How Environmental Variables Affect Insulation System Design
Knowing the ambient temperature, wind speed and relative humidity will help you select the proper insulation thickness.
For many years industrial insulation designers worked for companies that had several manufacturing locations throughout one region of North America. However, things have changed in the 21st century. Growth in the chemical process industry is increasingly moving offshore to China, India and other Asian locations. While roughly 25 percent of the world’s chemical production is still located in North America, that number will fall dramatically in the next 20 years. How will this change affect insulation design? It will have an impact on everything from material availability to the way insulation thickness is calculated. Let’s take a look at how environmental variables have an impact on the design of insulation systems.
We must first define environmental variables. With respect to insulation thickness calculation, there is ambient temperature, wind speed and relative humidity to consider. Ambient temperature is important because it has an influence on heat transfer,
and therefore affects the insulation thickness required. Humidity has an influence on the design of low-temperature systems because it controls the dew point temperature. Wind speed is important in both low- and elevated-temperature systems because it controls convective heat transfer and, again, will influence the required insulation thickness. We can use the computer program 3E Plus© to evaluate easily how these variables impact insulation thickness.
First, let’s consider the impact of ambient temperature on insulation thickness. In this sensitivity analysis, we will calculate how changing ambient temperature affects the personnel protection (PP) temperature of a system while all the other variables are held constant. The PP temperature is the maximum surface temperature allowed for insulation jacket that can be easily touched by personnel working in the area. This temperature is often 140 F. We will assume a process temperature of 350 F, an average ambient wind speed of 10 mph and will calculate the PP thickness for ambient temperatures ranging from 10 F to 100 F. The insulation jacket material is oxidized aluminum; the insulation material is mineral fiber, and the insulated substrate is a 4-inch diameter pipe of 300 series austenitic stainless steel.
The mineral fiber thickness required to achieve a PP temperature of 140 F on this 4-inch pipe is 0.5 inches, regardless of the ambient temperature up to a maximum ambient of 104 F. In this case, whether the plant is built in Siberia (really cold) or Singapore (hot, average year-round ambient high temperature of about 85 F), the PP thickness would be just 0.5 inches.
What about changing the size of the pipe? If we increase the diameter of the pipe to 8 inches, the maximum ambient temperature that still allows 0.5-inch thickness drops from 104 F to 88 F. Now location does matter; the Siberians would still be safe, but in Singapore it would be a good idea to increase the thickness to 1 inch.
This analysis shows that for similarly sized equipment, the range of average ambient high temperature found around the world probably won’t have much of an impact on insulation thickness. There are extremes: For instance, desert environments where average highs may be above 100 F would likely be different than more temperate locations. But overall, most locations where chemical plants are likely to be built (with the exception of Saudi Arabia) will require insulation of similar thickness when equipment size is ignored. Interestingly, in this example, moving to very cold locations has little impact on the insulation thickness required. Even at 10 F we must still use 0.5 inches of insulation—the lowest thickness available in 3E Plus—to obtain a safe surface temperature.
Next, let’s consider the impact of wind speed on insulation thickness. Wind speed controls convective heat transfer and is important regardless of whether we are looking at an elevated temperature system or a cold system. In this example, we will use the same operating conditions as our earlier example and will set the ambient temperature at 75 F.
In our first example, the PP thickness was 0.5 inches when the wind speed was 10 mph. With the wind speed at 0 mph, the PP thickness jumps up to 1.5 inches. At a thickness of 0.5 inches, as determined in the first example, the surface temperature is 189 F, well above the PP cutoff of 140 F and 77 F above the temperature when the wind is blowing at 10 mph. Using a wind speed of just 1 mph drops the PP thickness to 1 inch and lowers the surface temperature at the 0.5-inch thickness to 169 F. So, just a little movement of air past the insulated surface can result in a useful reduction in surface temperature. At 3 mph the PP thickness is back to 0.5 inches.
Clearly wind speed has a significant effect on surface temperature and must be considered when designing for personnel protection. What about efficiency? As you might expect, when the wind is not blowing, there is no convective cooling and the system does not lose as much heat. At 0 mph the heat loss at 0.5 inches is 157 Btu/hr/ft while it rises to 194 Btu/hr/ft at 3 mph. Again, just a little wind has a significant impact on heat loss. It is obviously important to understand what sort of air movement will exist around the asset that is to be insulated. Indoor locations may require more insulation for personnel protection but less insulation to achieve the desired efficiency target. It is also important to understand why insulation is being used in each case.
Finally, let’s consider the effect of relative humidity on insulation thickness. This variable becomes important in the design of low-temperature systems because it has an impact on the dew point temperature and the condensation control (CC) thickness. The CC thickness is the thickness required to raise the insulation jacket surface temperature above the dew point, thereby preventing condensation. In this example we will again use a 4-inch stainless steel pipe, but we will change the operating temperature to -50 F. The ambient temperature is 75 F; the wind speed is 3 mph, and the material is ASTM C591-00 polyisocyanurate closed-cell rigid insulation. With the relative humidity at 10 percent, the dew point is a low 14.8 F and the CC thickness is just 0.5 inches, yielding a surface temperature of 58 F, well above the dew point temperature. Raising the wind speed to 10 mph raises the surface temperature even more; so wind is helpful with respect to condensation control.
But, what does wind do for heat gain? In this case, not much: It increases about 4 Btu/hr/ft due to convective heating of the jacket surface. Raising the humidity to 50 percent raises the dew point to 55 F but does not change the surface temperature; so 0.5 inches still works as a CC thickness. When the humidity reaches 55 percent, the dew point and surface temperature converge at 58 F, and the thickness must be increased to prevent condensation. With this insulation material, increasing the thickness to 1 inch raises the surface temperature to 66 F, well above the dew point. Raising the humidity to 85 percent raises the dew point to 70 F, and the CC thickness goes to 2.5 inches! If your low-temperature system is installed in locations with high relative humidity, the condensation control thickness will be significantly higher than it would be in the desert.
These very simple analyses demonstrate that the environment in which a system operates plays an important role in system design. Using a specification from last month’s project in Saudi Arabia probably won’t work for this month’s project in Chicago. It is important to make the effort on each project to define the environment and use that information to properly size the insulation. It is also important to understand why insulation is being installed. Personnel protection may require a very different thickness than simply designing for an energy-efficiency target. The materials selected obviously also make a difference. This sounds like a good topic for a future column.