Paddlewheel aerators:
Paddlewheel aerators are the most common types used in large ponds. Paddlewheels consist of a hub with paddles attached in a staggered arrangement. The aerator is powered by a tractor power take-off (PTO), self-contained diesel or gas engine, or electric motor.
Electric paddlewheel aerators are usually mounted on floats and anchored to the pond bank. There are many different designs for PTO-driven paddlewheels (Figs. 1 and 2). Gearboxes and automobile differentials have been used for gear reduction. Paddlewheel diameters (paddle tip to tip) range from 2 to 4 feet; paddles are 2 to 10 inches wide and may be rectangular, triangular or semicircular (concave) in cross section.
Paddles are welded to the hub in various spiraled or staggered arrangements. Large-diameter paddlewheels transfer more oxygen than smaller diameter aerators, and flat paddles are less effective than other designs. For a given design, oxygen transfer can be increased by increasing paddle depth and hub rotation speed. Increased diameter, paddle depth and speed also increase the power required for operation.
Tractor PTO-powered paddlewheels can have high SOTRs— 90 pounds O2/hour or more. Thus, they are particularly useful in emergencies. However, tractors develop more power than is applied to the PTO and aerator drive shaft, and considerable energy is lost through the drive train, so PTO-driven paddlewheels are not particularly energy efficient.
But they are more flexible than other aerator designs. Because they are portable, they can be moved from pond to pond as needed and placed anywhere along the bank.
The aerator also can be operated in different modes, depending upon needs. For general pond aeration, PTO driven paddlewheels are usually operated with paddles submerged 3 to 4 inches in the water. Paddle depth can be increased to 5 to 6 inches for greater oxygen transfer, but this increases fuel consumption and produces a stronger water current, which forces fish to expend more energy (and consume more oxygen) when swimming behind the aerator.
Nevertheless, the increased paddle depth may be needed when maximum oxygen transfer is important during acute dissolved oxygen depletions. Depending on their diameter, PTO-driven paddlewheel aerators require 15 to 30 hp to operate at paddle depths of 3 to 6 inches.
Tractors with PTO power ratings of 45 to 60 hp are typically used as the power source. Larger tractors may be needed to move aerators around on farms with steep, eroded pond levees, but large tractors are an inefficient power source because most of the fuel consumed is used to run the engine and power train rather than to turn the paddlewheels.
When used at paddle depths of 3 to 6 inches and driven by a trac tor with a PTO power rating of 45 to 60 hp, most PTO-driven paddlewheels perform best when the tractor is operated at about half throttle (1,200 to 1,500 rpm engine speed). If the tractor has a standard 540-rpm PTO shaft and the aerator gear reduction is about 6 to 1, the PTO shaft speed will be about 500 rpm and the paddlewheel speed 80 to 90 rpm.
Tractor PTO-driven paddlewheels also can be operated with paddles almost fully submerged to mix ponds or provide a current of oxygenated water to fish held at high densities in harvest socks. When used in this manner, the tractor is operated just above idle speed (300 to 400 rpm engine speed) so that the paddlewheel speed is 20 to 30 rpm.
Some PTO-driven paddlewheels, called sidewinders (Figs. 3 and 4), use an in-line pinion-and-bullgear system to reduce PTO shaft rotation speed. Benefits of sidewinders are that they are durable and produce a current of oxygenated water parallel to the pond bank where stressed fish congregate. Sidewinders also are commonly used to produce a current of oxygenated water to sustain fish concentrated in harvest socks.
Personnel who operate PTO-driven aerators should be trained in placing and using them properly to ensure safe operation, optimal usage in critical situations, and long life of the equipment.For routine, everyday use—where efficiency is important—most culturists prefer paddlewheel aerators powered by electric motors (Figs. 5 and 6). Electric paddlewheels used on commercial catfish farms are usually powered by 10- to 15-hp motors and have hubs 10 to 15 feet long.
Electric paddlewheels are permanently anchored in each pond and individually controlled by switches on the pond bank. On some commercial aerators the paddle depth can be adjusted for optimum energy use and performance of the motor.
