Pipe Insulation - Heat Flow Calculations and R-value

Heat Flow Calculations and R-value

Heat flow passing through pipe insulation can be calculated by following the equations set out in either the ASTM C 680 or EN ISO 12241 standards. Heat flow is given by the following equation:

Where:

• is the internal pipe temperature,
• is the external ambient temperature, and
• is the sum total thermal resistance of all insulation layers and the internal- and external-surface heat-transfer resistances.

In order to calculate heat flow, it is first necessary to calculate the thermal resistance ("R-value") for each layer of insulation.

For pipe insulation, the R-value varies not only with the insulation thickness and thermal conductivity ("k-value") but also with the pipe outer diameter and the average material temperature. For this reason, it is more common to use the thermal conductivity value when comparing the effectiveness of pipe insulation, and R-values of pipe insulation are not covered by the US FTC R-value rule.

The thermal resistance of each insulation layer is calculated using the following equation:

Where:

• represents the insulation outer diameter,
• represents the insulation inner diameter, and
• represents the thermal conductivity ("k-value") at the average insulation temperature (for accurate results iterative calculations are necessary).

Calculating the heat transfer resistance of the inner- and outer-insulation surfaces is more complex and requires the calculation of the internal- and external-surface coefficients of heat transfer. Equations for calculating this are based on empirical results and vary from standard to standard (both ASTM C 680 and EN ISO 12241 contain equations for estimating surface coefficients of heat transfer).

A number of organisations such as the North American Insulation Manufacturers Association and Firo Insulation offer free programs that allow the calculation of heat flow through pipe insulation.