3. PROCESSES AND EQUIPMENT

3.1 Sources of Cold

3.1.1 General
3.1.2 Mechanical refrigeration
3.1.3 Liquefied gases

3.2 Specific Processes and Equipment

3.2.1 General
3.2.2 Air-blast freezers
3.2.3 Horizontal plate freezers
3.2.4 Vertical plate freezers
3.2.5 Direct contact

3.3 Packaging and Glazing

3.3.1 Packaging
3.3.2 Glazing

3.1 Sources of cold

3.1.1 General

3.1.1.1 Basic refrigeration systems:

Refrigeration is a process whereby heat is removed and rejected, and this can be achieved by any of the following methods:

Vapour compression
Vapour absorption
Air cycle
Thermoelectric

The most widely used is the vapour compressions system and only in exceptional circumstances would another method be contemplated in a modern fish processing installation. Even applications which apparently do not use this method, such as chilling with ice or freezing with a cryogenic liquid, are indirect uses of the vapour compression system, since such a system would have been used to make the ice or liquify the gas.

Vapour absorption is still used in some domestic refrigerators and there has been a revival of interest in this system for some commercial applications since it can be operated from a waste heat source. The absorption system does not require mechanical power, therefore, there is no requirement for an electrical supply of direct drive engine. The only requirement is an input of heat, therefore, this type of refrigeration system may be considered for limited applications in situations where a heat source is cheaper or more readily available than an electrical or mechanical drive.

The air cycle system is inherently safe and is now almost exclusively used for cabin cooling in aircraft.

The thermoelectric cooling system is confined to use with applications which require very small refrigeration effects, such as instrument cooling and laboratory use.

Other methods have been used, but they either have not been suitable for commercial operations or have now become obsolete for a variety of other reasons.

Cryogenic refrigeration, as mentioned above, is an indirect application of the vapour compression system and liquid nitrogen and liquified or solid carbon dioxide cryogenic systems have a limited application in fish processing.

A high purity fluorocarbon refrigerant, R12, is also used in immersion or spray cooling systems, but again there is only a limited use in the fishing industry.

3.1.1.2 Refrigerants:

Although there are a wide range of refrigerants available, many have properties which suit them for special purpose applications only. The refrigerants listed in Table 12 are those commonly used in the fish industry. and a number of secondary refrigerants, such as ethyl alcohol, methyl alcohol, glycol solutions and salt/sugar solutions, have also a limited use.

The choice of refrigerant is usually based on technical requirements, but other considerations, such as safety, high costs or import controls, may result in a compromise choice being made.

Table 12 Refrigerants

The cost of refrigerants depends on the unit quantity (size of cylinder) and the quantity (weight) ordered. The costs listed in Table 13 are 1983 UK values for the quantities and unit containers specified. Initial costs may also include an additional deposit for the refrigerant containers.

Many of the physical properties of a refrigerant are important to the design engineer and this information is now readily available in suppliers catalogues and in text books. Only some of their attributes and likely applications are, therefore, given in the following notes.

1. Refrigerant 12 has a relatively low latent heat value and this is an advantage in small machines since the higher flow rates required allow for better control. R12 is usable down to lower temperatures, but below -29.8�C negative pressures will prevail on the low pressure side of the system with potential problems resulting from leaks of air and moisture. R12 is completely miscible with oil at all likely operating temperatures, therefore, oil return systems are relatively uncomplicated.

Table 13 Refrigerant costs

2. Refrigerant 22: This refrigerant is mainly used as a replacement for R12 at lower evaporating temperatures since a positive pressure will prevail throughout the system down to a temperature of -40�C. It is, therefore, more likely to be used than R12 when there is a requirement to freeze and store fish at temperatures of -25�C or lower. R22 is completely miscible in oil down to a temperature of -9�C, but at lower temperatures there is a separation with oil collecting on the surface of the refrigerant. With the flooded systems used for most fish-freezing operations, special measures will therefore be necessary to return oil from the low-pressure receiver to the compressor.

3. Refrigerant 502: This azeotropic mixture of R22 and R115 combines some of the good properties of R12 and R22. For instance, it allows positive pressure operation at lower temperatures like R22, but it has also some of the oil miscibility qualities which are desirable and similar to those of R12. It is, however, more costly and this, together with availability, may be major factors relating to the use of this refrigerant.

4. Refrigerant 717 (ammonia): In spite of some of its unfavourable qualities, ammonia is still the most widely used refrigerant for larger installations. In terms of evaporator-operating temperatures, its properties are between those of R12 and R22, and since leaks into an R717 system do not have the long-term damaging effects as those experienced with halo-carbon refrigerant systems, R717 is suitable for nearly all fish-freezing and cold storage operations. R717 has a highly irritant effect on the eyes and nose, therefore, small leaks can be readily detected and repaired without the need for systematic testing, as is the case with other refrigerants. In the presence of water it is corrosive to many non-ferrous metals, therefore, copper, which is widely used with other refrigerants, cannot be used in pipes, evaporators, instruments and controls. R717 forms an explosive mixture when mixed in the right proportions with air, therefore, special precautions should be taken to avoid the presence of nearby sources of ignition when major leaks occur. The use of R717 may be restricted; for instance, it is not used on fishing vessels in the UK. Oil separates out from R717 at low temperatures and, with the oil being the heavier, it has to be periodically removed from the bottom of the evaporators and low-pressure receivers.

5. Calcium and sodium chloride brines: The main difference in the choice between these is that sodium chloride is cheaper and more readily available whereas calcium chloride brines can be used for lower temperatures. Although still used for some applications, such as tuna freezing, they are now seldom required for other fish-freezing and storage requirements since the development of primary refrigerant, pump circulation systems. The advantage of a secondary refrigerant is that it allows a finer control and more balanced circulation to multiple-unit installations. In larger installations where the refrigerant change in the evaporator or heat exchanger may be considerable, secondary refrigerant systems may be generally cheaper since they reduce the charge of the more expensive primary refrigerant. There is also no need to contain the bulk of the refrigerant in a system which has to withstand the higher standing pressure of a primary refrigerant; therefore, cooler construction will be cheaper and refrigerant losses less expensive.
   
   The use of secondary refrigerant, however, means that the system is inherently less efficient since an additional temperature difference must exist between the primary and secondary refrigerants for heat exchange. The present unpopularity of calcium and sodium chloride brines is also partly due to their corrosive nature and the unpleasant and messy effects of spillage.

6. Trichloroethylene: This secondary refrigerant was widely used in UK freeze: trawlers. It has far from the ideal qualities desirable 1n a secondary refrigerant, but its use avoided the distribution and leak problems associated with using a primary refrigerant in a multiunit plate freezer installation on board a fishing vessel. It also allowed lower temperatures to be used than would be the case if a calcium chloride brine had been used. Trichloroethylene vapour is toxic and consideration should be given to potential health hazards before contemplating its use.

3.1.1.3 Power and consumption factors:

The power required for a given refrigeration effect changes with operating conditions and this is illustrated graphically in Figure 3 where the power requirement for a nominal refrigeration effect of 20 000 kcal/h is given.

Figure 3 Compressor power requirement for a 20-kcal/h refrigeration plant

For any given compressor the capacity and power change significantly with changes in operating conditions, particularly those relating to the pressure/temperature of the refrigerant at the suction inlet to the compressor, and this is shown graphically in Figure 4.

The capacity of the compressor reduces at a greater rate than the power, therefore, the power per unit refrigeration effect increases and the corresponding values to the changes shown in Figure 4 are given in Figure 5.

In many situations it may be necessary for the factory to generate its own electricity, either because it is cheaper to do so or supplies from outside sources are unreliable. For this purpose a diesel generator would normally be used and the power output and electrical generation, characteristic for a range of units, is shown graphically in Figure 6. Fuel consumption for this range of diesel generators is shown in Figure 7 and, for budget purposes, ex-factory costs are given in Figure 8.

3.1.2 Mechanical refrigeration

3.1.2.1 Refrigeration systems:

Mechanical refrigeration can be achieved in a variety of ways with various degrees of refinement used to achieve greater versatility, more accurate control, improved economy and other objectives.

The basic mechanical refrigeration system is shown in Figure 9 and, in simple terms, it is designed to take in heat at the evaporator and thereby reduce the temperature of the surroundings.

The heat taken in is then raised to a suitable temperature by compression of the refrigerant gas and this allows the heat to be rejected to a fluid, such as water or air, at the condenser together with the heat of compression.

Figure 4 Variation in power and capacity of a compressor with changes in compressor suction conditions

Figure 5 Power requirement per unit refrigeration effect with changes at compressor suction operating condition

Figure 6 Horse power and output of diesel electric generators

Figure 7 Fuel consumption of diesel electric generators

US$

kVA

Figure 8 Cost of diesel electric generators

Figure 9 Basic mechanical refrigeration system

Refrigeration is therefore a continuous process with the refrigerant changing from liquid to gas, to liquid, as heat is added and lost.