The motor should draw about 90 percent of the full load amperage rating to provide maximum oxygen transfer and extend motor life. Electric paddlewheel design was systematically studied by Ahmad and Boyd (1988), who found that the best design consists of a 3-foot diameter paddlewheel with paddles that are triangular (135-degree interior angle) in cross section.
Paddles are about 4 to 6 inches wide, with four paddles attached per row and spiraled in a staggered arrangement around the hub. Paddle depth is 4 to 6 inches. Paddlewheel speed should be about 90 rpm. Commercial designs similar to that proposed by Ahmad and Boyd have SAE values of 4.5 to 5.5 pounds O2/hp?hour, which is very good for surface aerators.
Pump-sprayer aerators:
Pump-sprayer aerators have pumps that discharge water at high velocity through pipes or manifolds. Pumps may be powered by the PTO of a tractor (Fig. 7) or by an electric motor. Pump-sprayers are simple and require little maintenance. Pump-sprayers have a wide range of effectiveness and efficiency.
Those driven by electric motors have SAE values of 1.5 to 3.5 pounds O2/hp?hour. Pump-sprayers powered by tractor PTOs have lower SAE values than electric aerators, but may have very high SOTR values (up to 160 pounds O2/hour). Aerators with higher SOTR values usually require large tractors (90 PTO hp, or more) or that the PTO be operated at a high speed (up to 1,000 rpm).
Vertical pump aerators:
A vertical pump aerator consists of a submersible motor with an impeller attached to the output shaft. The motor and impeller are suspended beneath a float, and water is sprayed into the air through an opening in the center of the float (Fig. 8). Vertical pump aerators can be relatively efficient— SAE values usually range from 2 to 4 pounds O2/hp?hour— but most vertical pump aerators manufactured for aquaculture have relatively small motors (usually less than 1 hp) and do not produce a large area of oxygenated water.
This limits their use to ponds of less than 1 acre, where they can be quite effective.
Diffusers or bubblers:
These systems use blowers or compressors to supply air to diffusers. The diffusers have many small pores that release bubbles on the pond bottom. Oxygen is transferred as the bubbles rise through the water column.
Diffusers for large-scale aeration are usually discs, plates or tubes constructed of glass-bonded silica, ceramic, porous plastic, or flexible perforated membranes. Diffusers are customarily arranged in a grid pattern over the bottom of the pond, with the number of individual diffusers determined by the oxygen transfer rate of the diffuser and the oxygen consumption rate in the water. Oxygen transfer increases with smaller bubble size, deeper bubble release point, and higher oxygen content in the bubbles.
Diffused aeration is common in wastewater treatment, where basins 15 to 30 feet deep can be constructed to optimize oxygen transfer. When bubbles are released in deep water, hydrostatic pressure from the overlying water increases the saturation dissolved oxygen concentration, so that for any value of ambient dissolved oxygen, the saturation deficit is increased compared to conditions at the water surface.
Deep water also creates a long contact time between bubble and water, so that more of the oxygen in the bubble is transferred to the water before the bubble reaches the surface. Aeration efficiencies are also high if very small bubbles are produced because they have a higher ratio of total surface area to water volume than large bubbles. Fine-bubble diffusers operated in deep water can have very high SAE values— some over 15 pounds O2/hp?hour.
Despite the potential for high SAE values, diffusers are seldom used in aquaculture ponds. In shallow ponds, diffusers are relatively inefficient because bubbles ascend to the surface too quickly for effective oxygen transfer. At diffuser depths of about 3 to 4 feet, SAE values of most diffusers are about 1 to 3 pounds O2/hp?hour.
Fine-pore diffusers operated at low airflow rates are more efficient, but these systems foul (clog) easily and must be cleaned often to keep them working properly. In addition to these problems, most culturists dislike diffused aeration systems because the network of supply lines and diffusers interferes with fish harvest.
Diffusers that use pure oxygen in the gas phase can have high oxygen- transfer rates, but operating costs are too high for routine aeration of large ponds. Pure-oxygen systems may, however, have important specialty uses in ponds, such as providing oxygen to fish held at high densities in live cars or socks during harvest (Torrans et al., 2003).
Author:
Craig Tucker
