Industrial process cooling systems can operate at very cold and even cryogenic temperatures. Creating liquefied natural gas (LNG), for example, requires refrigeration systems to operate at temperatures of -265°F (-165°C) or less. At these temperature extremes, any flaws in the design or installation of the insulation system will quickly become apparent and could potentially damage the system. This is most commonly seen in the form of condensation forming on the outer jacketing or, in particularly bad situations, ice formation on (or in) the insulation. System design flaws also can cause material loss or pressure buildup because the liquefied material reverts back into its gaseous state.
This is why it is critical to design any industrial process cooling system to:
- Limit heat gain.
- Maintain a consistent temperature.
- Operate efficiently.
To accomplish these objectives, virtually all process cooling applications utilize mechanical insulation systems as a primary method for:
- Controlling the temperature.
- Limiting heat gain into the system.
- Minimizing surface condensation.
The most common insulation materials used in non-food/beverage industrial process cooling applications are polyisocyanurate (PIR) foam insulation and cellular glass insulation. Both these insulations offer benefits that system designers can utilize to ensure their systems operate as effectively as possible.
PIR insulation — a rigid, polyisocyanurate foam — can be used in applications ranging from extremely low cryogenic temperatures to those operating at 300°F (149°C).
PIR insulation is a rigid, polyisocyanurate foam that can be used in applications ranging from extremely low cryogenic temperatures to those operating up to 300°F (149°C). When compared to cellular glass, PIR insulation has a much lower (better) thermal conductivity (K factor), so it can be used at thinner thicknesses while still achieving the desired performance. Additionally, PIR offers mid-level compressive strength, which can help prevent damage to the system under abuse or impact.
Cellular glass is a rigid, glass insulation that can be used at cryogenic temperatures as well as in high temperature applications up to 900°F (482°C). When compared to PIR, cellular glass has a higher (worse) K factor. This means that the insulation will have to be thicker than PIR to deliver the same performance. Cellular glass offers very low water and water vapor permeability — zero or very close to zero for both. Like all insulation materials used at cryogenic temperatures, however, cellular glass requires the use of a high quality, continuous vapor retarder applied to the outer surface of the insulation.
When selecting a mechanical insulation material, system designers should keep several insulation characteristics in mind prior to specifying one insulation over another. These include:
- Insulation thickness.
- Vapor retarder requirements.
- Compressive strength.
- Metal jacketing options.
- Environmental conditions.
These considerations should be compared to the needs of the application to ensure that the insulation system’s characteristics meet the unique requirements of each project.
Metal jacketing should have a moisture barrier that is factory heat laminated to the interior surface to reduce the propensity of jacket corrosion.
For any set of specified conditions, the insulation thickness required is based on the thermal conductivity (K factor) of the insulation — the lower the K factor, the better the material is at insulating. This means that materials with a higher K factor will need to be thicker to achieve the same performance as materials with a lower K factor.
Insulation thickness can impact overall cost in several ways. Greater insulation thickness requires the use of more insulation as well as additional accessory materials such as joint sealant, vapor retarders and metal jacketing. In addition, the larger the outside diameter of the insulation system, the larger the pipe supports and, possibly, the pipe racks, will need to be.
Keep in mind that the K factor of a material cannot be accurately represented by a single value. (It varies considerably with temperature.) Typically, the K factor value at 75°F (24°C) mean temperature often is used as a rough estimate of performance when comparing materials.
When the K factors of PIR and cellular glass are compared for operating in the cryogenic temperature range, cellular glass has a thermal conductivity; that is, 25 to 65 percent worse than that of PIR. When this thermal performance is converted into a required thickness of insulation, cellular glass will need to be approximately 25 to 35 percent thicker than PIR to achieve the same performance.
While most PIR has a lower compressive strength than cellular glass, its compressive strength typically is still adequate to prevent most damage.
