Introduction
Classification of ice plants
Types of icemaker
Capacity of ice plants
Ice plant requirements
The refrigeration system
Storage of ice
Handling, conveying and weighing
Making ice at sea
Cost of ice plant
Ordering ice plant
This note briefly describes the design and operation of icemaking plants, for the general guidance of fish processors and fishermen. Space, power and refrigeration requirements are discussed, and the main types of icemaker are described. Methods of handling, transporting and storing ice are outlined, and the note also sets out the argument for and against making ice at sea.
The note is intended to serve as an introduction to ice manufacture for the prospective purchaser of plant, and to augment the information in Advisory Note 21 'Which kind of ice is best?'. Manufacturers' catalogues and instruction books give lengthy and detailed accounts of individual plants, and these should be referred to for more precise planning of an installation once the type of plant required has been settled on.
The term ice plant is used in this note to mean a complete installation for the production and storage of ice, including the icemaker itself, that is the unit that converts water into ice together with the associated refrigeration machinery, harvesting and storage equipment, and the building.
Ice plants are usually classified by the type of ice they produce; hence there are block ice plants, flake ice plants, tube, slice or plate ice plants and so on. Ice plants may be further subdivided into those that make dry or wet ice. Dry ice here means ice at a temperature low enough to prevent the particles becoming moist; the term does not refer in this note to solid carbon dioxide. In general, dry subcooled ice is made in plants that mechanically remove the ice from the cooling surface; most flake ice plants are of this type. When the cooling surface of an icemaker is warmed by a defrost mechanism to release the ice, the surface of the ice is wet and, unless the ice is then subcooled below 0°C, remains wet in storage; tube ice and plate ice plants are of this type.
Block ice
Tapered rectangular metal cans filled with water are immersed in a tank containing refrigerated sodium chloride brine. The dimensions of the can and the temperature of the brine are usually selected to give a 24 hour production time, and batches of cans are emptied and refilled in sequence during that period. Ice block weight can range from 12 to 150 kg depending on requirements; 150 kg is regarded as the largest size of block one man can conveniently handle. A block ice plant requires continuous attention and is labour intensive. The icemaker and the store require a good deal of floor space and impose heavy loads on the building structure. For these reasons block ice plants are going out of use, and more modern automatic plants are replacing them.
Rapid block ice
It is possible to reduce the freezing time for blocks considerably, and thus reduce the space required for the icemaker. This is done by reducing the thickness of ice to be frozen; in one type of rapid icemaker this is achieved by passing refrigerant through tubes around which the ice forms and fuses into a block. The blocks can be released by defrosting and harvested automatically, thus markedly reducing the labour requirement, but the storage space required is slightly larger than for the same weight of conventional block ice because the blocks have hollow centres after the tubes are removed.
Flake ice
A sheet of ice 2-3 mm thick is formed by spraying water on the surface of a refrigerated drum, and scraping it off to form dry subcooled flakes, usually 100-1000 mm2 in area. In some models the drum rotates against a stationary scraper on its outer surface; in others the scraper rotates and removes ice from the inner wall of a double walled stationary drum. In some models the drum is horizontal, but more usually it is mounted vertically. No water is sprayed on that part of the drum immediately before the scraper, so that the ice becomes dry and subcooled prior to removal.
Refrigerant temperature, drum or scraper speed, and degree of subcooling are all variable within designed limits so that the capacity of the icemaker and the thickness of the ice can be altered. Typical refrigerant temperature in a flake ice machine is - 20 to - 25°C, lower than in most other types of icemaker, to give rapid cooling and thus make the machine compact. The low operating temperature requires more power, but this is to some extent compensated for by the absence of a need to defrost.
FIG 1. Flake ice machine
Key: 1. Water supply pipe.
2. Rotating drum.
3. Scraper bar.
4. Ice subcooling zone.
Tube ice
Water is frozen on the inner surface of vertical refrigerated tubes to form hollow cylinders of ice about 50 mm in diameter and with walls 10-12 mm thick. The ice cylinders are released by defrosting the tubes automatically, and are chopped into pieces about 50 mm long by a rotating cutter as they slide out. The cylindrical pieces can be subcooled by storing them at - 5°C, but they may require further crushing before being suitable for some applications in the fish industry.
