Insulation thickness

The cost of a thermal insulator as well as its moisture resistance and thermal, chemical and mechanical properties must be considered in selecting any insulating material. 

Make a rough guess at the surface temperature and select two other points, say 25 to 50 degrees above and below this value. 

Find the corresponding insulation thickness for each of the three points by two ways, first 

Plot these points with A to for the X-axis and the insulation thickness for the V-axis and connect them with two smooth curves. The intersection of the two curves indicates the optimum thickness. 

Example 3. Find the optimum insulation thickness on the vertical pipe given in Example 2. The following data are available Cost of heat loss, Ch = 2.0/million Btu cost of installation, Ci = 4 depreciation fraction, f = 0.1 operating time, t = 8,000 hrs./year. 

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The interface between conductor shield and insulation is the region of the highest stress in the cable insulation stmcture. Any imperfections at this interface, especially sharp protmsions of the conductor shield into the insulation, will cause high local electrical stress that may reduce the dielectric strength of finished cable. Calculation of the stress enhancement, for a 15 kV cable with a 4.4 mm (175 mil) insulation thickness, indicates that the common round 50 p.m (2 mil) radius protmsions increase the electrical stress by a factor of 30 and a sharp 5 fim protmsion will increase the electric stress by as much as 210 times (11,20). 

An example in support of the first point is the case of optimum insulation thickness. A tank, optimally insulated when first installed, can fall below optimal if the value of heat is quadmpled. This change can justify twice the old iasulation thickness on a new tank. However, the old tank may have to function with its old iasulation. The reason is that there are large costs associated with preparation to iasulate. This means that the cost of an added increment of iasulation is much greater than assumed ia the optimum iasulation thickness formulas (Fig. 15). 

Table 6. 7 -Values vs Insulation Thickness for Selected Plastics ...

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. 

Recommended Thickness of Insulation Indoor insulation thickness appears in Table II-2I, and outdoor thickness appears in Table 11-22. These selections were based upon calcium silicate insulation with a suitable aluminum jacket. However, the variation in thickness for fiberglass, cellular glass, and rockwool is minimal. Fiberglass is available for maximum temperatures of 260, 343, and 454°C ( 500, 650, and 850°F). Rock wool, cellular glass, and calcium sihcate are used up to 649°C (I200°F). 

TABLE 11-21 Indoor Insulation Thickness/ 80" F Still Ambient Air ... 

Pipe size, in Insulation thickness, in Minimum pipe temperature, ... 

The tables were based upon the cost of energy at the end of the first year, a 10 percent inflation rate on energy costs, a 15 percent interest cost, and a present-worth pretax profit of 40 percent per annum on the last increment of insulation thickness. Dual-layer insulation was used for 3l/2-in and greater thicknesses. The tables and a full explanation of their derivation appear in a paper by F. L. Rubin (op. cit.). Alternatively, the selected thicknesses have a payback period on the last nominal l/2-in increment of 1.44 years as presented in a later paper by Rubin [ Can You Justify More Piping Insiilation Hydrocarbon Process., 152-155 (July 1982)]. 

Number Cross- Conft- Minimum insulation Thick- Approx. Thick- Approx all dia. weight length type weight d.c. resist- ance per citance Direct hi In air rating... 

Figure 5-23. Interior of the nose cone showing terminus of the cooling steam line. The interior shielding consists of a 2-in. (50.8-mm) insulation thickness.

Insulation thickness on the duct section must be increased. 

AX = insulation thickness, ft K. = insulation thermal conductivity, Btu/hr-ft- F = 0.03 for mineral wool... 

Table 15-7 shows recommended insulation thicknesses from API RP I4E. 

Nominal Pipe Size (in.) Nominal Insulation Thickness (in.) >5... 

IT Select from company insulation standards (or prepare, if necessary) the insulation codes to be applied to each hot or cold pipe or equipment. Note that insulation must be applied in some cases only to prevent operating personnel from contacting the base equipment. See Table 1-1 for typical insulation thickness from which code numbers can be established. 

Insulation thickness While the actual thickness of insulation needed to comply with the Building Regulations will vary depending on insulation type, thermal conductivity and the structure into which it is fitted, some general guidelines can be given ... 

Insulation of hot water storage vessels The requirements for these tanks and cylinders will be met if the heat loss is not greater than 90W/m. The thickness of insulation needed will therefore vary not only according to its thermal conductivity but also to the temperature of the water being stored. In practice, as long as the water is not hotter than 100°C the insulation thickness needed is likely to be of the order of 20-35 mm. 

BS 5422 (which also gives insulation thickness for protection against freezing) recommend thickness of 32 mm and 38 mm, respectively, for pipes of 48 mm outside diameter or less. It must be emphasized that these thicknesses only give protection for a relatively short time period (i.e. overnight). It is not possible by means of insulation alone to protect permanently static water. 

Insulation thicknesses used are 50, 75, 100, 125 and 150 mm, but insulants can be obtained in non-standard thicknesses for special applications. A general guide to determine the possible thickness for a required temperature difference is to design for a conductance of 9 W/m. This gives the values in Table 15.1. 

The environment factor F is used to account for vessel protection from insulation. A number of values for various insulation thicknesses are shown in Table 9-2. 

The effect of insulation thickness on total cost (x = optimum thickness). Insulation can be purchased in 0.5-in. increments. (The total cost function is shown as a smooth curve for convenience, although the sum of the two costs would not actually be smooth.)... 

In specifying the insulation thickness for a cylindrical vessel or pipe, it is necessary to consider both the costs of the insulation and the value of the energy saved by adding the insulation. In this example we determine the optimum thickness of insulation for a large pipe that contains a hot liquid. The insulation is added to reduce heat losses from the pipe. Next we develop an analytical expression for insulation thickness based on a mathematical model. 

The rate of heat loss from a large insulated cylinder (see Figure E3.3), for which the insulation thickness is much smaller than the cylinder diameter and the inside heat transfer coefficient is very large, can be approximated by the formula... 

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