5. Ice manufacturing equipment
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Classification of ice plants
Other than by description of the ice produced, there is no simple way to classify the different types of ice makers; hence we have block, slice, plate, tube, slush ice and so on. A further sub-classification may be made depending on whether they produce a "dry" subcooled ice or a "wet" ice. Generally, subcooled ice is produced in machines which mechanically remove the ice from the cooling surface. Most flake ice machines are examples of this type. On the other hand, "wet" ice is usually made in machines which use a defrost procedure to release the ice. The defrost partially thaws the ice where it makes contact with the cooling surface and, unless it has been reduced to a temperature substantially below 0° C (subcooled) the surfaces will remain wet; tube ice and plate ice systems are examples of this type.
In some machines the ice is formed and harvested concurrently to produce what is sometimes known as "slush ice" because it contains a good deal more unfrozen water than other forms of "wet" ice which have been harvested using a defrost procedure.
Types of ice maker
Block ice. The traditional block ice maker forms the ice in cans which are submerged in a tank containing circulating sodium or calcium chloride brine. The dimensions of the can and the temperature of the brine are usually selected to give a freezing period of between 8 and 24 hours. Too rapid freezing results in brittle ice. The block weight can vary from 12 to 150 kg, depending on requirements; 150 kg is considered the largest size of block one man can conveniently handle. The thicker the block the longer the freezing time. Blocks less than 150 mm thick are easily broken and a thickness of 150 to 170 mm is preferable to prevent the block toppling. The size of the tank required is related to the daily production. A travelling crane lifts a row of cans and transports them to a thawing tank at the end of the freezing tank, where they are submerged in water to release the ice from the moulds. The cans are tipped to remove the blocks, refilled with fresh water and replaced in the brine tank for a further cycle (Fig. 10). This type of plant often requires continuous attention and a shift system is operated by the labour force which may be 10 to 15 workers for a 100 t/day plant. Block ice plants require a good deal of space and labour for handling the ice. The latter factor has been the main reason for the development of modern automatic ice-making equipment.
Block ice still has a use, and sometimes an advantage, over other forms of ice in tropical countries. Storage, handling and transport can all be simplified if the ice is in the form of large blocks; simplification is often obligatory in small scale fisheries and in relatively remote situations. With an appropriate ice crushing machine block ice can be reduced to any particle size but the uniformity of size will not be as good as that achieved with some other forms of ice. In some situations, block ice may also be reduced in size by a manual crushing method.
Rapid block ice. The rapid ice plant can produce blocks in only a few hours and this means that the space requirements are considerably reduced compared with a conventional block ice plant. Block sizes vary with 25, 50 and 150 kg each being typical. In one type of machine, the relatively quick freeze is obtained by forming the block in a tank of water, around tubes through which the refrigerant is circulated. The effective thickness of ice to be frozen is a good deal less than in a conventional block ice machine. The tubes are arranged so that as the ice builds up it fuses with the ice on adjacent tubes to form a block with a number of hollow cores. These blocks are released from the tubes by a defrost procedure and they can then be harvested automatically from the surface of the tank. Some manual effort is required for storage or feeding to a breaker if the ice is required in the crushed form. In another type of rapid ice machine, the refrigerant is circulated through a jacket around each can of water and also through pipes running through the centres of the cans. Ice then forms simultaneously both at the outside and at the centre of the cam Blocks are then removed by gravity after a hot gas defrost.
An advantage of a rapid block ice machine is that it can be stopped and started in a relatively short time, since there is no large tank of brine to be cooled initially as in the conventional block ice machine in which the refrigeration system is often kept in continuous operation even when ice production has ceased.
