In general all materials offer some resistance to the flow of heat. But those materials, which offer heavy resistance to the flow of heat through them, come in the category of insulating materials. And the heat-conducting behavior of a material is indicated by the property named as thermal conductivity. The letter ‘k’ denotes it. Thermal conductivity of a material is equal to the amount of heat in kcal transmitted per hour per square meter area per degree Celsius difference in temperature for one-meter thickness. So, if we know the thermal conductivity of an insulating material, k and all other conditions i.e. area of heat flow, A, thickness of insulation material t and temperature inside and outside the cold room in T i and T o , we can calculate the amount of heat passed in kcal/hour through the insulating material as per the given formula:
Heat passed in kcal/hour =((T o - T i ). k. A)/t
We can judge from this formula that heat flow will decrease if the value of k is less, heat flow area is less and thickness of insulating material is more. The factor t/ k is called heat resistance of a material and it should be higher for an insulating material. However, in actual the wall of a cold storage or any refrigerated cabin is not made of insulating material only. It is made up of more than one material.
For example in the wall of a domestic refrigerator outside layer is of sheet metal and inside layer is of plastic and in between the insulating material such as glass wool is fixed as shown in figure.
The calculation for overall thermal conductivity of this type of wall is made like:-
Where, k 1, k 2, k 3 are the conductivity of different material used and t 1 , t 2 , t 3 are the thickness of layers of those materials.In this way, rating of insulation is done and the calculation for heat entering to cold space through all the walls, roof, floor, etc. is made.
Heat passed in kcal/hour =((T o - T i ). k. A)/t
We can judge from this formula that heat flow will decrease if the value of k is less, heat flow area is less and thickness of insulating material is more. The factor t/ k is called heat resistance of a material and it should be higher for an insulating material. However, in actual the wall of a cold storage or any refrigerated cabin is not made of insulating material only. It is made up of more than one material.
For example in the wall of a domestic refrigerator outside layer is of sheet metal and inside layer is of plastic and in between the insulating material such as glass wool is fixed as shown in figure.
The calculation for overall thermal conductivity of this type of wall is made like:-
Where, k 1, k 2, k 3 are the conductivity of different material used and t 1 , t 2 , t 3 are the thickness of layers of those materials.In this way, rating of insulation is done and the calculation for heat entering to cold space through all the walls, roof, floor, etc. is made.
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