FISHING VESSEL REFRIGERATION 
and 
SEAFOOD HANDLING

by Chris Johnson

INTRODUCTION

The international seafood markets have experienced tremendous growth in the last ten years. One factor contributing to this growth is the utilization of seafood resources by less developed countries as foreign exchange. These countries have found that their seafood products can bring in large amounts of hard currency from exports. The three largest customers for these products are the European Community, Japan and the USA. 

In addition, wide-body jet aircraft can carry fresh products from almost any country in the world to the available markets. A good example is the fresh "sashimi" grade tuna arriving in Japan from island countries like Fiji, Samoa, Tonga and Tahiti. The fishing industry has one of the fastest returns on investment for export products. 

A fisherman can unload his catch today and in a short time, the products are sold in markets like Tokyo, Los Angeles or New York.

The quality standards being required by these markets are very specific as demonstrated by HACCP in the USA and ISO 900X in the European Community. 

Because of demands for high quality, if fishermen want to export their products, they must have the ability to rapidly lower the temperature of their catch, when it is caught. 

This requires that virtually all fishing vessels must have sufficient, onboard refrigeration, if only in the form of ice. Many fishing vessels in the past made short trips and sold their catch locally. The seafood was consumed the same day or preserved by either salting or drying. While this practice will continue, many fisherman want to improve the standard of living for their families by exporting a higher value product to lucrative foreign markets. 

Therefore, fishing vessel refrigeration is one of the most important aspects of the growing global seafood economy.

SEAFOOD SPOILAGE

Fish, shrimp, crabs, lobster, squid and octopus all begin to spoil the minute they die. Death can occur in the water or onboard the vessel. Spoilage takes place by bacterial, enzymatic or chemical action. Bacteria are not very concentrated in the meat of the living animals. Bacteria are found in high numbers on the gills, in the intestines and in surface slime. These bacteria do little harm to a healthy, live fish. However, the minute the fish is dead, bacteria invade the fish flesh, which begins to spoil. Enzymes in the living fish continue actively after death and cause unwanted flavor changes during the first few days of storage.

In addition to bacterial and enzymatic changes after the fish dies, chemical changes involving oxygen in the air reacting with the fat in the fish create unwanted, rancid flavors. Bacteria, enzymes and chemical reactions all have something in common; they spoil fish faster with higher temperatures. Preventing rapid spoilage is accomplished in three ways; care in handling, cleanliness and chilling.

Care in handling includes anything that can be done to prevent bruising, cutting or scraping of fish which allows easy invasion of the flesh by bacteria. This can also include choosing a fishing method to prevent such damage. Hook and line fishing techniques produce fish with far less damage than fish caught in nets or traps.

Cleanliness means removing the intestines and sometimes the gills followed by careful washing with clean water. Cleanliness continues with constant attention being paid to hygiene until the fish is consumed.

Chilling the clean seafood to 0°C, as rapidly as possible, is the single most important factor to insure top quality products.  Even when the seafood is to be frozen, rapid chilling is critical for quality. 

Freezing does not improve quality but only slows or stabilizes the spoilage. Bacterial, enzymatic and chemical actions continue after freezing at temperatures above -30°C. Some chemical reactions, such as fats becoming rancid, continue even below -30°C. Seafood with high fat content should be kept at -40° to -60°C, if held for long periods of time. For fresh product, delays of a few hours can shorten the shelf life by several days.

When temperatures are above that of melting ice (0°C), bacteria multiply very rapidly as the temperature increases. For example, fish with a shelf life of 15 days @ 0°C will keep for 6 days at 5°C and only 2 days at 15°C before being considered inedible. This may vary depending on the temperature of the water from which the fish were captured and the size of the fish. For example a 100 Kg tuna takes many times longer to remove the heat from the backbone than does a 5 Kg bottom fish. These times can be improved by chilling in an ice and seawater slurry. It is most important "TO CHILL THE CATCH AS FAST AS POSSIBLE." Chilling the catch requires refrigeration and, unless supplied by ice produced on-shore, all fishing vessels need refrigeration equipment onboard.

