In part four of this series on combustion efficiency, we identify various flow technologies, specially mass flow meters and differential pressure flowmeters. There are many flow technologies that can be used to measure gas flow. While these meters are suitable for measuring natural gas flow, because of the higher volumes typical for combustion air and the larger duct sizes, most of the technologies are not suitable for combustion air flow measurement.
Coriolis flow meters provide a direct mass flow measurement based upon the deflection force of the fluid moving through a vibrating tube. These meters are highly accurate with high turndown capabilities and are independent of fluid properties. They are also expensive to purchase and install, and are not suitable for larger pipe sizes.
Positive Displacement meters require fluid to mechanically displace components, and measure volumetric flow at the operating pressure and temperature. While they have acceptable accuracy, pressure and temperature compensation would be required to achieve mass flow, and since they have moving parts gas cleanliness needs to be considered.
Thermal Mass flow meters measure the mass flow based on heat transfer from a heated element. The measurement is in mass flow, and additional pressure and temperature correction is not required. They also provide excellent accuracy and repeatability and are easy to install.
Turbine Flow meters measure volumetric flow based on fluid flowing passed a free-spinning rotor; each revolution corresponding to a specific volume of fluid. The meters have high turndown and accuracy. Unfortunately, because of the moving parts the meter’s service is limited to clean dry gases only, and pressure and temperature compensation would be required.
Ultrasonic Flow meters measure the difference in transit time of pulses that move from a downstream transducer to the upstream transducer, compared to the amount of time from the upstream transducer back to the downstream transducer. This style of meter is extremely accurate but extremely expensive, and pressure and temperature measurement would be required.
Vortex Flow meter has a bluff object or shedder bar that is placed in the flow path and as gas flows around the shedder bar, vortices are cyclically generated from opposite sides of the bar. The frequency of vortex generation is a function of the gas velocity. The frequency of vortex shedding is independent of fluid composition. The meter, however, requires pressure and temperature compensation and requires a minimum flow rate to produce vortices.
Differential Pressure Meters
Differential pressure flow meters calculate flow by measuring pressure drop over an obstruction inserted in the flow path. Common types of flow elements are orifice plates, flow nozzles, venturi tubes and averaging pitot tube. This technology can be used for both natural gas flow and combustion air flow.
Orifice Plate is a differential pressure meter frequently used for natural gas measurement. It measures volumetric flow, not mass flow. Limitations of this meter include: poor low flow sensitivity, limited turndown, creates a pressure drop impacting operating costs and requires correction of pressure and temperature to obtain mass flow.
Averaging Pitot Tube is a differential pressure flow measurement device commonly used for combustion air measurement. The instrument has limitations with gas flow; especially low flow sensitivity and turndown. The measurement is contingent upon achieving velocity pressure, and it is possible to have insufficient momentum to obtain a acceptable signal at low flow rates.
For combustion air flow measurements, other flow elements or obstructions have been used; these include measuring the pressure drop across an elbow in the duct work, across a venturi built into the duct or even across turning vanes in the duct.
Combustion Air Flow Measurement
While many flow technologies excel in natural gas measurement, they do not excel in combustion air flow measurement. In part five, the final section of this series, we reveal that thermal mass flow meters are ideally suited for both natural gas flow and combustion air flow measurement.
If you are interested in reading this article in full, see the Sage Metering white paper, Combustion Efficiency and Thermal Mass Flow Meters.
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Natural Gas Flow Meter by Sage Metering for Energy Conservation, https://sagemetering.com/applications/energy/natural-gas-flow-meter-for-conservat (accessed October 14, 2013).