Freezing Technology

3.2.7 Secondary refrigerant

• Substances that take away heat from the medium to be cooled and give it to the boiling refrigerant are called secondary refrigerants. They do not change their physical condition
• Cheap and harmless, chemically neutral to metals and packing material
• Should have a low freezing point and large heat capacity
• The refrigerants are brine which is used as intermediate fluid between evaporator and the substance or space to be cooled. They cool the substance and the space by absorbing their sensible heat, which is also being called having indirect expansion system.
• Eg. Brine solution made of calcium chloride or sodium chloride
• Choice of brine depends on temperature to which a material is to be cooled and industrial process in which it is to be used
• For lower temperature (-18 to -23^oC), CaCl₂ is used, it is very costly
• NaCl is used when it is desired to have direct contact between brine and product.
• Freezing point of brine depends on its concentration
• Cheapest secondary refrigerants are water and air but their application is limited. Since, water has a high freezing point (0^oC) and air has a low heat capacity.