If you are using liquid nitrogen (LN2) in your processing operation and your transfer lines are more than 20 years old, you might be losing money every day. However, you probably won't see a puddle on the floor to warn you that something is amiss. Instead, what typically happens is that you will gradually use more and more LN2 with each passing year -- so gradually, in fact, that you probably won't even notice the increase.
For decades, the accepted method of insulating cryogenic transfer pipe (typically copper) was to use foam insulation covered by a protective polyvinyl chloride layer. This type of insulation is inexpensive and works well when new, providing typical heat transfer rates above 20 BTU/hr/ft at LN2 temperatures. However, its performance deteriorates rapidly within about five years, and it becomes ineffective in preventing vaporization of the liquid as the product ages.
The technology in transfer piping for LN2 and other cryogenic fluids has made substantial advances since the early 1990s. Not only are today's transfer lines better insulated to minimize the loss of LN2 through evaporation, but they are also easier to install and are virtually maintenance-free.
Two types of vacuum-jacketed piping systems (also referred to as vacuum insulated) -- rigid and flexible -- are available for process plant installations where long runs of piping are required to transfer LN2 from a bulk storage vessel at the back of the plant to one or several use points.
Rigid Vacuum-Jacketed PipeWith rigid vacuum-jacketed piping, each pipe section is constructed of rigid inner and outer pipes. The inner pipe, which carries the cryogenic liquid, is wrapped with multiple layers of super-insulation consisting of alternating layers of a radiant heat barrier material and a nonconductive spacer material. The space between the two lines is evacuated to the highest industry standards. The vacuum annulus contains getter materials to adsorb outgassed molecules, thereby further improving the vacuum.
Rigid vacuum-jacketed piping is easier to install than foam-insulated copper pipe and is designed to be maintenance-free for a minimum of 10 years, with no deterioration of the system performance over that period. Pipe sections are joined with vacuum-insulated bayonet connectors that provide frost-free connections (figure 1). Bayonet fittings are used to simplify installation while maintaining the integrity of the insulating system.
The thermal barrier between the inner and outer lines is so effective that the outer pipe remains at room temperature even while -320°F (-195°C) liquid nitrogen is flowing through the inner line. For ease and speed of installation, rigid vacuum-jacketed piping also can be routed across the plant roof and can be exposed to direct sunlight without affecting the system's performance.
This type of system is typically used in food plants that use cryogenic liquids in their processing operations, pharmaceutical plants that require long supply runs of cryogenic liquid from the bulk storage tank to multiple use points, and processing facilities with long exterior pipe runs that are exposed to outdoor elements (figure 2).
Flexible Vacuum-Jacketed PipeFlexible vacuum-jacketed piping is constructed of convoluted inner and outer tubing. The inner tube, which serves as the LN2 carrier, is insulated with alternating layers of reflective foil and nonconductive spacer material and is loaded into the outer tube, which contains the vacuum jacket. The flexible system uses the same bayonet connectors as the rigid piping systems.
Flexible piping has several advantages over the more traditional rigid vacuum-jacketed piping, all of which translate into cost savings and improved system options for growing plant operations. For example, the flexibility of the pipe reduces the necessity for precise system layout measurements and allows much or all of a system to be reused if use-point locations are changed due to changes in the plant layout. The existing system can be expanded without major rework expenses, and the bayonet connections can be taken apart and reassembled. Additionally, flexible pipe systems are coiled and crated for shipment by motor or airfreight, which reduces delivery costs.
Flexible vacuum-jacketed piping is most often used in startup operations where cost is an important consideration, as well as in operations where substantial plant growth is anticipated. Other applications include laboratories where future rerouting of piping system may be necessary, new product launch facilities that need to be brought online quickly, and the replacement of old LN2 piping.
Although the initial purchase price of a vacuum-jacketed piping system can be higher than for non-vacuum systems, vacuum-jacketed piping systems typically provide a quick payback (less than 14 months in most cases) by reducing operating costs. They can significantly minimize the liquid losses caused by heat leaks, provide a longer lifecycle than foam-insulated copper piping, and do not decrease in performance over time. They also can increase production efficiency by delivering colder LN2 at the use point, and can improve process safety by eliminating frosty and dripping conditions.
However, not all vacuum-jacketed piping systems are the same. When selecting a vacuum-jacketed piping system, carefully examine all components for quality and look for a strong warranty. If possible, you should also visit the manufacturers of vacuum-jacketed piping systems to see the differences in the way the systems are built.
Choose the right system, and you will quickly reap the benefits of a more efficient process. PCE
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