SmartMeasurement Thermal Mass Flow Meters
SMARTMEASUREMENT™ THERMAL FLOW METERS
Thermal flow meters
The challenges of measuring gases are much greater than measuring any other fluid because a small change in pressure and/or temperature can affect measurement accuracies greatly. These challenges are outlined in detail at https://www.smartmeasurement.com/gas-flow-meters/
Thermal Mass flow meters do a better job of alleviating most of these challenges versus other gas flow measurement devices. With the exception of Coriolis gas mass flowmeters, all other gas flowmeters are volumetric flow measurement devices, which means that their accuracy can be affected by changes in media concentration, density, pressure, or temperature. Coriolis gas measurement has proven to be effective in high-pressure applications, but lower-pressure gases still pose challenges.
Traditionally, gas flow measurement was done with volumetric gas flow meters. These non-mass meters must be used with temperature and pressure sensors and a flow computer to correct for these change in these parameters when mass flow measurement is required. Compensating for dynamic changes in pressure and temperature are expensive and also inaccurate as one must add the inaccuracies of the volumetric flow meter, pressure, and temperature sensors to determine the gas flowmeter’s overall accuracy.
The ATMF series of Thermal Mass flowmeters from SmartMeasurement have been successfully installed in a wide variety of industries and applications including:
Why choose a Thermal Mass flow meter
There are many process gas flow measurement applications. Examples include biogas or waste gas, which require the ability to measure very low flows, measuring combustion air and natural gas for boilers or hydrocarbon gases in refineries, stack gases for pollution monitoring and any other applications that require accurate gas flow measurement. Applications such as these commonly involve conditions where the process temperature and pressure can be varying. This can create challenges for older, traditional flow measurement technologies such as turbines, differential pressure, vortex, ultrasonic, and positive displacement, because they only measure volumetric flow and require other instruments to determine mass flow. Thermal mass flow meters measure mass directly without the need to take into account of changes of process temperature, pressure and density as volumetric flow meters do. They have excellent low-flow sensitivity, virtually no pressure drop, and easy-to-install with insertion-mounting. Measuring gas mass flow without other instruments to compensate for dynamic process changes makes SmartMeasurement’ s ATMF series of gas mass flowmeters the instrument of choice.
The ATMF series of thermal mass flow meters feature accuracies of ±0.5% of Full Scale ±1% of Reading, greater than 100:1 turndown ratio, ability to measure from the smallest pipes of ¼” (6mm) to large ducts of unlimited sizes with our multipoint mass flow meters or the patented FAT (Flow Averaging Tube) probes. Flow velocities of 4-40,000 fpm (0.02-200 mps) at temperatures of 932 °F (500 °C) with operating pressures of up to 1000 psig (69 barg). The ATMF series are most popular for measuring air, natural gas, biogas, and industrial gas measurement applications. For applications with limited straight runs https://smartmeasurement.com/wp-content/uploads/2021/02/Straight-Run-Requirements-of-Various-Flow-Technologies.pdf flow conditioners of various sizes are offered.
The case for Insertion thermal mass meters the ATMFIS
SmartMeasurementTM offers a wide range of insertion thermal mass meters that can be used in pipes of 2″ (50mm) or greater. This makes thermal mass-meter installation less expensive than inline meters because they are installed on the top of the pipe with only one fitting. Moreover, insertion meters can be “hot tapped” with various ball valve retractors to allow for insertion/removal of the meter for and/or evaluation without shutting down the process.
The Case for thermal mass flow averaging tubes the ATMF-FAT
In situations where there are limited straight piping runs avaialble, SmartMeasurement’ s ATMF-FAT can reduce straight-run requirements to as few as 3 diameters upstream. The ATMF-FAT uses a flow averaging tube in conjunction with a thermal mass transducer placed inside the tube which has a number of large diameter inlet ports along the length of the upstream section. The pressure at each inlet port is averaged inside the tube to create the axial flow through the tube, providing a stable and conditioned flow to be measured across the flow transducer. The gas returns to the main flow stream through the ports located near the sensing elements. Anomalies in the actual flow profile or installations in non-circular ducts may still some require minor adjustments to achieve the best accuracy. The biggest advantage of this design is that only 3 upstream straight pipe diameters are required for stable and accurate thermal flow measurement. Please see the diagrams below. The ATMF-FAT comes both in inline and insertion configurations; the ATMF-FAT-IL for sizes from 2-4” and the insertion style ATMF-FAT-ISL for larger diameters. The ATMF-FAT also can come with an auto-purge fitting using the customer’s own compressed air to periodically clean the sensor and the flow averaging tube’s port holes for wet and dirty gases.
