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Produced by: Torry Research Station Title: Stowage of fish in chilled sea water...
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Introduction Design of fixed tanks Portable tank system Portable or fixed tanks?
Introduction This note discusses the advantages and disadvantages of stowage in chilled sea water in comparison with stowage in ice, and briefly describes the design and operation of suitable systems. Sea water can be cooled on board the catching vessel in two ways, by the addition of ice or by means of mechanical refrigeration, and shipboard tanks can be either built into the hull or portable for easy transfer to and from road transport at the quayside. Chilled sea water or ice? Stowage in chilled sea water offers the following advantages over stowage in ice; the catch is cooled more rapidly, less effort is required to stow and unload it, and there is less likelihood offish being crushed or losing weight. In addition sea water can be safely lowered in temperature to about -1°C without freezing the fish contained in it. Other advantages are effective washing and bleeding, and a tendency to firm the flesh of the fish, which can aid further processing. Disadvantages of the method which preclude its general adoption are as follows; some species, herring for example, keep as well or a little better than in ice for 3-4 days, but thereafter spoil more quickly, and some species take up unacceptable amounts of water and salt when kept in sea water. Other species, capelin for example, are reported to keep better in ice even during the first few days. For these reasons the method is usually confined to short term storage of particular species that are caught in large quantities within a short time, for example herring, mackerel, sprats and blue whiting in the UK fishery, particularly since these fish are usually too small and too numerous for gutting or sorting for size to be practicable prior to stowage. Seawater systems The two most practicable methods of cooling sea water are by mechanical refrigeration or by the addition of ice. Although both systems are dealt with in this note, under the general heading of chilled sea water, csw, that term is by custom also used more specifically to mean sea water cooled by the addition of ice, in order to distinguish it from sea water cooled by mechanical refrigeration and known as refrigerated sea water, rsw. Either method can be used for a fixed tank installation, but mechanical refrigeration is not generally practicable for portable tanks. Fixed tank system A number of UK purse seiners 25-30 m in length have fixed seawater tanks; most have 3 tanks, each holding 20-40 tonnes, built as an integral part of the hull between the engineroom and a much reduced fishroom, but a few have 6 tanks and no fishroom. Both csw and rsw are employed for cooling the catch. Refrigerated sea water The layout of a typical rsw system is shown in Figure 1. Operation of the system is outlined here; construction and insulation of tanks, and other design matters common to both rsw and csw systems, are discussed in more detail later. Fig. 1 One tank, usually the centre one, is filled with sea water soon after leaving harbour, and the water is cooled by the refrigeration plant on the outward passage to bring the temperature down to 0°C before the first fish come aboard. A rough estimate is made of the size of each haul while the fish are still in the net, in order to decide how many tanks are required; each selected tank is one quarter to one third filled with precooled sea water from the centre tank before putting in fish. The water in each tank offish is then circulated by drawing it from the top of the tank, passing it through a shell and tube heat exchanger, and returning it to the bottom of the tank, thus forcing it up through the mass offish to promote rapid and uniform cooling. The water should be circulated continuously until the vessel is unloaded to prevent temperature layering and the formation of pockets of warm water. Where the water can be circulated in only one tank at a time, and more than one tank is filled, the water in each tank in turn should be circulated for 1-2 hours at a time. Chilled sea water The layout of a typical csw system is shown in Figure 2. The first requirement is to estimate the amount of ice needed to cool a tank full of fish and water, making allowance for ice meltage during the outward passage; the amount will depend on the size of the tank, the degree of insulation, the sea and air temperature and the length of trip. For UK vessels in summer the amount can vary between 4 and 10 tonnes for each tank. The estimate can be altered on subsequent trips in the light of practical experience, but if there is any doubt it is better to take too much ice rather than too little. More detailed advice on estimating ice meltage in insulated and uninsulated tanks is given later in this note. FIG. 2
Once the tanks have been loaded with the correct amount of ice, they should be left undisturbed until fish are about to come on board. When fish are alongside in the net, the meltwater in each tank should be pumped overboard; this flushes the pipes and removes any pieces of fish remaining from a previous trip; the amount of cooling wasted by doing this is negligible. The congealed mass of ice is broken up so that it will mix easily with fish and water, and clean sea water is added; ice and water together should constitute one quarter to one third of the volume of the tank. It is difficult to judge how much water to add, but as a rough guide there is enough when the ice just floats and no more. Fish can then be loaded into the tank; when the tank is full, the proportions of sea water, ice and fish should be about 1:1:4. The water in the filled tank is circulated by pump, but not continuously; this is because the pump usually also serves to fill and empty the tank and possibly does duty as a standby bilge pump; it is therefore powerful, up to 7 kW, and the heat generated during continuous running would cause excessive ice meltage. It is recommended that after an initial circulation period of about 2 hours the pump is stopped for 2 hours and run for 1 hour alternately; this will be sufficient to prevent temperature layering in the tank, but at the same time keep down the heat input from the pump. Ideally there should be one large pump for rapid filling and emptying that could also be used for other ship duties, and small individual circulating pumps on each tank, but this arrangement is not always practicable. Tank cleaning After fish have been unloaded, all remaining water should be pumped out, and the tank scrubbed and hosed with clean water and a suitable detergent; high pressure hosing makes cleaning easier and more efficient. Filters should be removed and cleaned. The tank is finally filled with clean water, not dock water, to a depth of at least 250 mm which is then pumped overboard to flush through the pipes. The tank is then ready to receive clean sea water or fresh ice for the next trip.
