Refrigerant is used to transfer the cold and heat generated by a refrigerator to the intermediate substance of the object being cooled or heated. The commonly used refrigerants include air, water, saline water, organic compounds and their aqueous solutions (the most commonly used are ethylene glycol and propylene glycol (food grade)),

A) The commonly used cooling agents for the specific heat density and freezing point of salt calcium chloride aqueous solution include sodium chloride (NaCl) aqueous solution and calcium chloride aqueous solution (CaCL2). Salt water solution has a strong corrosive effect on metals, which can corrode pipelines and equipment. Its corrosiveness is related to the purity of the salt and the oxygen content in the solution. Possible measures: The contact part between the low-temperature freezer and the refrigerant must be made of corrosion-resistant materials,

A1 uses high-purity salt to prepare a saline solution;

A2: Reduce the chance of contact between solution and air, and use a closed circulation system or a cover on the brine tank;

A3: Add a certain amount of corrosion inhibitor to make the solution weakly alkaline, and maintain its pH value between 7.5 and 8.5. Commonly used are sodium hydroxide (NaOH) and sodium dichromate (Na2Cr2O7), with a mass ratio of NaOH: Na2Cr2O7=27:100.

A4: Differences between sodium chloride and calcium chloride as refrigerants: If making frozen saline water, use a 30% calcium chloride solution with a freezing point of -50 ℃ (the lowest); Use a 23% sodium chloride solution with a freezing point of -21 ℃ (the lowest).

A5: Use corrosion-resistant materials for the contact part of the refrigerant carrier.

B) Organic compound refrigerants are commonly used as ethylene glycol aqueous solution and ternary solution (mass percentage 40% ethylene glycol, 20% ethanol, 40% water, freezing point -64 ℃). These refrigerants have no corrosiveness to equipment, but have low volatility. Therefore, it is recommended to make them into a closed circulation system.

C) Comparison of the above three commonly used refrigerants: sodium chloride and potassium chloride have lower costs and are suitable for the use of large refrigerators. In the early stage, the main investment was in equipment refrigerators, while ethylene glycol solution is suitable for the use of small refrigerators, with low equipment requirements and higher operating costs than sodium chloride and potassium chloride.

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