FEATURE
INCORPORATING COLD CHAIN
a secondary coolant, limiting the R290
charge to the plant room area.
“In Europe, large R290 systems offer
a relatively simple alterative for MT
applications and it seems set to increase
significantly in future,” says La Grange.
A concern in our local market is that
R22 (an HCFC) is still almost half the price
of most HFCs in spite of the fact that it is a
refrigerant with an ODP and illegal to use
in new installations, says La Grange. One
factor seems to be the very low purchase
cost of R22 from the manufacturers in the
east by refrigerant importers.
REGULATIONS
Ammonia refrigeration plants are common options for a cold store, but come with strict
regulations and requirements in terms of health and safety.
The type of product being stored will define the requirements of the refrigeration system.
efficient in future (providing this was not
originally addressed during design and
commissioning),” he says.
Large DC or storage facilities have
traditionally often been R717 (ammonia)
installations, says La Grange. “Ammonia
as a refrigerant has few equals.”
Ammonia has no ozone depleting
potential (ODP) and a global warming
potential (GWP) of 0. With its excellent
thermodynamic properties, it makes
for a very energy efficient solution that
has no direct adverse effects on the
environment.
R717 solutions operating at low
temperature are at some disadvantage
in terms of COP though, says La Grange.
If you use reciprocating compressors on a
R717 system, they typically need two-stage
compression to operate at low suction
conditions, adversely affecting COP.
The MT side of the installation could
also be done using a secondary coolant.
In such cases the secondary coolant
(water and glycol mixture) is circulated
from the machinery room to the room
where the heat absorption is required.
With a good design, there is no electrical
defrost required for the air-coolers
(unlike an evaporator with an expansion
device). The saving in coil defrost energy
is significant, says La Grange. With a
secondary coolant, it is also possible to
26
modulate the cooling capacity of the air
cooler effectively. This is done with a three-
way “mixing” valve, meaning no defrost is
required in many cases.
Some large DC/industrial applications
favour R717/R744 (CO 2 ) cascade solutions.
The R744 refrigerant is a very good
alternative for LT applications and the
R717 has a good COP at MT conditions,
explains La Grange. The R744 systems heat
rejection forms part of the R717 system
heat load. In these cases, the R717 system
is always required to operate for the R744
system to be operational. That is suitable
in most cases, but it could be a limiting
factor in others.
In retail applications, the R717/R744
cascade solution uses a secondary
coolant circulated to take care of the
MT load, explains La Grange. That way,
the R717 system has a low charge and
the charge is contained in a machinery
room with all the required safety measures
in place. With the R744 system being
sub-critical, it functions very similar to an
HFC system but for the higher operating
pressure.
A cascade system with hydrocarbons
(HC) like R290 and R744 also offers many
advantages. In such cases, the R290
system is less costly than the R717 to take
care of the MT load, according to La
Grange. The MT side could be done with
You can’t simply build a cold store
whichever way you’d like. There are
various regulations that need to be
followed to obtain an Occupation
Certificate for a building which
insurance companies are starting to
demand on new build projects, explains
Van der Merwe.
Some of these regulations include:
• SANS 10400 (structural steelwork,
building works, building efficiency,
and fire engineering)
• SANS 10147, SANS 347 (refrigeration
installations)
• SANS 10142 (electrical installations).
There are also various product-dependent
regulations, explains Ford. For example,
pharmaceuticals and flammable
products must adhere to strict health and
safety regulations, including that electrical
work must have a certain amount of
protection to avoid sparks should a
malfunction occur.
Temperature logging and control is
also key to ensure the product doesn’t
go off, making temperature monitoring
(especially for long term storage) very
important.
Also, shelving comes with its own
regulations, and weight and volume need
to be taken into consideration so as not
to affect flows and even temperature
throughout the cooling space.
INSTALLATION DO’S AND DON’TS
So, you’ve selected your system – now it’s
time for the installation. La Grange says it’s
important to look at more than just initial
cost. “Before considering your options,
apply some business acumen.” What
would the expected payback period of
the project be? In most cases the project
is financed and the operation would
need to recover sufficient benefit from the
installation in a short enough time period
for the project to be financially viable.
A more costly installation might save
significantly on running cost (energy
consumed to operate) though, he
says. The ratio of the installation cost vs.
operating cost vs. maintenance cost is
becoming more widely used to compare
installations.
Use well-established guidelines like
the Total Equivalent Warming Impact
(TEWI) to compare alternatives, advises
La Grange. This approach factors in
direct as well as indirect global warming
www.coldlinkafrica.co.za
factors. With a lower index total, the
installation is also going to have a lower
running cost. Should the energy supply
be complemented with a renewable
option like solar, the TEWI index could be
significantly reduced.
“Another crucial factor to your project
is to ensure your heat load calculations
are correct,” says La Grange. “Few things
can save or waste more energy than
heat load calculations that don’t consider
all relevant factors.” For instance, what
would the potential payback period be
of using better, but more costly insulation?
Few consider that.
Size also matters – especially when
it comes to the refrigeration plant. If
you are too conservative and the plant
ends up being oversized, you might
adversely affect the viability of the
project, explains La Grange. Too small
and your customer’s room will struggle
to reach operating set point during the
warmer ambient periods. A system that is
oversized for the application might end
up being less reliable when it is required
to operate a part load conditions in the
winter. Compressors are designed to
operate for prolonged periods of time, so
at part load conditions they could short
cycle (stop and start regularly) resulting
in a shortened life cycle of a very costly
piece of equipment.
Before getting started, La Grange also
advises that you 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. That
way your condenser or evaporative
condenser selection process for the
site is informed properly. “Verify your
hypothesis with sound facts. Cross-check
your work with existing operations if
possible. Good case study principles
need to be applied.”
Van der Merwe emphasises the
importance of looking at the total cost of
ownership of a refrigeration system with
these tips:
• Budget for regular maintenance.
• Consider installing mobile racking
to ensure that the size of cold store
is minimised for the pallet holding
capacity with the subsequent
reduced running costs associated
with a smaller cold store.
• Be careful of going for the lowest
cost solution without considering the
possible differences in equipment
capacities/specifications and the
effect on the running cost. The short-
term benefit of a low first cost will be
quickly overcome by high running
costs and poor plant performance or
production or product losses.
• Consider the contractor’s track
record and the calibre of clients that
they serve regularly.
Ford advises:
• Either you know how to do it or you
don’t – don’t overcommit.
COLD LINK AFRICA • July/August 2019