Thermal mass flow meters are one of many energy-saving technology solutions that can be used in a variety of applications, including the treatment of wastewater. Various types of processes are used by wastewater treatment plants to remove organic pollutants. Activated sludge systems are currently the most widely used biological treatment. In the activated sludge process, a portion of the activated sludge (frequently from the secondary clarifier) is returned to the aeration basin. Wastewater flows continuously into the aeration basin where air is injected into the wastewater to mix it with the activated sludge. This also provides the oxygen needed for the microorganisms to break down the organic pollutants.
Compressed air is normally used to provide air into the basins. Controlling the amount of air that is released is very important since it controls the growth and the health of the microorganisms. Flow meters are typically installed in the pipes to measure and control the amount of air to run the system properly.
The cost of energy to produce compressed air has increased tremendously due to the high cost of fuel. Regulating and controlling the air injection not only reduces the amount of energy consumed but also optimizes the operation of the plant. While there are many technologies to measure the flow rate of air, most of these methods measure the flow rate at the actual operating pressure and temperature, and require pressure and temperature correction to obtain the mass flow. Traditionally, the most common benefit of thermal dispersion mass flow measurement is the inherent ability to directly measure the mass flow without the need for pressure and temperature correction, as required with volumetric gas flow measurement. This not only provides a more useful flow measurement, but also makes thermal very cost-effective.
Thermal Dispersion Technology
Thermal dispersion technologies are based on the operational principle that states the rate of heat transfer by a flow stream is proportional to its mass flow. The flow measurement is accomplished by precisely measuring the cooling effect as the mass (molecular) flow passes the heated sensor. The sensor consists of two elements: the reference, which measures the temperature of the gas, and a second element, which is heated at a variable power to maintain the desired temperature difference between the two sensors. The illustration below shows the amount of power required to maintain a constant temperature difference between the two sensors. Under low mass flow conditions, there is minimal cooling and little power is required. As the mass flow increases, more power is required. The thermal mass flow meter provides excellent low flow sensitivity and high turndown capabilities.
Thermal mass flow meters offer many advantages over traditional technologies:
- Mass flow measurement based upon heat transfer. No correction of the gas flow rate for pressure or temperature is required.
- Excellent low flow sensitivity. Sensitive to velocities down to 10 standard feet per minute.
- Excellent turndown. Turndown of 100:1 or more depending upon the application requirements and calibration of the instrument.
- Low pressure drop. The insertion probe has little blockage of the pipe, creating very low pressure drops.
- Ease in installation. The insertion probe can easily be installed in a pipe or duct.
- Low installation cost. When considering options to measure mass flow, thermal dispersion has the lowest installed cost while providing excellent performance. No additional instrumentation is required to obtain a mass flow measurement.
Improved process optimization and reduced energy consumption are the main benefits of selecting the proper flow meter for your plant. There are multiple ways to measure air and gas flow rates; thermal mass flow meters should be considered as one of the proven and acceptable methods of measuring air and gas flows in the wastewater industry. For more information on flow instrumentation for this and other applications, visit flow.magnetrol.com.