The popularity of ammonia and C02 began in the 1870s and continued through to the 1940s. During this time they were used in a wide range of freezing applications, even cooling a number of cinemas in Sydney right up to the year 1966.
Their use rapidly declined with the advent of CFCs but fast forward to the 21st century and CO2 is making a timely comeback.
Australia’s CO2 revival began with the installation of a CO2/NH3 cascade plant by Qantas.
Since then there have been a number of installations ranging from small CO2/HFC and medium size CO2/NH3 cascade systems to two transcritical CO2 plants in applications ranging from supermarkets to small food processing plants.
This article, which is based on a report by Kav Consulting and later published by AIRAH, profiles the local pioneers who paved the way for others to follow.
While Australia’s two largest supermarkets, Coles and Woolworths, have made the move to CO2 cascade systems, smaller retailers have been just as innovative.
The move by Australia’s supermarket giants was part of a broader commitment to reduce total greenhouse gas emissions and reduce refrigerant gas leakage.
The average leakage rate for most supermarket refrigeration systems is 23 per cent, according to the Australian National Greenhouse and Energy Reporting guidelines.
The rising cost of HFC refrigerants has also contributed to the revival.
By reducing the HFC charge in a supermarket cascade refrigeration system, both the cost and environmental impact are sharply reduced.
Leakage problems are removed because CO2 isn’t costly and it certainly doesn’t have a devastating impact on the environment in such small amounts.
CO2 has a global warming potential (GWP) of one versus 1000 to 3000 for commonly used HFCs.
Energy consumption is also reduced with these systems, especially if sound ammonia design practices are followed.
Compared to CO2, ammonia is a well known refrigerant so sound practices are well established and understood.
Refrigeration uses about 55 per cent of a supermarket’s electrical energy with lighting and air conditioners consuming about 20 per cent each.
Both Coles and Woolworths have a policy to retrofit natural refrigerant refrigeration plants into stores with conventional HFC refrigeration systems.
CCN understands that both supermarkets are installing an increasing number of liquid recirculation CO2 systems, which is a big step toward integrating all refrigeration and cooling functions into one plant.
Another opportunity for energy efficiency is the integration of refrigeration and air conditioning in supermarkets, especially if the rejected heat is used for reheat to reduce the dew point of the air in-store.
Using part or all of the refrigeration plant as a heat pump could provide hot water to heat the store during cooler weather and for domestic sanitary and cleaning purposes.
Another well known brand, Aldi, which has more than 200 stores in Australia, has also installed a small cascade system at one of its stores for testing and evaluation.
The earliest pioneer in this sector was SNAP Fresh in Queensland, having installed its CO2/ammonia cascade plant 10 years ago.
It features four independent single stage CO2 vapour compression systems each with a capacity of 14kW at -28°C evaporating temperature and 0°C condensing temperature.
Two cold stores are each served by two of these independent systems, providing each with a capacity of 28kW to maintain the cold storage temperature at 20°C.
Each independent system consisted of one 14kW DX evaporator, one suction accumulator, one compressor, one oil separator with automatic oil return to the compressor crank case via a manually set needle valve and a propylene glycol brine cooled CO2 cascade condenser with brine entering at -8°C after being cooled by the central ammonia plant.
Flowing from the CO2 receiver, which is located in the cold store, liquid travels to the TX valves via a filter drier and a solenoid valve.
The CO2 evaporator and its drip tray are electrically defrosted as required.
The system was relatively trouble-free after initial problems with oil return to the compressor crank case and mechanical TX valve superheat settings.
One compressor failed due to an electrical fault about one month after the plant was commissioned.
The compressor manufacturer replaced the unit under warranty and the system has performed well ever since.
One company that has pioneered the development of CO2 equipment locally is Bitzer Australia.
Bitzer has shipped more than 122 factory-assembled CO2 systems including a transcritical system, 69 direct expansion (DX) systems and 40 liquid recirculation/DX cascade systems.
In these systems the freezing is accomplished by TX valves supplying the evaporators being fed under pressure with liquid from the CO2 receiver.
A glandless pump pumps saturated CO2 liquid to the chiller evaporators where 50 to 70 per cent of the liquid CO2 is evaporated.
The wet vapour leaving the evaporator returns to the cascade condenser, where the vapour is re-condensed together with the hot CO2 gases from the discharge of the CO2 compressors.
More detailed examples of the current C02 revival appear in the October edition of CCN.
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