Plate ice
Water is frozen on one face of a vertical refrigerated plate, and the sheet of ice is released by running warm water on the other face of the plate. The size of ice particle is variable, but the optimum thickness is 10-12 mm. The plates are usually mounted in banks, often above the refrigeration machinery, to form a self contained unit. Water for defrosting has to be heated if its temperature is below 23°C. Like most other icemakers the plate ice machine will operate unattended on an automatic timing cycle.
Other icemakers
Machines are available that make ice by methods other than those described here, but the size of unit is usually small, producing at the most only a few hundred kilograms of ice a day; these are suitable for retail and catering services, but are unlikely to be of interest to those providing icemaking services to the catching and processing sectors of the fish industry.
Manufacturers usually quote a wide range of daily output for specific icemaker units, because their capacity can be affected by a number of factors, but this flexibility usually exists only at the planning stage; once the icemaker has been matched to suitable refrigeration machinery under given operating conditions, there is little scope for changing the capacity of the installed unit. Changes in demand are best catered for by reducing running time or by installing multiple units and operating only as many as arc needed.
Since the capacity of both the icemaker and the refrigeration machinery is lower in warmer weather, the size of the plant should be selected for warm weather operation, when demand for ice is also likely to be greatest.
Space
Modern icemakers arc compact in comparison with block ice equipment, but it is not always possible to compare directly the space occupied by different types; for example they may not be available in the same unit sizes. However some guidance on the space requirements for icemakers with a nominal capacity of 50 tonnes a day is given in Table 1; the figures are for icemakers only, and the space for refrigeration machinery, handling and storage will usually amount to far more than for the ice-maker.
Table 1 Space required for an icemaker producing 50 t/day
|
type of ice |
floor area m2 |
height m |
|
block |
190 |
5·0 |
|
rapid block |
30 |
3·5 |
|
tube |
3·3 |
6·6 |
|
flake |
2·7 |
3·7 |
Average power and peak power requirements may be different, and both have to be considered at the planning stage. The average power relates to the energy consumed in making a tonne of ice, and this is important in calculating operating cost. Peak power is important to the designer since it will determine what electrical supply is required, and may also affect operating cost if a peak demand factor is applicable.
The energy required to make a tonne of ice is not constant. It varies widely depending on a number of factors, the most important of which are
type of icemakerEnergy consumption figures quoted by manufacturers for unspecified operating conditions should be used only as a general guide. The values given in table 2 show how energy requirements can increase considerably in warm climates.
operating temperature
make-up water temperature
cooling water temperature
air temperature
size of plant
utilization of plant
method of refrigeration
Table 2 Energy required to manufacture ice kWh/tonne
|
type of ice |
temperate area |
tropical area |
|
flake |
50-60 |
70-85 |
|
tube |
40-50 |
55-70 |
|
block |
40-50 |
55-70 |
Water
In addition to water for making ice, water may be required for cooling, as in a refrigeration plant condenser, or for heating, as in a warm water defrosting system.
The amount of water required for making ice is roughly equal to the amount of ice being produced plus some allowance for wastage and for prevention of build up of solids in the water circulating system.
Fresh water for making ice for use with fish must satisfy the requirements for drinking water. In addition, the chemical composition of water for making ice must meet the equipment manufacturers' requirements; hard water containing excessive amounts of solids may foul the icemaker and may also yield a soft wet ice. On the other hand pure water may cause problems, particularly in flake ice plants, because the ice sticks hard to the drum; the remedy is to fit a dosing device that puts 200-500 g salt into each tonne of water to improve release of the ice without making the ice detectably salty when used on fish.
It is inadvisable to use shell and tube condensers in a refrigeration system where cooling water is run to waste, unless a plentiful supply of cheap water is available, independent of the domestic drinking water supply; otherwise water costs may be prohibitive, since 15 tonnes of cooling water at 10°C or 60 tonnes at 25°C are required for each tonne of ice produced. Other factors can affect cooling water consumption, and manufacturers' precise figures should be used at the detailed planning stage.
Air cooled condensers can be used on small plants, but for most commercial installations evaporative condensers, or shell and tube condensers with a cooling tower, are more likely to be supplied. Evaporative condensers and cooling tower cooling systems normally use less than 1/2 tonne of water for each tonne of ice, plus some small additional allowance if an overspill is necessary to prevent build up of solids in the recirculated water.
Water for defrosting plate icemakers has to be of the same high quality as water for making ice. About 2 tonnes of water is required for each tonne of ice if the water is run to waste, but consumption can be reduced to almost nothing by making a closed circuit and reheating the water between defrosts.