Flake ice. This type of machine forms ice 2 to 3 mm thick on the surface of a cooled cylinder and the ice is harvested as dry subcooled flakes usually 100 to 1,000 mm² in area. In some models, the cylinder or drum rotates and the scraper on the outer surface remains stationary. In others, the scraper rotates and removes the ice from the surface of a stationary drum, in this case, built in the form of a double-wailed cylinder. It is usual for the drum to rotate in a vertical plane but in some models the drum rotates in a horizontal plane. One distinct advantage of the rotating drum method is that the ice-forming surfaces and the ice release mechanism are exposed and the operator can observe whether the plant is operating satisfactorily (Fig. 1 1). The machine with the stationary drum has the advantage that it does not require a rotating seal on the refrigerant supply and takeaway pipes. However, this seal has been developed to a high degree of reliability in modern machines. The ice is subcooled when harvested, the degree of sub-cooling depending on a number of factors but mainly the temperature of the refrigerant and the time allowed for the ice to reach this subcooled temperature. The subcooling region of the drum is immediately before the scraper where no water is added for a part of the drum's rotation and the ice is reduced in temperature. This ensures that only dry subcooled ice falls into the storage space immediately below the scraper. The refrigerant temperature, degree of subcooling and speed of rotation of the drum are all variable with this type of machine and they affect both the capacity of the machine and the thickness of the ice produced.
Fig. 10. Block ice maker
Other factors such as ice make-up water temperature also affect the capacity of the machine. Thus, the optimum operating conditions will depend on both the local conditions and the thickness of ice preferred. The normal refrigerant temperature in a flake ice machine is -20 to -25°C, a good deal lower than in other types of ice-maker. The low temperature is necessary to produce higher ice making rates, thus keeping the machine small and compact. The extra power requirement resulting from operating with a lower temperature in the ice maker is somewhat compensated for by the fact that the method does not require a defrost. There is therefore no additional refrigeration load incurred by the method of releasing the ice from the drum. The range of unit sizes for this type of machine now extends from units with a capacity of 0.5 to 60 t/24 h. However, rather than use a single unit, it is often expedient to use two or more. This gives a better arrangement for operating at reduced capacity and also provides some degree of insurance against complete breakdown. This advice is also applicable to other types of automatic ice maker.
Tube ice. Tube ice is formed on the inner surface of vertical tubes and is produced in the form of small hollow cylinders of about 50 x 50 mm with a wall thickness of 10 to 12 mm. The tube ice plant arrangement is similar to a shell and tube condenser with the water on the inside of the tubes and the refrigerant filling the space between the tubes. The machine is operated automatically on a time cycle and the tubes of ice are released by a hot gas defrost process. As the ice drops from the tubes a cutter chops the ice into suitable lengths, nominally 50 mm, but this is adjustable (Fig 12). Transport of the ice to the storage area is usually automatic, thus, as in the flake ice plant, the harvesting and storage operations require no manual effort or operator attendance.
Tube ice is usually stored in the form it is harvested, but the particle size is rather large and unsuitable for use with fish. The discharge system from the plant therefore incorporates an ice crusher which can be adjusted to give an ice particle size to suit the customer's requirement. The usual operating temperature of this type of plant is -8°C to -10°C. The ice will not always be subcooled on entering the store but it is usually possible to maintain the store at -5°C since the particle size and shape allow the ice to be readily broken up for discharge, especially with a rake system described in Chapter 4.
Fig. 11. Flake ice maker
Plate ice. Plate ice is formed on one face of a refrigerated vertical plate and released by running water on the other face to defrost it. Other types form ice on both surfaces and use an internal defrost procedure. Multiple plate units are arranged to form the ice-making machine and often these are self contained units incorporating the refrigeration machinery in the space below the ice-maker. The optimum ice thickness is usually 10 to 12 mm and the particle size is variable. An ice breaker is required to break the ice into a suitable size for storage and use (Fig 13). Water for defrost requires heating if its temperature is less than about 25°C; below this value the defrost period is too long, resulting in a loss in capacity and an increase in cost. This machine, like the tube ice machine, operates on an automatic timed cycle and the ice is conveyed to the storage area or if the machine can be located directly above the storage space, harvesting can be achieved using gravity flow.
Fig. 12. Tube ice maker
Fig. 13. Plate ice maker
Slush ice. The cooling unit for making "slush" ice is called a scraped-surface heat exchanger. It consists of concentric tubes with refrigerant flowing between them and water in the inner tube. The inner surface of the inner tube is scraped using, for example, a rotary screw. The small ice crystals formed on the tube surface are scraped off and mixed with unfrozen water. This results in a slurry of ice and water, which may contain up to 30% water by weight. This mixture may be pumped or, after removing most of the water in a mechanical separator, used as a 'dry' form of ice.