Design of fixed tanks Fixed tanks are normally made of steel plate, and should be well insulated. The interior of the tank should have no protrusions or awkward corners that cannot easily be cleaned; otherwise small pieces of fish and other debris may remain lodged there to contaminate subsequent catches. Strainers or gratings within the tank should be removable for cleaning, and wells or sumps should be capable of being hosed and pumped dry. Distant reading thermometers should be provided for each tank with the indicators in a prominent position in the wheelhouse. In an rsw system, the heat exchanger should be protected by a filter to prevent it becoming fouled by debris from the tank. Twin filters are ideal; each can then be opened in turn for cleaning without stopping water circulation. In a csw system, where there is no heat exchanger, fine filters are generally unnecessary, but a strainer is necessary on the pump suction pipe within the tank to prevent whole fish from being sucked in. A simple and efficient design is a perforated steel plate with holes not more than 15 mm diameter for herring or 8 mm for sprats; where a variety of species is likely to be handled, the holes should be small enough to stop entry of the smallest fish. The total area of the holes in the plate should be at least four times the area of the suction pipe and should occupy not more than 40 per cent of the area of the strainer plate. Tank insulation Ideally all tanks for sea water stowage should be fully insulated to keep heat leakage to a minimum. In most UK. commercial installations the tanks are built onto the ship's hull frames, and insulation is foamed in place to a thickness of 100-150 mm between tank wall and hull plating. This method of construction is not entirely satisfactory, because the steel hull frames short circuit the insulation, so that the heat gain is much higher than in a fully insulated tank. Nevertheless partial insulation in this manner is favoured by shipbuilders and vessel owners because construction is easier and cheaper than for a fully insulated tank. But it must be borne in mind that poor insulation has to be compensated for, either by greater refrigeration capacity in an rsw system or by more ice in a csw system, to cope with the additional heat leak. TABLE 1 Heat Leakage and Ice Meltage in Shipboard Sea Water Tanks Surface
Area
Temp diff
Fully insulated tank
Partially insulated tank
Uninsulated tank
Heat transfer coeff.
Heat leak
Equiv. ice meltage
Heat transfer coeff.
Heat leak
Equiv. ice meltage
Heat transfer coeff.
Heat leak
Equiv. ice meltage
m2
°C
W/m 2 °C
W
kg/h
W/m 2 °C
W
kg/h
W/m 2 °C
W
kg/h
Deckhead
29·6
20
0·243
144
1·6
3·65
2160
23
32·1
19000
203
Floor
29·6
20
0·239
141
1·5
3·58
2120
22
9·1
5390
58
Engineroom bulkhead
19·0
30
0·238
136
1·4
3·57
2040
21
8·2
4680
50
Forward bulkhead
19·0
5
0·238
23
0·2
3·57
340
4
8·2
780
8
Ship side above waterline
9·3
20
0·243
45
0·5
3·64
680
7
32·1
5970
64
Ship side below waterline
9·3
15
0·245
34
0·4
3·67
510
5
435
60680
647
523
5·6
7850
82
96500
1030
Total
115·8
Estimating heat leak The heat leak into a tank on a particular vessel can be estimated fairly accurately; comparative figures are given below for three typical purse seiners each fitted with three 25 t tanks; one ship has uninsulated tanks, one has partially insulated tanks welded to the hull frames, and the third has fully insulated tanks. The calculations have been made for an outboard tank, which for simplicity is considered to be rectangular, 3-80 m long, 780 m wide and 2-44 m deep. The outside seawater line is taken to be at half the tank depth. The results for the fully insulated and the uninsulated tank are calculated, but the result for the partially insulated tank is based on the observation in practice that heat passes through its walls at a rate 15 times greater than through those of a fully insulated tank. The fully insulated tank has 100 mm of polyurethane foam between the 6 mm steel hull plating and the 5 mm steel tank wall, with no steelwork penetrating the insulation. The uninsulated tank is an integral part of the hull, with the ship's side and the forward and after bulkheads forming the walls of the tank. The following temperatures have been assumed: outside air 20°C, sea 15°C, engineroom air 30°C, fishroom air 5°C and tank interior 0°C. Table 1 gives the heat transfer coefficient for each surface of the tank, that is the measure of the rate at which heat passes through the surface, the total heat leak through each surface and the equivalent rate of ice meltage. Table 2 shows the total cooling required in terms of mechanical refrigeration for rsw, and ice consumption for csw systems. The figures are based on a typical herring purse seiner on a normal fishing trip which entails a 12 hour journey to the grounds, a 24 hour period for searching and catching, and a 12 hour journey home. For longer voyages the quantities of ice will need to be increased. TABLE 2 Total Cooling Capacity Required for a Partially Insulated Tank Tank Capacity tonnes