Most modern icemakers are designed to work unattended 24 hours a day with only routine inspection and maintenance. The system is therefore designed for reliability, with safeguards against failure or malfunction. Most manufacturers recommend the refrigeration system best suited to their icemakers, but where local installation engineers propose a system, the purchaser should ensure that the contractor is aware of the need for continuous automatic running and for rapid repair after breakdown.
The refrigeration system for an icemaker should be independent of any other refrigeration requirement; it should not be shared for example with a freezer or a cold store. The only exception to this rule is when a complex system is installed and a competent engineer is in fulltime attendance. Manufacturers often recommend a separate system for each icemaking unit, so that in a multiple unit installation there is considerable flexibility, and a reasonable guarantee that at least some of the units are always in production. Choice of refrigeration machinery and of refrigerant is a job for the refrigeration expert, and the advice of the ice plant manufacturer or competent consultant should be sought before making any decision.
Manufacture of ice can seldom be matched to meet immediate demand; therefore storage is necessary to cater for peak demand and to allow the icemaker to be operated continuously. Storage also acts as a buffer against interruption of production due to breakdown or routine maintenance.
The size of store required will depend on the pattern of operation, but it is never advisable to store less than 2 days' production, and in most installations it is useful to be able to store 4-5 times the daily production.
Stowage rates vary with the kind of ice being made, and Table 3 gives the storage space required for the principal types.
Table 3 Storage space for ice
|
type of ice |
space m³/tonne |
|
flake |
2·2-2·3 |
|
plate |
1·7-1·8 |
|
tube |
1·6-2·0 |
|
crushed block |
1·4-1·5 |
Silo Storage
Silos are generally used only for freeflowing subcooled ice, such as flake ice, and an independent refrigeration system for the silo is essential to keep the ice sub-cooled in storage.
It is usual to provide an air cooler to refrigerate the jacket space between the inner lining of the silo and the outer insulated structure; typically the air cooler is located next to the icemaker on top of the silo, and cold air either falls by gravity into the jacket or is circulated by fan.
Ice is removed from the bottom of the silo, gravity flow being assisted by an agitator, usually a rotating chain; this means the oldest ice is always used first. Ice adhering to the silo wall needs to be freed periodically; otherwise this wall of ice becomes permanent, and only the central core of ice in the silo remains freeflowing.
Silo storage is expensive for small amounts of ice; units have been made to hold as little as 10 tonnes, but silos are best suited for storing 40-100 t.
Bin storage
Bins can be used to store any kind of fragmented ice, and may be of any size from a simple box to hold 1/2 tonne to an installation holding 1000 t or more. Refrigeration of the bin is not always essential but, whatever the size, adequate insulation is necessary to reduce meltage; 100-150 mm of cork, or an equivalent thickness in many other suitable insulating materials, should be used.
A simple bin system is suitable for factories making ice for their own use. The icemaker can be mounted above the bin, so that ice flows by gravity to a take off point at the bottom of the bin; thus the oldest ice is used first. Where ice has to be distributed to customers, bins with a capacity of up to 50 tonnes can be made with a sloping floor and so mounted that rapid discharge direct to lorry or conveyor is possible. Some means of access to the bin is advisable in order to be able to dislodge any compacted ice.
FIG 2. Silo ice store for 10-100 tonnes.
Key:
1. Ice makers.
2. Concrete or steel silo
3. Agitator with chain
4. Sliding hatch.
5. Screw conveyor
6. Ice discharge.
7. Jacket cooler unit.
Depth of ice storage in a bin is limited to about 5 m to avoid fusion of ice under pressure; therefore large bins occupy considerable floor space, and usually require some mechanical means of unloading. Rakes, mechanical shovels and movable screw conveyors have all been used to remove ice from large bins. Rakes and shovels normally remove the topmost layer of ice in the bin, leaving older ice untouched at the bottom. It is therefore necessary to clear the bin periodically to remove all the old ice. Screw conveyors work at the bottom of a bin and remove oldest ice first, but an additional mechanism is required to distribute ice uniformly throughout the bin, and the screw drive takes up some space outside the bin area.
FIG. 3. Small ice store for 5-15 tonnes.
Block ice storage
Block ice can be crushed and stored in the same way as other fragmented ice, but it is more usual to store the blocks and crush them as required before delivery of the ice. Because of their weight and shape it is difficult to store blocks other than in a single layer; thus a considerable floor area is required. However there is usually some extra storage available in the icemaker itself, since all of the ice cans are normally kept full.
