Refrigeration on ice: Basics of industrial icemakers
Ice, in addition to keeping a product cool, also keeps it moist and prevents the drying that may accompany other chilling methods. Ice melts at a fixed temperature, so there is some control over product temperature.
With good contact between product and ice, cooling can be quick.
Applications for industrial iceIndustrial ice is used in many industries for processing, storage, transport, and manufacturing. The following are some of the many applications for industrial ice:
Fishing industry. Ice is used to ice product on-board vessels and for in-plant processing. For on-board requirements, the ice plant can be located on shore and ice conveyed to the dock side for loading into fishing vessels. Icemakers can be located on board as well.
Poultry production. Ice is used in spin chillers (tank with large auger, which moves the birds along slowly) to remove body heat from poultry after slaughter and dressing. Ice is also used for icing birds in packages, top icing of shipping containers, trucks and rail cars.
Produce icing. Field heat is removed from many vegetables using ice. A product called “liquid ice,” which consists of a solution of water with 40% to 50% ice, may also be used to ice wax impregnated boxes that have holes in the sides and bottom.
Baking process. During the mixing of dough, shear heating occurs and can be minimized with the addition of ice, which fulfills the water requirement as well. The yeast responsible for the leavening of the dough can be sensitive to this heating as well as some of the other dough ingredients.
Chemical processes. Application of ice has been used in the manufacture of colors, dyes, optical brighteners used in soaps, and the manufacture of certain herbicides.
Concrete making. Ice can be used, rather than water (up to 100%) during the mixing of concrete. Ice is required for critical pours where there is a requirement for rapid removal of the heat generated during the curing process.
Meat production. Ice is mixed with sausage to absorb the heat of friction generated during the blending process. Ice also provides moisture, which can enhance the quality of the product.
Thermal energy storage (TES). Ice builders and ice harvesters are used to store refrigeration capacity in the form of ice. The ice builders consist of an insulated tank of water in which tubes are immersed carrying refrigerant or cold brine.
Ice harvesters typically consist of icemakers positioned over an insulated tank of water into which the ice is harvested. In both cases, the stored refrigeration capacity can be built during off-peak hours when demand is low and electricity may be cheaper.
Packaged ice industry. Ice that is used for drinks and coolers, which is usually sold in small bags (e.g., 5 to 20 lb).
Types and design of industrial icemakersIce making has evolved over the years from cutting blocks of various sizes from lakes and storing in sawdust in barns, to using mechanical refrigeration to make blocks of ice.
Block ice plants, however, are being gradually replaced by more modern, automatic ice-making equipment.
There are six basic types and size of ice that can be used for industrial applications:
- Block ice made in plants;
- Shell ice;
- Flake ice;
- Tube ice;
- Plate ice; and
- Slush, slurry or binary ice.
The four main types of fragmented ice (shell, flake, tube, and plate) are illustrated in Figure 1.
Block ice manufacture. Block ice is produced by placing water in galvanized iron cans or molds that are immersed in a secondary coolant, brine (calcium or sodium chloride), which is kept cold by refrigerant (often ammonia) expansion coils. Cans vary with the weight of ice blocks required (from 25 to 182 kg).
Ice cans are usually filled by means of a filler device, generally a multi-faucet arrangement that is constructed as to automatically shut off the water supply when the can is filled to the proper height. Once the water in the cans is frozen, the ice is removed by lifting the cans out of the brine and spraying them with or dipping them in warm water.
This loosens the ice so that when the can is inclined on its side, the block slides out. Ice cans are often tapered to facilitate ice removal. The can is refilled and the process repeated.
Freezing tanks can be made of such materials as wood, steel, or concrete. Tanks made of wood have a relatively short life and are subject to leaks.
The freezing tank contains direct-expansion freezing coils equally distributed throughout the tank and submerged in brine. The tank is provided with a suitable hardwood frame for supporting the ice cans and an agitator for keeping the brine in motion to promote good heat transfer.
The brine in the tanks acts as a medium of contact only; the refrigerant evaporating in the freezing coils extracts the heat from the brine, which again absorbs the heat from the water in the cans, thereby freezing it. The brine temperature is usually maintained at 10Â° to 20Â°F (-12.2Â° to -6.6Â°C), which is equivalent to temperature of 5Â° to 15Â°F (-15Â° to -9.4Â°C) in the coils.
Freezing takes from 8 to 24 hours, so large production space is required. The rate of freezing is time regulated to prevent too rapid freezing, which could result in a brittle product. The freezing cycle takes about 8 hours for three harvests per day for a total daily capacity of 3 to 10 tonne (2.7 to 9.07 ton).
Shell ice manufacture. Shell ice is produced by freezing a falling film of water on the inside and outside of a stainless steel tube. The freezing cycle is normally between 8 to 15 min, with the final ice thickness being from 3 to 20 mm following the curvature of the tube.
Shell icemakers use a sump and recirculating pump, where the excess water not converted to ice is collected and applied to the freezing surface for ice making. Harvesting is accomplished using a hot gas defrost.
The application of hot gas to the refrigerant circuit of the tubes causes the ice surface touching the tube to reach its melting point and thereby releasing the ice.
The ice falls by gravity from the tubes to a storage bin, beater or cutter bar, or an auger that can further reduce the size of the ice. Freezing time, harvest time, and the water, pump, and refrigeration can be controlled by adjustable electrical timers and relays, or electronic devices such as programmed logic controllers.
