Tech Briefs Magazine - October 2021 - 18a

DESIGN OPTIMIZATION
FIGURE 2              
                
                 
              
              
them a popular choice in the refrigeration and air conditioning
industry.
Heat exchangers sound like a simple concept, but they can
actually be quite challenging to design. The essential task
in cooling a product is to get rid of unwanted heat so that
thermal energy from perishable goods is extracted. This is
where the refrigerant of a refrigeration cycle comes into play.
By changing the refrigerant phase from a liquid to a vapor
state, the heat exchanger is removing heat from its ambient
surroundings. This heat then has to be passed over to a
second heat exchanger, which emits this energy to the outside
environment.
In transcritical CO2
refrigeration cycles, a so-called gas cooler
chills the refrigerant inside a heat exchanger. Often, people
get confused by the name " gas cooler " , as if it uses gas to chill
its surroundings. Designing heat exchangers in general, and
gas coolers in particular, presents a fair amount of difficulties,
according to Heik. When striving for better, more energyconserving
refrigeration cycles, well-engineered heat exchanger
designs serve as a main contribution.
Like many cooling systems, gas coolers are designed to have
a minimal direct impact on the environment, so they use the
natural refrigerant CO2
CO2
is now used almost exclusively because it is classified as
a nonhazardous gas (safety group A1). Due to its properties,
however, it must dissipate its heat at air temperatures above
20-25°C, in the so-called transcritical range. That is why
these systems have a large temperature difference, consist
of many different circuits, and are made up of a wide range
of materials. Using simulation, Heik is able to efficiently and
simultaneously analyze the airflow and material properties of
these devices.
Designing the inner finned tubes presents another unique
18a COMSOL NEWS
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. For example, in the supermarket sector,
challenge when developing
heat exchangers. These tubes
are used in heat exchangers
to transform a hot fluid into
a cold fluid or vice versa.
The arrangement, diameter,
material (stainless steel is
required if using ammonia),
and fin spacing of these
finned tubes all depend on
the type of heat exchanger
in which they are being
used. " There is not a lot of
measured data available on
how these tubes work, " said
Heik. With simulation, he can
get a better understanding
of how finned tubes affect
a heat exchanger design
by modeling multiple tube
geometries and investigating
their inner and outer heat
transfer capabilities. The
finned tube geometries that
offer the best performance
are built and tested at an
in-house experimental
station. " We look to see if the
calculations and results are
the same or similar, and then
we take the best tube for our
industrial line, " said Heik.
» COLD STORAGE
ROOM SIMULATIONS
In addition to performing
simulations of heat exchanger
technology, thermofin GmbH
also simulates their customers'
cold storage warehouses. For
one specific project, a customer
asked for help designing a
meat storage room, which
would include several robotic
machines that hold the meat.
In this storage room, meat
enters at room temperature
and needs to be cooled before
it can be brought into a
different cold storage room.
" It was important that the
air velocity in the room was
not too high so that the meat
would not fall off the robotic
[machines], and on the other
hand, it was really important
that every area in the room
gets the same or similar
amount of air, " said Heik.
When performing cold
storage simulations like
this one, there are several
criteria that need to be
taken into account, including
temperature distribution,
airflow distribution, relative
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Tech Briefs Magazine - October 2021

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