R12, R22, etc., such refrigerants are generally used in ordinary single-stage compression and two-stage compression piston refrigeration compressors.
High-pressure cryogenic refrigerant.
Condensing pressure pk>20kg C (absolute), t0