The TechnologyAcoustic pyrometry is a proven technology that uses sound waves to provide real-time temperature measurements in utility and industrial boilers, kilns, furnaces, and stacks.Sound Waves
Sound is a powerful tool in the measurement and characterization
of certain physical characteristics of air and of gases in
general. Recognizing the potential of sound, SEI has focused
its expertise in acoustical physics, electronics, digital
signal processing, and software development to create powerful
innovative products that use sound to solve real-world problems
in a variety of industrial applications.
Sound waves can be used to accurately measure the temperature
of a gas. Our full-featured Boilerwatch® MMP
Systems are best for multiple paths, contour maps, and temperature
distribution mapping.
Additional information and applications of
SEI’s patented acoustic technologies have been reported
in a number of industry Publications,
including published articles and technical conference papers.
Also see our literature
page for Feasibility Tests, Reports, and Data Sheets.
Gas Temperature MeasurementGas temperature is one of the most important parameters to measure in a number of industrial processes. For example, the progressive design and operation of modern utility power boilers fired by coal, oil, and natural gas, black liquor chemical recovery boilers, and refuse-fired boilers, depends increasingly on the critical monitoring and evaluation of gas temperature conditions in the furnace and superheat sections of the thermal process.
Successful boiler operation and performance optimization requires attention to the boiler temperatures, and the proper and complete combustion of the various fuels involved. Small levels of over-temperatures to which any boiler tube metallurgy is subjected, result in corresponding reductions in your equipment’s life expectancy, and consequentially lower unit availability rates, plus forced outage repairs. Direct and accurate measurement of the gas temperature can provide valuable control of these expenses, as well as information for the design and operation of boilers, furnaces, and many other industrial processes.Theory of Acoustic Temperature MeasurementIt is known that the speed of sound is a function of the temperature of the medium through which the sound wave travels. In ranging systems, this variation in sound speed is treated as an error that requires appropriate correction. In acoustic pyrometry the changes in sound speed provide the desired measurement.
The measurement of flight time of sound for distance calculations in meteorological, hydrological, and industrial applications has been known for some time. The determination of temperature, on the other hand, requires measuring the flight time of an acoustic pulse over a known distance. This measurement yields the average temperature of the gaseous medium along the entire acoustic path.
The principles involved in acoustic pyrometry are clear. The speed of sound (c) in a gas is related to gas temperature by the equation: c = sqrt[rRT/M] (1) where: r = ratio of specific heat of the gas at a
constant pressure to that of a constant volume R = universal gas constant (8.314 J/K-mol) T = absolute temperature (degrees Kelvin) M = molecular weight of the gas (kg/mol) In theory, an acoustic pyrometry system simply requires a sound source (transmitter) to be placed on one side of a furnace, and a receiver or microphone be placed on the opposite side. The transmitter emits a pulse of sound and the receiver detects it. By measuring the time taken by the sound wave to travel from the transmitter to the receiver, and since the distance is known and fixed, we can then simply compute the average temperature of the gas in the path traversed by the acoustic pulse. This method, however, proves challenging in practice. For example, it has been determined that the practical frequency range for an acoustic pulse within a power boiler furnace region is between 500 Hz and 2000 Hz. Also, since the temperature of the gas involved can range up to 3000 degrees F, the acoustic velocity may be upwards of 880 meters/second, and the wavelength of the sound signal is about 1 meter. The sound pulse flight-time must be resolved to a fraction of one wavelength in order to obtain practical temperature resolution and an acceptable system accuracy. Problems also arise because the acoustic path is usually disturbed by severe thermal and velocity gradients as well as by cavities between tube banks located at various points throughout a boiler. The speed of sound is determined by measuring the flight time of an acoustic wave, then dividing it into the distance traveled: c = d/t (2) where: c = speed of sound (meters/second) d = distance sound wave travels (meters) t = flight time of acoustic wave (seconds) Once the speed of sound is known, the temperature may then be computed by combining equations (1) and (2) to yield an expression that relates gas temperature to distance and flight time: T = (d/Bt)2 - 273.16 where: T = gas temperature in degrees C d = distance(meters) B = acoustic constant of the gas = sqrt(rR/M) SI units t = flight time (seconds) For measurement of gas temperature in a furnace the distance (d) between the acoustic transmitter and receiver is fixed and easily determined, the acoustic constant (B) is calculated from an analysis of the fuel or flue gas, and the flight time (t) is measured by the acoustic pyrometer. From this information, the average gas temperature across the path is computed.
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