PROJECT
INCORPORATING COLD CHAIN
reclaim. The medium-temperature cycle
will operate in a trans-critical manner in
high-ambient temperatures where the
condensing pressure is above 72 bar or
31°C. When operating in this state, the
refrigerant is unable to condense into a
liquid. Below a condensing temperature
of 31°C, the medium-temperature cycle
will operate in a subcritical manner; this
means that the refrigerant will condense
to a liquid and the discharge pressure will
vary between 55 and 70 bar. The head
pressure varies according to ambient
temperature and is controlled via the
Danfoss controllers installed.
When the system is unable to condense
below 72 bar and to condense CO 2 to a
liquid, a high-pressure expansion valve is
used. In the case of Danfoss, the device is
known as the ICMT valve. The pressure in
the gas cooler leaving as well as the outlet
temperature are calculated to vary the
opening of the ICMT. The pressure in the
receiver is maintained at 38 bar, or 3.3°C
where CO 2 exists as a liquid.
The liquid is then fed to the evaporator
coils where direct expansion occurs.
The medium-, low-, and intermediate-
temperature cycles are equipped with a
common oil management system, which
ensures that the refrigeration compressors
stay lubricated. The system is of a closed
loop configuration type and only requires
maintenance should contamination occur.
The oil management system has built-in
safeties that automatically switch the
compressors off when it senses low or no
oil levels and automatically resets once
minimum oil level has returned.
All switch gear for the packs are situated in
the main board within the plant room and
within the pack boards that are situated on
the packs themselves.
All controls that operate the evaporator
coils in the store are situated within the
store at each individual coil respectively.
Each compressor is fitted with a manual
overload and thermistor, which will protect
the compressor electrically; should this
fail, it usually indicates a faulty compressor
and a technician will be required before
resetting.
The heat reclaim to supply hot water
can be connected through the discharge
of the medium-temperature cycle and
is piped in such a manner that the heat
exchanger could be bypassed with the
help of a three-way ball valve. A signal is
received when heating is required and in
turn, this opens the three-way ball valve so
that hot water is generated and supplied.
The hot water can be controlled to a
temperature close to boiling point.
The medium-, low-, and intermediate-
temperature racks are fitted with low- and
high-pressure safety switches, which will
stop the cycle if the operating pressures
are out of the desired set-points. Should the
physical safeties trip, a technician will be
required to manually reset and check that
the system is safe to operate.
Due to the high operating pressures of
CO 2 , the pack controller is set up so that
it will stop certain compressors should the
discharge pressure and/or temperature
of the cycle exceed the safety limits and
eventually, it will stop the whole cycle
should the problem not resolve itself.
Bitzer compressors were used in the CO 2 refrigeration packs.
Hot water exchanger.
Woolworths Kyalami chose to go with CO 2 for their refrigeration needs.
The controllers are fitted with pressure
transducers, which the controller reads to
calculate the operating parameters.
When the pressures are within the
desired range, the cycle will start up
automatically.
Every pack and its specific controllers are
connected to a main safety circuit that will
stop all refrigeration completely if none of
the compressors are safe to run. The safety
circuit will be de-energised, preventing
migration of the refrigerant. With CO 2 , this
can occur in a very short period and can
cause excessive pressures, relief valve
blow-offs, and very high start-up loads in
the system.
Pressure relief valves are fitted on
critical main pipelines and receiver to
relieve pressure when necessary; this is to
ensure the safety of the valves and the
components fitted in the refrigeration cycle.
CO 2 sensors are installed in the plant room
and on the floor area, which will detect
a leak and sound an alarm if the CO 2
concentration gets too high. The sensor in
the plant room will activate an extraction
fan when the concentration of CO 2 within
the plant room gets too high. CLA
List of professionals
Owner Woolworths
Developer Iguana
Architect / Designer R&L
Project manager
Consulting engineers
Contractors
Product suppliers
40
Woolworths
Electrical RWP
Mechanical CRS | WSP
Wet services and civil Iguana
Main building Iguana
HVAC&R Iguana
Wet services Iguana
Electrical RWP
Refrigeration CRS
Refrigeration plant CRS
Condensers HC Heat
Compressors Bitzer
Electronics Danfoss
Underfloor heating Pioneer Floor Heating
A Danfoss system manager, case, and pack controllers have been used
to ensure that the refrigeration cycle is continuously run at optimum
performance.
www.coldlinkafrica.co.za
COLD LINK AFRICA • January/February 2019