Four basic systems using mechanical refrigeration are used for fish freezing and cold storage, and these are shown diagrammatically in Figures 10, 11, 12 and 13, with notes on the type of application where they are likely to be used.

1. Dry expansion system: Used in all the small installations and in installations where there are not likely to be problems with refrigeration distribution or the temperature fluctuations induced by the cycling of the thermostatic expansion valve.

Figure 10 Dry expansion system

2. Flooded system with natural circulation: The flooded system gives a better heat transfer than the dry expansion system since there is more liquid present in the cooler. A flooded system also ensures better refrigerant distribution, therefore, they are appropriate when there are a number of parallel circuits for the refrigerant flow.
   
   The reservoir in a natural circulation system is situated above the coolers, therefore, it is not suitable for widely separated multiple units.
   
   The most appropriate application likely in fish freezing is with horizontal plate freezers which are single units with a number of parallel circuits formed by the freezer plates.

Figure 11 Diagram of natural convection flooded refrigeration system

3. Flooded system with pump circulation: Pump circulation allows a flooded system to be used with its advantage in good heat transfer and refrigerant distribution, in a multiple unit system with the low-temperature liquid reservoir situated, if necessary, away from the immediate vicinity of the coolers.
   
   An example of this kind of application is a number of vertical plate freezer units supplied from a common liquid receiver sited in a separate engine room.

Figure 12 Flooded refrigeration system with pump circulation

4. Secondary refrigerant system: This has all the advantages of a pump-circulated flooded system without the need to have a pipework and cooler system suitable for the higher refrigerant pressures. The system would therefore be appropriate when there is a high potential for leaks such as on a fishing vessel. The primary refrigerant is confined to the condenser unit and heat exchanger only, and this may be located in a separate engine room.
   
   A secondary system also avoids the potential problems that may result from having a large charge of a volatile refrigerant in a working space such as a factory floor or in a cold store (Figure 13).

Figure 13 Diagram of secondary refrigeration system

3.1.2.2 Compressors:

The function of a compressor is to draw refrigerant vapour from the evaporator and thereby create a low pressure so that the liquid refrigerant boils and achieves the desired heat exchange at a low temperature. The compressor also raises the pressure and thereby the temperature of the refrigerant vapour, so that it can transfer its heat to the cooling air or water at the condenser and, as a result, the refrigerant is liquified.

A compressor is in effect a pump which also creates the necessary conditions for heat transfer at the evaporator and condenser.

Compressors used in refrigeration may be divided into two main classes:

1. positive displacement compressors, and
2. kinetic displacement compressors.

Only positive displacement compressors are likely to be considered for fish freezing and storage applications and, of this class, reciprocating compressors are by far the most widely used. Screw and rotary compressors, however, may also be used with advantage under certain circumstances.

Reciprocating compressors can be further divided into three categories:

1. hermetic compressors,
2. semi-hermetic compressors, and
3. open compressors.

Hermetic compressors are part of a totally sealed system which also includes the drive motor, condenser and evaporator and they are widely used in small units since they require little maintenance.

Semi-hermetic compressors may be part of a sealed or open system and their main feature is that the drive motor and compressor are combined in a single unit so that there is no requirement for a drive shaft seal. The compressor and drive motor, however, can be separated for maintenance and repair. Their use is confined to relatively small installations requiring a compact compressor/motor unit.

Open type compressors are separate from their drive motor and are operated by means of a shaft with a rotary seal. Motor and compressor are completely accessible for maintenance and repair as separate units and the size range available covers most likely applications in fish freezing and cold storage.

Open type reciprocating compressors are made as units which may have up to 16 cylinders with six and eight cylinders being usual in commercial fish-freezing operations. Cylinder arrangements can be vertical and in-line or, in more modern compressors, arranged in W, V or WV formation to achieve a better balanced and a more compact unit.

Table 14 Size range of reciprocating compressors

Multiple-cylinder units can be operated with a manual or automatic arrangement which involves off-loading some of the cylinders to give a stepped capacity control. This off-loading arrangement is often used during start-up procedures and larger units also use off-loading rather than frequent stopping and starting in order to achieve a controlled condition.

Compressors used for applications which give rise to high compression ratios (usually the result of low evaporator pressures) may require to be operated with a two-stage compression arrangement. This improves the economy of the compressor and also prevents excessively high refrigerant discharge temperatures, which may result in a breakdown of the oil present.

Two-stage compression can be accomplished by using two compressors and, in large installations, a rotary booster compressor is often used for the first stage. However, it is more likely that this is done in a duplex or compound compressor with some of the cylinders in a single machine forming the low-pressure (LP) stage and the remainder the high-pressure (HP) stage.

Manufacturers catalogues usually clearly define when two-stage operations should be used with their machines by only quoting capacities for single-stage machines within the limits of a single-stage operation.

Most fish-freezing operations involving evaporating temperatures of -35�C or lower, should have a two-stage compression.

The oil in a compressor does not remain entirely in the compressor sump, but is carried over with the refrigerant to the rest of the system. Oil separators are used with larger compressors to reduce, but not eliminate, this distribution of oil throughout the system, therefore, precautions still have to be taken to ensure that oil is returned to the compressor and not allowed to accumulate in other parts of the system, such as in the low pressure receiver or evaporator.

Once a system has been charged with oil and a balance maintained, with the oil leaving the compressor being matched by the oil returning, there should be little change in the oil level in the compressor sump. Some systems operating with ammonia refrigerant, however, may have oil drained from time to time from parts of the system and this has to be made up as necessary. Most other refrigerants, however, operate with a sealed system with no total loss of oil and the only requirement is the manufacturers' recommendations for oil changes. This usually only requires a change of oil shortly after commissioning and a routine change at the time of a major overhaul.

Any drop in the oil level therefore indicates a fault in the oil return system or in the operation of the machine.

Oil for refrigeration compressors has special properties for operating at low temperature, therefore, only the grades recommended by manufacturers should be used.

If the cost of a compressor or condensing unit is to be related to its refrigeration capacity, it is necessary to specify the following operational conditions:

• refrigerant used
• compressor speed
• compressor suction saturation temperature
• compressor delivery saturation temperature
• compressor suction superheat
• liquid subcooling at condenser outlet

A compressor cost can therefore only be determined when the exact operating conditions are known, and the unit identified after consulting manufacturer's capacity tables of the type shown in Figure 14.

General costs for a range of compressor sizes are therefore only listed for some arbitrary set of operating conditions and, with experience, this list may be interpreted to give a rough guide suitable for elementary costing at the planning stage.

The compressor prices in Figure 15 are for conditions which relate to likely fish-freezing and cold-storage operations but, when possible, budget prices should be obtained from the manufacturer or supplier when operating conditions are known.

3.1.2.3 Condensers:

The condenser is the part of a refrigeration system where the heat taken in at the evaporator, together with the heat added by compression, is lost by the refrigerant to the surrounding air or water coolant.

1. Air condensers :

Air is a cheap collant, but since the rate of heat transfer from a surface to air is poor, air condensers tend to be large. Ambient air may also be at a high temperature and this will result in higher condenser pressures with added operational costs.

With smaller plants, the cost of operating with higher condensing pressures may not be prohibitive and the advantage of not having a piped water supply and waste-water requirement may be attractive. Air condensers are therefore widely used with smaller units.

One difficulty with air-cooled condensers is that they must be placed where cool air can be readily taken in and hot air leaving the condenser rejected.

Small domestic refrigerators and chill-display cabinets may have "static" air condensers which are usually finned pipe grids, fixed to the rear of the cabinet with natural circulation of air doing the cooling. These units must therefore be sited where air is allowed to circulate freely.

Air condensers with fan-assisted circulation can also be fitted to small chill cabinets, freezer cabinets or cold rooms as part of a self-contained unit and again they require correct sitting in order to give adequate ventilation of the condenser air.

Figure 14 Manufacturer's compressor capacity table refrigerant 502

Notes:
1) Capacity in thousands of kcal/h
2) Power in kW absorbed at the compressor shaft
3) Capacity and power values given are for compressor speed of 1 450 rev/min
4) Compressor speed range 850-1 750 rev/min
5) Capacity at other speeds directly proportional to speed
6) Power at other speeds obtained by the percentages shown:

7) Compressor suction superheat 80�C
8) Subcooling at condenser outlet 5�C .

Figure 15 Compressor prices (capacity at -35�C suction) +35�C delivery)

Commercial and industrial applications with larger air condensers usually have the condensers remote from the compressor and out with the building, usually on the roof. This, however, may result in long runs of delivery pipework and under certain circumstances this leads to difficulties with partial condensation before the condenser.

In some climates, seasonal changes in ambient air temperature may also give rise to problems.