Icemakers located directly above the store feed the ice by gravity. Where an ice-maker produces wet ice, it is advisable to drain off excess water before storing it; this is normally done on a conveyor between icemaker and store. Large bins require some means of distributing ice evenly throughout the storage space; silos and small bins do not require such an arrangement.
Both dished belt and screw conveyors are used extensively for transporting ice. Screws allow both horizontal and vertical movement, but can operate only over a limited distance. Belts are generally used for long hauls, and special ribbed belts can be used on an incline. Delivery into lorry or fishing vessel is by means of a chute that can be moved to distribute the ice evenly.
Pneumatic systems have been used for moving ice, but the method is unsuitable for ice that is to be stored again. The energy used in moving the ice is dissipated as heat which can cause some meltage, and more heat is transferred to the ice from the blown air, unless the air is precooled. In addition the ice is broken down into smaller particles by impact on the duct walls, so that a proportion of the ice at the delivery point appears as wet snow that cannot be stored, for example in a trawler fishroom. The use of pneumatic systems should therefore be confined to distributing ice to the point of use, for instance into fish boxes.
Ice can be weighed automatically on a conveyor belt to within ± 2 per cent. Elsewhere ice is usually measured by volume, the contents of a standard container having been weighed to determine the density. Weight of crushed block ice supplied is checked by counting the number of blocks delivered to the crusher.
A number of icemakers arc suitable, with little modification, for use with either fresh or salt water at sea.
Many factory vessels carry icemakers because it would be impracticable to store on board sufficient ice from a shore plant to meet their needs throughout a long voyage. Other vessels carry icemakers where a permanent shore plant would be uneconomic, perhaps because of the seasonal nature of the fishery. Yet other vessels make their own ice because of difficulty and delay in obtaining a regular supply from the port ice plant. But although there are a number of valid reasons for considering manufacture of ice at sea, the prospective owner of shipborne ice plant should bear the following points in mind.
It is rarely possible to match production to demand; therefore some ice storage is still required, and the amount of valuable ship space occupied by the ice plant and the store should be carefully worked out.
There is sometimes insufficient spare power available on board for making ice, and space may have to be found for an additional generator; 30-35 kW would be required for an ice plant producing 6 tonnes of ice in 24 hours, a production rate that would be reasonable for many boats making weekly trips.
The cost of making ice at sea can be higher than the price of ice ashore. A vessel owner who opts for making his own ice at sea may not always be able to draw quickly on a shore supply when his own equipment fails. Subsequent delays may be just as frustrating as those incurred in queucing at the shore plant. Seawater ice is somewhat less suitable than freshwater ice for storing fish, and some ice plant manufacturers now offer desalinators to produce fresh water for making ice on board. Although cheap to operate, more space is again required. Finally, water for ice manufacture cannot be taken from a dock or from inshore areas that might be contaminated.
It is impossible to generalize about cost of ice plant, since so much depends on local conditions. When a new installation is being planned, it may be necessary to take into account cost of land, buildings, roads, electrical and water supplies, drainage and so on as well as the cost of the icemaking and storage equipment. Annual fixed costs are likely to include depreciation, maintenance, interest on capital, tax, insurance and other overheads, while the main operating costs will include power, labour, water and possibly transport.
Cost estimating at an early stage may well influence the choice of plant size, since many of the costs are largely independent of size, and it may prove more economic for the plant operator to make more than he needs himself and become a supplier.
Maintenance cost may be of great importance in remote areas; although modern plants operate with minimum attention, regular professional maintenance is necessary. Direct comparison of capital and running cost of different types or makes of ice plant is not possible except on so general a basis as to be of little value to a prospective purchaser; each particular project has to be costed individually, using current prices.
The more information the prospective buyer gives about local conditions and requirements, the easier it is for manufacturers to submit comparable tenders for the equipment. Initial planning may have enabled the buyer to make some decisions about the type of ice required, location of plant, building layout and so on, and the following check list indicates the kind of information the prospective customer should make available to the manufacturer or supplier.
Type of ice required, intended use, and production rate.
Ice make-up water: temperature, mains pressure, purity and so
on.
Cooling water: details of supply if from a separate source.
Electrical supply.
Ice storage capacity required: type required, space available,
and ancillary handling equipment needed.
Sketch of site with preferred layout.
Note of any local restrictions on buildings and services that
may affect the design of the installation.
