INDUSTRIAL COOLING
Secondary refrigeration
According to Ecomesh, environmental concerns over ozone depletion and global warming of the refrigerants as well as the risk involved circulating refrigerants like ammonia for chilled and frozen refrigeration applications, have prompted a reconsideration of some of the old established refrigeration technologies in order to minimise the refrigerant usage.
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n many refrigeration applications, heat is transferred to a secondary fluid, which can be any liquid cooled by the primary refrigerant and then used to transfer heat without changing state. These fluids are also known as heat transfer fluids, brines, or secondary refrigerants. Secondary refrigerants generally consist of anti-freeze Solution, corrosion inhibitor(s) and biocides where applicable to satisfy the temperature and application range. A wide range of glycol, potassium acetate and potassium formate based energy efficient secondary refrigerants has been applied for both chilled and frozen food applications.
Indirect refrigeration systems can have some significant potential advantages over direct refrigeration systems. In an indirect system, for example, it is possible to design and manufacture factory-built compact refrigeration units with an extremely small primary refrigerant charge.
Direct vs indirect refrigeration systems However, an indirect system with a secondary refrigerant circuit means an extra cost for the pump and heat exchanger with an added temperature difference. If a secondary refrigeration system is not designed correctly, this may lead to higher total energy consumption in comparison with a direct system. Therefore, it is vital to choose the right secondary refrigerant for the application in order to provide an economical and energy efficient system.
In principle, water is an excellent secondary refrigerant for mainly air conditioning and many other applications for temperatures down to around +3°C (37.4 °F). The main problem,
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however, is to find suitable fluids for chilling and freezing applications below the water freezing point of 0°C (32 °F).
Secondary refrigerant options
There are several fundamental requirements that any secondary refrigeration must satisfy: ■ Low viscosity ■ High specific heat ■ Good thermal conductivity ■ Good chemical corrosion inhibitor ■ Chemically stable, no separation or degrading ■ Non-toxic ■ Non-flammable ■ Food grade for food refrigeration.
Furthermore, any secondary refrigerant must be compatible with commonly used materials in terms of corrosion and long-term stability. It is also vital to satisfy health and safety and environmental requirements.
The freezing point can be considered as the starting point to choose a secondary refrigerant, and it should be below the operating temperature of the system with a comfortable safety margin. The physical properties of secondary refrigerants are also very important. It is essential to find the right or balance between the viscosity, specific heat and thermal conductivity for optimum design efficiency.
Some of the most important paramereters are corrosivity, environmental pollution, toxicity, and flammability, handling security and cost levels. The commercially available secondary refrigerants can be divided into two categories, namely aqueous (i.e. water based) and non- aqueous solutions .
Aqueous solutions are mixtures of various salts and waters. The mixtures of such compounds as magnesium and calcium chloride have been used extensively since the early days of refrigeration. More recently, mixtures of potassium acetate and potassium formate have been introduced to the market to overcome some of the corrosion and physical property problems of the old mixtures, in particular for low temperature applications.
Non-aqueous liquids are marketed under many different brand names and they have comparatively poor heat transfer ability and transport capability. They are also quite expensive and have practical application problems in terms of cross contamination, corrosion and operating pressures.
Some of the design issues related to the fluid selection and applications are as follows; Viscosity strongly influences the type of flow that occurs inside the heat exchanger. It also has a significant impact on the pipework pressure drop. However, higher specific heat capacity reduces the mass flow requirement for a given cooling load with identical system circulation temperature differences.
Low viscosity may not necessarily offer smaller pipes and less pressure drop for the system as a whole if it has a very poor specific heat capacity. Therefore, it is vital to strike a balance between the specific heat capacity and viscosity of the fluid for the intended operating temperature range, in order to achieve optimum pipe velocities.
Materials compatibility: It is vital to check all system components’ compatibility with the intended secondary refrigerants. The majority of
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