Pumpable ice technology (PIT) uses thin liquids, with the cooling capacity of ice. Pumpable ice is typically a slurry of ice crystals or particles ranging from 5 micrometers to 1 cm in diameter and transported in brine, seawater, food liquid, or gas bubbles of air, ozone, or carbon dioxide.
Terminology Beyond generic terms, such as pumpable, jelly, or slurry ice, there are many trademark names for such coolant, like "Deepchill", "Beluga", "optim", "flow", "fluid", "jel", "binary", "liquid", "maxim", "whipped", and "bubble slurry" ice. These trademarks are authorized by industrial ice maker production companies in Australia, Canada, China, Germany, Iceland, Israel, Russia, Spain, United Kingdom, and the US.
Technological process Pumpable ice can be produced in one of two ways: either by mixing crushed ice with a liquid or by freezing water within a liquid.
The primary way is to manufacture commonly used forms of crystal solid ice, such as plate, tube, shell or flake ice, by crushing and mixing it with water. This mixture of different ice concentrations and particle dimensions (ice crystals can vary in length from 200 μm to 10 mm) is passed by pumps from a storage tank to the consumer. The constructions, specifications and applications of current conventional ice makers are described in this reference: The secondary way is to create the crystallization process inside the volume of the cooled liquid. This crystallization inside can be accomplished using vacuum or cooling technologies. In vacuum technology, very low pressure forces a small part of the water to evaporate while the remaining water freezes, forming a water-ice mixture. Depending on the additive concentrations, the final temperature of pumpable ice is . The large volume of vapor and an operating pressure of about 6 mbar (600 Pa) require the use of a water vapor compressor with a great swept volume. This technology is economically reasonable and can be recommended for systems with a cooling capacity of 1 MW (300 ton of refrigeration; 3.5 million BTU/h) or larger.
Crystallization by cooling can be done using direct or indirect systems.
Direct pumpable ice technology In direct methods, a refrigerant is directly injected inside the liquid. The advantage of this method is the absence of any intermediate device between the refrigerant and the liquid. However, the absence of heat loss between refrigerant and liquid in the process of thermal interaction (heat transfer) may cause problems. The safety measures that have to be implemented, the need for the additional step of refrigerant separation, and difficulties in producing crystals are further disadvantages of this method.
Indirect pumpable ice technology
In indirect methods, the evaporator (heat exchanger-crystallizer) is assembled horizontally or vertically. It has a shell tubing assembled with one to a hundred inner tubes and containing a refrigerant that evaporates between the shell and the internal tubing. Liquid flows through the tubing of a small diameter. In the inside volume of the evaporator, cooling, super cooling, and freezing of the liquid take place due to heat exchange with the crystallizer-cooled wall.
The idea is to use a well-polished evaporator surface (dynamic scraped surface heat exchanger) and appropriate mechanisms to prevent tubing from adhering to the ice embryos, and to prevent growth and a thickening of the ice on the inside cooling surface. A whip rod, screw, or shaft with metallic or plastic wipers is usually used as a mechanism for removal.
Indirect pumpable ice technologies produce pumpable ice consisting of 5 to 50 micrometer crystals and have a number of advantages: they can produce 1,000 kg of crystal ice at the low energy expenditure of 60 to 75 kWh instead of the 90 to 130 kWh required to produce regular water ice (plate, flake, shell types). Further improvements are expected to lead to a specific energy expenditure for ice production of 40 to 5 5kWh per 1,000 kg of pure ice and a high specific ice capacity per an area value at the evaporator cooling surface (up to 450 kg/(m2·h)).
Commercial evaporators of the double-pipe type used in the food and fish industries have an inside diameter of interior tube and length in a range of 50–125 mm and 60–300 cm. For the dewaxing lubrication oil, evaporators are widely used with the following dimensions: internal diameter of the inner tube is 150–300 mm; the length is 600–1,200 cm.
Sometimes a gas can be added to the liquid flowing through the evaporator. It destroys the liquid laminar layer on the cooled surface of the heat exchanger-crystallizer, increases flow turbulence, and decreases the average viscosity of the pumpable ice.
Different liquids, such as sea water, juice, brines, or glycol solutions of additives with more than 3–5% concentrations and a freezing point less than −2 °C are used in the process.
Typically, the equipment for the production, accumulation and supplying of pumpable ice includes an ice maker, a storage tank, a heat exchanger, piping, pumps, and electrical and electronic appliances and devices.
Pumpable ice with maximum ice concentration of 40% can be pumped straight from the ice maker to the consumer. The final possible ice concentration of pumpable ice in the storage tank is 50%. The maximum value of cooling energy of pumpable ice accumulated in the storage tank in a homogeneous phase is about 700 kWh, which corresponds to 10–15 m3 volume of a storage tank. A high-shear mixer is used to prevent the separation of ice from the cooled liquid and keeps the ice concentration unchanged over time and unaffected by the tank height. Pumpable ice is transported from the storage tank to a place of consumption that could be hundreds of meters away. The practical ratio between the required electric power of the submersible mixer motor (kW) and the "kneaded" pumpable ice volume (m3) is 1:1.
