Increased capacity and improved quality

The 64m stern freezer trawler is one of two in the Irvin and Johnson freezer fleet which is based at Cape Town harbour.

The Boronia has an onboard plant to freeze and process its catch of hake caught off the coast of South Africa.

The main refrigeration plant has two Sabroe screw compressors, which were installed when the vessel was built in Norway in 1991. As part of a major refit, while the Boronia was in dry dock during 2013, the refrigeration plant was upgraded and additional flow-ice generators installed.

As part of the upgrade of the ammonia refrigeration plant, South African commpanyPam Refrigeration replaced the condensers, with two shell and tube sea water condensers with titanium tubes.

The accumulator was also replaced and installed at a higher level to increase the head on the ammonia circulation pumps. “The upgrade was done not only to improve the cooling efficiency of the plant but also to make it fully compliant with the Pressure Equipment Regulations (PER), which became a legal requirement October 2009”, said Evert Potgieter from Pam Refrigeration.

The two additional flow-ice packaged units each produce 5 tonne/hour of slurry of 25% ice concentration. The slurry is pumped into four receiving tanks into which the hake is placed to cool with the least delay after being caught. Pam also installed a flow-ice ice delivery and temperature management system to regulate bleeding buffer tanks in the factory to achieve the best quality fish.

The company also installed a unique HDPE header system to spray slurry ice above the fish while in the holding tanks before being processed. The addition of the slurry ice generators begins the cold chain soon after leaving the sea.

The cleaning, filleting and freezing in plate freezers is carried out onboard and the frozen product is stored in the hold at -28⁰C until discharged. A normal sea trip averages 20-30 days before the Boronia returns with its catch.

The compact flow-ice generators are each fitted with Bitzer OSKA 8561 open drive screw compressors with 75kW motors and a cooling capacity of 236kW at -9/32⁰C.

Each flow-ice generator has an ammonia charge of only 60kg and the central plant has a charge of 1,500kg. Safety features to minimise possible risks to the crew of 65 include extractor fans in the plant room plus emergency fans, multi-sensor automatic leak detection system with sensors in all areas with ammonia piping, and discharge from safety relief valves at a high level above the deck.