154 Insulation Thickness Determination

This section presents formulas and graphical procedures for calculating heat loss, surface temperature, temperature drop, and proper insulation thickness. Over the last years, computer programs that perform these calculations are more readily available to customers (see section 15.5.4). But still, it is important to understand the basics for their use. Although the overall objective is to determine the right amount of insulation that should be used, some of the equations use thickness as an input variable rather than solving for it. However, all the calculations are simply manipulations or further refinements of the equation in Section 15.1.2:

2n 2

Following is a list of symbols, definitions, and units to be used in the heat-transfer calculations.

ta = ambient temperature, °F

ts=surface temperature of insulation next to ambient,

th = hot surface temperature, normally operating temperature (cold surface temperature in cold applications), °F

k = thermal conductivity of insulation always determined at mean temperature, Btu-in./ hr ft2 °F

tm = (th + ts)/2 = mean temperature of insulation, °F

th = (tin + tout)/2 = average hot temperature when fluid enters at one temperature and leaves at another, °F

tk = thickness of insulation, in.

fi = actual outer radius of steel pipe or tubing, in.

r2 = (ri + tk) = radius to outside of insulation on piping, in.

Eq tk = r2 Ln (^/r^ = equivalent thickness of insulation on a pipe, in.

f = surface air film coefficient, Btu/hr ft2 °F

= tk/k = thermal resistance of insulation, hr ft2 °F/Btu qf = heat flux through a flat surface, Btu/hr ft2

Btu/hr lin. ft A = area of insulation surface, ft2 L = length of piping, lin. ft QT- = Qf x A or Qp x L = total heat loss, Btu/ hr

Cp = specific heat of material. Btu/lb- °F p = density, lb/ft3 M = mass flow rate of a material, lb/hr A = difference by subtraction, unit less RH = relative humidity, % DP = dew-point temperature, °F

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