The ability to adjust these parameters makes the shell icemaker very flexible in that different thicknesses and degrees of hardness can be achieved. By adjusting the thickness and/or hardness, shell ice can be made to suit the application required.
Softening of the ice is possible by adding salt and/or adjusting the amount of subcooling applied during the refrigeration cycle. The shell icemaker can make a reasonably thin (adjust thickness and subcooling) soft ice (adding salt) used for icing fish without damaging the delicate flesh or a very thick, hard clear ice, desirable for the packaged ice industry.
The curvature of shell ice helps prevent bridging in storage. The curvature also enables the ice to conform to the shape of many products that are to be cooled, thus facilitating effective heat transfer.
As noted earlier, a type of fresh water slush ice called liquid ice, which is a solution of water and 40% to 50% shell ice, can also be made.
Using a special tank and mixing device, the shell ice is kept in suspension so it can be transferred with a specially designed pump through a 4- to 6-in.-dia hose.
A new modular shell ice-maker design, the “Ice/Berg LS” can produce a lot of ice in a small amount of space. The design allows for expansion as well.
The modularity is achieved through an arrangement of four main system components: Ice making on the low side, refrigeration on the high side, the condenser and a new SC5 controller. By adding icemaking modules in nominal 5-ton increments, the user can boost production while using the same SC5 controller. The installer or contractor can supply their own refrigeration unit.
The ice-making section of the modular shell icemaker is 304 stainless steel. Front discharge minimizes space requirements, and an optional conveyor is available for multiple module configurations.
Flake ice manufacture. Flake ice is made by freezing water in thin layers either on the inside or outside of a smooth refrigerated surface. The surface is usually a drum that is oriented horizontally or vertically and may be stationary. The ice is removed by mechanical action, for example by a scraper on a cylindrical surface.
The ice thickness can be varied marginally from 1.5 to 3 mm by adjusting the speed of the rotating drum, varying the evaporator temperature, and regulating the water flow on the freezing surface. The ice is produced on a continuous basis as opposed to a freeze-and-harvest cycle typical of shell, tube, and plate icemakers.
This continuous operation without a harvest cycle results in less refrigeration capacity per ton of ice than any other types of manufactured ice if similar makeup water and evaporating temperatures are compared.
Flake icemakers are operated at lower evaporating temperatures than shell, tube, or plate icemakers and the result is that the ice is colder as it comes off the ice-making surface. The rapid freezing of flake ice results in the opaque to white appearance, which is caused by entrained air.
Tube ice manufacture. Tube or cylindrical ice is formed by freezing a falling film of water either on the outside of a stainless steel tube with evaporating refrigerant on the inside of the tube or freezing water on the inside of tubes surrounded by evaporating refrigerant on the outside.
Ice is harvested as a cylinder by introducing hot discharge gas into the refrigerant in the freezing section, which releases the ice from the tubes. The ice falls to a motor-driven cutter plate, which is adjusted to cut the ice cylinders to the length desired.
Typical dimensions of the pieces are 40 mm dia with a hole 10 mm dia and 40 mm long. This shape makes tube ice ideal for the hotel drink industry. It has a bulk density of about 35 lb/cu ft. The shape, however, is such that voids are present when this tube ice is used to ice fish and produce.
The large size and shape is a disadvantage for icing fish and produce, as the tube ice does not conform to the shape of the product, thus inhibiting good and effective heat transfer.
Plate ice manufacture. Plate ice is made on icemakers that build ice on a flat vertical surface. During the freezing cycle, water is applied above and flows by gravity over the freezing plates. Liquid refrigerant is contained in the internal circuiting inside the plate.
The freezing cycle time governs the ice thickness. Cycle times from 12 to 45 minutes producing ice of a thickness in a range of 6 to 20 mm are typical.
The water is recirculated by a sump system and a hot gas defrost is used. The freezing time, harvest time and the water, pump and refrigeration are controlled by adjustable electric timers and relays or PLCs.
The ice produced is flat and usually comes off the evaporator surface at a temperature that is such that, combined with the shape of the ice, can lead to possible bridging. The flat shape of the ice allows the ice surfaces to interface and bond together forming masses of ice. This is not desirable when using the ice to cool food products that require full contact and filling of void spaces for effective cooling
Slush, slurry or binary ice manufacture. Slush, slurry or binary ice consists of ice from a brine or seawater solution, which forms small ice crystals in a solution that is supercooled.
The slush ice generator consists of a standard refrigeration unit that supplies refrigerant to a scraped surface heat exchanger. Ice does not form on the cooling surface as in shell, flake, tube, and plate icemakers. Rather, it is formed inside a solution that is cooled below the solution’s freezing temperature in the form of small ice crystals.
The ice crystals are generated by cooling the inner surface of a stainless steel cylinder through which brine or seawater is passed. A scraper mechanism removes the ice crystals, which become suspended in the fluid and form a liquid, pumpable ice.
The slush ice may be stored in a reservoir and possibly recirculated back through the ice plant to increase the percentage of ice crystals in the mixture (“ice fraction”). The ice fraction can be as high a 60% ice before it becomes too viscous and pumpability is lost.
This is known as the limit of pumpability. These ice generators require operator adjustment and attention so as to avoid possible freeze up.
The temperature of the slush ice is lower than that of conventional fresh water ice and is dependent upon two factors: The brine strength or seawater salinity and the percentage of ice fraction in the slush ice mix.
The lower temperatures that can be achieved with slush ice are such that partial freezing of product surface may occur. This could be disadvantageous in certain applications. Uptake of salt is a consideration as well.