Cooling per tank rsw kW csw tonnes ice
20
58
5
25
73
6
30
87
7
35
102
8
40
116
9
Portable tank system The main advantage of a portable tank is that the fish can be held undisturbed in chilled sea water at 0°C from the time of catching until they reach the processing factory. On long overland journeys in warm weather it is possible to run off some water and add more ice. Summer herring can be kept in csw at 0°C for up to 84 hours and still give a good product. Size of container is important; it should be large enough to hold the desired quantity of fish plus the correct proportion of water and ice, but not so large and unwieldy that it cannot be easily manoeuvred into position aboard ship or be handled easily on shore. Tanks holding up to about 1 -5 tonnes of fish have been found to be practicable. A mixture of roughly one part each of ice and sea water to three parts of fish by weight is sufficient in a fully insulated tank to cool the contents rapidly and uniformly to 0°C and to keep them at that temperature for about 3 days with an outside temperature of 20°C. Trials have been made in the UK with light alloy tanks insulated with polyurethane foam protected by a removable jacket of glass reinforced plastics; each tank had a capacity of 2-1 m 3 and when full held 1350 kg of fish, 500 kg of sea water and 450 kg of ice. These tanks were well suited to the purpose, could be manhandled when empty and, with suitable lifting equipment, could be handled comfortably when full. Herring reached their destination in good condition after 3 days' stowage, and there was little or no damage due to crushing of the fish. The method of operation is as follows. The hold of the vessel is loaded with as many tanks as possible, each charged with the prescribed amount of ice. When fish are in the net alongside the vessel, the catch is estimated and the required number of tanks made ready by metering in the correct quantity of clean sea water. As the fish are taken aboard they are directed by chute into each tank in turn. Once a tank is full, the lid is closed and the contents agitated for at least 6 hours by injecting compressed air or nitrogen into the bottom of the tank, to ensure rapid and even cooling; the gas flow rate should be 2-4 kg/h at a pressure of about 35 kN/m 2. Failure to circulate the mixture of ice and water in this way will result in fish at the bottom of the tank remaining unchilled. On arrival in port the tanks are unloaded by crane and transported unopened to the processing factory, while the vessel takes on another batch of clean empty tanks charged with ice and goes fishing again with minimum delay.
Portable or fixed tanks? Where there is a long overland journey from quayside to factory or inland market, fish in portable tanks are more likely to arrive in better condition than fish taken from fixed tanks, principally because the fish remain undisturbed during unloading and transportation. On the other hand fixed tanks make better use of stowage space than portable ones; a purse seiner can hold at least three times as much fish in fixed tanks, unless the portable tanks and the ship are tailormade for each other, when it is possible to improve the stowage rate considerably. Removal of full portable tanks from a properly fitted ship and replacement with empty ones ready for sea can be a quicker, cheaper and easier operation than unloading the same amount of fish from fixed tanks and icing it in boxes on a lorry, but at least three sets of portable tanks are needed to operate a single ship continuously; while one set is at sea, a second is in transit ashore and a third being cleaned and iced ready to load at the quayside, although a tank pool serving several ships would reduce the capital outlay. Thus a portable tank system can reduce the turnround time of the ship in port, but at the cost of some loss of stowage space and a greater outlay in equipping the ship. If you have any queries, write, phone or call at either of the addresses given, below: The Director
The Officer in Charge
Torry Research Station Humber Laboratory PO Box 31
Wassand Street
135 Abbey Road
Hull
Aberdeen
HU3 4AR
AB9 8DG Tel: 0224 877071
Tel: 0482 27879
Other recent Notes in this series, which are available free .of charge in the UK from the above addresses are: 61 Gaping of fillets, by R. M. LOVE. 62 The freezing time of fish, by F. J. NICHOLSON. 63 Fishing ports in the UK, by J. J. WATERMAN 64 Fish silage, by I. TATTERSON and M. L. WINDSOR. 65 Fishworking machinery, by S. MAIR 66 Handling and processing mackerel, by J. N. KEAY. 67 The haddock, by J. J. Waterman. 68 Icemaking plant, by J. GRAHAM. 69 Cook-freeze fish products, by J. N. KEAY. 70 Advice for the fish industry; who does what, by J. J. WATERMAN. 71 Processing cod the influence of season and fishing ground, by R. M. Love. 72 Reducing odour in fish meal production. Earlier notes in the series, most of which are still available, are summarized in 60 Key to Advisory Notes 1-59, by J. J. WATERMAN. Crown Copyright Reserved