Numerous thermal and acoustic insulations are available for use in industrial pipe applications. As stated in both the National Commercial & Industrial Insulation
Standards Manual and the IIAR Ammonia Refrigeration Piping Handbook, the choice is a balance of matching process temperatures, environment, condensation control, installation and operational costs, and other factors. Insulation manufacturers often attempt to cast their nets as broadly as possible to secure industrial pipe business. This can lead to some confusion when choosing the best insulation for a specific industrial pipe application.
To navigate the range of insulation choices, it is important for specifiers to calculate insulation thickness using thermal curves and process temperatures. It also is essential to understand a little-understood phenomenon that affects rigid-foam insulation in low temperature applications.
The Insulation Material Selection Process
The first step in specifying pipe insulation is to narrow the material selection based on the specific application. The temperature range of the pipe typically is the starting point for this process. Figure 1 shows the basic temperature range for the most common insulation materials: elastomeric, cellular glass, polystyrene, mineral wool and fiberglass.
Once the proper material is selected based on temperature range, it is necessary to factor in costs. Cost of the insulation material is a function of several factors, including:
• The amount of material required to meet performance requirements — a function of the thickness of the insulation.
• The amount of mastic, joint sealers, vapor retarders and protective jacketing needed. This amount depends on the pipe circumference — again, a function of thickness.
Thermal Conductivity Thickness.Ultimately, insulation cost is a matter of thickness. Specifiers know that the difference in thickness between materials is driven by the materials’ thermal conductivity (k-factor). Each rigid-foam material has a unique k-factor that is a function of the insulation material, the blowing agent used, and the size and shape of the cellular structure. For simplicity of comparison, k-factors are often reported at a default of 75°F (23.8°C) mean temperature. The lowest k-factor will yield the best insulation value and, thus, require the smallest thickness (i.e. lowest cost). This scenario works for many materials used primarily on the “hot” application side because they share similar thermal-curve shapes. However, this is not always the case in low temperature applications. This is where that little-known phenomenon with certain rigid foams comes into play.
Temperature. Figure 2 shows the k-factors for several insulation materials over the range of their temperature limits. As one can see, the shapes of the thermal curves change depending on the mean temperatures at which they are tested. Because not all of the curves are similar in shape, the use of a single point for selecting insulation material could lead to an improper insulation choice, depending on the temperature range of the process. For instance, notice that the k-factors of one rigid extruded polystyrene (XPS) and a polyisocyanurate rigid insulaton (PIR Grade 2) are almost identical at 75°F but significantly different at 0°F (-17.8°C).
An example shows how the insulation thickness changes based on the material selected when inputting criteria for condensation control into an insulation thickness program, 3E Plus Software. The initial conditions were:
• Application: Ammonia refrigeration.
• Process Temperature: -50°F (-45°C).
• Ambient Temperature: 80°F (26.7°C).
• Mean Temperature: 15°F (-9.4°C).
• Relative Humidity: 80 percent.
• Wind Speed: 0 mph.
• Jacket Type: PVC.
• Pipe Size: 6".
With those inputs, the software recommends a material thickness for the rigid extruded polystyrene at 2.0" thick while the polyisocyanurate rigid insulation is greater at 2.5".
If the selection process had been made based on the single k-factor point at 75°F mean temperature, the specifier could have assumed the thermal performance of these two rigid-foam insulation materials was the same across the entire temperature range of the process. The specifier may never have realized there was a thinner option.
As the example illustrates, specifiers should ask prospective insulation manufacturers to provide current thermal data of their material for use with 3E Plus Software.
How Blowing Agents Affect Insulation Performance
One must consider the physical properties of the specific blowing agents used to understand why thermal curves do not have similar shapes. The various curves
in figure 2 have differing shapes due to the blowing agents used in the manufacturing of the respective rigid foams.
Both the rigid extruded polystyrene and polyisocyanurate rigid insulation use blowing agent gases that are trapped in the cells created during the manufacturing process. The blowing agent inside the cells typically is in a gas state at room temperature and above, but it can condense at lower temperatures. This is what creates the shape of the curve.
Does the blowing agent last forever? The simple answer is no. However, it continues to diffuse out of the foam over time — actually over a very long period of time. The diffusion rates of various blowing agents can extend over decades. Foams such as extruded polystyrene generally reach a state of equilibrium within the first 180 days after manufacture. That is why material standards for rigid extruded polystyrene and polyisocyanurate rigid insulation foams require k-factors to be tested and reported on material that is 180 days old. From a design perspective, k-factors that are at least 180 days old should be used to ensure long-term performance of the refrigeration system.
Can you stop the diffusion process? No. Diffusion is a natural process that follows the laws of physics. Adding facers such as vapor retarders or jacketing materials can alter the diffusion rate but will not stop the diffusion process.
In conclusion, to navigate the wide range of insulation choices, it is important for specifiers to calculate insulation thickness using thermal curves and process temperatures. It also is essential to understand how low temperature applications affect rigid-foam insulation thickness. Those who select insulation materials by comparing data-sheet k-factors at 75°F (mean temperatures) may be paying for more insulation than is needed.
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