Thermal insulation economic thickness

A pipeline of 100 mm outside diameter, carrying steam at 420 K, is to be insulated with a lagging material which costs 10/m3 and which has a thermal conductivity of 0.1 W/m K. The ambient temperature may be taken as 285 K, and the coefficient of heat transfer from the outside of the lagging to the surroundings as 10 W/m2 K. If the value of heat energy is 7.5 x 10 4 /MJ and the capital cost of the lagging is to be depreciated over 5 years with an effective simple interest rate of 10 per cent per annum based on the initial investment, what is the economic thickness of the lagging ... 

While thermal insulations are selected first for their resistance to heal flow, their other properties need evaluation for each application. Hence there is no "best" insulation because a material well suited to one service may be poorly suited to another. Economics must be studied in detail because in some services the cost of a highly efficient material per unit of thickness may be overcome by additional thickness of a less efficient material, provided there is room for the grealet thickness. All properties of materials must he considered for each exposure, even within the same system. Recognize that when different materials are used or different parts nl insulation sv -.tents that arc close together, the probability of using the wrong material on a particular surface is increased appreciably. Unless there is specific reason for wide use ol multiple types of materials, it may be prudent to accept some compromises of properties. 

SlafT Ho lo Deternane Economic Thickness of Insulation. Thermal Insulation Manutacturers Association. Ml. Kivcn. New York. NY. [Revised periodically). Turner. W.C. and J.F. Mtdloy Thermal insulation, Krtcger Publishing Company. Melbourne. FL. 1991). 

A critical design variable for MLI-insulated vessels is the most economic number of layers of insulation per unit thickness. If the insulation is compressed too tightly, the increase in solid conductivity outweighs the decrease in radiative heat. A typical dependency of the apparent thermal conductivity on layer density for MLI is presented in Fig. 7.5. 

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