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Slurry ice is aphase changingrefrigerant made up of millions of ice "micro-crystals" (typically 0.1 to 1 mm in diameter) formed and suspended within a solution of water and afreezing point depressant. Some compounds used in the field aresalt,ethylene glycol,propylene glycol,alcohols likeisobutyl andethanol, and sugars likesucrose andglucose. Slurry ice has greaterheat absorption compared to single phase refrigerants likebrine, because the meltingenthalpy (latent heat) of the ice is also used.
The small ice particle size results in greaterheat transfer area than other types of ice for a given weight. It can be packed inside a container asdense as 700 kg/m3, the highest ice-packing factor among all usable industrial ice.
Thespherical crystals have good flow properties, making them easy to distribute through conventional pumps and piping and over product in direct contact chilling applications, allowing them to flow into crevices and provide greater surface contact and faster cooling than other traditional forms of ice (flake, block, shell, etc.).
Its flow properties, high cooling capacity, and flexibility in application make a slurry ice system a substitute for conventional ice generators andrefrigeration systems, and offers improvements inenergy efficiency: 70%, compared to around 45% in standard systems, lowerfreon consumption per ton of ice, and lower operating costs.
Slurry ice is commonly used in a wide range ofair conditioning,packaging, and industrial cooling processes, supermarkets, and cooling and storage of fish, produce, poultry and other perishable products.
Slurry ice can boost by up to 200% the cooling efficiency of existing cooling or freezing brine systems without any major changes to the system (i.e.heat exchanger, pipes, valves), and reduce the amount ofenergy consumption used for pumping.
Slurry ice is also used in direct contact cooling of products in food processing applications in water resistantshipping containers. It provides the following advantages:
Slurry ice is generated using a unique type of ice-making technology. Conventional ice generators produce sharp edged, dry ice fragments, not the small, spherical crystals found in slurry ice. In traditional brine chiller systems, crystals forming inside the solution would block or damage the system.
The world’s firstpatent for a slurry ice generator was filed bySunwell Technologies Inc. ofCanada in 1976. Sunwell Technologies Inc. introduced slurry ice under the trade name deepchill ice, in the late 1970s. Slurry ice is created through a process of forming spherical ice crystals within a liquid. The slurry ice generator is a scraped-surface verticalshell and tube heat exchanger. It consists ofconcentric tubes with refrigerant flowing between them and the water/freezing point depressant solution in the inner tube. The inner surface of the inner tube is wiped using a mechanism which in the original Sunwell design consists of a central shaft, spring-loaded plasticblades,bearings, andseals. The small ice crystals formed in the solution near the tube surface are wiped away from the surface and mixed with unfrozen water, forming the slurry.Other slurry ice generators adapted the first idea of wiping the surface using anauger originally designed to create flake ice. Wipers also can be brushes orfluidized bed heat exchangers for ice crystallization. In these heat exchangers, steel particles circulate with the fluid, mechanically removing the crystals from the surface. At the outlet, steel particles and slurry ice are separated.
Animmiscibleprimary refrigerantevaporates tosupersaturate the water and form small, smooth crystals. With direct contact chilling, there is no physical boundary between the brine and the refrigerant, increasing the rate of heat transfer. However, the major disadvantage of this system is that a small amount of refrigerant stays in the brine, trapped in the crystals. This refrigerant is pumped with the slurry out of the generator and into the environment.
Pure water issupercooled in a chiller to −2°C and released through a nozzle into a storage tank. Upon release, it undergoes aphase transition, forming small ice particles with 2.5% ice fraction. In the storage tank, it is separated by the difference in density between ice and water. The cold water is supercooled and released again, increasing the ice fraction in the storage tank.However, a small crystal in the supercooled water or a nucleation cell on the surface will act as a seed for ice crystals and block the generator.
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