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November 10, 2015
Dr. Claude Boyd on Paddlewheel Aeration
Shrimp News: I’ve removed some of the technical aspects from this long, Global Aquaculture Advocatearticle on aeration, but you can find them all by clicking on the link in the source below.
Dr. Boyd Writes: During a recent visit to a shrimp-farming area in Thailand, I saw ponds aerated at 24-36 horsepower per hectare (hp/ha), requiring 18-27 kilowatts of electricity (kW/ha). Aerators in Thailand are often operated for 20 hours a day. At a farm with 24 hp/ha of aeration and a 100-day crop, about 36,000 kW per hour of electricity would be used for aeration.
Shrimp production for successful crops of 14 to 18-gram shrimp was reported to be around 7,000-9,000 kilograms per hectare. Electricity costs about $0.10 per kW per hour in Thailand. Thus, electricity costs $0.41 to $0.53 per kilogram of shrimp produced!
Much improvement in aeration efficiency could be achieved by modifying the designs of Asian paddlewheel aerators through adoption of paddle and paddlewheel design features already shown to be efficient through testing.
In the late 1980s, an effort was initiated at Auburn University to develop an efficient aerator for channel catfish ponds. Several types of commercially available aerators were tested, and we discovered that the paddlewheel aerator had the most potential for improvement.
A device was fabricated that allowed the testing of an array of paddle shapes, paddlewheel diameters, numbers of paddles per row around the aerator hub, paddlewheel speeds, paddle depths, paddle positioning on the hub and the amount of power input necessary for each combination.
The optimum operating conditions established in the tests varied with aerator size, but a 10-hp paddlewheel with multiple rows of staggered paddles has become the standard aerator for catfish farming. This paddlewheel usually is operated at 80 to 90 rpm with paddle depth submergence around 8 to 12 centimeters. Apparently, because of its greater initial cost, this type of aerator is not commonly used in Asia.
The paddles of Asian paddlewheel aerators have many holes in them, reducing the amount of water splashed into the air, where it can absorb oxygen. There are typically six or eight paddles per row on Asian aerators, while tests have revealed that four paddles per row are more efficient.
Another problem with many paddlewheel aerators used in shrimp ponds in Asia is that the power of the electric motor or diesel engine used to power the devices often is not well matched with the load imposed by the rotating paddlewheel. The mismatch of power units and paddlewheels is particularly evident for the long-arm aerators used in Thailand.
Because aeration is a major expense on shrimp farms, it makes good sense to improve aeration efficiency and lessen costs. It also is important to note that it is usually more costly to use diesel-powered aerators than electric ones.
Aeration often is not applied at high enough rates to prevent dissolved-oxygen concentrations from falling low enough to stress shrimp at night. Conversely, excessive aeration may be used in the daytime, when dissolved-oxygen concentrations usually are adequate. Thus, greater effort should be devoted to developing better operational strategies for aerators.
Those involved in aquaculture eco-label certification should consider including the efficiency of aeration in their standards.
I will provide the procedure for measuring the standard aeration efficiency (SAE) of mechanical aerators to anyone interested in conducting aerator performance tests.
Source: The Global Aquaculture Advocate (The Global Magazine for Farmed Seafood). Editor, Darryl Jory (firstname.lastname@example.org). Efficiency of Mechanical Aeration. Claude E. Boyd (email email@example.com, School of Fisheries, Aquaculture and Aquatic Sciences, Auburn University, Auburn, Alabama 36830, USA). Posted Online on November 5, 2015.
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