Insulation economic thickness

Optimal economic insulation thickness may be determined Iw various methods. Two of these are the minimum-total-cost method and the incremental-cost method (or marginal-cost method). The minimum-total-cost method involves the actual calculations of lost energy and insulation costs for each insulation thickness. The thickness producing the lowest total cost is the optimal economic solution. The optimum thickness is determined to be the point where the last dollar invested in insulation results in exactly 1 in energy-cost savings ( ETI— Economic Thickness for Industrial Insulation, Conservation Pap. 46, Federal Energy Administration, August 1976). The incremental-cost method provides a simplified and direcl solution for the least-cost 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 ... 

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). 

Derive an expression for the optimum economic thickness of insulation to put on a flat surface if the annual fixed charges per square foot of insulation are directly proportional to the thickness, (a) neglecting the air film, (b) including the air film. The air-film coefficient of heat transfer may be assumed as constant for all insulation thicknesses. 

Optimum economic thickness of insulation, in., for nominal. pipe diameter of... 

The total-cost method does not in general provide a satisfactory means for making most insulation investment decisions, since an economic return on investment is required by investors and the method does not properly consider this factor. Return on investment is considered by Rubin ( Piping Insulation—Economics and Profits, in Practical Considerations in Piping Analysis, ASME Symposium, vol. 69,1982, pp. 27-A6). The incremental method used in this reference requires that each incremental in of insulation provide the predetermined return on investment. The minimum thickness of installed insulation is used as a base for calculations. The incremental installed capital cost for each additional V2 in of insulation is determined. The energy saved for each increment is then determined. The value of this energy varies directly with the temperature level [e.g., steam at 538°C (1000°E) has a greater value than condensate at 100°C (212°F)]. The final increment selected for use is required either to provide a satisfactory return on investment or to have a suitable payback period. 

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The total annual cost of insulation is the sum of the cost of heat energy lost and fixed charges. A plot of costs versus the insulation thickness will determine the most economical thickness. A dimensionless factor can be used to calculate insulation thickness, depending on the ratio of insulation thickness to pipe diameter (Figure 12-4) in the following equation ... 

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