The North American Insulation Manufacturers Association (NAIMA) offers a free, readily available software tool called 3ePlus® that performs heat transfer and temperature related calculations for insulated pipe, tanks, and equipment. It uses the calculation methodology and algorithms contained in ASTM C680 for heat transfer and insulating ability.
The first version of 3ePlus was written more than two decades ago by NAIMA for fiberglass and mineral wool insulation manufacturers in North America. Understandably, with fiberglass and mineral wool as the primary focus for 3ePlus, it calculates thickness tables for hot applications for fibrous insulation materials very well. You can even download a version of NAIMA’s 3e Plus on the JM website that will provide thickness tables for JM high temperature mechanical and industrial insulations.
However, for cold applications where the design criteria include condensation control or when using closed-cell insulation materials made with blowing agents, 3ePlus has two key shortcomings.
First, when performing condensation control calculations, 3ePlus includes a 0.75°F "safety factor" for the comparison of insulation system surface temperature to dew point. This means that instead of determining the insulation thickness required to yield an outer insulation system surface temperature above the dew point, 3ePlus provides the thickness required to yield an outer surface that is 0.75°F greater than the dew point. A safety factor of 0.75°F might sound very small, but it can make significant differences in the calculated insulation thickness required.
In one case we calculated for a 10-inch NPS pipe at -40°F, the required thicknesses with and without using the safety factor were 5.5 and 4 inches, respectively – a 38% increase in required thickness. The use of this safety factor is not necessary since insulation thickness calculations are already inherently conservative for two reasons.
a. Insulation thickness calculations done for most (maybe all) insulation materials use the conservative ASTM k-Factor values rather than the actual test results. For JM Trymer 2000XP PIR insulation, for example, the conservative ASTM values include a k-Factor of 0.19 Btu-in/hr-ft2-°F at 75°F mean temp, which are the same values that JM publishes on our data sheets and are consistent with the values we recommend system designers use in their calculations. However, Trymer 2000 XP PIR has actual test results with a k-Factor of 0.168 Btu. Our published k-factor values are 13% worse than the measured k-Factor because they include this built-in 0.75°F safety factor.
b. The ambient conditions assumed in the industry are VERY conservative for condensation control. We assume very high relative humidity (typically 85-95%) which is rarely reached unless it is about to rain, just finished raining, or a heavy dew is occurring - all situations where the insulation system is getting wet anyway.
For these reasons there is no need to add an additional safety factor of 0.75°F. It’s important to realize that the 0.75°F safety factor is integrated into the 3ePlus program without notifying the program user. No mention is made of it in the program instructions, and there is no way to turn it off or to even change its value.
Second, 3ePlus does not properly calculate the thermal conductivity of materials that contain blowing agents. A very critical part of all insulation thickness calculation processes requires the modeling of the thermal conductivity vs. mean temperature curve. 3ePlus allows only the use of a single 2nd order polynomial to model the curve. This works great for open-cell, fibrous, or granular materials that do not contain blowing agents. A single 2nd order polynomial very accurately models the curve for glass fiber, mineral wool, perlite, and calsil. However, a single 2nd order polynomial does a poor job of modeling closed-cell insulation materials containing a blowing agent because the k-Factor curve of these type materials is S or Z shaped. The error introduced by this issue is not predictable. It can yield a required insulation thickness that is too small, too large, or just right depending on various factors. NAIMA is aware of these issues and is considering addressing them in a future version of 3ePlus.
The ASTM C680 calculation methodology upon which 3ePlus is based provides a solution to this curve modeling problem. It recommends to simply use a piece-wise curve fit with three "pieces." This is the modeling method JM uses in our low-temperature and cryogenic insulation thickness calculation tool, and it allows very accurate modeling of the S/Z shaped curve.
Those are the reasons that 3ePlus is not recommended for calculation of required insulation thickness or heat transfer in cold applications or when the insulation is a closed-cell material made using blowing agents. If you need assistance establishing the appropriate insulation thickness for your cold application, please contact us to speak with our technical team.
