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DeepChill® vs. RSW

How does the cooling compare?

Refrigerated Seawater (RSW) is a common method of chilling fish onboard fishing vessels. For pelagic fish that is caught in large quantities in a short period of time, RSW provides better turbulence and more uniform cooling compared to flake ice. The question is however, is there a better way?

DeepChill® provides similar operational advantages to RSW such as ease of pumping and distribution; simply put, DeepChill® flows like water! The distinction however, comes from the cooling power.

A study by Slurry Ice Centre in the Danish Technological Institute shows that cooling rate of DeepChill® Slurry Ice is 3-7 times higher than that of [single phase] chilled brine. As shown in the diagram, with increase of ice concentration, the Heat Transfer Coefficient and therefore the Rate of cooling increase.

In a nutshell, DeepChill® slurry ice cools a product at a much higher rate than Chilled seawater. Moreover, DeepChill® with a higher crystal concentration carries a higher cooling capacity.

Rate of Heat Transfer = Heat Transfer Coefficient x Surface area x Temperature difference

heat-transfer-coefficient-img

The fundamental difference lies in energy accumulation and storage. Before a fishing vessel reaches fishing grounds, all the cooling energy required is accumulated in DeepChill®. Once fish is caught, it can be immediately cooled with DeepChill®. RSW, on the other hand, is unable to accumulate energy and will only start cooling once fish is caught and already deposited to the fish hold.

This distinction in cooling is especially more noticeable in low turbulence environments. Naturally, RSW requires high agitation and circulation in order to cool a product- which could cause abrasion to the fish skin. In contrast, DeepChill® has the same cooling effect in low or non-agitated settings.

Let’s take a closer look and compare chilling process of fish in RSW and slurry ice. The following diagram illustrates test results for chilling bonito fish (average weight ~ 5kg) in slurry ice and refrigerated seawater over 5 hours.

chilling-rsw-dc2-img

As you can see, water temperature rises during the process of cooling fish, thus reducing temperature difference between the fish and water, resulting in much longer cooling time.

Moreover, fish temperature can hardly reach close to 0°C unless chilled water temperature is sub-cooled to minimum -3°C. This can result in partial freezing of fish and loss of its quality.

In contrast, DeepChill® remains constant at about -2°C and continues chilling the fish until it reaches sub-zero temperatures. In this test, bonito fish in slurry ice reached 0°C in 3 hours while fish in chilled seawater was only cooled to 4°C within the same period of time.

To compensate this low cooling power, RSW systems run continuously to recirculate chilled water through the fish holds using high horsepower pumps. This is especially necessary when newly-caught warm fish is added to the fish hold, causing temperature fluctuations. While this helps with better fish cooling, it results in higher power consumption and lower energy efficiency compared to a DeepChill® system.

When choosing the right chilling system for your operation, it is important to consider all parameters. On the surface, RSW systems seem to be the obvious choice for fisheries. With easy operation and small compressor power, RSW systems have become an integral part of many fishing vessels. On the other hand, low cooling capacity, potential to bruise and damage fish, paired with high power requirements for a complete circulation system should not be overlooked.

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