Refrigerant.
Refrigerant has the meaning of "carrying" from the literal meaning of **, but in fact it is a substance that "carries cold capacity", in the indirect refrigeration system, through the refrigerant to accept the cold capacity of the refrigerant and then cool other substances. It plays the role of transferring the cold capacity of the refrigerant. In the indirect cooling system, the refrigerant plays the role of cold capacity transmission and distribution.
1.The requirements of refrigerant, the factors that should be considered when selecting refrigerant include freezing point, specific heat capacity, corrosivity to metal, etc. The specific requirements are as follows:
1) The specific heat capacity is larger. The larger the specific heat capacity, the greater the cooling capacity, which can reduce the circulating capacity of the refrigerant.
2) Low viscosity and high thermal conductivity.
3) The freezing point is low and suitable, because the freezing point is too low, which will lead to a decrease in specific heat capacity and an increase in viscosity.
4) Odorless, non-toxic, safe to use, and less corrosive to metal.
5) Inexpensive and easy to buy.
2.There are many types of refrigerants, which can be gases, liquids or solids. Commonly used refrigerants are air, water, and brine solutions.
1) Air and water.
Air and water are the cheapest and most readily available refrigerants. Both have the advantages of low density, safety and harmlessness, and almost non-corrosive to equipment. However, the specific heat capacity of air is small, so air is used as a refrigerant only when air is used for direct cooling; Although water has the advantage of having a large specific heat capacity, the freezing point of water is high, so it can only be used as a refrigerant above 0, and brine should be used as a refrigerant below 0.
2) Saline solution.
Brine is the most commonly used refrigerant and is made from salt dissolved in water. The commonly used brine mainly includes sodium chloride aqueous solution and calcium chloride aqueous solution. The nature of salt water is related to the amount of salt in the solution. In particular, it is important to note that the freezing point of brine depends on the concentration of brine. The eutectic point of different brine solutions is different, such as the eutectic temperature of sodium chloride brine is -212 The eutectic concentration is 224%;The eutectic temperature of calcium chloride brine is -55 and the eutectic concentration is 299%。
Although brine has the advantages of abundant raw materials, low cost and adjustable freezing point, because the concentration of brine has a great influence on the properties of brine solution, the concentration of brine as a refrigerant should be reasonably selected, and the corrosion of brine on equipment and pipelines should be paid attention to, and the proportion of brine must be regularly tested.
3) Organic refrigerant, organic refrigerant is divided into the following 3 types.
Methanol, ethanol, and their aqueous solutions.
The freezing point of methanol is -978. The freezing point of ethanol is -1141℃。Because their pure liquid specific gravity and specific heat capacity are lower than those of salt water, they can be cooled at lower temperatures**. Methanol is slightly more viscous than ethanol in aqueous solutions. They are all relatively liquid. Because both methanol and ethanol are volatile and flammable, it is necessary to pay attention to fire prevention during use, especially when the machine is stopped and the system is at room temperature.
Aqueous solutions of ethylene glycol, propylene glycol and glycerol.
The characteristics of ethylene glycol and propylene glycol aqueous solutions are similar, and their eutectic temperature can reach about -60 (the corresponding eutectic concentration is 0.).6 or so). They have a large specific gravity and specific heat capacity, a high viscosity of the solution, and are slightly toxic, but not harmful. In the process refrigeration use below -20, in order to reduce the viscosity of the solution, ethanol is often added to the ethylene glycol solution to become a ternary mixed solution, and the general formula is ethylene glycol: ethanol: water = 40:20:40, the freezing point of the solution is -64, the specific gravity is 1, and the specific heat capacity is 3The kinematic viscosity at 14kj (kg·k) and -35 is 45x10-6m2 s. Glycerol (glycerol) is an extremely stable compound, and its aqueous solution is non-corrosive to metals; It is non-toxic, can be in direct contact with food, and is a good refrigerant.
Pure organic liquids.
Pure organic liquids include dichloromethane (CH, C1,) with refrigerant number R30, trichloroethylene (C, HC1) with refrigerant number R1120 and other freon liquids. They have a low freezing point (around -100 or lower). It is characterized by specific gravity, low viscosity, and small specific heat capacity, which can be used to obtain a lower cooling temperature.