Document 3Na1yEnj1482738X5MgR699Jy

612 CHAPTER 57 1962 Guide And Dafa Boot for dairy products, one for fruits and vegetables, one for meats, and a fourth for frozen foods, each would have its own Refrigerant 12 condensing unit with associated cooling unite. This provides maximum protection of perishables in of mechanical failure of any important component. A hetter ar rangement will provide one or more standby condensing units. By proper piping and valving, the standby unit can be connected to substitute for any unit which has failed. Unit installations axe those in which a ringto condensing unit is applied to each fixture, an arrangement preferred by some engineers. Cooling units. The choice of cooling units for the refrig erated rooms is somewhat controversial in the manno field, During and since World War II, there has been a growing tendency to use packaged forced-air cooling units with finnaH tubes to speed and reduce the cost of the installation. The dunce has also been prompted by the safe non-toxic halo* genated hydrocarbons. Automatic defrosting, where average room temperatures are about 34 F and above, is considered practical. Air distribution should be worked out so that the force of the air blast is directed above the food and allowed to diffuse downwardly, returning to the cooling unit, for re circulation. One or more of the units may be inatellAd in a room after careful consideration of air distribution. If finned tube forced air units are used for temperatures below 34 F, some positive means of defrosting must be pro vided. Hot gas, electric and water defrost are successful if properly applied and used. When wall coils are used, paneled should be jnxfadlari over them-to prevent the produce from contacting the low temperature surfaces and to promote positive air circulation. In designing direct-expansion wall coils, the circuits must not be so long that excessive superheat of the expanded refrigerant results. On the other hnH a shortage of eonling surface results in excessive differences between refrigerant end room air temperature, with consequent dehydration of stored products. The proper layout of wall coils requires that the expansion of the refrigerant be upward in the first series of coils and downward in the last coil to assure correct oil return to the compressor. Where coils are connected into a common overhead suction main, the piping from the coil c*n be ar ranged, which will avoid collecting the oil draining from the suction line. Where suction lines must be run along or under the deck, a trap should be placed at the end of the run where tiie line rises to the compressor. If possible, the line should elope to the trap. Controls. The most reliable system is one with a minimum of automatic apparatus which affords the utmost in sim plicity. For marine applications, liquid controls should be of the automatic expansion type. Either constant pressure or thermostatic type valves are suitable; the latter is preferable for most applications. Rolling, pitching, trim, and list of vessels make it difficult for float valve controls to function cor rectly. Expansion valves, being spring operated, are not af fected by these conditions. Capillary tubes are commonly used in unit refrigerators and will function on board vessels. If a float type control is unavoidable, its vertical plane of opera tion should be fore-and-aft rather than athwartship. Float controls are unsuitable in vessels with propulsion machinery aft or which normally trim aft. Temperature controls using thermostats, low prepare controls, evaporator regulating valves, and solenoid valves perform satisfactorily in ships' service systems. Such equip ment, however, must be designed to be unaffected by vibration or ship's motion. Mercury bulb controls should not be used. Direct-expansion controls should be located adjacent to their respective units, be installed so that they are clear f ordinary hazards of damage due to handling stores, ha^ strongly built guards, and be fitted with locked covers to prevent tampering with the Sensitive apparatus by uq. authorized persons. Controls should be installed outside the refrigerators. On closely coupled systems using a central plant they may be installed in the machinery room without benefit of guards. On vessels outfitted with alternating current electric m. terns, the use of factory assembled refrigerators is possible. Load Calculations The calculation of refrigerant loads of a ship's service re frigerator system should follow the mme procedure indicated (or cargo systems in the section Marine Cargo Refrigeration. A larger margin for miscellaneous heat gain, such as frequently opened doors, should be allowed. An arbitrary figure of from 10 to 25 percent of the total load may be used. PART HI: FISHING BOATS As fishing vessels are forced to travel longer distances to find desirable and profitable varieties of fish to satisfy the market, the need for refrigeration becomes more acute. Fish is a very perishable commodity and must be quickly lowered to and kept continuously at 32 F or below if undesirable bacterial action and spoilage are to be kept to a minimum On long trips most of the gmaller fishing vessels use crushed ice, supplemented in some cases with mp^hani^Al refrigera tion. When properly used, ice (or ice and salt) will bring the temperature of the fish down to 32 F, and in favorable cir cumstances as low as 26 F. AMERICAN PRACTICE This section deals with the refrigeration required on fish ing boats for the purpose of preserving the fiah in a condition from the time they are caught until the vessel re turns to port. Freezing fish at sea in the ordinary fishing ves sels, except in the case of the tuna clipper, is not a common American practice though it has been successfully used in France. Plate freezers for shrimp, and freezer storage bolds, aboard vessels have been tried out in the Gulf of Mexico fishery with considerable success. One of the latest developments has been the use of re circulating refrigerated sea water, instead of ice, for holding the fish in a satisfactory condition after being caught. The sea water is continuously circulated by mAana of pumps, through the fish and over baffled cooling coils placed along one side of the insulated tank, or through external brine coolers. Welded reinforced aluminum tanks and aluminum coaling coils have been used successfully. Ammonia and Refrig erants 12 and 22 have been used. The mUpr trailers, thus equipped, require about 1 ton refrigeration capacity for each 2 tons of fish capacity, while the larger fishing vessels re quire about 