Other ice makers. A number of ice makers operate with systems that are different from those described above, but these are normally made with capacities of no more than a few hundred kilogrammes of ice a day, mainly for retail and catering services.
Refrigeration systems for ice plants
The modern continuous ice plant is designed to operate 24 hours a day, most of this time unattended. The refrigeration system, which includes the compressor, condenser, pipework, control equipment and the ice-maker itself, should therefore be designed for reliability, with safeguards against any foreseeable failure or malfunction. Most ice plant manufacturers specify the refrigeration system to be used with their ice making plant but, inevitably, individual requirements mean modifications, and installation engineers not directly connected with the ice machine manufacturer may design their own particular systems. The purchaser should therefore ensure that the system installed is suitable for unattended automatic operation, other than for routine checks and maintenance, and the control system should cater for all eventualities, with fail-safe arrangements which allow the plant to be quickly made operational again when a fault has been remedied.
The refrigeration system for the ice maker should normally be a separate unit which only needs a simple control system to maintain it at the appropriate operating conditions. In contrast, a centralised plant serving a number of separate refrigeration requirements needs an elaborate control system, particularly where the refrigeration requirements are independently variable. Centralised units usually have lower capital costs but any shortcomings in their operation compared with individual units may result in revenue losses in other areas such as quality loss in chill stores or associated freezers and cold stores. These losses may offset the saving in capital costs.
Most of the common refrigerants such as ammonia and the halogenated hydrocarbons, known by trade names such as Arcton, Freon and isceon, are normally quoted for ice-making plants. Most ice makers are suitable for use with any one of these. Refrigerant trade names are still widely used, but more correctly they should be named according to the internationally agreed numbering system. Thus, ammonia is known as R717, and the more common halogenated hydrocarbons es R12, R22 and R502. In some cases, the choice of refrigerant will depend on local availability and cost. However, there are many other complex factors to be considered when selecting a refrigerant, and the choice of refrigerant together with the type of compressor and the refrigeration system adopted should be left in the hands of a competent engineer. The ice plant manufacturer, knowing the particular requirements of his own machine, will also be able to help and the potential buyer should supply him with all the information he can about the project.
At the time of writing this document, firm decisions have been made to phase out most of the widely used halogenated hydrocarbons or chlorofluoro-carbon refrigerants (CFCs) because of concern that they are making a significant contribution to depletion of the ozone layer of the Earth's atmosphere. The current status of individual national programmes for the phasing out and availability of refrigerants should therefore be ascertained before a decision is made on the choice of refrigerant.
In multiple unit installations, special care has to be taken with refrigerant distribution to ensure that each ice maker is adequately supplied with sufficient refrigerant. Pump or gravity circulation systems, for instance, should have the refrigeration pipework designed to ensure that unequal pressure drops do not lead to dissimilar refrigeration operating conditions at each ice maker.
In all refrigeration systems, oil is carried over from the compressor sump. This oil may eventually find its way into the ice maker and foul the refrigerant side of the cooling surfaces, thus reducing the capacity of the machine. Oil separators are incorporated in a refrigeration system to minimise this carryover, but it is also necessary to ensure that there is an effective oil return from the ice maker to prevent an accumulation of oil in the mixture. This function is usually incorporated into the design of the unit but with some machines it is also necessary to follow the manufacturers instructions to clear oil from the ice maker at frequent intervals.
Capacity of ice plants
As mentioned elsewhere, a number of factors affect the capacity of an ice maker and its associated refrigeration plant. The tables produced below put into perspective, the consequences of variations in some of the operating conditions in terms of ice making capacity.
Table 5. Variation of ice maker capacity with refrigerant temperature for a small flake ice plant
Table 6. Variation in ice maker capacity with water temperature
The relationship in Table 6 will apply to most types of ice plant and it can clearly be seen that the higher temperature of the make-up water encountered in the tropics will significantly reduce the capacity of an ice plant. Prechilling the water from 35 to 5°C will increase the capacity of a plant by about 20 percent. When supply water temperatures are particularly high, a separate cooling unit should be considered, which will prechill the water more efficiently than the ice maker and may therefore be an economical addition to the plant.