Air condensers are designed to match the highest ambient temperature likely and when it falls substantially below this value, the pressure at the expansion valve inlet may be insufficient to ensure the required flow of refrigerant. If this condition is likely, special precautions, such as reducing the condenser capacity or changing to another expansion valve, may have to be taken.

In spite of difficulties that may arise with air condensers and the extra costs involved, they are extensively used, particularly where there is no regular supply of cheap water of suitable quality.

Air-cooled condensers are usually rated for about 15�C temperature difference between the air and condensing temperature. Air velocities, measured as the average velocity over the condenser face area, are normally between 2.5 and 5.0 m/s. Heat transfer per unit surface area varies with a number of factors, but a value of 250 W/h per m� of extended surface would be typical.

Table 15 gives the likely physical dimensions and power requirements for a range of air condensers.

Table 15 Dimensions and power requirements for air-cooled condensers

Note: The above ratings are for R12 with 3�C subcooling

It should be noted that correction factors may have to be applied depending on the refrigerant, and this is clearly stated in manufacturers. catalogues.

2. Water-cooled condensers

Water from a main supply is expensive and should only be used where the plant is small and air-cooling is impractical.

Water from natural sources, such as rivers, lakes and wells, requires to be of a suitable quality so that it does not corrode the condenser or foul the heat exchange surfaces. Chemicals, silt and biological contaminants such as algae are potentially damaging and maximum levels recommended by manufacturers may require the water to be filtered or treated chemically.

Uncertainty about the continuity of water supplies should also influence whether a water con- denser is used, especially when it is considered that water supplies are likely to be more restricted during hot weather when the cooling demand is greatest.

The most popular type of water-cooled condenser is the shell-and-tube type where the water passes through the tubes. End covers are required to be removed periodically to clean the tubes, therefore, condensers should be sited to give access for this operation (Figure 16).

Figure 16 Water requirement of shell-and-tube condensers

Shell-and-tube condensers can either be mounted with the tubes horizontal or vertical and the available floor space and roof clearance may dictate which type is used.

Different flow patterns are also used and again the choice is mainly based on convenience. For instance, there may be a requirement for the water entry and exit connections to be at the same end of the tube.

Water-cooled condensers are more compact than the air-cooled due to the improved heat transfer between the water and the tube surface, which is about 15-20 times better than the air to surface heat transfer in an air-cooled condenser. Water-cooled condensers, however, do not always have fins to extend the heat transfer surface, therefore, the difference in size is not as striking as the difference in heat transfer rate would suggest.

If the temperature of the water supply is likely to change sufficiently to affect the performance of the thermostatic expansion valve or other refrigerant flow control, condenser pressure can be readily controlled by fitting a pressure-operated water valve which reduces water flow when the condenser pressure is reduced by circulating colder water than the design value.

Condensers are designed for a water flow rate of about 1.5 m/s through the tubes, and a temperature rise between inlet and outlet of about 5�C, but this may vary from one manufacturer to another, or it may be raised if water is not plentiful.

Table 16 gives the likely physical dimensions and water consumption for a range of shell-and-tube condensers.

Table 16 Shell-and-tube condensers

Notes:
(1) sw = salt water; fw = fresh water
(2) based on a 12�C temperature difference between inlet water and condenser saturation temperature, and a 5�C water temperature rise

3. Evaporative condensers

Air and water cooling are combined in an evaporative condenser with an advantage over air cooling, both in terms of heat transfer and lower condensing temperatures, and over water condensers with a greatly reduced water consumption.

In an evaporative condenser air is drawn upward over the condenser pipes and water is sprayed over the surfaces from above (Figure 17). Heat transfer results in the evaporation of some of the water flowing over the coils and the resultant water vapour is discharged to atmosphere with the exit air.

Figure 17 Evaporative condenser

As with air condensers, evaporative condensers require to be positioned so that air can be circulated freely and since the discharge air also contains a good deal of water vapour, the condenser must always be sited outside or the air ducted outside.

Theoretically, the water evaporated is approximately 2.7 kg/1 000 kcal of refrigeration effect, which is well below the nominal value of 200 kg required for a shell-and-tube condenser. Some water, however, is lost by small droplets being discharged with the air and a regulated overflow from the condenser reservoir is necessary to keep solids and other contaminants at a suitably low level of concentration.

Water consumption in an evaporative condenser in practice is, therefore about 4-5 kg/1 000 kcal of refrigeration effect.

The likely dimensions, power requirements and water consumption for a range of evaporative condensers is given in Table 17.

4. Other condensing systems:

Some of the benefits from different types of condenser can be combined when two or more of the basic systems of heat transfer are used.

A shell-and-tube condenser can be used with circulation of the cooling water if it is piped to either an evaporative cooler or air cooler at a suitably remote site (Figure 18). This offsets the need for a long run of refrigeration pipework, thus avoiding the associated refrigeration problems and, also, avoiding having high pressure pipes where they are vulnerable to damage or where leaks would create a safety hazard.

Table 17 Forced draught evaporative condensers

Note: This table is based on a wet-bulb temperature of 27�C, a condensing temperature of 40�C, and an evaporating temperature of -40�C

Figure 18 Condenser with remote water cooling

3.1.2.4 Evaporators:

The word evaporator, by common use, now includes all coolers even when no evaporation takes place, such as in a system using a secondary refrigerant.

The evaporator is in effect the heat exchanger where heat from the product, or medium to be cooled, is transferred to the refrigerant.

Evaporators are made in many different forms depending on the refrigeration system and the application. In some cases they may form an integral part of the main equipment, such as the plates of horizontal and vertical plate freezers or the drum of a drum freezer. In other systems, the evaporator is an intermediate heat exchanger or liquid receiver, such as in flooded secondary and primary refrigeration applications. Many of these special cases are dealt with, or mentioned else where, and therefore only the main type of evaporator used, which is for an air-to-refrigerant heat transfer, is considered in detail.

Evaporators for air-to-refrigerant heat transfer in air-blast freezers and cold stores are either made from plain pipe or finned pipe. Plain pipe evaporators are now little used for transfer of heat from air since they are more expensive, require a substantial supporting structure, take up more space and require a large standing charge of refrigerant. Plain pipes, however, may be used when heat transfer from the pipe surface is high, such as from a refrigerant to a brine in a secondary system.

By far the most widely used are finned pipe evaporators which are built as compact units with fan-assisted circulation of air. Plate evaporators have also a limited application for air-cooling systems, particularly when the heat transfer requirement is not high and frosting of finned tube evaporators may be critical.

The materials used for evaporators must be compatible with the refrigerant and also with the environment in which they are sited. For instance, copper must not be used with R717 (ammonia) and special consideration should be given in a salt laden marine environment.

Finned tube evaporator units are made for wall, roof or floor mounting and the choice would depend on the convenience of placing them in any of these positions.

Defrost is an important consideration for evaporators operating below 0�C. Frost build-up on the heat exchange surfaces greatly reduces the heat transfer capability of the evaporator and this condition would become critical when the frost bridges the gap between adjacent fins and greatly reduces the surface area available for heat transfer.

In smaller units, electrical defrost is used with the heater elements built in as part of the unit. This is expensive and for larger units other methods are used, which may either be external or internal. Water or an anti-freeze solution can be periodically poured over the surface and drained away. This method, however, is not always convenient, especially when low temperature conditions have to be maintained at all times as they are in a cold store. Internal defrosting by means of hot gas redirected from the condenser discharge is more usual, but in order to do this effectively more than one evaporator must be operated from the same condensing unit and the evaporators defrosted in sequence. Reverse cycle defrosting, where the functions of the evaporator and condenser are reversed, is another method used with small units.

Fin-spacing is also an important aspect of design and a compromise has to be made between a close spacing giving a compact unit and a wider spacing which results in a larger unit but allows more frost to accumulate before a defrost is necessary. Fin spacing of 6, 8 and 12 mm are available in some standard evaporator ranges.

Each case must be considered individually but, in general, a spacing of 6 mm may be used when frost build-up is not critical, a spacing of 12 mm when there is likely to be a heavy accumulation of frost, and 8 mm for a compromise situation.

Another important consideration in selecting an evaporator is the characteristics of the fan. If the evaporator is free standing, as it is in an open cold store, and there is no need for the air to be circulated over a large area, propeller type radial fans may be used: These have: simple blades shaped from flat, sheet metal and they are inefficient when there is a resistance to air flow.

In air-blast freezers and in cold stores where the air is required to be "thrown" over a distance, aerofoil blade fans are required. The blades are designed with a well shaped profile similar to the wing of an aeroplane and they are a good deal more efficient than the cheaper propeller type.

Standard ranges of wall/roof and floor mounted evaporators are listed in Tables 18, 19 and 20, together with their overall dimensions, weights and fan power requirements.

