Applications in the food, beverage, rubber deflashing, biological, material processing and electronic industries require liquid nitrogen. In most processing facilities, the liquid nitrogen storage tank is located between 50 and 500' away from the point of use. Therefore, the liquid nitrogen must be piped from the bulk storage tank to the process point. Specifying the right type of pipe requires an understanding of the four main types of cryogenic pipe that are available as well as initial cost and operating cost of a system. Specific application use and pipe layout also must be considered.
Cryogenic Pipe OptionsThe four main types of pipe that are used currently in cryogenic installations include urethane-foam-insulated pipe, static-vacuum insulated pipe, dynamic-vacuum insulated pipe and static-vacuum pipe with foam-insulated joints.
Urethane-Foam-Insulated Pipe. Foam-insulated pipe is the most common. The pipe is purchased in standard lengths of 20' and can be modified easily during installation to accommodate the required layout. The pipe is composed of copper pipe surrounded by urethane foam for insulation. A PVC jacket surrounds the foam to provide a protective barrier against the environment. Key benefits of foam-insulated pipe include the low initial capital cost and the ability to complete an installation with lead times as short as a couple of weeks. Although the initial cost will be lower, the cost of operation will be higher than the other types of pipe due to the higher heat leak rate. Also, the pipe can degrade over time as moisture penetrates the urethane-foam insulation.
Dynamic-Vacuum Pipe. With dynamic-vacuum insulated pipe, a vacuum pump is used to constantly pull a vacuum between the pipe and the outer jacket of the piping system. Benefits of dynamic-vacuum pipe include its bendable nature that simplifies the installation process. Its improved insulating ability as compared to foam-insulated pipe also is an advantage. However, the requirement for a vacuum pump is the main drawback to this type of technology. The electric cost to operate the pump increases the processor's operating expenses. Also, the pump must be maintained. If the pump stops operating or if the pipe becomes damaged, the entire system could become inoperable. The high maintenance cost of this pipe system and the inability to know when the vacuum pump might fail must also be considered.
Static-Vacuum Pipe. In simple terms, a static-vacuum pipe is a stainless steel pipe within a stainless steel pipe. Static-vacuum insulated pipe is available in rigid and flexible sections in lengths up to 40'. A vacuum is pulled in the space between the pipes and sealed at the factory, so a vacuum pump is not required at the manufacturing facility. Each pipe end contains bayonets that slide together to allow for a simple installation that does not require welding. Benefits of this type of pipe include the lowest amount of heat leak and the greatest durability of any piping system. Lead-times of four to five weeks to custom build the pipe sections must be considered.
Static-Vacuum Pipe with Foam-Insulated Joints. With this lower cost option to static-vacuum insulated pipe, static-vacuum insulated pipe sections up to 20' long are stocked at the factory, and elbows and tees are added as needed during installation. The vacuum-insulated sections are welded together at the customer's location. The joints are insulated with a clamshell-type foam insulation covered by a stainless steel sheath. This type of pipe with foam-insulated joints provides some of the benefits of static-vacuum pipe with shorter installation lead times and lower initial capital costs.
Application ConsiderationsThe initial cost of an installed pipe system is an important factor to take into account. This includes the cost of both the pipe and the labor to install the pipe. Typically, the least expensive piping system is foam-insulated pipe. The installed cost of a foam-insulated piping system is about half the cost of a vacuum-insulated piping system. A majority of the cost for a foam system is the installation expense. At the other end of the spectrum, a majority of the cost of an installed vacuum system is the cost of the pipe. The use of bayonet fittings, which simplifies the installation, keeps labor costs to a minimum with static-vacuum piping systems. Customers that have limited capital budgets may need to consider foam-insulated pipe.
Another point to ponder is operating cost. Both foam-insulated pipe and vacuum-insulated pipe must be cooled prior to operation. The cool-down losses for vacuum-insulated pipe are less than one-half of the losses from foam-insulated pipe. The differences in the steady-state losses due to heat leak are far more dramatic. The factors that have the greatest effect on steady-state losses include:
- Amount of time that the line is charged.
- Price of nitrogen.
- Length of pipe.
As these factors increase, the benefits of vacuum pipe as compared to foam increase. Thus, production facilities that occasionally use liquid nitrogen and have a short pipe run may be well served with foam-insulated pipe. By contrast, production facilities that use nitrogen during two shifts per day and have long pipe runs may be better served with vacuum-insulated pipe.
The operating cost savings of static-vacuum pipe can be illustrated with the following example. Suppose a manufacturing plant operates 16 hours per day, five days per week. The plant has 506' of pipe and pays $0.41 per HCF for liquid nitrogen. The cool-down and steady-state losses per year for 1.5" foam-insulated and static-vacuum pipe are summarized in table 1. Given these operating parameters, the processor would save approximately $50,000 per year with static-vacuum piping.
Specific Application Considerations, Pipe LayoutLiquid nitrogen is stored at its boiling point, -320oF (-196oC). As the nitrogen is transported through the pipe, it absorbs heat. This decreases the quality of the liquid as some of the liquid is converted to gas. Many applications such as environmental test chambers, nitrogen food freezers and noncarbonated beverage pressurization systems require high quality liquid on demand. If the usage point is more than 100' away from the bulk storage tank, vacuum-insulated pipe should be considered.
As the pipe run becomes longer, more of the liquid is converted to gas. The gas will reduce the pipe's liquid flowability. In many applications, lower storage pressures for liquid nitrogen also are desired. The use of vacuum-insulated pipe will enable a processor to operate at lower pressures. Lower pressures equate to added efficiencies in cryogenic applications.
Many processing plants also need to eliminate the likelihood that frost will form near the use point. Frost can be a vehicle for biological contamination in a bottled water facility. The use of vacuum-insulated pipe will prevent the formation of frost.
The location of the pipe also should be considered. Foam-insulated pipe will last much longer in a dry environment. However, if the foam-insulated pipe is located in a very wet environment, the foam's insulating ability will degrade over time.
When specifying the pipe that is right for your cryogenic application, key points to consider include application type; the requirement for high quality liquid; length of the pipe run; environmental concerns; hours of operation per month; price of the cryogen; available capital budget; and payback period for vacuum-insulated pipe. Engineers from your industrial gas supplier and cryogenic service providers can help you qualify the right pipe for your application.