Australia is home to the world’s first multifunction, two-stage transcritical CO2 system. Klaas Visser explains that while the project has been a huge success, there were some challenges on the way.
Food processing company Exquisite made the decision to install a refrigeration plant to replace 22 independent systems at its food processing facility in Melbourne, which manufactures high-end frozen dairy desserts.
The decision was supported by a 50 per cent grant from AusIndustry under the ReTooling for Climate Change program.
Initially, the company considered a CO2-ammonia cascade plant but with residential properties bordering the site, a decision was made not to use ammonia. There was also a potential problem with plant noise.
The new two-stage transcritical CO2 refrigeration plant is used to carry out all of the required blast freezing and cold and chilled process water cooling.
The system heats all potable tap water for sanitary and factory cleaning purposes.
Process hot water is partially generated to provide AC reheat and space heating for the office and factory.
This is in addition to underfloor heating provided via a secondary ethylene glycol circuit for the door jamb heating of two large cold store and three blast freezer doors.
The plant includes nine Bitzer CO2 compressors comprising two AC and three high-stage transcritical compressors, plus one common standby and three boosters including one standby.
All compressors have VSD drives, allowing the machines to run between 30 and 60 Hertz speeds, delivering 60 to 120 per cent capacity of the 50 Hertz ratings.
The AC transcritical compressors are used as water heating machines because of their high COP at +5°C saturated suction temperature conditions.
The three high-stage machines are also able to run in a transcritical condition to heat water when required.
The six AC and high-stage compressors can also discharge to an oversized two-stage Guentner gas cooler, which is adiabatically assisted on the second stage to achieve a low gas cooler exit temperature when running in transcritical mode during hot weather.
When not heating water the machine runs in subcritical mode at as low a condensing temperature as possible, limited by the compressor manufacturer’s requirements to achieve high operating efficiency.
Visser says one of the most critical parts of the design is oil management.
Each of the six transcritical compressors is equipped with an oil separator while the three boosters share one unit.
He says two refrigerant heated with electrical assistance oil stills are also employed because no oil separator is 100 per cent effective.
“We found this to our great chagrin with oil consumption being 200 times as great as specified; this caused substantial delays,” Visser says.
Despite swift and decisive action taken by Temprite, HB Products and Bitzer Australia to resolve the problem, Visser says there are still areas where oil is causing intermittent problems.
“They are in line for rectification and will be fixed.”
Problems were also experienced with the control of flash gas between the +10 and +5°C and the +5 and –10°C vessels.
Visser says the refrigerant heated oil still from the –5°C vessel is totally ineffective, but the suction trap in the –40°C booster suction collects all system oil effectively.
A –5°C liquid CO2 coil in the –40°C acts as a liquid subcooling coil in the liquid feed to the –40°C evaporators when boiling off liquid carried CO2 over from the evaporators.
He says the new system comprises four individual systems for blast freezing and cold storage, one of each, and two chillers.
Then there is one independent chilled water system, one evaporative cooler used for factory cooling, four reverse cycle office AC units, six air to water heat pumps, three gas-fired mains pressure hot water systems and four electrical underfloor and freezer door circuits.
Calculated savings amount to a 33 per cent reduction in electrical energy consumption, a 60 per cent drop in natural gas consumption, a 44 per cent fall in direct and indirect global warming emissions and a 40 per cent reduction in cooling water consumption.
“The big challenge is achieving high efficiency operations at a relatively large plant with low production levels, which are only 25 per cent of design levels,” he says.
“That is the reason for the liberal use of variable speed drives on the compressors and the fans on the gas coolers, in the blast freezer and in the office and factory cooling air handling units.
“The electrical energy savings are understated by 25 per cent for that reason.”