Thermal insulations normally consist of the following basic
materials and composites :
- Inorganic, fibrous or cellular materials such as glass, rock or slag wool; and calcium silicate, bonded perlite, vermiculite, and ceramic products. Asbestos insulations used to be applied, but asbestos has been shown to be a carcinogen. Extreme caution must be used if it is encountered;
- Organic fibrous materials cuch as cellulose, cutton, animal hair, wood, pulp, cane or synthetic fibers, and organic cellular materials such as cork, foamed rubber, polystyrene, polyurethane, and other polymers;
- Metallic or metallized organic reflective membranes. These surfaces must face an air gas-filled, or evacuated space to be effective.
Some of the more common materials used for insulation
are compared in Table below with their relative insulating values and the
advantages and disadvantages of particular types. In general, the more
expensive materials, such as the polyurethane foams are more efficient
insulators for given thicknesses. Using the “R” system of grading (see
definitions in paragraph 5.1.2), it is possible to arrive at equivalent “R
values” for a variety of insulating material types.
Figure 5.4 shows the comparison of typical thicknesses
of different insulation materials used for chill rooms and ice stores,
operating on shore, in temperate and tropical areas, at average ambient air
temperatures of 20, 30 and 40 °C. Some designers indicate that the thermal
conductance coefficient (l) for shore-based chill and ice stores should not
exceed 0.26 kcal m-2 h-1 °C-1 (equivalent to
an R-value = 18.8 ft2 h °F Btu-1). However, the setting
of this value depends basically on the energy costs, therefore it may be
reduced if, in the future, energy costs increase.
Common insulating materials, “R” values, advantages and disadvantages
Insulating material
|
“R” value per inch (2.54 cm)
|
Advantages
|
Disadvantages
|
Polyurethane, board
|
6.25
|
Very good R-value, can be used with fibreglass
resins
|
Not always easily available, relatively expensive
|
Polyurethane, spray on
|
7.0
|
Very good R-value, can be used with fibreglass
resins, easy application with spray equipment
|
Not always easily available, expensive, requires
special spray equipment
|
Polyurethane, poured (two-part chemical)
|
7.0
|
Very good R-value, can be used with fibreglass
resins, relative ease of application
|
Not always easily available, expensive, requires
very careful volume calculations
|
Polystyrene, sheets (smooth)
Trade name “Styrofoam” |
5.0
|
Readily available, low cost, reasonable R-value
|
Cannot be used with fibreglass resins unless
protected, easily damaged
|
Polystyrene, foamed in place and expanded moulded
beads. Known as Isopor, Polypor, etc.
|
3.75 to 4.0
|
Reasonable R-values, lower cost than smooth surfaced
sheets
|
Cannot be used with fibreglass resins unless
protected, easily damaged
|
Cork board
|
3.33
|
Availability in many markets, reasonable cost, can
be covered with fibreglass
|
Lower R-values than polyurethane for styrene foams
|
Fibreglass wool batts
|
3.3
|
Low cost, ease of installation
|
Readily absorbs water or other fluids, loses
insulating value when wet
|
Rock wool batts
|
3.7
|
As above
|
As above
|
Wood shavings
|
2.2
|
Readily available, low cost
|
Absorbs moisture and loses R-values when wet, decays
|
Sawdust
|
2.44
|
Readily available, low cost
|
Absorbs moisture and loses R-value when wet, packs
down under vibration
|
Straw
|
Readily available, low cost
|
Absorbs moisture and loses R-value when wet, host to
insects, etc.
|
|
Air space
|
1.0 approx.
|
No cost
|
Has to be completely sealed to prevent air
circulation causing heat infiltration
|
The selection of the optimum insulation thickness for
fish holds will depend on factors such as the insulation costs (materials and
installation), ice costs (or power and equipment costs according to the
refrigeration requirements), annual cost savings in refrigeration due to
improved insulation efficiency, and local conditions (type of fishing
operations and vessel, species caught, fish prices, borrowing costs).
Therefore, the optimum thickness of insulation should be selected on an
individual basis. However, taking into account the local environmental
conditions in which the fishing vessel is likely to operate, which do not
depend on economic calculations, a minimum recommended thickness of insulation
should be determined. In practice, a compromise should be reached between the
optimum economic insulation thickness and the ice/refrigeration costs.
It is also important, for planning purposes, to take
into consideration the heat gains from radiation and conduction, to select the
optimum insulation thickness.
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