The primary function of thermal insulation materials used in small fishing vessels using ice is to reduce the transmission of heat through fish hold walls, hatches, pipes or stanchions into the place where chilled fish or ice is being stored. By reducing the amount of heat leak, the amount of ice that melts can be reduced and so the efficiency of the icing process can be increased. As has already been discussed, ice is used up because it removes heat energy from the fish but also from heat energy leaking through the walls of the storage container. Insulation in the walls of the container can reduce the amount of heat that enters the container and so reduce the amount of ice needed to keep the contents chilled.
The main advantages of insulating the fish hold with adequate materials are:
to prevent heat transmission entering from the surrounding warm air, the engine room and heat leaks (fish hold walls, hatches, pipes and stanchions);
to optimize the useful capacity of the fish hold and fish-chilling operating costs;
to help reduce energy requirements for refrigeration systems if these are used.
Because hold space is often at a premium on small vessels and the costs of insulation can amount to a significant proportion of the costs involved in construction, the choice of insulation material can be very important.
Several thermal insulation materials are used commercially for fishing vessels, but few are completely satisfactory for this purpose. The main problems are lack of sufficient mechanical strength and moisture absorption. The latter is a particularly significant problem in fishing vessels, where melting ice is used as a chilling medium. Thermal insulators work by trapping bubbles or pockets of gas inside a foam structure. When these cells of gas are filled with moisture, there are significant losses in insulating efficiency.
The thermal conductivity of water (at 10 °C) is 0.5 kcal m-1 h-1 °C-1 and that of ice (at 0 °C) is 2 kcal m-1 h-1 °C-1 (about four times the value of water). In comparison, dry stagnant air is about 0.02 kcal m-1 h-1 °C-1. Figure 5.1 shows the thermal conductivities of R-11, dry air, water vapour and ice within an insulation material and illustrates the significant increase in thermal conductivity that can occur if air/gas is replaced by water vapour in the insulation.
Absorption of moisture by the insulating materials can take place not only by direct contact with water leaking into the hold walls, but also by condensation of water vapour in the walls where the dew point is reached in the temperature gradient through the walls.
The proper design of water vapour barriers is therefore of utmost importance for protecting the insulation from gaining moisture. In most climates the transmission of water vapour will tend to be from the outside to the inside of the hold walls, as the external temperature is likely to be higher than the internal temperature. This requires an impervious moisture-proof layer on the outside of the insulation, as well as a waterproof barrier on the lining to prevent liquid melt water entering the insulation. The vapour barrier can be achieved either through watertight surfaces of prefabricated insulation panels (sandwich-type panels, with one face being the vapour barrier of light-gauge galvanized steel sheets and the other face being the internal finish of plastic-coated aluminium or galvanized iron sheets), reinforced plastic materials, polythene sheets, plastic films of minimum thickness of 0.2 mm or aluminium foil of minimum thickness of 0.02 mm, laminated with a bitumen membrane. The minimum thickness of aluminium or galvanized sheets should be 0.3 mm.
One of the best commercially available choices of insulation material for fishing vessels is polyurethane foam. It has good thermal insulating properties, low moisture-vapour permeability, high resistance to water absorption, relatively high mechanical strength and low density. In addition, it is relatively easy and economical to install. The main features of polyurethane foams are shown in Table 5.1.
Polyurethane foam is effective as an insulator because it has a high proportion (90 percent minimum) of non-connected closed microcells, filled with inert gas. Until recently, the inert gas most commonly used in polyurethane foams was R-11 (trichlorofluoromethane). However, the Montreal Protocol on Substances that Deplete the Ozone Layer has called for the phasing out of the use of CFCs such as R-11. Replacement foaming agents are being investigated at the present time, with hydrocarbons, hydrofluorocarbons and inert gases such as carbon dioxide being developed as substitutes.
The main ways polyurethane foams can be applied and used are as rigid boards/ slabs and pre-formed pipes, which can be manufactured in various shapes and sizes. The main applications of these types of foam are in chill rooms, ice stores and cold stores. Structural sandwich panels incorporating slabs of foam can be produced for prefabricated refrigerated stores.
Foam can also be produced in situ by a variety of means, as follows:
It can be poured in place. This involves mixing the chemicals either manually or by mechanical means and pouring in open moulds or spaces where insulation is required. The mixture creates a foam and solidifies. If necessary, the solidified foam can be cut to the required size or shape.
It can be sprayed directly onto a solid surface using guns that mix and atomize the foam as it is being applied. For example, fish holds or tanks can be sprayed directly on the outside surface and inaccessible areas may be sprayed on and built up without the need of moulds. The foam will adhere to itself and most metals, wood and other materials. It can also be injected into a cavity (e.g. it can be used for moulded insulated boxes). Spray and injection techniques are becoming the most widely used for the installation of rigid polyurethane foam in ships and fishing vessels.
In frothing, the mixture of chemicals is dispensed partially pre-expanded, like an aerosol cream. Appropriate equipment, including an extra blowing agent, is required for immediate pre-expansion. The final phase of expansion takes place as the chemical reaction reaches completion. This technique is used when rigid foams/panels with a high strength-weight ratio are required.
Through polymerization styrene can be made into white pearls/beads of polystyrene plastic. These beads can then be expanded to form a foam known as expanded polystyrene. There are two main ways of making of expanded polystyrene: by extrusion and by moulding of slabs.
Extruded foams are made by mixing the polystyrene with a solvent, adding a gas under pressure and finally extruding the mixture to the required thickness. The extrusion process improves the characteristics of the final foam, such as its mechanical resistance, producing non-interconnecting pores and a more homogeneous material. The mechanical resistance of expanded polystyrene foams can vary from 0.4 to 1.1 kg/cm2. There are several grades of foams available with densities from 10 to 33 kg/m3, with thermal conductivities that are lower with the increase in density, as shown in Table 5.3.
Expanded polystyrene foams have a number of technical limitations:
they are flammable, although fire-retardant grades are available;
they break down gradually when exposed to direct sunlight;
they react with solvents used in the installation of fibreglass-reinforced plastic (such as styrene-formulated polyesters) as well as with other organic solvents (petrol, kerosene, acetone, etc.).
This last characteristic makes them unsuitable for use in fish holds/fish containers that have a lining of fibreglass-reinforced plastic where the fibreglass is applied in situ directly onto the insulation material.
Rigid board panels can be made with expanded polystyrene of different densities, various thicknesses and sizes.
Fibreglass matting is also used as insulating material and offers the following advantages:
high resistance to fire;
high resistance to microbiological attack;
good resistance to most chemicals;
high heat resistance;
available in a variety of presentations (e.g. blankets, mats, loose fill and boards);
low thermal conductivity (see Table 5.4).
Fibreglass insulation is available in rolls of different thickness, also called blankets and mats. The width of the blankets and mats will depend on the way they are to be installed and some come faced on one side with foil or Kraft paper, which serve as vapour barriers.
However, the main technical limitations of fibreglass matting as insulation are:
poor structural strength or compression resistance;
a tendency to settle after installation if not properly installed;
its permeability to moisture.
Rigid board panels can be made with compressed fibreglass. These lightweight insulation boards have relatively high R-values for their thickness.