As energy costs continue to rise, plants with process cooling requirements are constantly seeking ways to increase process efficiency and reduce energy consumption. A common area for efficiency losses is the insulation surrounding pipes and vessels. Moisture infiltrates through all insulation vapor barriers, even when they are installed correctly and are in good condition. As a result, even the best insulation absorbs moisture over time and loses its ability to insulate efficiently (figure 1). A plant with insulation that was installed more than seven or eight years ago could be losing thousands of dollars in system inefficiencies due to poor insulation.

One option is to replace all of the insulation periodically to ensure continued high levels of efficiency. However, in a typical plant with miles of piping and numerous vessels, the costs for installing new insulation on all systems can be staggering. Unless losses through insulation have been identified as a substantial source of wasted energy in the plant, the economics of this solution are difficult to justify.

An alternative is to purge existing moisture from the insulation using dry-air injection. Dry-air injection removes moisture from beneath the vapor barrier, protects the piping or vessel from moisture migration and rusting under the insulation, and reduces thermal loss — all without requiring system shutdown, and at a lower cost than the expense of insulation replacement.

Enlarge this picture Figure 1. Wet or frozen insulation is much less effective at preventing heat gain compared to dry insulation.

Dry-air injection uses compressed air (either an existing or dedicated air supply) that is directed to an air-drying system. Using a desiccant- or membrane-type dryer, the technology reduces the moisture content of the air to a dewpoint that is suitable for the application. The dry air is directed to one or more air-distribution stations, which monitor the airflow to each piping run through adjustable flowmeters, and the air is fed to each run through distribution lines. From there, the dry air enters smaller injection lines that are installed in machined slots along the outside of the insulation and under the vapor barrier (figure 2). In retrofit applications, the air-distribution tube is inserted into a trail that is cut into the outer layer of insulation. The trail is then covered, and the compressor/dryer is installed.

Enlarge this picture Figure 2. Dry-air injection uses compressed air that is directed first to an air-drying system, then to one or more air distribution stations. From there, it is fed through distribution lines into smaller injection lines that are installed in machined slots along the outside of the insulation and under the vapor barrier.

Air-purge points are installed on top of the insulation, and each check valve is located at a predetermined place to achieve the proper dry airflow throughout the vessel or pipeline. After the system is installed, dry air will flow under the vapor barrier at a low flow rate to ensure that no moisture migrates into the piping system. In systems that already have excessive moisture, the airflow first is set at a higher rate until the dry-air system removes the excessive moisture; then, once the moisture is removed, the airflow is reduced to a maintenance level.

Brent Cottingham, staff engineer at IND Industries, has seen substantial benefits achieved with dry-air injection on numerous process cooling operations. The Comstock Park, Mich.-based company specializes in dry-air injection systems that can be used for both new installations and retrofits.

“Dry-air injection can return water-compromised insulation to its original R-value, which is a measure of thermal resistance or how well the insulation holds back heat. With the water removed, corrosion is also halted. Dry-air injection protects both the insulation and the pipe underneath it,” Cottingham says.

The progress of the drying operation can be monitored by temporarily inserting an air-moisture probe into the vents, which allow the moist air to escape. Where the insulation is water-free, the escaping air will be nearly as dry as that which is in the distribution tube. An additional option involves remote sensing to monitor the progress. Small, microchip-based humidity meters can be installed in vents or buried in the insulation itself. These meters are sampled remotely and the results tabulated. The amount of time required to achieve complete drying varies, depending on the insulation type and the level of failure. In most cases, though, dry-air injection can provide significant results in a few months.

Applications and Actual Results

Enlarge this picture In retrofit applications, the air-distribution tube is inserted into a trail that is cut into the outer layer of insulation. Air purge points are installed on top of the insulation, and each check valve is located at a predetermined place to achieve the proper dry airflow throughout the vessel or pipeline.

A number of plants have used dry-air injection successfully to remove moisture from insulation, thereby halting corrosion and improving system efficiencies.

For example, in one bread-freezing operation, insulation on a -20°F (-29°C) ammonia liquid line failed due to long-term exposure to water vapor. The operation ran 24 hours per day, seven days per week, and could not be shut down long enough to thaw out and replace the failed pipe. The plant installed a dry-air system and injected a blanket of extremely dry air under a new vapor barrier to remove moisture from the insulation. After 29 months of operation, the system had stopped any visible sign of condensation and had removed ice deeply embedded in the insulation.

In a poultry processing plant, ice had formed on the insulation encircling a -10°F (-23°C) ammonia vessel and associated piping. The ice was stressing the external piping and increasing system inefficiency. Water dripping from the frosted areas fell on the pump motors and on the floor, causing additional problems for the plant. This operation also ran seven days a week and could not be shut down long enough to thaw, remove and replace the existing insulation. Instead, the plant increased the room temperature and removed the ice, jacketing and vapor retarder. It then installed dry-air injection tubing in the insulation, along with a 0.25" thick polyfoam overlay (to provide a clean, dry surface for the new vapor retarder), new vapor retarder and vents. No system shutdown was required.

After the initial installation, some frost and condensate formed on the outside of the new vapor retarder. However, four months later, the vessel showed no sign of external frost and little condensation, resulting in reduced piping stress and a reduction in heat intrusion.

As manufacturing costs continue to rise, today’s plants must find new ways to increase efficiency. For operations that are facing energy losses due to saturated insulation, dry-air injection can provide a cost-effective solution.

For more information from IND Technologies LLC, Comstock Park, Mich., a manufacturer of dry-air injection system for insulation, call (616) 785-5374, e-mail bcottingham@rcicold.com or visit www.insuldry.com.