Gaseous nitrogen has been used to expel oxygen and increase product shelf life. Liquid nitrogen can serve the same purpose while reducing nitrogen consumption by 80% over traditional gas tunnels. Handling liquid nitrogen on a production line poses challenges, whether pressurizing or inerting food or beverage containers.
Equipment Overview
Liquid nitrogen has a boiling temperature of -320°F (-196°C) and will boil away rapidly when exposed to room temperatures. Therefore, insulated equipment must be used to ensure efficiency and safety. This equipment includes an injection device capable of metering small doses of liquid nitrogen into food or beverage containers, storage vessels or tanks, and piping to transport the liquid nitrogen to the injection location.
Storage vessels generally come in two forms: large bulk tanks and small portable tanks called dewars.
Both designs feature a double-wall construction with a vacuum space separating the inner and outer walls. This vacuum “jacket” allows the tank's outside surface to remain at ambient temperatures while maintaining cryogenic temperatures inside. The nitrogen can be held in liquid form for quite a while, but even with this vacuum barrier, the insulation isn’t perfect, and tank losses can range 0.5% – 2% per day. Large bulk tanks are typically installed outside and require longer piping systems to transfer the liquid to the use point. Dewar tanks are portable and can be situated closer to the use point, requiring a shorter hose length.
Insulated piping should be used for either type of storage vessel to limit losses and improve efficiency. There are multiple types of insulated piping, but they can generally be categorized as vacuum-jacketed and non-vacuum-jacketed.
Vacuum-jacketed piping is like the insulation found in bulk tanks or dewars. An internal pipe is surrounded by a vacuum annulus that provides insulation between the cryogenic temperatures in the pipe and the ambient temperature outside it.
This vacuum space significantly reduces heat losses, improving the efficiency of the pipe. Vacuum jacketed piping is more efficient than non-jacketed piping and offers completely frost-free operation. The vacuum jacket on this type of pipe is generated by attaching the pipe to a vacuum pump. In a dynamic vacuum system, a vacuum pump continuously pumps, and the vacuum quality consistently improves. With the need for a dedicated vacuum pump running all the time, the operating costs are slightly higher with this style. The vacuum on a sealed-vacuum system is typically evacuated at the factory and then sealed off. Over time, this vacuum will gradually degrade, resulting in increased heat losses and decreased performance.
Either type of vacuum jacketed piping can come in rigid or flexible sections. Rigid piping must be accurately dimensioned to ensure a proper fit in the field. Flexible piping is fabricated in sections, making it easier to install and more adaptable for routing around obstructions. Vacuum Barrier Corporation custom manufactures sealed and dynamic- vacuum versions in various materials and sizes. Non-vacuum jacketed lines are often insulated with foam and are not as efficient, with heat losses as high as 20 times that of vacuum jacketed piping. As the foam degrades over time, it loses its insulating qualities. These piping systems also have larger outer dimensions, making routing through tight spaces difficult.
Nitrogen dosing equipment is the main component of a liquid nitrogen system. It’s often what production facilities are most interested in, as it directly affects their ability to meet pressurization or inerting goals. Typically, these are called dosers and must operate frost-free and efficiently during dosing or idle times.
The reliability of a doser on a production line is critical as losses are calculated in minutes of downtime. As with any cryogenic device, internal exposure to moisture must always be limited as it’s a doser’s biggest enemy. Care must be taken during nozzle changes and maintenance to prevent contamination by moist air. In specific industries, there may be a requirement that liquid nitrogen be delivered aseptically, and therefore, the unit must be capable of being sterilized.
Production Goals
Aside from operating frost-free, a doser must also meet the goals of the production facility. Any bottling or canning operation will look for consistent pressurization or inerting of their containers. This requires the doser to consistently output an accurate dose of liquid nitrogen, whether dosing discretely or steady streaming. Too small of a dose can lead to unstable containers and the possibility of collapse. For inerting processes, this could lead to food spoilage. If dosed with too much nitrogen, there is a risk of containers bulging or bursting, which could cause jamming and downtime.
The challenge for the dosing equipment is to reliably and accurately control the liquid nitrogen dose for each container up to speeds of 2000 bottles per minute. To meet the production goals of pressurization, a doser relies on consistent fill heights from the filler. Even a small change in fill height can lead to under- or over-pressurization.
The doser can adjust to changes in the filler's line speed. As the line ramps up or down, timing is adjusted automatically to ensure each dose enters the container. Likewise, dose compensation adjusts the amount of LN2 dispensed as the line speed changes. For example, as a line slows down, there is more time between filling and capping, which means more time for the nitrogen to boil off. Therefore, a larger dose is dispensed to maintain consistent pressures.
Other factors on the production line must also be considered to ensure proper pressurization. Travel time from the doser to the seamer or capper should be minimized to prevent excess boiling or loss of nitrogen. Shaking or bouncing containers on the conveyors can force nitrogen and product out of the package before closure. Reliable sealing closures are also needed to maintain the pressure within the container after dosing.
VBC offers a range of dosers to accommodate a variety of line speeds and budgets.
Safety
Addressing worker and machine safety when dealing with liquid nitrogen is essential. When boiling from liquid to gas, nitrogen expands roughly 700 times. Safety relief valves are installed on tanks, piping, and dosers to prevent over-pressurization and potential equipment ruptures. Where there are shut-off valves in a system, there is potential for nitrogen to be trapped. A safety relief valve must be placed between any two such valves. On bulk tank-fed systems, the lowest-rated relief device is typically placed outdoors. If a safety relief valve does relieve, it is safer if it happens outdoors rather than inside where workers are present.
Costs
Cost is another key consideration of production facilities, and it’s important to look at the full picture when measuring the costs of an LN2system. The upfront purchase price, installation, and operating costs must be considered jointly. When evaluating tank options, large bulk tanks cost more initially, but nitrogen is less costly in bulk. The need to continually change out dewars during a production run can also add hidden costs.
There are more options for cost reduction when it comes to piping. Inexpensive foam-insulated piping can reduce the initial price, but the operating costs associated with using more nitrogen can add up over the life of a system. Vacuum jacketed piping is more expensive up front, but it’s more efficient and will reduce nitrogen consumption and operating costs. As mentioned earlier, reliable operation is a key component for liquid nitrogen dosing systems, and downtime caused by an inexpensive, foam-insulated dosing system can offset any perceived gains from a low initial purchase price.
A production facility considering liquid nitrogen dosing must consider costs, both upfront and operating, the reliability of the system from tank through piping to doser, and the safety of its workers. To succeed in this and collect the proper information, please consult Vacuum Barrier Corporation.
Starting as an Application Engineer in 2016, Jim gained valuable experience and knowledge of LN2 piping and dosing systems in the domestic market. In 2023 he stepped into his current role with a focus on Europe and Asia, bringing with him an education in interdisciplinary engineering and management and a strong technical sales background.
