Thermal resistivity
سه شنبه 9 بهمن 1397
11:03
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Thermal resistivity
The thermal resistivity is the reciprocal of the k-value (1/k).
Thermal resistance (R-value)
The thermal resistance (R-value) is the reciprocal of l (1/l) and is used for calculating the thermal resistance of any material or composite material. The R-value can be defined in simple terms as the resistance that any specific material offers to the heat flow. A good insulation material will have a high R-value. For thicknesses other than 1 m, the R-value increases in direct proportion to the increase in thickness of the insulation material. This is x/l, where x stands for the thickness of the material in metres.
Coefficient of heat transmission (U) (kcal m-2 h-1 °C-1)
The symbol U designates the overall coefficient of heat transmission for any section of a material or a composite of materials. The SI units for U are kcal per square metre of section per hour per degree Celsius, the difference between inside air temperature and outside air temperature. It can also be expressed in other unit systems. The U coefficient includes the thermal resistances of both surfaces of walls or flooring, as well as the thermal resistance of individual layers and air spaces that may be contained within the wall or flooring itself.
Permeance to water vapour (pv)
This is defined as the quantity of water vapour that passes through the unit of area of a material of unit thickness, when the difference of water pressure between both faces of the material is the unit. It can be expressed as g cm mmHg-1 m-2 day-1 or in the SI system as g m MN-1 s-1 (grams metre per mega Newton per second).
Resistance to water vapour (rv)
This is the reciprocal of the permeance to water vapour and is defined as rv = 1/pv.
5.2 Why insulation is necessary
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.
5.2.1 Insulating materials
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 in
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