Accurate, repeatable measurement of air and process gases with thermal mass flowmeters is key to improving accountability and energy-management processes.
Although manufacturers have made significant improvements in process heating efficiency, the U.S.'s total energy use for process heating is expected to increase. With overall thermal efficiency of process equipment varying from 15 to 80 percent (compared to the thermal efficiency of steam generation, which varies from 65 to 85 percent), there is clearly an opportunity to achieve significant energy savings, improve productivity and enhance competitiveness. The U.S. Department of Energy's Industrial Technologies Program (www.eere.energy.gov) has identified improved burner control systems as a significant opportunity for reducing energy operating costs, waste and environmental emissions.
Plant managers seeking to improve combustion performance and product quality must balance fuel- and energy-saving measures with emissions-reductions priorities. In many process heating operations such as drying, incineration and heat treating, excess air is a process requirement. In these cases, combustion air and excess air used to suppress emissions must be heated, which increases fuel consumption and may result in incomplete combustion. One of the most effective techniques for improving efficiency and reducing emissions in these applications is a precise control strategy based on mass flow measurement of fuel and airflow.
Accurate, repeatable measurement of air and gas, at low and varying flow rates, is a critical variable in combustion control. Unlike orifice plates, turbine meters and other volumetric flow devices, the thermal mass flowmeter (TMFM) is unaffected by changes in temperature and pressure. Because of this characteristic, it is capable of providing a more accurate measurement of mass flow rate.
As shown in table 1, most conventional flowmeters measure volumetric flow and require additional measurements of pressure and temperature to calculate density and mass flow. Because the thermal mass flowmeter measures mass flow directly, it provides a reliable, repeatable and accurate measurement. The thermal mass flowmeter also provides rangeability and a lower pressure drop than conventional flowmeters.
Sophisticated burner-control systems optimize the air/fuel ratio control to obtain peak thermal efficiency over the entire range of the burner and to facilitate proactive emissions control. Mass flow control of air and fuel is used to compensate automatically for changes in temperature or pressure that affect combustion performance. Many systems also use fuel totalizing and other data outputs for distributed control system (DCS) interfacing and remote system monitoring.
Thermal mass flowmeters are designed for installation in fuel gas and air feed lines found in process heating and utility operations. In addition to the primary benefits of direct mass flow measurement, low-flow sensitivity and fast response, the thermal mass flowmeter's lack of moving parts also helps reduce maintenance costs.
Meeting Air-Quality Management Requirements
The federal Clean Air Act (CAA) requires the U.S. EPA to set national ambient air quality standards to ensure public health. State agencies as well as regional and metropolitan air-quality management districts are responsible for ensuring attainment and maintenance of these standards. These agencies have published rules and regulations regarding NOX and carbon monoxide emissions from steam generators, process heaters and industrial, institutional and commercial boilers.
Owners or operators of units subject to these regulations may install a non-resetting totalizing fuel flowmeter to measure the total fuel used by each individual unit (figure 1). The regulations specify mass-flow measurement of fuel usage, and if a volumetric flowmeter is installed, it must compensate for pressure and temperature using integral gauges.
The thermal mass flowmeter's ability to deliver a direct reading of mass flow rate of natural gas and other fuel gases -- without temperature and pressure compensation -- provides a simple, reliable and cost-effective method for tracking and reporting fuel consumption.
The thermal mass flowmeter has several analog and digital output signals to easily interface with the emissions management system and an integrated, non-resetting totalizer that helps manufacturers meet air quality management equipment requirements. The instrument also includes broad measurement range (100 to 1 typical) suitable for low velocity flow measurement.
Rising energy prices have made a daily accounting of natural gas usage a priority for large industrial facilities with multiple processes and/or buildings. Fuel gas flowmeters are used to analyze demand, improve operating efficiency, reduce waste and adjust for peak usage. A thermal mass flowmeter can be used for these energy-accounting applications. In addition, these instruments can help plant managers provide accurate usage reports for environmental compliance as well as compare measured usage to billing reports from gas providers.
Insertion-type thermal mass flowmeters can be mounted throughout the facility to provide a reading of natural gas consumption by plant or process. In constrained areas, inline meters reduce the traditional requirements for straight, unobstructed upstream piping and simplify installation (figure 2). Typical applications include:
- Sub-metering by process or department to help manufacturers assess inefficiencies, assign costs and implement conservation measures.
- Used to monitor and bill for fuel consumption on skid-mounted generators and compressors.
- Document usage, negotiate rates and resolve billing disputes.
- Allocate fuel costs to various buildings and/or tenants.
In addition to the primary benefits -- direct measurement of mass flow rate, low-flow sensitivity and fast response -- the design of the thermal mass flowmeter (with no moving parts) also helps reduce maintenance costs. One manufacturer replaced a fuel-measurement system, which consisted of turbine meters with ancillary pressure and temperature transducers, with thermal mass flowmeters. The flowmeters reduced the time and expense associated with servicing the turbine meters as well as provided a wide turndown for a more accurate measurement at low and varied loads, ensuring accurate cost-allocation and improving combustion control capability.
In conclusion, thermal mass flowmeters provide the real-time measurement required for sophisticated combustion control systems as well as other critical flow measurement applications such as purge monitoring and flare gas and vent gas measurement, hydrogen or landfill monitoring, and wastewater aeration. Based on the thermal-sensing principle, thermal mass flowmeters offer a repeatable and reliable method for measuring the flow rates of air and gases.
Sidebar: How It Works
The thermal mass flow meter uses a constant temperature differential (ΔT) technology to measure mass flow rate of air and gases. The flow sensor consists of two resistance temperature detectors (RTDs). The sensor elements are constructed of a reference-grade platinum wire wound around ceramic mandrels that are inserted into stainless steel or Hastelloy tubes.
As shown in the figure, the reference RTD measures the gas temperature. The instrument electronics heat the mass flow sensor, or heated element, to a constant temperature differential (ΔT) above the gas temperature and measure the cooling effect of the gas flow. The electrical power required to maintain a constant temperature differential is directly proportional to the gas mass flow rate. The instrument's microprocessor then linearizes this signal to deliver linear output signals.