Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants. Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems. When used in compression or absorption systems, these fluids provide refrigeration by undergoing a phase change process in the evaporator. As the name implies, secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes. of course, if the operating temperatures are above 0oC, then pure water can also be used as secondary refrigerant,
for example in large air conditioning systems. Antifreezes or brines are used when refrigeration is required at sub-zero temperatures. Unlike primary refrigerants, the secondary refrigerants do not undergo phase change as they transport energy from one location to other. An important property of a secondary refrigerant is its freezing point. Generally, the freezing point of a brine will be lower than the freezing point of its constituents. The temperature at which freezing of a brine takes place its depends on its concentration. The concentration at which a lowest temperature can be reached without solidification is called as eutectic point. The commonly used secondary refrigerants are the solutions of water and ethylene glycol, propylene glycol or calcium chloride. These solutions are known under the general name of brines.
How secondary refrigerant used in ice plants:
The usual brines used in freezing tanks are sodium chloride or calcium chloride. sodium chloride is used where the cooling surfaces will not be exposed to damage if freezing occurs, such as on pipe coil coolers and trunk evaporators. calcium chloride brine is preferred in shell tubular coolers but where the brine flows inside he tubes, sodium chloride brine may be used.
Chromate treatment against corrosion of the galvanizing on ice cans is universal. Ammonia leaks in brine are very destructive if this treatment has not been used. nearly 160 kg of sodium dichromate for every 100 $m^3$ of calcium and 300 kg per $100 m^3$ of sodium chloride are used. the exact concentration of the dichromate varies with different plants but the best condition is obtained with the chemical indicator for pH of 7 when the solution is just neutral. An alkaline solution having higher pH value has a tendency to remove galvanizing and to cause the growth of sediment on the cans which acts as insulation and reduces heat transfer rates. it is observed that corrosive action with calcium brine is less because of more difficulty in introducing air into the brine.
The brine temperature for ice making should be maintained at - 12 degree C to - 7 degree C and th back pressure in the ammonia coils from 1.3 to 2 bar which is equivalent to a temperature of 9 15 degree C to -10 degree C.