REFRIGERATION EQUIPMENT COSTS AND PLANNING

The following is to help the fishing vessel owners estimate their refrigeration needs. This does not treat all possibilities or conditions, which may be present.

Equipment Costs and Product Loads

Equipment cost comparisons (approximate and without installation)

ABOVE FREEZING $ PER TON OF REFRIGERATION

RSW (Chilling only)
225 TR @ -4°C

$1,270/TR.
BELOW FREEZING $ PER TON OF REFRIGERATION

Ice Machines
2 tons per 24 hrs
3 TR @-30°C

$7,600/TR.

Dry Hold Evaporator Plates
(storage only)
2.5 TR @ -29°C

$8,614/TR.

Plate Freezer
4.5 TR @ -35°C

$8,980/TR

Brine Freezer
3 TR @ -26°C

$7,786/TR

Air Blast
3 TR @ -32°C

11,560/TR

These comparisons are only meant to show a relative cost between different evaporators using marine industrial condensing units. The refrigerant is Freon R22 except for the RAW system which is Ammonia R717 For the purpose of pricing equipment for your fishing vessel or your fisheries, refrigeration loads would have to be calculated.

ICE requirements on a fishing vessel vary greatly with ambient water and air temperatures and the method of handling the ice onboard a particular vessel. In general one or two Kg of ice is needed for one Kg of seafood. It doesn’t matter whether you make ice onboard or carry the ice with you. For onboard ice making equipment install a machine large enough to produce 2 Kg of ice for each Kg of fish captured per 24 hours. Example: A tuna longline boat with an average daily catch of one ton should have an ice machine with a capacity of two tons per 24 hours.

RSW (Refrigerated Seawater) systems can be calculated as follows:

Example : To determine the total refrigeration capacity per hour to lower the temperature of 200 MT seawater from 20°C to 5°C in 6 hours.

LOAD CALCULATION FORMULA #1

The 155.4 tons of refrigeration still has to be adjusted for various other losses and heat gain, which could be estimated at 10%.

FREEZING equipment capacity is calculated as follows:

Example: To freeze 500 Kg of shrimp in 3 hours with a double contact plate freezer. The entering temperature of the shrimp is 25°C and the final desired temperature is -18°C. The freezing point of shrimp is -2°C. When products are frozen, it is important to understand that, at the temperature of freezing, a large amount of refrigerating capacity is required to freeze without a further drop in temperature. This capacity is known as the LATENT HEAT OF FUSION. Once frozen, the product will begin to decrease in temperature but at a different rate than above the freezing temperature.

Other considerations for choosing this equipment are thickness of box (assumed here to be 70mm) and the quantity of boxed shrimp per plate (station). In this example we assume that the plate freezer will have a sufficient number of stations to freeze 500 Kg per 3 hr. cycle.

LOAD CALCULATION FORMULA #2

For accurate equipment selection, there are many other details to be considered. Your refrigeration company should be willing to help you with choosing the correct operating conditions, such as the suction and discharge (condensing) temperatures.

These examples are for estimating equipment size and budget requirements during the early planning for refrigeration. You must use a professional for final selection.

REFRIGERATION FUNDAMENTALS:

The purpose of the refrigeration system is to remove heat from your seafood products as fast as possible into the evaporator and discharge the heat overboard through the condenser.

It is important that fishing vessel owners, captains and engineers understand the basic refrigeration system if they are to make informed choices about the equipment best suited to their respective fishery and vessel. As with the main propulsion and auxiliary engines, proper preventative maintenance of refrigeration equipment will give longer, breakdown-free service.



