by M. E. Esmaeili, aquaculture engineer and shrimp farmer, AFTM Ltd., Iran
Venturi aeration is a very simple system for the aeration and circulation of small ponds with high performance. Let us have a view of such a device with the related calculations simply.
Basic equations
When a fluid passes through a pipe with a diameter reduction, the velocity will be raised and the pressure come down based on the Bernoulli Equation. When we reduce the diameter of a part of a pipe, the water velocity rises, and the pressure comes down. It is known as Venturi effect. (Figure 1-4)
How to measure the parameters in a Venturi
We may use the equations shown In Figure 5 to calculate discharge in relation to Venturi diameter and water velocity. The pressure drop in the middle of the Venturi can be found using the equation in Figure 6. The amount of (P1-P2) is the pressure difference and shall be considered as the suction pressure. This negative pressure is the source of air sucking into the Venturi. You do not need any blower to send air bubbles into the water, but the Venturi can do it as a side effect while running. Simply, Venturi works as a water jet to move the water and as a bubbler aerator. Two very important functions in a single simple device: Aeration + Circulation.
Aeration by Venturi
Venturi suck air, mix with the moving water and give a jet of water and bubbles to the pool. It can inject pure gas too. No need for a blower or other bubbler machine.Water dissolved oxygen will be increased by adding small bubbles into the water column.By adding the depth of Venturi, the oxygenation will not be increased accordingly.A small bubble, suppose 2 mm in diameter, has a vertical velocity in the water column at 20-30 cm/sec. Most oxygen transactions will be in the first 2 seconds.The most efficient depth of Venturi is 50-60 cm under the water surface.It is better to keep Venturi at 20-30 cm above the bottom of the tank.Do not install a Venturi less than 50 cm from the water surface level, as the efficiency comes down. Depth, more than 60 cm is allowed, just the efficiency of aeration will come down but the aeration and current creation at the lower layers will be increased.
Water circulation by Venturi
Water movement is the second performance of the Venturi jet. Circulation is important for the tank to make water parameters uniform. Water circulation can accumulate the wastes in the tank center to be drained easily. Continuously draining the wastes reduces the oxygen consumption of the pond and reduces the risk of diseases. Dual drain is very essential for separating wastes from clean water.
Advised notes for water circulation
- Tanks: D/h 5:1 to 10:1, D: Tank diameter, h: Tank height
- Water velocity: 0.5-2.0 x fish length/sec
- For removing wastes: 15 to 30 cm/sec
- Central drain: 0.1 lit/sec/m2
- Tank water exchange every 30-60 minutes
- Water must rotate around the axis of a culture tank once in 60-90 seconds
- Vr = (QNVJ/125Dh)0.5 (for round tank)*
- Vr: Side velocity of rotating water, Q: Discharge of each Venturi
- N: Number of Venturies VJ: Jet velocity
(*: this equation is from the author as a rough calculation)
Venturi in comparison with other aerators
- It has a higher Standard Aerator Efficiency (SAE), in comparison with other aerators
- Good water current
- Low price, simple installation and operation
- Easy connection with pumps and pipes
- No need for special equipment, only pump and pipe
you should follow the hydraulic and fluid mechanics rules.)
Published in the October 2024 issue of International Aquafeed.
