by Larry Gallagher, Specialty Gas Products Manager, CONCOA, Virginia Beach VA, lgallagher@concoa.com
The growth of storing and preserving biological materials in cryogenic liquid nitrogen freezers has resulted in many options and challenges for how to effectively supply these systems with the cryogenic liquid they require. Evaluating options, sizing and safety considerations is critical to determining the path a university laboratory or research facility should choose for implementation.
Liquid Nitrogen Supply Options
In new construction of facilities where dedicated freezer storage areas can be designed into the building plan, a large low-pressure bulk liquid nitrogen supply connected to a vacuum jacketed pipeline is typically the best option. This type of installation allows for the most cost-efficient delivery of nitrogen by the supplier and potentially the lowest evaporation loss of cryogen to the freezers because of the low heat transfer properties of vacuum insulated piping systems.
The deciding factor for these facilities is often the result of two considerations:
1. If the permitting and space for the bulk storage tank are allowed.
2. If the funding is available to cover the considerable cost involved with these types of installations.
ROI and reliability of supply are typically only appealing if the number of freezers involved is greater than a few dozen and if they can be located in a common area. Since the evaporation rate from the bulk tank is typically less than 1 percent per day, sizing the system is dependent on the cumulative demand of the number of freezers connected to it and the delivery cycle of the vendor.
Piping should include appropriate shutoff valves and safety relief devices piped to an appropriately sized vent line, and as with any liquid nitrogen use area, it should have oxygen-deficiency monitoring that integrates into an alarm system that can shut down supply when oxygen deficiency is detected in those areas. When the number of freezers or their location requires piping to multiple floors with only a few freezers, the cost and potential efficiency of this type of an installation make it less appealing or financially viable.
When the requirement for cryogenic nitrogen supply is throughout an existing laboratory or where the number of freezers involved does not justify the cost to install a bulk system, other options must be considered. It is common to find the method of supply in existing facilities to be a single low-pressure liquid nitrogen cylinder connected to each freezer with a four- or six-foot-long uninsulated hose.
Though common practice, this is typically the least cost-effective method of supply because each time the freezer requires liquid, the uninsulated transfer hose must be cooled down to the liquid cryogenic temperature of -196° C (77 K) before any liquid is transferred to the freezer. In addition, the “hot” nitrogen gas that is transferred to the freezer during this cool-down cycle results in evaporation and loss of the liquid already in the freezer.
This type of installation can cause several safety concerns resulting from two occurrences:
1. Moisture condensation on the uninsulated hose that creates puddles of water around the area after each fill.
2. Potential for the “hot” nitrogen gas release to reduce the oxygen content to a level that could result in an oxygen-deficient condition in the area.
The other option is to routinely change out liquid cylinders that still have significant contents left with full ones that may have already been sitting in reserve venting 2 percent of their contents per day. Neither of these options is considered the best or most efficient, though they usually can be found in virtually every university laboratory or research facility.
A better option for these types of installations is to connect multiple supply sources to each freezer or group of freezers with a system that can automatically change to a backup reserve cylinder when it senses that the primary is empty. The system should be insulated to minimize evaporation from heat transfer as well as to control condensation, and the hoses and/or piping should generally be vacuum jacketed with appropriate safety relief devices between any areas where liquid could be trapped.
To minimize “hot” gas transfer to the freezers, the system should have a means to purge any gas that is generated during cool down cycles away to an appropriately piped vent line along with the system safety relief valves. Their use and storage areas should still be monitored with an appropriate oxygen deficiency monitor and any alarm should be able to be integrated into an automatic shutdown of the supply. This not only ensures a more efficient delivery of the cryogen to the freezers, but also improves the overall safety of the installation. Sizing the number of liquid cylinders connected should be determined by the overall average weekly demand for liquid the freezers connected to the system.
The number of liquid cylinders connected to the primary and reserve side of the system should each be capable of supplying the freezers for a minimum of seven days.
This ensures that the freezers can be supplied without interruption over an extended weekend and that the money spent on liquid nitrogen is not wasted by discarding partially full containers or inefficient transfer of the cryogen to the freezers. (See Figure 1 above, a typical installation incorporating these system features.)
Though the cost for installing the necessary piping and system may be significant, the benefits of ensuring safe and uninterrupted supply of cryogenic liquid to irreplaceable specimens is typically worth the investment when a bulk installation is either not permitted or is too expensive an undertaking.
Sustained and consistent supply [whether in gas phase at room temperature or cryogenic fluids at -196° C (77 K), precision control of pressure, and temperature are indeed critical to the further growth of cryogenic applications.
CONCOA is actively engaged in developing the next level and generation of control for cryogenics, gas or liquid in what now appears to be the cryogenic century.