Table 7. Variation in the relative capacity of a refrigeration plant with operating conditions
|Condensing temperature (°C)||Evaporating temperature (°C)|
Table 7 gives comparative values for the capacity of a refrigeration compressor operating over a range of conditions which may be encountered in ice manufacturing plants. The lower the cooler (evaporating) temperature and the higher the condensing temperature the lower will be the capacity of a refrigeration unit. The cooler temperature is often fixed by the requirements of the ice maker and this may only be varied slightly, whereas the condenser temperature depends almost entirely on the locality and the prevailing climatic conditions. A larger compressor will therefore be required to produce a given amount of ice in a warmer country than in a temperate one.
It can be seen from Tables 5, 6 and 7 that the ice maker and refrigeration plant have to be matched to give the required ice making capacity at the appropriate operating conditions.
The higher ice making capacities shown in Tables 5 and 6 may, therefore, only be achieved if the associated refrigeration equipment is increased in size to give the appropriate refrigeration capacity.
Making ice from seawater
When seawater is frozen slowly, freshwater ice crystals are initially frozen out of the mixture. The whole solution will not be frozen until the temperature has reached -22 C, the eutectic point. (The eutectic point is a physical constant for a mixture of given substances.) At higher freezing rates, the ice crystals will be salt-contaminated from the very beginning but this salt will eventually migrate to the outer surface and separate during storage. As the crystals are made mainly of fresh water, the residual liquid will contain an ever increasing concentration of salt as the temperature is reduced.
The special structure of seawater ice gives it different properties from freshwater ice. Seawater ice is rather soft and flexible and, at normal subcooled ice temperatures of -5 to -1 0°C, it will not keep the form of flakes; in fact, at -5°C, seawater ice will look rather wet. For this reason, seawater ice is usually produced at lower temperatures than freshwater ice, and often this adjustment has to be made to the ice maker. Otherwise the plant required is basically the same. Some difficulties have also been experienced with the pneumatic transportation of seawater ice. Even when subcooled, the conveyor raises the temperature sufficiently to make the ice soft, sticky and difficult to move.
Making ice at sea
A number of ice plants are suitable for operation at sea with little modification to their design and they may use either fresh or seawater supplies. Many vessels which process their catch at sea have ice makers installed for cooling the fish during processing. Since they are often at sea for many months at a time, it would be unreasonable for them to carry ice from a shore-based plant. Some fishing vessels have ice plants installed where it may not be economical to have a permanent shore-based plant; for example, because demand for ice is only seasonal due to the type of fishery. Other fishing vessels operate their own ice plant because of difficulties in getting regular supplies without incurring unacceptable delays in port. However, the argument is not clear-cut. The shipborne ice plant occupies valuable space on the vessel and space is also required for ice storage, since a plant which will produce ice to match the peak catching rate without buffer storage would be excessively large. The power requirement is also considerable and, if enough power is not available on board, space will be required for an additional generator. The power to produce 6 tonnes of ice in 24 hours, suitable for a vessel likely to be making weekly trips, is about 30 to 35 kW. The true cost of making ice at sea should be compared with the cost of purchasing ice from a shore-based ice supplier. Even if this is found to be unfavourable, the cost of delays in waiting for ice supplies may sway a vessel owner toward a shipborne plant. The economic factors and the question of continuity of supply, along with the need to avoid contaminated seawater (Chapter 2) have also to be considered before a decision is made.
Solar energy ice plants
In areas where there may not be an on-line energy source to operate a refrigeration plant, the energy from the sun can be used in conjunction with an absorption type refrigeration plant to provide sufficient ice to support a modest operation.
The solar energy refrigeration plant is made as a self contained unit and, for ice manufacture, the only requirement is a suitable water supply. Equipment currently available manufactures block-ice with the blocks weighing approximately 10 kg each. The standard module provides 200 kg of ice per 24 furs, but units for up to 1000 kg per 24 hrs are also available. Output will obviously depend on the hours and intensity of the sunlight each day, therefore an insulated storage facility is also supplied as part of the package, to act as a buffer against daily fluctuations. Fortunately, unlike other refrigeration systems, this unit is more efficient and productive when the ambient conditions dictate that more ice is required.
Since there are no moving parts the equipment needs no maintenance other than weekly cleaning.
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