1 ton refrigeration capacity for each 5 to 8 tons fish capacity. REFRIGERATION WITH ICE The most common method for preserving fish at sea in fishing vessels is by means of ice. Bin or pen boards may be installed to divide the hold as desired (Fig. 10). Ice is usually stored in alternate bins so that it will be handy for packing around the fish as it is loaded into the adjacent bin. The Marine and Air Transport 613 gashed ice varies in size all the way up to lumps the size of a 6st. ^ the fish are stowed with crushed ice, each pen is generally divided horizontally by the additional insertion of horizontal hoards so that the bottom fish wiii not be crushed by the weight of ice and fish above. The compartmented sections jkould not be more than 30 in. high if undesirable crushing and bruising of the bottom layer of fish are to be avoided. ' The approximate amount of ice required is 1 ton for each 2 tons of hah capacity in summer, and 1 ton to each 3 tons of fob capacity in winter, based on a voyage of about 8 days. Leg ice is needed if the ship has mechanical refrigeration. The method of stowing the fish in ice. is very important'to the keeping qualities. The depth of ice on the floor of the pen gfrw.M be a minimum of two inches at the end of the voyage. Experience has shown that this is obtained by having the initial- biding of ice one inch thick for each day of the voyage. In stowing, one or two layers of fish are laid upon the b~jr|ing ice in such a way that the ice is just covered com pletely. In no case should the layer of fish exceed 12 in. in ftirlmpga- The top covering layer of ice is about 9 in. thick, heaped up higher in the center -than along the sides. This method of stowing permits the pile to adjust itself to subse quent melting and settling, and results in good drainage of water and fish from tiie pile. Most mn fishing vessels are constructed of wood and the holds are not insulated. A few of the newer vessels are of steel construction with insulated holds. Mechanical refrig eration is used on some of the vessels to keep the ice from netting go fast, and to maintain lower temperatures. The most common type of mechanical system employs direct- expaction cooling coils under the deckhead of the hold and nmatimpa amimH the entire shell. Another system uses a double wall construction with cold air being circulated around the entire hold between the insulated hull and deck, and an mnpf fnatal lining. In the Atlantic offshore fighariaa tiie ice system is exten sively used for cod, haddock, rosefish, flounders and other varieties of bottom fish. On both Atlantic and Pacific Coasts, many tuna trailers use ice for tiie preservation of their catch. On the Pacific Coast refrigerated sea water seems to be gradually Higpla^ing tiie ice in many areas. Ice is used ex tensively for the stowage of headed shrimp aboard trawlers operating in the Gulf of Mexico. Tig. 10 .... Typical Layout of Pens in Hold of New England.Trawler MECHANICAL REFRIGERATION Tuna Seiners Most of the refrigerated California tuna seiners are dual purpose ships used for both sardines and tuna. Sardines are caught on short hml *nH refrigeration is not used. In order not to interfere with Bardining, the floor coils of the hold are flanged and maHa removable so that they can be taken out and stored ashore during the sardine season. Sardines are brought in at a temperature of approximately 70 F. Upon unloading and weighing, they are flumed into storage tanks and cooled in 30 F brine at the cannery for a period of eight hours before processing and canning. When fishing for tuna, the cooling system is used to keep the crushed ice from melting on the outgoing trip. On the re turn trip when the hold is loaded with tuna packed in ice, the refrigeration is operated continuously to bring the hold tem perature down to 25 F or to 20 F and partially freeze the fish. To prepare for unloading just before reaching port, clean sea water is pumped into the hold to loosen the fish from the ice so that they can be removed from the hold. On the larger seiners, the refrigeration equipment is usually powered by electric motors. On tiie smaller vessels small diesel Anginas are used, or the refrigeration compressor may be Vbclted from the main propulsion engine. When a separate Angina is used for powering the refrigeration, the condenser pump is normally driven by the same engine. The usual marine practice of having duplicate equipment to prevent cargo loss in case of breakdown is not as a rule carried out on seiners. There arc several reasons for this, one of which is the cost. Another reason is that the ordinary seiner does not bold as much fish as the larger clipper and consequently does not go as far or stay out as long. The ice carried on a seiner* will prevent loss of the fish on a run back to port in case of serious breakdown of the refrigeration machinery. The addition of steel cargo wells on seiners, the same as are used on tuna dippers, came as a natural development as larger seiners were built for longer trips in search of tuna. One reason for this is the large fuel oil bunkerage space required for the longer trips. The steel cargo wells serve a dual purpose, as auxiliary fuel oil tanks on the outgoing trip, and as fish holds after being thoroughly scrubbed down for the return trip. In order not to reduce the holding capacity for the sar dine operation, large manholes are provided through tiie deck for loading each tank, and several huge removable bolted steel plate patches are provided in the tides of each tank. The patches are removed during the sardine season. Most large long range seiners today have at least two tanks in the hold space, and some have the entire hold space devoted to tanks like the all brine tuna clipper. In the larger brine seiners, multiple refrigeration compressors are provided as in the case of the tuna dippers. As the main purpose of the refrigeration on the small tuna seiner is to keep the ice from melting, best results are ob tained by blanketing all surfaces of the hold with cooling coils. Standard practice is to use in. steel pipe coils hot dipped galvanized on the outside only and refrigerated by direct expansion ammonia. Refrigerant 12 systems generally use one inch pipe to obtain higher refrigerant velocities ami better oil return. The greatest source of heat gain is the deck surface exposed to the heat from the sun. Therefore, the in. overhead coils are closely spaced from 5 to 8 in. on centers (4 to 6> in. for 1 in. pipe). Less heat comes through the skin of the ship so the lK hr- coil spacing on the free-board is from