By Bobby Baird, SASHE, CHFM, CHSP, CHC
As the United States and other countries are racing to develop and distribute a COVID-19 vaccine, many facilities will need to increase their storage capacity. Given the potential need to store vaccines in a cryogenic (or ultra-low temperature) environment, this article will provide the reader with baseline information to help discern which type of system will work best given their facility’s existing infrastructure.
There are two basic types of cryogenic storage (CS) available on the market today, compressor-driven refrigerant and liquid nitrogen (LN2) cooled. Each option has pros and cons that should be taken into account when planning a space for use. Evaluating the cryogenic temperature required, whether -85 C or -145 C, will be necessary in the selection process.
Whichever system your team selects, there are several considerations that must be taken:
1. Perform a multidisciplinary risk assessment.
2. Evaluate the utilities needed.
3. Develop or review safety procedures.
4. Evaluate the space.
5. Evaluate life cycle costs.
6. Ensure mechanics and staff are trained on proper use and handling.
Compressor-driven CS require either a dedicated 120 volt 20 amp or 220 volt electrical supply, which should be on the critical branch if possible, but could be on the equipment branch if necessary. Given the critical nature of what is being stored, it is not recommended to use the normal branch. The electricity cost to keep the CS cold should be evaluated. The typical measurement is kilowatt hours per day (kWh/day). Your local cost of electricity may provide a return on investment for a more expensive but higher-efficiency model, so do your homework. Compressor-driven CS requires a smaller footprint since LN2 dewars are not required. (A dewar is a double-walled container for storing LN with a vacuum between the wall like a thermos.)
Many manufacturers are using flammable refrigerants to move heat in these CS units. While the amount of flammable refrigerants is typically within ASHRAE standards, placing multiple CS units in one room or smoke compartment may prove unsafe. For additional guidance, please refer to this Health Facilities Management June 2020 article.
Compressor-driven CS room air conditioning must be evaluated to ensure the rejected heat from the compressors can be managed with the building HVAC system. Also, be aware that the compressor does add noise into the room; check with each manufacturer to determine whether the dBA sound levels will interfere with employees working in that space. Manufacturers use single and twin compressors to cool. While both are acceptable, recognize the complexity of the system and ensure you have adequate support should you need it. Some manufacturers offer LN2 backup to their compressor-driven CS units, which provides significant redundancy to protect the stored material in the event of power loss or compressor failure.
LN2 CS has no compressor; the product is cooled in either a bath of LN which has a boiling point of -196 C and can be adjusted to maintain temperatures between -145 C and -45 C, or it is cooled in a hybrid liquid/vapor. The lack of a compressor does not necessarily mean the LN2 unit is quieter, however, because the dewar will boil off and vent vapor nitrogen, causing a loud hissing sound. Depending on the environmental conditions and the frequency of opening the CS unit, this vapor discharge could occur hourly or more frequently.
LN2, when changing phase to gas, expands 695 times (e.g., one cubic foot of LN2 will become nearly 700 cubic feet of pure nitrogen vapor). This vapor is cold and dense, and typically gathers at floor level. If an adequate amount of nitrogen is spilled, asphyxiation may occur as the oxygen levels drop below 19.5%. This supports the necessity of oxygen monitoring and alarms to warn staff that a dangerous condition exists, especially considering nitrogen vapor is colorless, odorless and gives no indication when a dangerous condition exists. Take additional precautions at start up, as this is when the largest amount of nitrogen will be used to bring the CS down to holding temperature from ambient temperature. Using LN2 for cooling creates similar risks as confined spaces may present, so proceed with caution.
Given these risks, why would a facility risk using LN2 CS? LN2 is known to be extremely reliable. Provided you have LN2 available and connected to the CS, you will maintain appropriate temperatures. This method has does not rely on compressors, which can malfunction, nor does it cause concern if the electricity goes out. Also, initial cost is often lower since LN2 is in essence one big insulated thermos bottle used to hold product. Additionally, the LN2 CS is the primary option for temperatures below -45 C.
Another worthy consideration as you are evaluating these systems is life cycle costs between compressor-driven CS and LN2-cooled CS. Compressor system manufacturers publish electric consumption data, which is useful for comparing compressor models to each other. As you compare these data, make sure the baseline data is equivalent, specifically the amount of CS access that is used in the data set.
Comparing compressors to LN2 is not as straightforward. You will need research local costs to lease dewars and have LN2 delivered to your site. The cost of LN2 varies widely region to region due to the process necessary to distill air into its base components, like nitrogen and oxygen. This process is electrically intensive, so local electricity rates will play a major role in LN2 pricing. Using this data combined with consumption data for LN2 CS units will provide a reasonable comparison to compressor systems.
Whichever system your team eventually decide works best in your organization, you can count on most manufacturers to provide a quality product. The materials stored in these CS units are often regulated, which drives the market towards a higher quality product. If you are part of a group purchasing organization or an independent hospital, you can negotiate discounts off list price. Lead times may increase as demand increases, making it imperative that you are knowledgeable and ready to be an invaluable resource to your employer.