Economic Thickness Insulation

5.4 Economic Thickness of Insulation (ETI)

Insulation of any system means capital expenditure. Hence the most important factor in any insulation system is to analyse the thermal insulation with respect to cost. The effectiveness of insulation follows the law of decreasing returns. Hence, there is a definite economic limit to the amount of insulation, which is justified. An increased thickness is uneconomical and cannot be recovered through small heat savings. This limiting value is termed as economic thickness of insulation. An illustrative case is given in Figure 5.3. Each industry has different fuel cost and boiler efficiency.

These values can be used for calculating economic thickness of insulation.

This shows that thickness for a given set of circumstances results in the lowest overall cost of insulation and heat loss combined over a given period of time. The following figure 5.4 illustrates the principle of economic thickness of insulation.

The simplest method of analysing whether you should use 1” or 2” or 3” insulation is by comparing the cost of energy losses with the cost of insulating the pipe. The insulation thickness for which the total cost is minimum is termed as economic thickness. Refer fig 5.4 The curve representing the total cost reduces initially and after reaching the economic thickness corresponding to the minimum cost, it increases.

The determination of economic thickness requires the attention to the following factors.

Cost of fuel
Annual hours of operation
Heat content of fuel
Boiler efficiency
Operating surface temperature
Pipe diameter/thickness of surface
Estimated cost of insulation.
Average exposure ambient still air temperature

Procedure for calculating Economic thickness of insulation

To explain the concept of economic thickness of insulation, we will use an example. (Refer Table 5.3) Consider an 8 bar steam pipeline of 6” dia having 50-meter length. We will evaluate the cost of energy losses when we use 1”, 2” and 3” insulation to find out the most economic thickness.

A step-by-step procedure is given below.

Establish the bare pipe surface temperature, by measurement.
Note the dimensions such as diameter, length & surface area of the pipe section under consideration.
Assume an average ambient temperature. Here, we have taken 30oC.
Since we are doing the calculations for commercially available insulation thickness, some trial and error calculations will be required for deciding the surface temperature after putting insulation. To begin with assume a value between 55 & 65 C, which is a safe, touch temperature.
Select an insulation material, with known thermal conductivity values in the mean insulation temperature range. Here the mean temperature is 111 C. and the value of k = 0.044 W/m2oC for mineral wool.
Calculate surface heat transfer coefficients of bare and insulated surfaces, using equations discussed previously. Calculate the thermal resistance and thickness of insulation.
Select r2 such that the equivalent thickness of insulation of pipe equals to the insulation thickness estimated in step 6. From this value, calculate the radial thickness of pipe insulation = r2-r1
Adjust the desired surface temperature values so that the thickness of insulation is close to the standard value of 1” ( 25.4 mm).
Estimate the surface area of the pipe with different insulation thickness and calculate the total heat loss from the surfaces using heat transfer coefficient, temperature difference between pipe surface and ambient.
Estimate the cost of energy losses in the 3 scenarios. Calculate the Net Present Value of the future energy costs during an insulation life of typically 5 years.
Find out the total cost of putting insulation on the pipe ( material + labor cost)
Calculate the total cost of energy costs and insulation for 3 situations.
Insulation thickness corresponding to the lowest total cost will be the economic thickness of insulation.

Note that the total cost in lower when using 2” insulation, hence is the economic insulation thickness.