Feature
The size of the
cold store will
influence which
refrigeration
system should be
selected.
According to Ford, the considerations when choosing a system
nowadays are linked to:
a. Mechanical systems:
i. Simplex – single compressor application
ii. Multiplex – multi compressor application
b. Refrigerant: Hydrocarbons, ammonia, CO 2, or
synthetic refrigerants (HFCs mainly).
c. A multi-rack also provides a more controlled
temperature range.
One of the main considerations for selecting a system
relates to the refrigerant used in the system. Locally, HFC
refrigerants are still the most common solution (and generally
the cheapest) but as the imminent global phase-down starts
taking effect, many are looking at more natural alternatives
such as hydrocarbons, ammonia, or CO 2 systems.
Van der Merwe explains that the most common systems
are as follows:
• Ammonia systems
Industrial ammonia is the most appropriate system for large
installations and has the best coefficient of performance (COP)
available. These systems are designed for a 30-year plus life
span and offer excellent reliability if maintained correctly. For
the smaller sized cold store installations, the first costs can be
considerably higher but over a 20-year life cycle costing, which
takes into account the total operating cost, the ammonia
option can be an attractive proposition.
• CO 2 cascade systems
CO 2 is established as a preferred ‘green’ solution for
supermarket applications and is being considered for certain
low temperature applications such as freezer stores. CO 2
(sub-critical) cannot match the COP of ammonia plants
though. CO 2 (trans-critical) COPs are just starting to match
www.hvacronline.co.za
typical air-cooled R404 COPs and the higher ambients in South
Africa must be taken into account when evaluating the total
operating costs of this solution.
• R404a, R507, and other HFC cocktails
Air-cooled synthetic refrigerant systems are a popular choice
for small cold storage applications, typically up to 150 or 200
pallets of frozen storage. These systems have a lower first
cost but the running costs are higher (due to a low COP and
high electrical costs) and often the maintenance costs can
be higher due to low quality installations with gas leaks and
equipment failures.
It’s important to calculate the medium temperature (MT)
load required compared to that of the low temperature (MT),
in particularly a distribution centre (DC) application, advises
La Grange. The COP of the LT load is significantly lower than
MT applications, regardless of the refrigerant used. “That is an
important ratio to consider when comparing one installation
with another.”
INSTALLATION DO’S AND DON’TS
So, you’ve selected your system – now it’s time for the
installation. Before you get started, La Grange advises that you
do the homework and review the potential payback period of
each system and make sure you have facts and figures to back
up your project planning. Get the recorded weather data (wet
bulb and dry bulb) from the area where you intend to do the
installation. “Verify your hypothesis with sound facts. Cross-
check your work with existing operations if possible.”
Everyone recognises the importance of not looking simply
at initial or CAPEX costs and also looking at what the expected
payback period of the project is. A more costly installation
might save significantly on running cost (energy consumed
to operate). “The ratio of the installation cost vs. operating
RACA Journal I June 2019
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