Figure 1 shows the basic system for all marine refrigeration. The refrigerant as low pressure vapor "A" is compressed to high pressure by the compressor "B". The hot vapor "C" passes through an oil separator "D" and then is cooled, which causes it to become liquid in the condenser "E". Liquid refrigerant, at high pressure "F", flows into a holding tank called a receiver "G". The liquid then passes through a filter-drier "H"and heat exchanger "I", which lowers the temperature of the liquid before reaching the expansion valve "J". The expansion valve meters the high pressure liquid as it rapidly drops to low pressure. This very cold low pressure liquid enters the evaporator "K" where it gains heat and boils into a vapor. As the low pressure vapor returns to the compressor it passes through a suction accumulator "L" where any unvaporized liquid accumulates. IF LIQUID REFRIGERANT RETURNS TO THE COMPRESSOR, SEVERE DAMAGE CAN OCCUR.

EVAPORATORS

There are many types of evaporators which include RSW (Refrigerated Seawater) CHILLERS, ICE MAKERS, DRY HOLD PLATES, PLATE FREEZERS, FORCED AIR FREEZERS, AND BRINE FREEZERS.

RSW CHILLERS are heat exchangers where seawater comes in direct contact with pipes or tubes containing refrigerant. Most frequently, these tubes are made of 90/10 cupronickel, stainless steel or titanium and are enclosed inside a large pipe (shell) or in a sealed galvanized steel box. The tubes can lay in the fish hold water, but this is very inefficient and subjects them to damage.

ICE MAKERS for shipboard can use plates, tubes, or drums through which refrigerant passes. The water, most often seawater, flows over the surfaces and forms ice. Several types of ice makers require cycles of freezing, defrost, and harvesting, which require electrical timers, relays and solenoid valves. For onboard fishing vessels, the defrost harvesting machines are not practical because of maintenance. The constant harvesting ice machines are much more reliable. An example of constant harvesting is the drum flake ice machine with a rotating blade. All shipboard ice makers should have stainless steel ice making surfaces. Most other materials, including aluminum, will have a reduced service life. When choosing an ice machine, it is most important to buy a machine which is unaffected by the ship pitch and roll of up to 35°. There are also liquid ice (slurry) generators now available where ice is formed on the inside surface of a liquid filled chamber. When the ice is concentrated in the slurry mixture it is pumped to the various fish holds. This equipment requires more complicated electric controls which are subject to the corrosive effects of saltwater and could possibly require more maintenance.

DRY HOLD EVAPORATOR PLATES are constructed of 3.5mm thick x 204mm wide marine alloy aluminum plates through which passes the refrigerant in one or two tubular spaces. These plates are banked together and can be custom made to fit any fish hold. This type evaporator requires no fan motors, switches or defrost timers. and is maintenance free for many years. These plate banks are used for maintaining ice at 0°C or frozen storage of product down to -30°C. Because there are no fans, there is virtually no dehydration of the product. This technology replaces coils of copper piping on the ceiling or walls of the fish hold.

PLATE FREEZERS are racks of aluminum or stainless steel plates, through which the refrigerant passes. These can be either single or double contact. When mounted in a refrigerated fish hold, they generally do not have insulated cabinets. Insulated cabinets allow them to be mounted on the deck of the vessel. The most common use of plate freezers is for boxed shrimp and blocks of fish fillets. Double contact plate freezers have hydraulically movable plates which close on the boxes or frames to give 2 plate contact, which shortens the freezing time for each batch by nearly 50% and increases processing capacities. Most plate freezers have horizontal plates but some large factory trawlers use vertical plate freezers.

BRINE FREEZERS are insulated tanks with at least two compartments. One contains stainless steel evaporator plates, which are manifolded together and protected with a zinc anode. The second compartment is for the freezing of product. A refrigerated brine solution of sodium chloride with a salt concentration of 18 to 23% by weight is circulated rapidly by a stainless steel agitator, preventing ice formation on the plates and providing maximum heat transfer of the product. The brine temperatures are -15° to -20°C. These brine freeze tanks can be deck mounted and are used extensively on shrimp boats in North, Central, and South America. The shrimp are bagged in 15Kg mesh bags and placed in the brine tank. Freezing is complete in 12 to 15 minutes. The shrimp do not have a heavy glaze and are IQF (individually quick frozen). They can be poured from the bag into boxes and stored in a freezer hold with dry hold evaporator plates. Brine freeze tanks can also freeze crab, lobster, and small fish.

