Location: Warmwater Aquaculture Research Unit
Title: Pumping performance of a slow-rotating paddlewheel for split-ponds Authors
Submitted to: NWAC (National Warmwater Aquaculture Center) Aquaculture Newsletter
Publication Type: Abstract Only
Publication Acceptance Date: November 4, 2013
Publication Date: N/A
Technical Abstract: Commercial catfish farmers are intensifying production by retrofitting ponds with variations of the partitioned aquaculture system. The split-pond system is the most common variation used commercially. The split-pond consists of a small fish-holding basin connected to a waste treatment lagoon by two conduits. Water is circulated between the two basins to remove fish waste and provide oxygenated water to the fish-holding basin Although much research has been devoted to algal and fish production dynamics in variations of the partitioned aquaculture system, little information is available on basic engineering considerations for devices to circulate water in these systems. This study determined performance characteristics for a slow-turning paddlewheel (SRP), which is one type of pump used to circulate water in split-ponds. We evaluated relationships among power input, rotational speed, and water flow rate. This study was conducted in 2012 at the Thad Cochran National Warmwater Aquaculture Center, Stoneville, Mississippi. The split-pond system consisted of a fish-holding basin (0.15 acres; 4.9-foot average water depth), a waste-treatment lagoon (0.55 acres; 3.0-foot average water depth), two open channels connecting the two basins, and one SRP pump. Channels had concrete foundations and cinder-block walls. The channel fitted with the SRP pump was 16.1-feet-long x 10.3-feet-wide and the other channel was 10.3-feet long x 10.0-feet wide. Both channels had a total wall height of 6.0 feet and an average water depth of approximately 4.0 feet. No fish were present in the system during this study. The SRP pump was powered by a 5.0-hp, 3-phase electric motor (Blador, Fort Smith, Arizona) that had a rated rotational speed of 1,750 rpm. A combination of gear drives and sprockets reduced rotational speed to approximately 2.94 rpm at 30 Hz. A variable frequency drive was installed to allow control of the paddlewheel rotational speed. The actual power supplied to the electric motor was obtained directly from the variable frequency drive. The estimated power required for maintaining flow in channels is due to friction head loss and was calculated using a serious of equations. Water flow rate at the four rotational speeds (1.0, 2.0, 3.0, and 4.0 rpm) was measured and ranged from 4,548 to 9,330 gallons/minute (gpm) with a measured power input of 0.11 to 1.80 hp (Figure 1). Measured power input was greater than the estimated power requirement at all water flow rates. This is likely due to mechanical losses associated with the gear drives and the sprocket and chain assembly. When rotational speed of the SRP was accelerated, water flow rate increased (Figure 2). However, there was a dramatic decrease in water discharge per unit power input (gpm per hp, a measure of pumping efficiency) as rotational speed increased. For example, at 1.0 rpm the water discharge per unit power input was 40,729 gpm per hp as compared to 10,749 gpm per hp at 4.0 rpm. The amount of power required to circulate water in a split-pond with a SRP pump depends on the maximum water flow rate required by the system which is a function of paddlewheel size and rotational speed. Because SRPs operate for long periods in the split-pond (12-18 hours per day during midsummer), design should account for the decreased pumping efficiency as rotational speed increases. That is, SRP pumps should be sized to achieve targeted flow rates using rotational speeds of 1.0 to 2.0 rpm rather than attempting to use an under-sized device at very high rotational speeds. Elevated rotational speed for this particular unit increased torque and field observations indicated that the SRP pump we tested should not operate above 2.0 rpm for extended periods of time to minimize the likelihood of paddlewheel cavitation, shaft torque surge, and reduced operational life. In addition, correct design of SRPs should be taken into consideration to reduce the possibility of mechanical failure. In summary, SRP pumps operated at 1.0 to 2.0 rpm are highly efficient, although efficiency decreases dramatically as rotational speed increases. The SRP that we tested, with a rotational speed of 1.0 rpm, will circulate approximately 4,500 gal/min at an annual operating expense of approximately $30 if electricity costs 12 cents per kWh. This flow rate will support approximately 23,000 pounds of catfish. Information in this study can be used to design SRP pumps for split-ponds and to adjust water flows throughout the season by using variable-speed motors to change rotational speed and flow rates in response to fish growth. Long-term studies are underway to compare operational issues and costs associated with the use of various pump types in split-pond aquaculture systems.