Alchemists of ancient times were occupied with finding ways to change metals primarily. Their most well-known endeavour is the effort to find a way to turn lead into gold. More modern chemistry has established that this is not possible. There are still many endeavours that include an element of turning metal into gold today. Or, in the case of heat recovery, turning grade 2 titanium and steel into lower energy costs to create more valuable lobster, crab, oysters, or finfish in RAS for example.
Well-designed heat recovery in RAS is a method of transferring heat to/from a waste stream into an input stream such as ambient water or air being introduced to the system.
For example, if a tank is being kept at 21°C and the ambient water supply is 1°C, there is a delta T of 20°C that the heating system must overcome. Using heat recovery, that delta T can be reduced to 2 °C - a 10x reduction in energy consumption for that input! How does it work? Once a tank is full of water, if you add 1 LPM of water, there will be 1LPM of effluent. This effluent is at the tank temperature (21°C). Using a heat exchanger (requires zero energy) the heat can be stripped from the effluent and used to heat the 1°C ambient new water up to 19°C, for example. In many cases, the heat recovery energy (passive) can actually be as large or larger than the active heating system input!
Because of the significant impact on the energy consumption, these systems are best left to subject matter experts such as aquaculture engineers who specialize in them. There are three 'secrets' to doing it right. Firstly, for best results, the influent and effluent flow should match exactly... typically not a simple thing to do. Secondly, plate exchangers are most commonly used in this application and are notorious for fouling to the point of dropping from 80-90% heat recovery to less than 10% in a matter of days. Solutions exist for this and pay for themselves in a matter of weeks. Thirdly, plate exchanger design is a whole field itself. Many 'practitioners' just blindly go around oversizing plate exchangers and selecting other features/benefits like improper materials or improper Reynolds' numbers that often create more problems and energy consumption.
For a heat recovery system that is sure to save you money on your energy bill and in almost all cases where such programs exist, will garner you an energy efficiency incentive, contact APS. We design, supply, install, and maintain many dozens of these each year in seawater RAS operations.
