Evaluation of methods for injecting oxygen gas
Many methods are available to inject oxygen into water, using both low and high pressure. To be able to evaluate the performance, evaluation methods are available for aerators. As pure oxygen is much more expensive than air, its utilization is very important. This is given by the oxygen transfer effi-ciency (OTE), which is also known as the absorption efficiency and is the amount of oxygen dissolved in water in relation to the amount of oxygen supplied:
Cin=concentration of oxygen before injection ofoxygen
Cout=concentration of oxygen after injection of oxygen
Qw= water flow
Mo2= mass flow of oxygen gas.
Performance values ranging from a few percentage points up to 100% have been reported.6
Another important parameter is oxygenation efficiency (OE) which is a measure of the amount of oxygen dissolved in relation to the power sup-plied. Normally excess pressure is used to achieve high oxygen concentrations in the water and for this power is necessary, for example to run a high-pressure pump. This can be described as follows:
Oxygenation efficiency = (Cin−Cout)Qw / Power supply
Units for OE are mg O2/kW.
Most important of course, is the amount of oxygen dissolved in the water in relation to the investment and running costs for the injection equipment; this is measured in mg O2 per unit cost.
It is important to remember that it is the amount of oxygen available for the fish in the production unit that is of interest. For example, the absorption efficiency directly after the injection unit, which may be very high, is of no concern; neither are values that are achieved in laboratories where all other factors are optimized in a way that is impossible to achieve under real farming conditions. Several design factors for the whole farm will have a great influence on the efficiency of the oxygenation system; these include the water transfer system and tank inlet design. No two farms will be exactly the same and this must be taken into account. Com-parison of systems is therefore not straightforward; tests must be performed under exactly the same conditions and if possible on the site where the equipment is to be used.
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