FORCED AIR FAN COILS are used extensively on land in walk-in coolers, freezers and freezing tunnels. For fishing vessels, fan coils are less practical because they require fan motors, switches, defrost heaters and timers, all of which do not survive well in saltwater conditions. Also when freezing product, the rate of heat transfer is much less for air than direct contact plates, ice or liquid. Air causes substantial dehydration of product and ice evaporation. There, are however, some shrimp fisherman using high velocity fan coils for rapid freezing of boxed product. For ultra low temperature freezing of -40° to -60°C ( for products like sashimi grade fish) forced air coils must be used.

COMPRESSORS

There are many good compressor manufacturers in the world. The brand is less important than selecting the type of compressor best suited to your application. As with all the refrigeration components, the ideal choice is the simplest available compressor, which can easily be repaired on the vessel and has a service life of 10 to 20 years. While not usually the cheapest in terms of initial cost, spread over 10 years with little maintenance, a quality compressor is the best value. These industrial compressors will reduce downtime, actually saving money in terms of product loss and fishing time. With this in mind, the hermetic and semi-hermetic compressors should not be used. The choices should be open drive, reciprocating compressors, direct driven by flexible couplings or belts. These compressors can be driven by electric motors, hydraulic motors or independent diesel engines. For the diesel drive units, accessory hydraulic pumps, water pumps, and A.C. power generators can also be installed. For very small compressors, up to 2 tons of refrigeration, the compressors can be belt driven from the main propulsion engine.

Screw compressors are another option for large vessels (30 meters to over 100 meters), which use very large refrigeration capacities. The main disadvantages of the screw compressors are; they are efficient only when operating near 100% capacity and they can not be disassembled and repaired on the vessel by the average engineer.

CONDENSERS

Condensers for marine application should be made of 90/10 cupronickel, stainless steel or titanium. They must be cleanable with sufficiently large seawater passes to prevent loss of flow due to fouling. The most popular type for this application is the shell and tube. Our company has manufactured and installed thousands of these condensers which have lasted 15 to 25 years. The only maintenance required is tube cleaning and changing the zinc anode every 6 to 12 months. The plate frame heat exchanger is also widely used and is made with titanium plates. They are cleanable, but the water passages are very small compared to shell and tube; hence, fouling can be a problem. An advantage of the plate heat exchanger is that it requires less space for mounting. Tube in tube spiral or coaxial condensers are much less expensive, but are not readily cleanable and their service life is short compared to shell and tube or plate frame condensers.

When comparing systems you should always be sure that the condenser is oversized. When the condenser is only large enough for the heat you remove from the product and the heat generated by the compressor, you will have reduced capacity and compressor life as the condenser becomes fouled or pump flow rates drop. In tropical waters, selecting a condenser near double the total heat rejection is cheap insurance. Not a considerable difference for the security it brings.

REFRIGERANTS

The two refrigerants used with most equipment manufactured for fishing vessels today are Freon R22 or Ammonia R717. There are future replacements for R22 but they are at least 5 times more costly. R22 is scheduled for global phase out in the year 2015. Present equipment will be easily retrofitted for the new refrigerants by complete purging of both the old R22 and oil. The new refrigerants require special oils.

Ammonia is a refrigerant with heat transfer capabilities many times superior to R22, R134a, R507, or R502. However, because of extreme toxic characteristics, ammonia should only be used on larger vessels with qualified engineers. All ammonia refrigeration equipment should be in a vapor proof compartment with special alarms, ventilation and water sprinkler systems. This requires special consideration for space when the vessel is under construction.

TYPICAL FISHERIES

SHRIMP

Shrimp fishing vessels are usually 15 to 28 meters long and primarily fish warm waters. Figure 2 shows the typical American shrimp boat using a diesel-drive condensing unit mounted in the engine room, a brine freeze tank mounted on the aft deck, and dry hold evaporator plates in the fish hold. The approximate installed price for the refrigeration equipment is $55,000 U.S. dollars. The first refrigeration on shrimp vessels has been ice brought from the dock. This is satisfactory for vessels returning to port each day. For multiple day trips the markets are demanding quality which can only be obtained by onboard freezing. The most common shipboard freezing is done in plate freezers or brine freeze tanks. High volume, forced-air blast-freezing tunnels or plate freezers can also be used.