Table 18 Wall/roof mounted coolers/evaporators- Propeller fans -

Table 19 Wall/roof mounted coolers/evaporators - Aerofoil fans -

Table 20 Floor mounted coolers/evaporators

Plate evaporators are either used in units consisting of a bank of plates on one framework and often incorporating a fan for air circulation. or as single plates which may be arranged against a wall or roof with natural circulation of the air. Types of plates used in plate evaporators are:

1. steel with an embossed serpentine pattern for the circulation of the refrigerant;
2. pipe grids contained between sheet metal which is kept in contact with the pipe by evacuation of the air in the space between the sheets;
3. extruded aluminium plates of the type now extensively used in plate freezers.

3.1.2.5 Refrigeration, instrumentation and controls:

In order to regulate the flow of refrigerant, maintain design operating conditions and allow equipment to be operated safely and economically, a number of controls are used with refrigeration systems.

The complexity of a control system usually relates to the size of the plant, ranging from no more than a capillary throttling device to regulate refrigerant flow in a small hermetic system to complex, computer-based control systems for the more recently installed large, multiple unit systems.

Some of the controls used for the size of plant more likely in the fishing industry are listed below. A brief indication is given of the requirement. function. whether spares are advisable and the likely cost.

1. Pressure gauges

Pressure gauges are used to indicate plant-operating conditions and they are therefore useful for routine inspections and, also, when fault-finding. Gauges are normally positioned at the compressor to indicate pressures on the high and low pressure sides of the system with additional gauges, as necessary, to indicate the intermediate pressure in a two-stage system and the delivery pressure of the compressor lubricating oil pump. An additional gauge may be used in a larger system to indicate the pressure at the evaporator and, also, the pump delivery pressure when a secondary refrigerant is used.

To cover all these requirements, three different pressure ranges may be required, and although they are not essential for the plant operation, spares should be available since a reliable gauge would help to reduce both operational and maintenance costs.

The cost of gauges will vary between US$ 5 and US$ 25.

2. Temperature gauges

Like pressure gauges, temperature gauges, or pocket thermometers, are used to monitor plant-operating conditions and to assist with fault-finding. Thermometers used with the refrigeration compressor are used to monitor temperatures at the same positions as the refrigerant pressure gauges since both readings are normally required to assess the plant-operating condition.

Additional temperature gauges are also helpful to measure cooling water temperature, the temperature of a secondary refrigerant or the temperature of a low pressure, primary refrigerant, liquid reservoir.

Dial gauges are also used to monitor the temperature in air-blast freezers, but for cold stores a recording thermometer is advisable since this information is often required for checking later.

Dial thermometers cost between US$ 15 and US$ 25.
Steam thermometers cost between US$ 3 and US$ 8.
Circular chart recorders cost about US$ 350.

At least two temperature ranges are required to cover all these requirements, and the availability of spares is not normally critical since thermometers can usually be interchanged without breaking into the system, or a hand-held thermometer, used in an appropriate way, can be substituted.

Figure 19 Pressure and temperature measurement

3. Refrigerant flow

Control of refrigerant flow is probably the most important control to be exercized in a refrigeration system. The following are four examples of control likely to be used:

1. Hand expansion valve: A valve which accurately controls the flow of refrigerant to exactly match the refrigeration load. Hand expansion would only be used when the refrigeration load is constant or the inertia of the system means that changes would only be small and progress slowly; a large cold store with constant attendance is the type of application suited to this method.
   
   Hand expansion valves are often fitted in parallel with other control devices so that they can be manually operated in the case of a failure. Cost US$ 30-60, depending on size.

2. Thermostatic expansion valve: This is an automatic device which gives a modulated control of refrigerant flow and it is the most widely used method with a variety of individual designs to suit particular requirements. The valve senses pressure and temperature at the evaporator and uses the information to supply only sufficient liquid refrigerant to match the evaporator's requirement.
   
   Thermostatic expansion valves (tev) are made in a wide range of sizes and models and the following list will, therefore, only give an approximate indication of the likely cost.

US$ 15 for tev with capacity of 1000 kcal/h
US$ 50 for tev with capacity of 9 000 kcal/h
US$ 135 for tev with capacity of 90 000 kcal/h
US$ 150 for tev with capacity of 165 000 kcal/h

A spare valve should always be available, but the installation of a hand expansion by-pass can be used for a short time in an emergency.

3. Low side level control: This can be a mechanical or electrical device which is used to control the level of liquid in the low pressure receiver of a flooded primary refrigerant system or the primary refrigerant level in the heat exchanger of a secondary refrigeration system.
   
   The level control is essential to the operation of the system, therefore, a spare should be available or, depending on the type used, spares for the more vulnerable parts should be held. Cost US$ 200-350.

4. High pressure level control: This is a mechanical or electrical device which is designed to maintain a constant level in a high-pressure liquid receiver. In a correctly charged flooded system this will ensure the correct level of refrigerant in the low-pressure receiver or heat exchanger. This type of control also ensures that the condenser is always empty of liquid refrigerant and thereby operates at its design capacity.

4. Safety devices

Safety devices come into two categories: those for protecting the equipment and those for the environment. Most small units operate without safety devices since replacement costs are low and irreparable damage less likely. Also, with smaller units, simplicity is always desirable. Larger units, however, from about 20 hp and upwards, would have one or more of the following devices:

1. High pressure cut-out: This limits the pressure in the condenser and other parts of the high pressure side of this system thus avoiding the possibility of damage and overheating of the refrigerant oil. Cost US$ 15-75.

2. Low pressure cut-out: This limits the minimum pressure at which the evaporator and other parts of the low pressure side of the system operate. Low pressures can be damaging to the machine and also give rise to excessive leakage into the system when the pressure is unnecessarily operated below atmospheric. Cost US$ 15-75.

3. Oil pressure cut-out: This ensures that the compressor is not operated if for some reason lubricating oil is not being circulated at the required rate. Cost US$ 50-75.

4. Motor overload: This device can protect both the drive motor and the refrigeration equipment if there is an excessively high load on the compressor, a blockage or ceasure.

5. Capacity control

Capacity control of a refrigeration system can be achieved in many ways and the following list gives some of the methods likely to be used in fish freezing and cold storage:

Off-loading of cylinders in a multicylinder compressor .
By-passing from de 1 i very to suction at the compressor
On/off-cycling of the compressor by either a temperature or pressure-sensing device,
Speed control of the drive motor either by electrical or mechanical means ,

A qualified person should be consulted on whether this requirement is necessary and on the choice of method used.

3.1.3 Liquified gases

Liquified gases are used almost exclusively for freezing smaller fish products in continuous freezers,

There are three liquified gases commonly used for this purpose:

Liquid nitrogen (N2)
Liquid carbon dioxide (CO2)
High purity R12 (liquid freon freezant or LFF)

Liquid nitrogen is sprayed over the product after it has been precooled or conditioned in the nitrogen gas during the early stages of freezing, Freezing by total immersion in liquid nitrogen cannot be used since it results in very quick temperature change in the product and this "thermal shock" results in physical damage,

Liquid carbon dioxide sprayed into the freezer results in the instantaneous formation of both solid and gaseous forms of CO2. The solid form is deposited as a covering of "snow" on the surface of the product and sublimation of this "snow" at -78,5�C is responsible for most of the refrigeration effect.

Liquid freon freezant is either sprayed on the product or used with total immersion of the product, or a combination of both of these methods can also be used, In the case of an LFF system, unlike the others, the evaporated refrigerant is recondensed by means of a mechanical refrigeration system and reused with only a small loss of up to 3%.

Table 21 lists the physical properties and other data of the three refrigerants.

Table 21 Liquified gas refrigerants used in open systems

3.2 Specific Processes and Equipment

3.2.1 General

Freezers for fish and fish products can be divided into four main groups depending on the method of heat transfer used:

Air blast
Contact
Immersion
Liquified gases

The above main groups are dealt with later, where more specific details are given about. their design and uses in the fishing industry, The following brief description therefore only gives a very broad outline of their main characteristics.

Air-blast freezers - widely used for nearly all fish and fish products since their main feature is a versatility to freeze products of all shapes and sizes, They are constructed for batch, continuous and batch/continuous modes of operation and therefore can be used for small operations as well as in large-scale production lines.

Contact freezers - this type of freezer has also many applications for fish-freezing with vertical plate freezers (VPF) and horizontal plate freezers (HPF) being the most familiar types. The main use of VPF and HPF is to freeze regular-shaped fish products such as blocks, packages and cartons.

Another type of contact freezer, which is a continuous process using a rotating drum, has also a limited use for freezing IQF products such as fish fillets.

Plate freezers are more familiar as units for batch freezing, but some designs for a continuous operation are available.

Immersion freezers - this type of freezer has now only a limited use and some of the applications, which previously used an immersion process, have changed to other freezing methods. Sodium chloride brine and salt/sugar solutions are two fluids which may be still considered for special purposes.