In the tanks with volumes larger than 15 m3, pumpable ice is not mixed and the cold energy of stored ice is only used by a heat transfer of liquid that circulates between a storage tank and the consumers of cold. The disadvantages of existing ice storage reservoirs include the following:
The chaotic uncontrollable upsurge of ice ridges which arise due to uneven sprinkling of warm fluid. This liquid is fed into the storage tank from the heat exchanger for further cooling by direct contact with the surface of the ice. The solution is sprayed unevenly in space. Moreover, the rate of supply is not constant. Therefore, the ice melts unevenly. Thus, the ice spikes rise above the ice surface, which leads to the destruction of the spraying devices. In this case, it is necessary to reduce the level of solution in the storage tank to avoid breakage of spray devices.
Ice accumulated in the tank turns into a large chunk. The warm liquid that comes from the air-conditioning system may generate channels through which the liquid could return to the system without being cooled. As a result, the accumulated ice is not fully utilized.
Ineffective use of the volume of the accumulation tank leads to a decrease in the achievable maximum of ice concentration and an inability to fill the entire working volume of the storage tank.
Research and development on overcoming these disadvantages is underway and is expected to lead to the mass production of cheap, reliable and efficient accumulating tanks. These tanks should ensure higher ice concentrations and allow full use of stored cold potential.
Applications Many ice maker producers, research centers, and inventors are working on pumpable ice technologies. Due to their high energy efficiency, reduced size, and low refrigerant charges, there are many applications for this technology.
Selection
There are different pumpable ice maker designs and many special areas of application. The choice is facilitated by computer programs developed by manufacturers.
A customer who intends to use pumpable ice technology should know: Required maximum/minimum cooling capacity (TR) Profile of energy consumption (TR•h) of the plant over 24h, one week, one season, and one year Temperature ranges of the products to be refrigerated (water, juice, liquid, food, and fish) Temperature conditions of the climate at the location of the customer Design limitations on equipment placement Characteristics of the power supply system Intentions and plans of future expansion
When designing storage tanks, several features are to be taken into account: Target of usage of PIT system: Applying pumpable ice for direct contact with a refrigerated product requires the installation of storage tanks with a mixer. To prevail over the tendency of ice to freeze in an iceberg form and to pump ice through the pipes over a distance of 100 m to 200 m, continuous mixing must be used. For pumpable ice applications in thermal energy storage systems, mixing is not needed. Available space: To determine the type of construction (vertical or horizontal ) and the number of storage tanks, site dimensions and permissible heights must be considered. Required daily and weekly stored energy: The cost of the storage tanks is a significant factor in the total cost of a pumpable ice system. Typically, storage tanks are designed with a stored energy value 10–20% higher than that required for production. Furthermore, it has to be remembered that 100% ice concentration in the tank is impossible.
The thickness of the wall of the evaporators is usually determined to ensure: High sustainable heat transfer flux during the process Tensile strength of the inside pipe adequate to withstand the external pressure Tensile strength of the outside pipe adequate to withstand internal pressure Enough room for corrosion Availability of spare parts
Evaporators are usually cheaper when they have a smaller shell diameter and a long pipe length. Thus, the evaporator of pumpable ice makers is typically as long as physically possible whilst not exceeding production capabilities. However, there are many limitations, including the space available at the customer site where the pumpable ice maker is going to be used.
Maintenance and service A pumpable ice maker has predictive maintenance and cleaning requirements. Operational conditions of the specific equipment determine the service intervals and types of service.
Proper refrigeration maintenance of a pumpable ice maker will extend its life, and routine maintenance can reduce the probability of an emergency service caused by major component failure, such as of a refrigeration compressor or the fan motor of the air condenser due to a dirty coil, and refrigerant leakage.
Possible problems caused by not maintaining the air-cooled pumpable ice maker are: Failure of fan motors caused by dirty coils restricting air flow Failure of the thermostat caused by high amp draw due to dirty condenser coils Failure of refrigeration compressor caused by dirty condenser coil and excessive head pressure Restriction of capillary tubing (metering device) caused by refrigerant oil overheating and fouling Wiring burning and failure due to excessive amperage caused by high head pressure and dirty condenser coils Increased electrical consumption due to longer-run times caused by dirty condenser coils Pollution and blockage of the line of condensate water.
In the pumpable ice maker liquid treatment is used to remove particles down to 1 μm in size and to minimize fouling of the heat transfer surface of the evaporators. Plate heat exchangers also need to be disassembled and cleaned periodically. Properly treating liquid before it enters the pumpable ice maker or the Plate heat exchanger will help to limit the amount of scale build-up, thus reducing cleaning times and preventative maintenance costs. Improperly sizing the liquid filter system leads to costly early change outs and poor performance.
Wastewater treatment