In many countries the use of R22 in new refrigeration systems is prohibited. Alternative refrigerants such as R507 and R404a are subject to very strict legislation and may be prohibited in the nearby future.
According to Grenco B.V. of the Netherlands, several Dutch fishing fleet owners have therefore decided to switch to refrigeration systems using environmentally-friendly, natural refrigerants such as Ammonia (NH3) and Carbon dioxide (CO2) which also have good thermo-dynamic properties.
CO2 has a high pressure (6.8 bar. at 50°C and 39.7 Bar at 0°C) so one needs to install a primary cooling system with another refrigerant (e.g. NH3 or R507) in order to maintain the CO2 pressures in the secondary system within limits.
CO2 offers lower and faster freezing temperatures, needs less equipment; it is safe, cheap and easily available.
The quicker freezing means that at --50 °C instead of at -38°C, the freezing time is reduced by 25%, so a 100 mm block of herring can go from +2°C to -20°C in approx. 150 minutes. So 25 per cent fewer plate freezers are needed.
W. van der Zwan B.V replaced six brine plate freezers with five CO2 plate freezers. The relatively small NH3/CO2 freezing plant was installed on their OCEAAN VII in October 2003.
Grenco also won an order in September 2003 from Parlevliet & Van der Plas for an NH3/CO2 system for their trawler "Dirk Dirk". While both vessels have similar refrigerants, the cooling principles are different. On the Ocean VII, CO2 is only used as an evaporative coolant. The Dirk Dirk will have a so-called cascade system where CO2 is used as a refrigerant, (with CO2 screw compressors in the low stage).
On the Dirk Dirk the R22/ CaCl2 brine, deep-freeze plant with 32 vertical plate freezers, with 20 or 26 stations will be replaced by a new 23 plate freezers with 26 stations each, while maintaining the same production.
On the Ocean VII, there are five vertical plate freezers with CO2 pumped through at --50 °C.
The primary ammonia (NH3) system consists of a two stage screw compressor unit and a CO2 cooler in which the evaporating temperature of the ammonia will be maintained at -53 °C. In the CO2 cooler the gaseous CO2 will be condensed at a temperature of --50 °C.
The secondary system (CO2) consists of a CO2 liquid separator, two CO2 pumps and the plate freezers. The plate freezers are defrosted with hot CO2 gas.
Cascade
On the Dirk Dirk, CO2 will be pumped through the 23 plate freezers at --50 °C . Furthermore the system is connected to the hold's cooling facility. In the cascade condenser the evaporating ammonia (at -15 °C) causes the CO2 to condens at a temperature of -10 °C.
The primary system has two compact duo-pack compressor units, each with two single-stage screw compressors and two cascade condensers. All ammonia-containing equipment is installed in a separate gas-tight engine room. The CO2 system also contains two duo-pack compressor units. Furthermore the system contains of CO2 circulating pumps, nineteen CO2 plate freezers and two CO2 / CaCl2 chillers.The CO2 / CaCl2 chillers are connected to the existing CaCl2 brine system for cooling the holds. The plate freezers are defrosted with hot CO2 gas generated by the hot discharge gases of the ammonia systems.