Liquified gases - these are used mainly where refrigerant supplies are readily available and are relatively cheap. The gases are used mainly for continuous freezing processes and nitrogen, carbon dioxide and a refined form of R12 are refrigerants which have been used for fish freezing applications.

3.2.2 Air-blast freezers

3.2.2.1 Types:

Air-blast freezers can be subdivided according to their mode of operation, method of loading and pattern of air flow. Some of the likely combinations can be derived from the block diagram in Table 22.

1. Batch, continuous and batch/continuous

Batch freezers are more versatile than continuous, therefore, if a variety of products are to be frozen, a batch freezer may be selected. Batch freezers are also likely to be used for products with longer freezing times since with a batch freezer there is better utilization of floor space due to the multi-layer arrangement of loading. It is difficult to decide on an exact line of demarkation but freezing times longer than one hour would usually require a batch mode of operation. (See Figures 20, 21 and 22.)

Continuous freezers are best used for freezing individual portions, such as single fillets and small shellfish, such as peeled prawns and scallops. The main advantage in using a continuous freezer for these smaller and/or thinner products is that since they freeze quickly they will also thaw quickly and the delays that occur with a batch-freezing operation may be overlong. Continuous freezing allows quick handling after freezing and a quick transfer to the cold store.

Batch/continuous freezers are usually batch type freezers operated with trolleys which are loaded in sequence at fixed-time intervals rather than all at one time as in the truly batch freezer.

Table 22 Types of air-blast freezer

The type of freezer used will depend on the specific requirements of each installation and the following brief notes will help with this choice

Figure 20 Batch air-blast freezer with side loading and unloading

Figure 21 Batch-continuous air-blast freezer with counterflow air circulation

Figure 22 Batch-continuous air-blast freezer with crossflow air circulation

This allows the produce to be quickly loaded into the freezer rather than have delays while waiting for full loads. The number of trolleys and the loading interval are selected to ensure that when the freezer is fully loaded a new trolley will be exchanged with one which has been in for the necessary freezing time and, thereafter, a one out and one in system is operated. This loading arrangement also ensures that the refrigeration demand is more uniformly spread than would be the case for a batch freezer (Figure 23).

A batch-continuous freezer layout and airflow arrangement should be designed to ensure that a new load of warm fish is not placed upstream of a partially frozen load.

2. Trolleys, pallets, shelves

Trolleys are more mobile but take up more space on the factory floor. Pallet loads can be moved directly from the freezer to a cold store for temporary storage without the need for taking expensive equipment out of service.

Fixed shelves within the freezer are not recommended since the freezer door must be left open during loading. This may take a considerable time and result in an unnecessary high ingress of heat and water vapour with the air entering the freezer. With a fixed shelf arrangement some of the air-blast freezer's versatility will also be lost.

3. In-line or spiral continuous freezers

Spiral freezers are designed to reduce the floor space requirement of the freezer but they, in turn, require a higher roof clearance and in some cases this may be equally critical. Spiral freezers also tend to distort the shape of some products due to the curvature of the belt path. In line freezers have been built with a multi-belt operation in order to reduce the floor space (Figure 26). Transfer of partially frozen fish from one belt to another. however, may be difficult and result in breakage or distortion of the product. This transfer, however, is achieved with some degree of success in a semi-fluidized freezer with some products by ensuring that the wet product does not adhere to the first belt (Figure 27).

Figure 23 The effect on temperature and refrigeration capacity when loading a batch air-blast freezer

Figure 24 Continuous belt air-blast freezer with crossflow air circulation(also constructed with countercurrent series flow air circulation)

Figure 25 Continuous air-blast freezer with the belt arranged in a spiral

Figure 26 A triple-belt air-blast freezer

Figure 27 Semi-fluidized flow freezer with double belt

Figure 28 A fluidized flow air-blast freezer

4. Mesh, link or flat belts

Mesh and link belts can be used with fish which are unskinned, if the skin is arranged to be in contact with the belt. Freezing skin-off fillets and other fish and fish products, which must be frozen with the flesh in contact with the belt may, however, prove to be difficult, with damage to the product and loss of yield due to breakage when releasing the frozen product. Flat belts made from sheet stainless steel are now used with in-line freezers, and most products can readily be released without damage. Also, with a flat belt, particularly delicate products can be released by defrosting the belt at the exit of the freezer by spraying the underside of the belt with water.

5. Crossflow and series flow

Whether one pattern of flow or another is used depends mainly on the configuration of the freezer and, before deciding on this aspect and other features of the design and layout, the following should be considered:

- total volume of air flow
- temperature rise of the air over the product
- flow only from colder to warmer products
- large-face area of the cooler required for minimum frost interference
- pressure differences within the freezer in relation to doors and openings
- pressure drop around the circuit and the effect on fan power

A knowledge of both the intended freezer operation and the elements of good engineering and heat transfer practice will, therefore, be required before a final decision is taken.

3.2.2.2 Costs:

Only a rough guide can be given on the cost of air-blast freezers since, unlike other types of freezer, it is the exception rather than the rule to buy them as standard designs from an established range of sizes, particularly in the case of batch freezers. Batch air-blast freezers are usually built on site to suit individual requirements and local costs may, thereby, account for a good proportion of the total cost.

Some guidance on costs, however, is presented in graphical form in Figures 29-33. In these costings the refrigeration machinery is included, but this may not always be the case.

Care should always be taken to ascertain what the cost includes when making comparisons between freezers.

3.2.2.3 Air speed:

A compromise has to be made in the design air speed of an air-blast freezer since high air speeds and good heat transfer also mean higher costs. Figure 34 shows a typical relationship between air speed and freezing time with long freezing times at low air speeds and little to be gained if higher air speeds are used. Air speed affects the surface heat transfer only, therefore, the influence on overall heat transfer depends on the relationship of this surface effect to the heat transfer, through the product and packaging. Improved surface heat transfer would, for instance, have less effect on the freezing time of thicker products or products which are wrapped before freezing.

Experience has shown that an air speed of 5 m/s is a good average value for batch air-blast freezers and this should ensure that there is an effective air speed throughout the freezer space. With longer freezing times, such as in freezers designed to freeze thicker products overnight, lower air speeds may be justified, but it is unlikely that this air speed would be less than about 3 m/s.

Figure 29 Batch freezers (including refrigeration)

Figure 30 Continuous spiral freezers (including refrigeration)

Figure 31 Continuous single-mesh belt in-line freezers (including refrigeration) ,

Figure 32 Fluidized freezers (including refrigeration)

Note: The above costs were for a PEA freezing operation. Although there will be some difference when freezing small shrimp the costs will be of much the same order of magnitude

kg/h

Figure 33 Flat belt continuous in-line freezer (including refrigeration)

Figure 34 Variation of freezing time with air speed in an air-blast freezer

Since continuous air-blast freezers can take up a good deal of floor space, there is an incentive to improve the heat transfer and reduce the freezer size by using higher air speeds.

An air speed of 10 m/s has been used in a continuous air-blast freezer and since the type of product frozen is either small or thin, this higher air speed has a significant effect on the overall heat transfer. High air speeds need only exist in the working section of the freezer, therefore, an arrangement such as that shown in Figure 24 will ensure that the fan power is not excessive.

Even in well designed freezers the heat introduced by the fan may account for 20-25% of the refrigeration requirement and in a badly designed freezer fan heat may even exceed the heat to be removed from the product.

3.2.2.4 Operating temperature:

The design temperature of an air-blast freezer should be related to the temperature of the cold store to which the product is transferred after freezing. Good freezing practice should ensure that the product is frozen down to an average temperature equal to the intended storage temperature, and in order to achieve this, the air temperature in the freezer should be at lease 5�C lower. For example, with a storage temperature of -30�C, the associated air-blast freezer will require to have an air temperature of -35�C and for -25�C storage, the air temperature should be -30�C.

Lower operating temperatures may occasionally be justified in order to achieve quicker freezing times, but this adds to the cost of freezing and there may be problems with the leaks into the refrigeration system due to the low pressures associated with the low temperatures.

3.2.2.5 Floor area:

The floor area required for a freezer will obviously depend on its freezing capacity, but it also depends on the type of the freezer and the product to be frozen.

Bigger freezers tend to use space more economically as can be seen from the following examples of batch freezers for freezing trays of fish on trolleys:

A slow freezing product will require a larger freezer than a faster product and this is illustrated by the following comparison:

Both freezers have the same capacity, but due to the longer freezing time in B, the freezer requires to hold double the load of freezer A which will be loaded and emptied twice during the two hour period.

A comparison is made between the floor area requirements of different types of air-blast freezer in Table 23. All freezers have a capacity for freezing 700 kg/h of IQF fillets.

Table 23 Floor space requirement of air-blast freezers

a/ Only likely application would be for cooked, shell-off small shrimp and other products of similar nature

All space requirements quoted in Table 23 are for the freezer cabinet only, but other space requirements must also be added.

