ASHRAE Journal - February 2022 - 15

TECHNICAL FEATURE
FIGURE 2 Simple cascade system.6
Condenser
Expansion
Device
Compressor
* A high-pressure low stage refrigerant, typically
either R-508B or R-170 (ethane), via a cold wall evaporator
design.
* Then to the high stage refrigerant, typically either
Expansion
Device
Cascade
Condenser
Compressor
Low-Temperature
Evaporator
Cabinet
R-404A or R-290 (propane), via an inter-stage cascade
condenser.
* Then to the ambient environment through a traditional
air-cooled or water-cooled condenser.
ULTs are designed with either thick-walled traditional
blown foam or advanced insulation technology such
as vacuum panels to minimize the heat load during
operation and provide extended warm-up time in the
event of a power failure. Many ULT designs include a
cold wall confi guration and do not incorporate forced
air convection due to diffi culty in fan operation at ULT
temperatures.7
Another type of system architecture that is used to
checked at the beginning and end of each trip to avoid
exposing the vaccines by frequent container openings
for accessing data loggers. For data loggers with an
outside reader, temperatures are checked at least twice
during the trip. More details of vaccine transport are
described in Refrigerated Vaccine Transportation later
in this article.
Vaccine Storage
Deep Freezer
Guidance from both the CDC and the National Science
Foundation (NSF) Joint Committee on Vaccine Storage
recommend the use of medical grade, purpose-built
cold storage products for use in the vaccine supply
chain. Medical grade storage is separated into ultralowtemperature
(-80°C to -60°C [-112°F to -76°F]), low-temperature
(-30°C to -15°C [-22°F to 5°F]), and mediumtemperature
(2°C to 8°C [35.6°F to 46.4°F]) refrigeration
categories. Viability and storage time for a given vaccine
varies by storage temperature category.
Ultralow-temperature (ULT) storage freezers are generally
defi ned with a temperature range from -80°C to
-60°C (-112°F to -76°F). When approved, vaccines can
typically be stored in ULT temperatures for much longer
periods of time due to increased stability and reduced
speed of chemical reactions as compared to higher temperature
ranges. A traditional ULT design includes a cascade
arrangement in the refrigeration system (Figure 2),
where heat load from the cabinet is transferred to:
operate in the temperature range of -80°C to -60°C
(-112°F to -76°F) is so-called auto-cascades or mixedrefrigerant
cascades. While auto-cascades are normally
used in applications requiring temperatures below the
glass transition temperature of water (-137°C [-215°F]),
some variants exist in the -80°C to -60°C (-112°F to
-76°F) range featuring this design.
In auto-cascades, the low temperature is achieved
through the use of a mixture of refrigerants with very
different normal boiling points, forming blends with
very strong zeotropic behavior. After achieving partial
condensation in the condenser, vapor and liquid are
separated. The liquid phase has a higher concentration
in the less volatile mixture components. This liquid is
subsequently expanded and absorbs, through another
heat exchanger, heat from the vapor that was separated
earlier, leading to condensation of the more volatile
components. Meanwhile, the vapor generated in that
second heat exchanger is sent back to the compressor.
The newly formed liquid phase is then expanded and
reaches even lower temperatures than the preceding
expansion stage.
This combination of subsequent condensation and
expansion processes can be performed in one or several
stages, depending on the boiling point that is targeted
in the evaporator and the mixture constituents. For
example, mixtures of four different refrigerants with
normal boiling points of approximately -130°C, -80°C,
-10°C, and +15°C(-202°F, -112°F, 14°F and 59°F) could be
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ASHRAE Journal - February 2022

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