Proper vaccine storage and handling practices are critical for ensuring that vaccines maintain their efficacy from the point of manufacture until they are administered. It has been widely publicized that the recently approved COVID-19 vaccines must be stored at very low temperatures to remain viable for extended periods.
The Pfizer vaccine requires the lowest storage temperature of -70 °C (-95 °F), which is drastically colder than a household freezer that stores food at about -18 °C (0 °F). Read more to learn why these low temperatures are necessary and how they are achieved.
WHY IS IT NECESSARY TO GET THE VACCINES COLD?
Molecular motion and chemical reactions occur more slowly at lower temperatures, but they don’t stop, even when the chemical components appear to be frozen solid. In the case of the COVID-19 vaccines, it is necessary to slow the rate at which enzymes break down the vaccine components.
HOW ARE VACCINES ABLE TO BE TRANSPORTED AT LOW TEMPERATURES?
In the case of the Pfizer COVID-19 vaccine, the Pfizer team developed innovative, temperature-controlled thermal shippers. These utilize dry ice to maintain the required storage temperature of -70°C±10°C for up to 10 days unopened. The boxes have a layer of dry ice on the bottom, a thermal container to hold the vaccines on top of that, then another layer of dry ice. Before the box is sealed, another, sensor layer goes on top, which contains both a temperature monitor and a GPS tracking device. Pfizer ships doses directly from its facilities in Michigan and Wisconsin. This process is well underway. Millions of doses have gone out, with the number increasing each week. Distribution is coordinated with Pfizer's strategic transportation partners to ship by air to major hubs within a country/region and by ground transport to dosing locations. Once the vaccine doses reach their destination they are stored in ultralow-temperature freezers. These freezers plug into a power source like a household freezer.
WHY CAN’T A HOUSEHOLD FREEZER COOL TO -70 °C AND STORE THE VACCINES?
One reason, but not the primary one, is that the insulation in household freezers is not sufficient.
The rate at which heat from outside the freezer is conducted to the freezer’s internal cold space is directly related to the temperature difference from the outside to the inside. Ultra-low temperature freezers use thicker insulation and/or better insulation, such as vacuum insulated panels.
However, no amount of insulation will get a household freezer to ultra-low temperatures. The fundamental issue is the way the system works.
The Vapor Compression Refrigeration Cycle
With some exceptions for niche applications, virtually all refrigeration systems utilize the vapor compression cycle. This includes everything from air conditioning for the Superdome to dorm fridges.
The vapor compression cycle uses four main components (see figure below).
These components are connected to each other with tubing in a closed loop.
The compressor and refrigerant gas
A refrigerant fluid, often incorrectly referred to as Freon, is contained within the system components. The compressor compresses the refrigerant gas to a relatively high pressure. This process also increases the temperature of the refrigerant to the point that it is hotter than the outside air.
The refrigerant flows into the condenser where ambient air is blown over the tubing to cool the refrigerant. (The condenser is the part of your home air conditioning system that is outside your house.)
When the high pressure refrigerant in the condenser is cooled to a temperature close to the ambient air it condenses into a liquid. The liquid high pressure refrigerant is then directed to the expansion valve.
The expansion valve and flow restriction
The expansion valve is just a flow restriction in the tubing. It's like a valve that is almost all the way closed. This flow resistance causes the refrigerant pressure to drop substantially, which results in some of the liquid refrigerant flashing to a vapor with a significant drop in temperature.
The refrigerant is now about 10 °C colder than the inside of the freezer. The tubing leaving the expansion valve is wrapped around the outer wall of the freezer compartment.
Since the refrigerant temperature is lower than the temperature of stuff stored in the freezer, the refrigerant absorbs heat from the stored materials. The refrigerant also absorbs heat that leaks from the surroundings through the insulation.
Although this part of the refrigeration system is referred to as "the evaporator," the refrigerant is actually boiling, albeit at a low temperature. By the time the refrigerant leaves the evaporator, all of the liquid has boiled off to a vapor phase where it then enters the compressor to be compressed again.
Importance Of Refrigerant Selection
The temperature that can be achieved in the freezer compartment depends on a number of design variables. One important variable is the refrigerant selection.
There are a number of different refrigerants with different properties for a range of applications. It turns out that a single refrigerant tends to work well in a somewhat narrow range of temperatures between the surroundings and the freezer compartment. A refrigerant that condenses well at room temperature tend not to work well in the evaporator below about -40 °C.
HOW DO ULTRA-LOW TEMPERATURE FREEZERS WORK?
There is more than one technology used for ultra-low temperature freezers, but the most common is the cascade system.
The cascade system stacks two vapor compression systems together in series (see diagram below).
The high temperature system uses a refrigerant that works well for cooling down to about -40 °C. This same refrigerant is able to condense in the condenser at about room temperature.
The low temperature system uses a refrigerant that works well in the evaporator to temperatures much lower than -40 °C.
However this refrigerant can’t condense at room temperature. Instead the refrigerant in the high temperature evaporator is used to cool the refrigerant in the low temperature condenser.
REFRIGERATION AND CRYOGENICS SOLUTIONS
Inventherm takes an innovative approach to fulfilling your cooling needs. From custom applications of conventional refrigeration systems to implementations of more exotic technologies, our has the expertise to provide the cooling you need.
We partner with our clients to identify the best approach to meet your cooling needs.
Our team has a wide range of experience including:
Consumer beverage coolers
Deployable military systems