Only in very small freezers will the refrigeration machinery be incorporated as part of the freezer unit, positioned either above or below the freezer cabinet. Machinery is therefore located out with the working area and preferably in a separate room.

Floor space is required for loading and unloading the freezer, and space may also be required for a complete spare set of trolleys or pallets so that they can be loaded and waiting for the next freezing cycle.

Batch freezers are often built on site as an integral part of the building, but previously constructed or free-standing freezers should be left with a space around them for inspection and maintenance of the insulated structure.

Space may also be required for releasing frozen fish from trays or frames and, in turn, space and facilities for cleaning and storage of the trays will also be required.

The space requirements of a freezer may therefore exceed the space occupied by the freezer cabinet and it is this total space requirement that should be used in making comparisons and calculations. With the other space requirements taken into account the relationship between the freezers shown in Table 23 may therefore be totally different.

3.2.2.6 Freezing time and cycle time

Freezing times are determined by the size, shape and packing of the product together with the temperature and air speed in the freezer. The freezing cycle depends both on the freezing time and the time taken to load and unload the freezer.

If other considerations permit it, freezing cycle times should be selected to give optimum use of the freezer. For instance, a change in product thickness may allow the cycle time to fit exactly into a working day and thus the freezer will be fully utilized, as shown in the following comparison:

3.2.3 Horizontal plate freezers

3.2.3.1 General:

Horizontal plate freezers (HPF) are used for freezing regular shaped packs of fish. The product is frozen in cartons, freezing frames or lidded trays which make direct contact with the plates through which the refrigerant is circulated.

Figure 35 Horizontal plate freezer

The horizontal plates are pressed together to a predetermined spacing, set by the product thickness, and the contact pressure is maintained by a hydraulic device during the freezing process.

The bank of freezer plates forming the unit is normally contained in an insulated enclosure which is lined internally and externally with galvanized or coated metal sheeting. Access to the freezer stations is either by means of vertically-lifted curtains or doors, at the front and rear.

The capacity of a HPF unit depends on the product thickness, plate size and the number of stations and, within the extreme limits of the various combinations available, freezers can be constructed to suit individual requirement.

Plate sizes may differ between one manufacturer and another and each may only offer a choice of three standard sizes for their complete range of freezers.

A manufacturer may have a range of freezer units with up to 20 stations but, obviously, the wider the spacing between plates the fewer would be the maximum number of stations in order to keep the freezer height reasonable.

When selecting the space required between plates, it is usual to have the maximum plate opening 25 mm more than the thickness of the product to allow easy loading. (Tables 24 and 25.)

When calculating the weight that can be frozen in a HPF the number of packages and the weight of fish contained in each package have to be taken into account.

Plate dimensions and package sizes should be matched to avoid excessive waste of potential freezing space and it is usual to achieve about a 75% plate coverate in practice.

Although freezers can be supplied with wider plate spacings, it is usual to freeze fish products in packages, trays or forming frames with a depth not greater than 50 mm to 75 mm.

Individual calculations will require to be made for each application to obtain accurate figures but an indication of the loading of individual freezer stations is given in Table 26 for guidance.

The use of freezer trays and freezer frames (Figure 36), depends on the type of product to be frozen. Cartons, other containers and blocks of fish with only nominal dimensions are normally frozen in trays. For the production of blocks with a requirement for precise dimensions, freezing frames are necessary, but again these may be placed on a base tray to facilitate loading and unloading.

Refrigeration systems used with HPF are:

1. Pump circulation system with a primary refrigerant
2. Pump circulation system with a secondary refrigerant

Table 24 Dimensions and shipping weights for horizontal plate freezers (See Table 25 for plate sizes)

a/ For shipping, the hydraulic cylinder can be removed. Maximum shipping height is therefore given by A and F

Table 25 Horizontal plate freezer-plate sizes and openings

Table 26 Horizontal plate freezers Average net weight of product per station (kg)

Figure 36

3. Gravity feed flooded system
4. Direct expansion system

The pump circulation system is recommended for large capacity freezers or multiple freezer installations in order to achieve a good, uniform heat transfer in all freezer plates. For secondary pump circulation systems calcium chloride brine or trichloroethylene are the most commonly used refrigerants, but due to the higher capital cost of this system it is usually confined to large marine installations.

The gravity feed, flooded system is the most widely used for medium and large, single unit freezers. The low temperature reservoir vessel is mounted directly above the freezer to give a compact and efficient layout and operation.

Direct expansion systems are more appropriate for smaller freezer units where there will be less difficulty in achieving good refrigerant distribution and hence uniform freezing.

Table 27 Pump circulation rates for horizontal plate freezers

Accurate freezing times are essential for calculating the capacity of a freezer and the times used should preferably be measured freezing times obtained by freezing samples of the product under the intended operating conditions. Before using freezing times from other sources. reference should be made to sources giving more information on this subject since the freezing time can vary considerably depending on a number of factors.

Table 28 Horizontal plate freezer -approximate freezing times (Pump circulating system evaporating at -34�C)

Table 29 Horizontal plate freezer -approximate freezing times (Gravity feed flooded system evaporating at -34�C)

Defrost time and the time taken to load and unload the freezer also have to be taken into account when calculating the freezing capability.

On most installations it is only necessary to defrost once per day if no liquid is spilled on the surface of the freezer plates. The daily defrost then may be achieved overnight by leaving both doors open although. on occasion, some assistance may be given by hosing the plates with clean water.

If it is necessary to operate with a quick defrost between each freezer load, it may be necessary to have a built-in defrost system. A quick defrost may also be required if the freezer was used 24 h/day with only 1-2 h allowed for defrost and cleaning.

Assuming trays are preloaded and a full load is available, the combined loading and unloading time for each HPF cycle under ideal conditions should be about 15-20 min for medium sized freezers and proportionally shorter or longer for other sizes.

3.2.3.2 Selection:

Before any selection can be made, basic information must be established

1. Quantity of frozen product required per day
2. Thickness of product
3. Nature of product
4. The refrigeration system to be used

The number of cycles per day is:

When selecting horizontal freezers for 24-h operation, allow 1-2 h less for defrost and cleaning.

The weight of product to be frozen during each cycle:

The number of stations required:

The nearest whole number is taken. Should this figure be above the maximum available, divide so that suitable equally stationed freezers can be selected.

3.2.3.3 Refrigeration capacity:

Calculate the total heat content of the product and add 20%.

Refrigeration capacity required:

Where multi-installations are selected, the above capacity may be reduced 5% for each additional equally sized freezer up to a maximum of 15%, provided the loadings are staggered.

Evaporating temperature to be used with the above refrigeration capacity is given in Table 30.

Table 30 Evaporating temperatures of horizontal plate freezers

The refrigeration requirement calculation for an HPF must take into account allowances for the type of product, initial and final temperatures, trays, frames and packaging, insulation heat gains, pump energy input and other possible factors. In the absence of detailed information a value of 115 kcal/kg can be used with some assurance for chilled fish frozen without added water. Added water will increase this value by 101 kcal/kg of water added.

Some other recommended values are given for typical fish products in Table 31.

Table 31 Horizontal plate freezer refrigeration requirements

Notes:
(i) Product initial temperature is 10�C and final temperature approximately -30�C
(ii) Capacity is for product only, excluding added water

Selection example

A customer requires to freeze 30 t of fillets in a 24-h working day. The fillets are to be frozen in 50 mm thick blocks and the refrigeration plant is to operate with an ammonia pump circulation system. The initial product temperature is 10�C and the final equilibrium temperature -30�C.

Freezing time	1 h
Weight loading/station (1 930 x 1 120-mm plates)	100 kg
Number of cycles/day	corresponding to 19 cycles say, 18 cycles allowing 1 1/2 h for defrosting
Weight frozen each cycle	30 000 kg corresponding to 1.666 kg 18 cycles
Number of stations required	1 666 kg/cycle corresponding to 17 stations 100 kg/station

The freezer selected will therefore be one 17-station freezer fitted with 1 930 x 1 120-mm plates.

At this point the maximum plate openings must be checked to assure that a clearance of at least 25 mm is available from a standard range. Should a greater opening be necessary, then a purpose-built unit will be required.

If 10% water was added this would increase by 1 666 kg x 0.1 x 101 kcal/kg, corresponding to 16 827 kcal/h

Refrigeration requirement 194 922 + 16 827 = 211 749 kcal/kg

Table 32 Cost of horizontal plate freezers .

3.2.4 Vertical plate freezers

3.2.4.1 General:

The vertical plate freezer (VPF) is ideally suited for bulk freezing of fish. and although originally designed for freezing fish at sea. it is now also used extensively on land mainly for freezing seasonal fish which are frozen in bulk for processing throughout the year.