The case for Multi Point thermal mass flow meters: the ATMF-MT
In applications where there are non-circular or large ducts, a single transducer insertion style mass flow meter may not provide stable flows due to uneven flow profiles which may pose flow accuracy and stability issues. Insertion meters must be placed at the point of average velocity within the flow profile and it would be impossible to determine this area on a non-circular pipe/duct in. In applications such as air ducts, combustion air ducts, flare or stacks, a multi-sensor thermal flow meter is needed to determine the average mass flow rate. SmartMeasurement offers our ATMF-MT multipoint Systems, designed with 2 to 5 individual transducers in a flow tube to accurately measure the average flow in these difficult applications. The transducers are field replaceable should one or more sensors become inoperative. In situations where the gases are wet and/or dirty such as air intake ducts or air exhaust and flue stacks, etc. the ATMF-MT is offered with an optional Air Purge System (APG) using the customer’s own compressed air system. The ATMF-MT with an APG ensures that the transducers remain clean ensuring accurate and stable mass flow measurement in dirty gas applications. The ATMFMT are design for applications as diverse as ambient air flows in HVAC ducts, exhaust gas in large diameter stacks, boiler NOx efficiency systems and municipal waste incinerators. See the diagrams below:c
Advantages of using Thermal Flow Meters
The primary application for thermal mass flow meters is measuring direct mass flow of gases without the need for pressure or temperature compensation. Another feature unique to thermal mass flowmeters that they can be used in with pipe sizes ranging from very small, as little ¼” (6mm), up to virtually unlimited large-sized of ducts of several feet or meters in diameter, as well as non-circular ducts.
Thermal meters’ versatile design allows them to be deployed in almost any type of duct or piping system where gas flow needs to be measured. For inline meters, the ATMF-IL is available with with several process connection options such flanges, threads, and VCR. For insertion style meters, the ATMF-IS is available with flanges, NPT threads, or a hot-tap ball valve.
Thermal meters may also be used in special situations with difficult conditions where older technology meters are not suitable. SmartMeasurement’s ATMF-FAT flow averaging tubes can be used in non-cylindrical ducts while the ATMF-MT allows for multi-point calibrations in large pipe applications. Thermal meters can to configured to detect fluid velocities ranging from 4-40,000 fpm (0.02-200 mps) at temperatures of up to 932 °F (500 °C) with operating pressures of up to 1000 psig (69 barg). Finally, the thermal mass flowmeter is also renowned for its’ sensitivity. This meter’s ability to detect extremely low velocities makes it an ideal choice for low flow/low velocity applications such as biogas or waste gas measurement as well as high velocity applications such as compressor output measurement.
Theories of Operation
Regardless of whether thermal mass flow meters are insertion or inline style, the probe of the meter supports two sensors that extend into the gas flow. The sensors are resistance temperature detectors (RTDs), consisting of durable reference-grade platinum windings protected in a 316 stainless steel sheath. One RTD is called the active RTD, or flow sensor and the other is a temperature sensor. As gas flows past the heated sensor, gas molecules carry heat away from the heated sensor, while the other compensates for the change in process temperature. The temperature difference between the two sensors is proportional to the mass flow. Since RTDs’ outputs are proportional to temperature, the changes in the resistance of the two RTDs transforms into a change in resistance or voltage. Thermal mass meters work best in dry gas applications. Wet gases can also be monitored but must be factory adjusted. It is important that users let manufactures know if there are wet conditions for the process gas. However, moisture condensation on either of the sensors will cause errors and it is advised that the sensors be periodically cleaned under these conditions.