ASHRAE Journal - February 2022 - 16

TECHNICAL FEATURE
used to reach product temperatures that are as low as
-80°C (-112°F).
When compared with conventional cascades that were
described earlier, auto-cascades require only one compressor,
i.e., there are only two pressure levels throughout
the entire system. Other potential advantages of
auto-cascades are seen in better lubricant oil return
because the oil stays with the liquid and never (at least
theoretically) reaches the system components operating
at the lowest temperature. Increased oil viscosity-associated
problems with oil return can be a serious concern
in conventional cascade systems, often requiring other
measures to ensure flowability of the oil in the coldest
sections of the system.
To achieve full redundancy, two fully independent
refrigeration circuits are often used in modern autocascade
systems. This certainly increases system reliability
by avoiding situations that could result in total loss of
cooling in case of one compressor failure or leakage from
one circuit. On the other hand, auto-cascades often have
reduced energy efficiency compared to conventional
cascade systems. Other challenges that equally affect
both system types are frost accumulation on the inside
walls in case of frequent door openings as well as the
limited possibility to operate the freezer at much higher
setpoints than the design temperature.
CDC guidance for medical grade refrigerators and
freezers typically includes microprocessor-based temperature
control, digital temperature sensors, temperature
data logging including minimum and maximum
temperatures, forced air convection to promote temperature
uniformity and recovery and safeguards like
self-closing hinges and door alarms to protect stored
products. Medical grade refrigerators and freezers typically
use traditional refrigeration systems. Design goals
typically focus on:
* Tight temperature uniformity and stability, which
may include incorporation of variable speed compressor
control;
* Tight evaporator temperature control to minimize
the potential for freezing in refrigerator applications;
* Fast temperature recovery during inventory and
product loading;
* Energy efficiency, which may include refrigerant
selection, intelligent compressor and fan control, improved
insulation and robust gasketing strategies;
* Low ambient noise generation;
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ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2022
* High reliability to minimize downtime and interruption
of product storage.
Refrigerated Vaccine Transportation
The cold chain, or refrigerated supply chain, is a critical
aspect in the transportation of vaccines from the
production site to the endpoint of use. Vaccines must
be maintained within the manufacturer's prescribed
temperature range throughout the delivery and storage
process to ensure product quality and potency. There
are multiple types of transport refrigeration products
that are used within the cold chain to accomplish this,
spanning transport via air, ship, rail and roadways. Each
of these products can play a key role depending on the
geographic shipping requirements. In addition to temperature
control, products often incorporate remote
temperature sensing and vehicle GPS tracking through
cellular or satellite telematics devices to monitor performance
and ensure security and quality compliance. An
overview of the various types of products available in the
market will be provided.
Air Transport Products
Unit load devices (ULD) are small (typically up to 5 m3
[177 ft3] volume) insulated containers used on aircraft
for a variety of products that require temperature control.
Figure 3 shows an example of an air transport insulated
container with refrigeration unit. These are often
classified as either passive or active systems. Passive
systems use onboard thermal storage through the use
of phase change materials to provide product cooling.
Active systems incorporate a vapor compression
refrigeration circuit that can be powered either through
standby electrical power, before loading on the aircraft,
or battery power during air transit.
Typical temperature control on ULDs ranges from 0°C
to 25°C (32°F to 77°F), in ambient temperatures up to
50°C (122°F). Autonomous run time on battery power
varies by product, typically ranging up to a maximum of
125 hours. Systems are also often equipped with telematics
devices, which can provide data for GPS tracking,
as well as continuous monitoring of internal box and/
or product temperature. Given the temperature range
of these systems (0°C to 25°C [32°F to 77°F]), care must
be used in selecting which vaccines could be stored
without supplemental cooling. Such devices could not
be used to directly control vaccines requiring ultracold
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