Figure 37 Twenty-station vertical plate freezer with top unloading arrangement

The product is loaded into spaces formed by refrigerated plates which form the stations of a vertical plate freezer unit. The plates are hydraulically closed thereby slightly compacting the product to a preset block thickness and also improving the contact between the fish and the plate surfaces.

After freezing, the cold refrigerant is turned off and a hot refrigerant supply is circulated through the plates to defrost them and break the bond between the plates and the product. This defrost procedure only takes a few minutes and, when complete, the hydraulic system is operated to open the plates and raise the blocks to the top of the freezer ready for removal.

Since the product is frozen into a symmetrical block, it is ideally suited for palletizing to give good utilization of cold storage space. The product can be frozen unwrapped and stored without packaging or frozen in paper or plastic bags which are inserted between the plates before loading the fish. Unwrapped products may, however, be glazed or inserted into cartons after freezing.

3.2.4.2 Freezer size:

The size of a VPF unit depends on the plate size, plate spacing and the number of stations.

A plate widely used has dimensions of 1 120 x 558 mm which produces full-sized blocks measuring 1 060 x 520 mm. Other plate sizes available from the same manufacturer give block dimensions of 1 180 x 490 mm and 800 x 806 mm. Other standard sizes may, however, be available from other manufacturers. The provision of a non-standard plate size will be expensive since this may require a special die to extrude the plate sections.

The standard block thicknesses produced in VPF are 50 mm, 75 mm and 100 mm, and by means of special adaptors it is possible to have more than one standard spacing in the same unit.

Any number of stations can be supplied up to a limit of about 30, but manufacturers will normally only supply five or six standard sizes. Special requirements, such as small units for freezing trials or laboratory use, can, however, be made on request.

When selecting the number of stations per freezer unit, account should be taken of the likely pattern of fish supplies. Unit sizes should be selected so that they are likely to be completely filled during each cycle, thus avoiding the possibility of freezing partial loads or having freezers waiting for further supplies to complete a load.

Overall dimensions and weights for a full standard range is given in Table 33.

Table 33 Vertical plate freezers Dimensions and shipping data (uncrated)

Note: Freezers have standard plates measuring 1 120 mm x 558 mm

3.2.4.3 Refrigeration system:

To ensure a good circulation of refrigerant, and hence uniform freezing in all stations, vertical plate freezers use pump circulation of refrigerant. Both primary and secondary refrigerants are used and all the refrigerants listed in Table 12 are suitable. For safety reasons, ammonia, however, may not be used at sea in some countries.

Refrigerant pump circulation rates are the same as those given in Table 32 for HPF.

Defrosting is an important operation and this is done at the end of each freeze to enable the block to be quickly removed from the freezer. Defrosted plates are also necessary for reloading the freezer. Loading fish between frosted plates results in the fish adhering to the plate surfaces to give low density blocks and the poor contact results in longer freezing times.

With a primary refrigeration system a hot gas defrost can only be achieved if two or more freezer units are operated, and they are defrosted in sequence.

3.2.4.4 Freezing times and freezing capacity:

The density of the block after loading is an important factor which affects the capacity of a freezer unit. This will vary depending on the product, and considerable variations can also exist for any given product. Fish size, fish freshness and whether they are still in rigor mortis are all factors which determine how the fish pack between the plates, and thereby determine the final block density.

Variations in block density affect the load that can be contained in the freezer, but this also has an influence on the freezing time since block density relates to the contact made between the product and the plate, and hence freezing time.

Knowledge of the product and hence the block density likely to be achieved is therefore important in the calculation of freezer capacity. There is no accurate method of determining this density other than from past experience or by tests, but in order to give some guidance and show how block weights can vary, some typical figures are given in Table 34.

Table 34 Vertical plate freezers -block weights (blocks- 1 120 mm x 558 mm x 100 mm)

As in other cases, accurate freezing times can only be obtained by measurement under closely specified conditions and even when known times appear to relate to the product under consideration, a variation in only one of the conditions can make a significant difference.

The times listed in Table 35 should therefore only be used as a guide during the early stages of planning or for comparison with figures obtained from other sources. The figures relate to good operating conditions and they therefore may not always be obtainable in commercial practice.

Table 35 Approximate freezing times for vertical plate freezers

3.2.4.5 Example calculation:

A customer requires 30 t of fish fillets to be frozen, over a working day of 24 h, as 50-mm thick blocks. The initial fish temperature is 10�C and the final temperature after freezing is assumed to be an average temperature of -18�C. The refrigeration plant is to operate with an ammonia pump circulation system.

Freezing time	1 h
Weight of block	27.3 kg
Number of cycles/day	corresponding to 18 cycles
Weight frozen each cycle	30 000 kg corresponding to 1.666 kg 18 cycles
Number of stations required	1 666 kg/cycle corresponding to 62 stations 27.3 kg/station

Two 30-station freezers may therefore be selected to give a slightly reduced capacity of 3 x 20-station units or three units with 12, 20 and 30 stations, respectively, may be more appropriate to match the likely pattern of fish supplies.

Refrigeration capacity:

Heat to be removed (Table 79)	75 kcal/kg
Freezing time	1 h
Load/cycle	1 666 kg
Refrigeration requirement	1 666 kg x 75 kcal/kg 1 h
	corresponding to 124 950 kcal/h

The figure is derived by taking account of the product load only. In order to allow for the higher refrigeration requirement at the start of freezing, and other refrigeration requirements such as cooling the freezer plates, insulation heat gains and pumping energy transmitted to the system, additional accurate calculations can be made or an arbitrary factor of 30'0 added to the above value.

Total refrigeration duty required 124950 x 1.3 corresponding to 162 435 kcal/h

Table 36 Cost of vertical plate freezers (freezer only)

3.2.5 Direct contact

The three refrigerants most commonly used in direct contact with the product are nitrogen, carbon dioxide and liquid freon freezant, and information on their properties and other data is given in 3.1.3.

The main advantage in using these refrigerants is that high rates of heat transfer can be achieved and they are therefore all used in freezers intended for a continuous mode of operation.

Another advantage compared with air blast freezing is that there is a reduced weight loss by evaporation and this is often taken into account in costing when comparing with air blast. However, many of the costings are not based on measured weight losses under comparable conditions and care should be taken to ensure that the conditions are relevant to the application under consideration.

3.2.5.1 Nitrogen (N2):

A nitrogen freezer is a total loss system with the nitrogen gas being vented to atmosphere after use. In some situations, natural ventilation may be sufficient, but in more confined spaces forced ventilation may be necessary to maintain the correct oxygen level in the factory air.

Figure 38 Liquid nitrogen freezer

Freezing can use from one to three times the frozen products weight of nitrogen with lower consumption rates where there is a strict control of the operation and a high freezer utilization value.

The cost of nitrogen freezing has been reported with wide variations depending on the criteria set down for the costing calculations. Care should therefore be taken that when a comparison is made, the conditions are related to the particular application. Calculations made under likely UK conditions show that the total cost of nitrogen freezing is about three times higher than the cost for continuous air-blast freezing and this relationship is even less favourable for nitrogen when compared with bulk air-blast freezing.

The budget costs for a range of nitrogen freezers are given in Figure 39.

Figure 39 Cost of liquid nitrogen continuous freezers (excluding storage tank)

Nitrogen freezers are more compact than any other type of continuous freezer, but account should also be taken of the space requirement for the on-site storage tank and the need for access for refilling.

Together with freezing costs, continuity of supplies of refrigerant is the major consideration when contemplating the use of nitrogen freezers. In many cases, unavailability of supplies will prohibit their use, but even when there is a suitable supply, other elements of delivery, such as access roads and their condition throughout the year, should be given consideration.

Nitrogen is usually a by-product from other processes, therefore, depending on other likely local uses, the nitrogen may be available at a low competitive price.

3.2.5.2 Carbon dioxide (CO2):

Carbon dioxide is also a total loss refrigerant and it is essential in this case that the gas is ventilated outside the building, since at concentrations of 2% in the working area it will become unpleasant, and at over 10% dangerous.

Carbon dioxide is supplied as a pressurized liquid, but when this is metered into the freezer, it converts to a mixture of solid and gas. Good heat transfer from the product depends on contact with this solid fraction, therefore, if distribution is poor, freezing times will be variable.

Like nitrogen freezing, the cost of carbon dioxide freezing depends on the source of supply since, again, it is also likely to be a by-product. Carbon dioxide freezing will be slightly cheaper than nitrogen freezing, with total costs approximately two-and-a-half times that of continuous air-blast freezing.

Continuity of supplies will also be a major consideration, and additional space for storage of the refrigerants on site will again be a factor to be taken into account.

3.2.5.3 Liquid freon freezant (LFF):

Unlike nitrogen and carbon dioxide, most of the refrigerant is recondensed and used again, thus costs are a good deal less at about one-and-a-half times the total cost of air-blast freezing.

