Power Consumption
Power Consumption for single phase system ( Liquid Phase )
The cost of a process depends on the power consumption. In order to ensure an optimum productivity, energy consumption must be optimized. Power consumption is related to fluid density ρ, fluid viscosity µ, rotational speed N and impeller diameter D by following formulae
P = NpρN3D5
Where, Np is the power number and mainly depends upon impeller design, number of impellers and and impeller Reynolds Number, The value of Np decreases with Re in the entire range of Re ,whereas in the presence of baffles, the value of Np remains constant when Re exceeds critial Reynold’s Number. Under these conditions, the power number depend upon impeller design.
Power number data for different type of impellers, under a give set of conditions are documented in technical literature. Typical values of power number are as under:
- Axial flow impellers such as propellers, hydrofoils : 0.3 < Np < 0.75
- Mixed flow impellers such as paddles, pitched blade turbines : 0.75 < Np < 3.0
- Radial flow impellers such as Rushton turbines : 3.0 < Np < 6.
Power Number for various impeller types are as shwon in following figure.
Power Consumption for Gas-Liquid Agitated Vessel
When gas is present, power consumption is given by following formulae
P = NpρDN3D5
where density of dispersion is given by ρD = ρLεL + ρgεg
Above Image shows various Regimes of operation in a gas-liquid agitated vessel with a pitched-blade downflow turbine with sparger placed below the impeller.
The sparger introduces the gas into the liquid in a vessel, Spargers are located at the bottom of the vessel and consist of a tube with tiny holes for the gas to escape through into the culture. The gas coming out of the sparger helps to aerate and mix the contents in the vessel.
The basic types of sparger:
1. Porous sparger
2. Nozzle sparger
3. Ring sparger
4. Pipe sparger
5. Concentric ring sparger
6. Spider sparger