PELAGIC LONGLINING

This fishery is primarily the high dollar value "sashimi" grade Yellow Fin, Big Eye and Blue Fin Tunas. Ten to forty years ago most of these fish were captured by larger, distant water fleets, primarily from Japan, Korea, and Taiwan. These fleets have been drastically shrinking due to both the establishment of 200 mile Fisheries Economic Zones and high operational costs. These vessels delivered their products to the market in frozen form, at temperatures of -50° to -60°C, which required forced air fan coils and 2 stage (compound) compressors. These systems are very expensive compared to -30°C temperature systems. Today large numbers of mid-sized vessels (15 to 30 meters) are entering the fisheries and operating from countries around the world. The "sashimi" grade fish are kept fresh and flown to Japan and the USA. This product brings a higher price because the consumer prefers fresh rather than frozen. Fish are chilled and stored in flake ice, which in most cases, is made onboard the vessel. Having ice making equipment onboard, allows the vessel to use all the hold capacity and properly chill the fish in a slurry tank. With the high price of this catch, a shipboard flake ice machine can pay for itself in less than one year, and sometimes in one trip.
       

Figure 2



LIVE FISH AND LOBSTER

These are also very high value fisheries products. In addition to keeping the animals alive, RSW chillers can be used to lower the metabolic rate, which increases the survival rate during holding and transportation. RSW chillers must be made with titanium to prevent toxic, metallic chemicals from killing the animals.

PURSE SEINING

This fishery is the supplier of raw material for the fish meal and fish oil industries. The quality of this product, in the past, was less important. Since Aquaculture is growing at 11 - 12% per year and is gradually replacing wild fish for human consumption, pressure is increasing on the fish meal industry to produce more superprime meals for shrimp and fish feeds. Superprime fish meal returns nearly U.S. $100/MT more than standard fish meal. Production of prime meals requires refrigeration of the raw material both onboard the fishing vessel and in the process plants. The present industry is shifting to refrigerated fishing vessels with RSW capacities of several hundred tons of refrigeration. The vessels have hold capacities of 350 to 650 MT in Peru and up to 1500 MT in Chile. The RSW works well for some species but does not work for anchovies, which in Peru supply the greatest amount of fish meal raw material. The answer for the anchovies and the system which is possibly best for all these vessels is an ice slurry created from a combination of RSW and Ice Making equipment. The primary reason is that ice has from 20 to 80 times more refrigeration capacity than the typical RSW system. The ice and water mixture provides the high capacity needed to rapidly chill large catches of 50 to 500 MT of fish per hour. The secondary reason is that this method would not require constant pumping, during which the anchovies disintegrate.

BOTTOM FISH

Many small artesian fishing vessels must carry ice from the dock to chill and store their catch. In warm waters (22° to 32°C) the ice must first chill the fish to 0°C. This requires a minimum of 500 Kg of ice per 1000 Kg of fish. Additional ice is used for storage. On small vessels in warm latitudes, a small RSW system (1 to 2 tons) may be installed for chilling the fish to 0°C. The total ice required for the fishing trip could be reduced by approximately 50%. This obviously allows a greater catch capacity and insures consistent
quality.

FACTORY TRAWLERS

These vessels are large (50 to over 100 meters) and of a wide variety. All the types of refrigeration systems described above are used onboard. Factory trawler systems are both very large and complex and a detailed description is beyond the scope of this report.

SUMMARY

In today's fishing industry, refrigeration equipment is necessary for high quality export fish. The choice of equipment is as important as selecting the main propulsion engine, electronics or fishing equipment. Therefore, equipment should last many years with little maintenance. Generally, paying too cheap a price for this equipment will, in the long run, cost the fishermen a lot of money because of lost fishing time and high maintenance.