Selecting the Correct Blower
TMC Fluid Systems’ Aquaculture Blowers are specially designed for aquaculture applications. These blowers deliver a constant flow rate of air, independent of the discharge pressure conditions. The flow rate is dependent, largely on the operating speed. Due to these constructional features it has the following distinct characteristics:
- The flow is largely dependent on the operating speed
- The input Power is largely dependent on the total pressure across the machine.
- The suction and Discharge pressures are determined by the system conditions.
- The temperature rise in air discharged is largely dependent on the working pressure.
Blower Selection: The right selection and installation of an air blower can result in substantial energy savings. For proper selection of blower, air volume and air pressure are the most important parameters.
Air Volume: Is the quantity of air required generally measured in m3/hr (or cfm). It is the total air requirement of the plant and depends on the plant capacity or the quantity of water to be aerated. Detailed calculations are needed to establish the required air flow rate, based on initial oxygen levels, final oxygen levels desired, BOD demand, and oxygen transfer efficiency, etc. However, as a rule of thumb, air requirement per 100-tons of water can be taken as approximately 100-m3/hr for medium pore diffusion aeration systems (i.e. using stones). Accordingly, the total air requirement can be calculated based on the total quantity of water that needs to be aerated.
Air Pressure: Is the minimum air pressure required to enable air to bubble through the water, through the immersed stones, after overcoming the total pipeline pressure drop from where the blower is mounted and installed all the way to the diffuser tank.
Power: The power consumed by the blower is directly proportional to the air pressure. Consequently, pipe sizing and installation, system resistance, pressure drop across valves and elbows and bends should be adequately addressed.
For a tank with a depth of 1.5-meters, the optimum pressure requirement should be about 2.5-psi (approximately 1,700-mmWg). A pressure gauge, installed at the discharge of the blower delivery line provides the total back pressure load on the blower. Higher back pressure indication may be due to:
- High line losses due to small diameter distribution grid / piping
- High air flow
- Choked suction filter
- Insufficient number of diffusers
A systematic step-by-step checklist can be made to establish the cause for excessive back pressure and the same should be corrected to optimize power consumption. High discharge air temperature is also an indication of excessive differential pressure across the blower.
There is no compression or change in volume within the blower. Instead, the blower works under system back pressure conditions.
To illustrate further, let us consider a case when the discharge of a blower is connected to the bottom of a tank with a water depth of ‘H’-mm. The air discharged out of the blower accumulates in the discharge line until sufficient pressure is built (slightly over ‘H’-mmWG), when it starts to escape out. The system resistance or the static load on the blower is thus ‘H’-mmWG.
The power consumed by the Blower depends upon the flow rate and the total pressure head on the Blower. A Blower is capable of resisting high pressures but the mechanical limitations arising due to increased power intake, temperature rise and increase in ‘SLIP’ restricts the working pressure head. The Blowers are generally selected for the maximum system pressure which they may encounter during operation and the prime mover is selected accordingly. When in operation, the Blower offers a considerable power saving since the power consumed by it depends upon the actual working pressure under which it operates and not the rated pressure. Consider a case when Twin Lobe Rotary Air Blower is selected for an application requiring a capacity of “Q”-m3/hr at “H”-mm of WG at which the power is specified as “P”-KW. Under the rated conditions it would consume “P”-KW, but if the system back pressure falls from the rated/design value, the Blower automatically starts working under lesser head and power requirement falls accordingly. These salient features make Rotary Air Blowers a versatile machine. They are ideal for applications requiring Constant Flow Rate at Varying discharge Pressures.