Water is the main enemy of cold insulation systems. When water vapor enters the system, it will condense and often freeze inside the insulation system. At the very least, this situation reduces the insulating ability of the insulation system. It also can lead to more frequent condensation on the outer surface and an increased heat gain. If enough water vapor enters the system, ice formation can occur both inside and on the surface of the insulation system, which can physically destroy an insulation system. As a result, in cold and cryogenic applications, vapor retarders are a crucial component. They help protect the system from water-vapor ingress caused by the large vapor drive that is present.
The vapor retarder is the main barrier to water-vapor entry into the insulation system. A well-designed system will have a continuous vapor retarder with a permeance no greater than 0.01 perms (0.57 ng/Pa•s•m2) applied to the outer surface of the insulation. Typically, this number is identified on the product data sheets for the vapor retarder.
In addition, in LNG applications where it is common to use a three-layer insulation system, a secondary vapor retarder usually is applied between the second and third insulation layers.
Typically, plant personnel can identify when the vapor retarder has failed on a system because, initially, excessive condensation will occur. As more water enters the insulation system and the thermal performance is further compromised, ice can begin to form on the outside of the metal jacketing. Because ice is vastly more thermally conductive than pristine insulation, its presence in or on the insulation system is a sign that there is likely more heat gain into the system than desired. Additionally, freezing water and its associated expansion can further damage the insulation and vapor-retarder systems if it is not addressed promptly.
Unfortunately, it is difficult to repair an insulation system that has failed to the point of exhibiting ice formation. Therefore, the best approach is to properly design and install the system with a robust, long-lasting and high performance vapor retarder in the first place.
Cellular glass is a rigid, glass insulation that can be used at cryogenic temperatures as well as in high temperature applications up to 900°F (482°C).
For an industrial process cooling insulation system to remain effective, the insulation must retain its specified thickness. The compressive strength of the insulation is relevant because it can help prevent the insulation system and vapor retarder from being damaged by an external force. Cellular glass exhibits a compressive strength of 100 psi, making it highly resilient against impact damage.
High compressive strength can make the insulation more challenging to fabricate, however. While most PIR has a lower compressive strength than cellular glass, typically, it is still adequate to prevent most damage, and the PIR is easier to fabricate. There also are higher density/strength grades of PIR available for use in locations such as pipe supports, where a higher compressive strength is needed.
No insulation system, hot or cold, would be complete without the metal jacketing. Metal jacketing helps protect the insulation system from damage, including helping protect the vapor retarder from damaging UV light exposure and animal assault.
The most common metal jacketings used in North America are stainless steel and aluminum. Stainless steel jacketing is more resistant to corrosion, which makes it the more common choice in corrosive environments such as near or on the ocean. Stainless steel jacketing also is more resistant to fire. Yet in most applications, aluminum jacketing is the preferred material because it is a less costly alternative while still providing acceptable corrosion resistance.
Regardless of the metal jacketing used, it should have corrosion protection. For instance, one type is a three-layer, 76-micron thick, polyfilm moisture barrier heat laminated at the factory to the interior surface. Such a moisture barrier helps reduce the propensity of jacket corrosion. After all, a metal jacket with holes in it from corrosion will not provide much protection.
Metal jacketing helps protect the insulation system from damage. It also helps protect the vapor retarder from UV light exposure and animal assault.
Typically, system designers should design their insulation systems and insulation thickness using near-worst-case ambient climatic conditions. This means high temperature, high humidity and low wind speed. Even though an area may only encounter 90 percent relative humidity and 90°F (32°C) ambient temperatures for a month out of the year, that month will test the system. It is far better to design the system with this in mind than to have the system fail under challenging conditions.
In conclusion, proper specification of mechanical insulation used in industrial cryogenic process cooling requires system designers to consider more than simply the required temperature of the application. The insulation system is a critical and complex part of the total project. It must be designed by trained experts familiar with the engineering aspects of such a system, and it must be installed by insulation contractors familiar with the materials and the systems used in these cryogenic systems.
In determining which insulation to use, system designers should keep in mind the physical characteristics of the insulation and compare those to the unique needs of the system as well as the project’s budget and cost constraints. PC