Continuous freezers

Taj Continuous Freezer:

Ice Cream Continuous Freezer converts mix into ice cream by simultaneously aerating, freezing and beating it, to generate the ice crystals, the air bubbles and the matrix. Simultaneous aeration, freezing and beating has been the basis of ice cream production since its invention and remains so today.

Modern ice cream Continuous Freezers belong to a class of equipment known to chemical engineers as scraped surface heat exchangers; These are designed to remove heat from (or add heat to) viscous liquids. Ice cream Continuous Freezers consist of a cylindrical barrel typically 670 mm brass made with high finished chrome, However, Continuous Freezers designed for different production rates have barrels with a wide range of sizes, Like : 200, 400, 600, 1000........A refrigerant, normally a liquefied volatile gas, Freon Gas, flows through a jacket and cools the outside of the barrel as it evaporates. Inside the barrel is a rotating stainless steel dasher driven by an electric motor. The dasher is equipped with scrapper blades that fit very closely inside the barrel. The dasher has two functions: to subject the mix to high shear and to scrape off the layer of ice crystals that forms on the very cold barrel wall (hence the term 'scraped surface heat exchanger'). The barrel is often made from nickel, covered on the inside with a thin layer of chromium. Nickel gives good heat transfer, and can withstand high pressures. The chromium coating provides resistance to wear from the scraping, and chemical resistance to the cleaningagents used between batches. Ice Cream mix at approximately 4c is pumped from the dump tank into the barrel, where it is aerated and frozen, before being pumped out from the other end. The operation of the factory freezer is controlled by several parameters. The pressure of the refrigerant sets the temperature at which it evaporates, and hence the wall temperature (typically -30°C). The mix and air in – flow and ice cream out – flow rates determine the time that the mix spends inside the barrel (known as the residence time, typically 30-60s), the overrun, the pressure inside the barrel (typically 5 atm) and the throughput (which can be as much as 3000 1h -1 in a large industrial freezer). All of these, together with the dasher rotation speed (typically 200 rpm), determine the outlet temperature. Modern factory freezers are computer – controlled, allowing easy monitoring and control of the process parameters.

Air is injected into the barrel through a system of filters to ensure that it is clean, dry and free from microbiological contamination. Initially the air forms large bubbles. It is essential to create (and maintain) a dispersion of small air bubbles to obtain good quality ice cream. The beating of the dasher shears the large air bubbles and breaks them down into many smaller ones: the larger the applied shear stress, the smaller the air bubbles. Long residence times also lead to small air bubbles. It is easier to whip air into foam that consists of a large volume fraction of liquid and a small volume.fraction of air than vice versa. The high pressure inside the barrel reduces the volume of the air that has been introduced, and therefore makes it easier to aerate further.

In a standard ice cream formulation, sufficient partial coalescence occurs to enable a stable air cell structure to be maintained at overruns up to about 120%. It can be difficult to obtain overruns of more than about 60% in products where fat and protein are not present, or only present in small quantities, such as sorbets. Similarly it is difficult to obtain high overruns if there is insufficient shear (for example because of a very short residence time) to produce partially coalesced fat. Extra shear, and hence increased de-emulsification, can be produced either by increasing the dasher speed or by using a closed dasher.

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