Figure 40 Liquid-freezant freezer

This refrigerant, however, may not be approved for use in direct contact with food locally, or not allowed for freezing foods imported into some countries, therefore, it may not be an option which can be considered.

Although LFF incorporates some of the advantages that are associated with other liquified gases, the need to have a mechanical refrigeration system for recondensing the refrigerant does not make it independent of a suitable electrical supply or other motive force as is the case with the others.

3.3 Packaging and Glazing

3.3.1 Packaging

3.3.1.1 Need (Table 37)

1. General: The main functions of packaging are envelopment, protection and identification. Packaging of frozen fish will reduce the main spoilage mechanisms, i.e., dehydration and oxidative rancidity. Packaging will also provide protection against mechanical damage and contamination, and it will also allow the product to be identified both for storage management and for customer information at the point of sale.

2. Package descriptions:

Carton - Any box of card or plastic which fully encloses the packaged material. "Carton" tends to be used for smaller boxes and "master carton" for large boxes used to hold collations of smaller packs.

Vacuum packing - packs which are evacuated then sealed so that virtually no air is inside the pack.

Boil-in-the-bag - Generally vacuum packed; the product is cooked by boiling. The packaging film used must be effective under boiling conditions.

Stretch wrap - The product is packaged in a thin elastic film which is stretched tight. Often a "cling" film is used which has good, dry adhesion to itself and to other smooth dry surfaces.

Shrinkwrap - The product is packaged in a film, generally medium to thick, which shrinks in one or both directions when heated.

Overwrap - The product is packaged in a film which is wrapped loosely and sealed.

3. Abbreviations:

The following abbreviations are sometimes used for packaging film materials.

Abbreviation	Trade/generic name	Chemical name
PA	"Nylon"	Polyacetal
PE	"Polythene"	Polyethylene
PVdC	"Saran" "Cryovac"	Polyvinylidene chloride
PVC	-	Polyvinyl chloride
PP	-	Polypropylene
OPP	-	Oriented polypropylene
PET	-	Polyester

3.3.1.2 Frozen packs:

There are many different packing materials used for fish, and it would be unrealistic to try to .list them all. Only the more widely used materials are therefore mentioned, and the tables give the more salient properties only. Quantities required should be determined after discussion with the makers of packaging machinery since wastage can be as high as 10%, particularly with preprinted material and high-speed machinery.

The two main properties, which must be considered in packaging material for fish, are water vapour permeability which determines the extent of dehydration and gas permeability which determines the extent of oxidative rancidity. As can be seen from the tables, there is no ideal single material and, generally, films are laminated to optimize the properties of two or more. Thicker films form better barriers but cost more, therefore, a compromise has often to be made. Thicker films, however, are essential for rigid-formed packs.

The basic films have a 2-1 cost spread, but the final cost will depend on lamination, printing and processing, and this will result in an even wider price range, depending on ,the specification.

Packaging should be carried out as soon as possible after freezing, but if delay is unavoidable, the- frozen product should be held in closed, lidded containers in a cold store.

The choice of packaging film will, to some extent, depend on availability. Local suppliers should be contacted for appropriate costs. It must not be forgotten that packing materials intended for other goods may not be suitable for frozen fish.

Table 37 Packaging methods and applications

Table 38 Typical laminates compared with PE and PA

Note:
(1) As can be seen from the Table, water vapour permeability properties should be compared with the excellent qualities of PE, and oxygen permeability with the good qualities of PA
(2) For both permeability figures low values are the better

3.3.1.3 Equipment:

Packaging equipment may vary between a simple hand-operated tool and a complicated machine, therefore, a range of methods are listed in Table 40 to suit all eventualities for a variety of packaging operations.

The type of machinery selected will depend on many factors, some of which will be greatly influenced by local conditions, therefore, independent advice should be taken for each application.

3.3.1.4 Labelling:

It is possible for all the label information to be incorporated at the pack manufacture stage. This is appropriate where large runs of a product in a fixed weight pack are used and where no date or serial markings are required. Secondary labelling or marking is used to apply extra information to a pre-printed bag or carton. For instance, the contents of a master carton or the date mark, or weight on a smaller package, could be marked by a separate printer. Labelling machines are available and, of course, labels and markings can be hand-applied. Table 40 gives examples of available equipment. The decision as to when labelling and marking is to be done depends on the type of equipment and materials being used, and many labelling machines are designed to be incorporated in packaging machinery or conveyors.

Prices of label dispensers vary from US$ 750 for a simple machine up to US$ 7 500 for machines with overprinting and programmable controls. Overprinters vary from US$ 300 upwards. A form of coding can be incorporated into bag-sealing apparatus at low cost.

3.3.1.5 Space:

The space requirements for packaging will depend on the methods used. Manufacturers should be consulted for information on machinery sizes. Plant layout should be determined, using flow charts and space allowed for conveyors and access.

At this stage of the operation it may also be necessary to weigh or check-weigh packages and to pass packages through a metal detector. Space must be allowed for this.

3.3.2 Glazing

Glazing is the application of a layer of ice to the surface of a frozen product by spraying on water or by dipping the product in water, and it is a widely used means of protecting frozen fish products from the effects of dehydration, oxidation and other changes during cold storage

Table 39 Properties of basic materials

Notes:
1/ Tensile, tear and water vapour (wv) qualities are relative to PE which has unit value
2/ Gas permeability is related to PA which has unit value

Table 40 Packaging machinery

Note: Data for typical machines. Local prices and availability may vary

An ice glaze is, in effect, a tight fitting wrapper, but unlike other wrappers it is not permanent since the ice is lost by sublimation in the cold store. Periodic inspection and reglazing, as necessary, is therefore required, particularly if no other wrapper is used.

Water used for glazing must be of a potable quality, and when a dipping method is used the water should be periodically changed.

On applying a glaze, it is necessary to ensure that all the surfaces are covered, and in circumstances where the amount of glaze added is important, the glazing operation should be done under strictly controlled conditions.

It is not practical to measure the thickness of an applied glaze, and usually glaze will not be of a uniform thickness over all surfaces. Glaze is therefore defined as a percentage of the product weight and this will vary considerably with the size and shape of the product, even when the glaze thickness is the same.

For instance, a large 45-kg block of cod, quickly dipped in chilled water, will have a glaze of about 1.5%, which corresponds to a mean thickness of 0.5 mm, whereas a single fillet dipped in a similar manner will have a glaze of about 8% with a mean thickness of 0.25 mm.

Under these conditions, the larger surface area per unit weight of the smaller fillet results in a higher percentage glaze, but the difficulty of handling the larger block quickly in and out of the water results in a longer immersion time and a thicker glaze.

When glaze is applied in an uncontrolled manner, large variations can result and values between 2% and 20% by weight have been measured in an IQF fillet-glazing operation. Even when the process is controlled, there are difficulties in keeping the glaze within specified limits, and this may result in complications when glaze has to be accounted for in the selling weight.

The factors which influence the amount of glaze taken up are as follows:

Glazing time
Fish temperature
Water temperature
Weight grade (product size)
Product shape

It is possible to control some of these factors, such as glazing time and fish temperatures, particularly if a continuous glazing operation is used immediately after freezing. In some climates, seasonal changes in the temperature of the domestic water supply can significantly change the weight of glaze added, but with a continuous system, glazing time can be varied to compensate for this. However, even when glazing conditions are strictly controlled, glaze variations will exist.

With larger products, such as the 45-kg block mentioned above, differences may be insignificant since the total glaze is only a small percentage of the product weight. With 100-g fillets, strict control of a continuous spray-glazing process resulted in a mean glaze of 6.2% with a statistical probability of 5% of the fillets having a glaze out with the range of 5.1-7.3%. This glazing operation also showed that with glazing times which result in less than 4% glaze, the product was not

completely covered and with a glaze and above 870 there was some partial thawing at the edge of the fillets. Therefore, even with good glazing practice, minimum and maximum glazing limits will apply, and possible variations should be confined within this range.

Glazing also adds heat to the product and excessive glazing will mean a considerable additional product heat load when it is placed in cold storage.

The heat added and subsequent equilibrium temperatures for normal and excessive glazing are given in Table 41.

Standard designs of continuous glazing machines are not readily available, and many are there-fore built to the processors specification only. Basically, a machine consists of an enclosed variable-speed open mesh conveyor belt with water sprays above and below to cover all surfaces of the product. Manufacturing costs should be between US$ 3 000 and US$ 6 000, and the following specification is typical for an in-line unit:

Capacity	2 000 kg/h of IQF fillets
Dimensions	2 m x 1 m x 1 m
Power requirement	less than 2 kW
Water consumption	300 litres/h
Cost	approximately US$ 5 000

Table 41 Heat added and equilibrium temperature after glazing (initial fish equilibrium temperature -30�C)