11. INSULATED CONTAINERS

11.1 Description
11.2 Calculation of Fish, Ice and Water Quantities

The use of insulated containers for stowage and transport of fish in iced sea water offers several advantages over traditional boxed and iced practises. It is particularly suited to small pelagic species that are not normally gutted at sea but which are caught in bulk and require rapid chilling because of their high fat content.

Advantages:

  1. Fish can be transferred rapidly from deck to the containers (by careful arrangement of the deck pounds and scuttles) with minimum delay and with minimum disruption to fishing effort.
  2. Improvements in quality of the fish can be achieved due to the nature of stowage and to the reduction in handling of catch.
  3. Rapid unloading of the vessels by removal of containers by crane and refitting with clean iced containers enables a quick turn about of the fishing vessel in port and substantially reduces the labour required for unloading.

Disadvantages:

  1. The major disadvantage of the portable insulated container is the reduction of carrying capacity of both the fishing vessel and road transport.
  2. For efficient operation of a container system the fishing vessel needs to be designed for the purpose. (Particularly scuttles. chutes. fishroom floors and unloading hatches.)
  3. Fishing vessels are restricted to ports that can provide a change of containers.
  4. A slight uptake of salt in fish flesh occurs with stowage in sea water.
  5. Containerized catches are a less attractive proposition if the fish is to be sold through a traditional market of small unit sale and are better suited for sale direct to the processing factory.

11.1 Description

The following specification relates to a container developed for use in the UK for stowage and handling of herring and is shown in Figure 40. For similar use in tropical areas the insulation thickness should be increased to a minimum of 75 mm.

Container dimensions: 1 170 mm wide x 1320 mm deep x 1950 mm high
Insulation: 50 mm rigid foam polyurethane
Construction: Aluminium inner skin with GRP outer skin; hinged lid at top and drain at bottom
Capacity: 2.1 m�
Empty weight: 175 kg
Aeration: By means of an aluminium lance. built into the container with coupling at top to manifold

 

Figure 40. Chilled sea water container

11.2 Calculation of Fish, Ice and Water Quantities

For any given container .the ice:water:fish ratios can be calculated from a knowledge of ambient temperature. sea water temperature, the insulative properties of the container and the period of holding. The requirement of the ice is to cool the sea water and fish to 0�C, to counter the heat input from aeration and to counter the heat gain through the container.

The requirement for water is to provide a medium that will enable rapid heat transfer from the fish and to hold the fish such that they are not subject to crushing.

Table 16 (Section 6.2) shows the weight of ice required to cool, one kilogramme of fish or water to zero degrees centigrade. over a range of temperatures. It is assumed for the purposes of estimation that the specific heats are equal. Table 23 shows the weight of ice required to offset the heat gain through the container previously specified. It assumes a figure of 0.085 kg of ice loss per degree centigrade above zero per hour based on trials with the container.

Table 23 Ice required for each day of stowage to counter heat gain through the walls of the container specified

Ambient temperature
(�C)

kg of ice/24h
of stowage

35
25
15
5

72
51
31
10

 

Example calculation

The following example calculation of ice:water:fish ratios assumes ambient temperature of 20�C. sea water temperature of 15�C. a holding period of three days and a container as previously specified.

Assume 1/5 volume of container for water

0.2 x 2.1 = 0.42 m�.

Ice required to cool 0.42 m� of sea water at 15�C from Table 16

=420 x 0.17 kg .71.4 kg

which occupies a volume of 0.08 m� according to Table 3.

Ice required to counter heat gain through container for 3 days from Table 23

=123 kg

which occupies a volume of 0.13 m�.

For purposes of estimation the heat input by aeration will be assumed negligible.

Volume of remaining space within container

Vr = 2.1 - 0.42 - 0.08 - 0.13 = 1.47 m�.

This volume will be the volume of fish plus the volume of ice required to cool the fish from 15�C to 0�C.

From Table 16 the ratio of fish:ice is approximately 1:0.17

volume of fish = 1.47 x 1/1.17 = 1.26 m�. and

volume of ice = 1.47 x 0.17/1.17 = 0.21 m�.

Total volume water = 0.42 m�

Total volume ice = 0.08 + 0.13 + 0.21 = 0.42 m�

Total volume fish = 1.26 m�

From this follows:

Ratio fish:ice:water = 3:1:1 (by volume).

For higher conditions of ambience and sea water temperatures the above calculation can lead to unacceptably low carrying capacities and in these circumstances what is done in practice is to put in less ice and water initially than calculated but to later drain off some of the water and re-ice. It is important when doing this not to drain off all the water or the fish will no longer be floating in an ice water mix but subject to its dead weight

which can lead to damage by crushing.