For almost 25 years, FCI’s constant power thermal flow meter technology has been the defacto, proven solution for moist gas applications such as WWT digester gas and other biogas. FCI’s constant power technology provides two key differences when compared to constant ΔT technology: 1) constant power is measuring a change in temperature which is slower to change and provides a natural dampening effect for moisture, and 2) the temperature difference at the sensors is 30°C to 50°C which creates a better drying effect on the sensors. Conversely, constant ΔT technology, reacts quickly to add or remove power to the sensors in response to a temperature change, so power, and therefore flow rate readings, react erratically to any condensation or water droplets striking the sensors. Further, the active sensor in typical constant ΔT devices run at just 10°C to 20°C above ambient and have less of a drying effect. In other words, FCI's constant power technique is significantly less reactive to condensation itself and will more quickly dissipates it.
Additionally, for moist, dirty gas applications, FCI flow elements provide open, equal mass sensors which will better shed the moisture and are most accessible for occasional cleaning. (Figure 1) Further, because this all normal mode for constant power, the full flow range capabilities, both low and high flow rates, the instrument’s low power draw and all agency approvals are intact. Conversely, constant ΔT technology are commonly thin-wall, non-equal mass sensors with a shroud which further contributes to its unstable and erratic readings from condensation and much more difficult and time consuming to clean thoroughly (Figure 2).
Figure 1: FCI "S" type flow element s On left is new condition. On right is element removed from digester gas flow stream after years of service. Still functioning and in calibration.
Figure 2: A constant ΔT flow element in moist gas application with water droplets and condensate on non-equal mass sensors and shroud.
For years, constant ΔT technology providers have tried various methods to minimize the erratic readings. The most typical approach being to slow down the reading by measuring several samples and then provide an averaged output. One major manufacturer of constant ΔT technology flow meters has gone so far as to cease selling their product into moist gas applications and instead suggest installing a very expensive, in both purchase price and installation costs, ultrasonic flow meter instead. Yet another approach from one constant ΔT technology flow meter supplier has been to boost up their sensor’s temperature to an extreme of 300°C [550°F] to cause any moisture to flash-off. While this method may help, there are significant caveats and performance trade-offs that must also be considered. At 300°C, that is the equivalent to sticking a soldering iron into the gas stream! If the application is in a potentially explosive gas atmosphere, like methane and H2S as found in biogases, is this acceptable, safe, best engineering practice? Further beyond the extreme temperature itself, the instrument's flow ranges are severely reduced, to as little as just 10% of normal, a larger higher power draw requirement and, if Ex agency approval, which should be required if measuring methane-based based biogas, severely restricted T-ratings.
Immersible thermal mass flow flow meters are a well-known and accepted technology applied in a wide variety air and gas flow measurements. The two technologies, constant power and constant ΔT, are both widely specified and successfully applied in numerous applications across many industries and processes. However, in moist gas applications, only constant power type has proven itself to be the effective, low power and safe technology solution for decades.
Read more... for articles, case studies and success stories in moist gas applications, please click on the links below
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Fluid Components International