The larger the diameter of a pipe, stack or duct, the greater the likelihood of significant flow velocity profile distortions. In these highly skewed flow conditions, a single point flow meter won’t do the job. So, what’s the best mass flow technology and measurement solution?
A frequent choice for greater measurement accuracy is the multi-point sensor array. In this case, two or more flow sensors are placed along an extended-length insertion element. With more sensor points across the interior expanse of the duct, the flow measurement will be more representative. And, if you install two of these arrays in crisscross fashion inside the duct (as diagramed below), you have a sensor array that will provide very good results. However, there are downsides to this scenario: The multi-point array is engineering-intensive at the front end, and maintenance-intensive down the line.
A smart option to measuring mass flow in larger pipes and ducts is to utilize multiple (four, or more as needed) thermal mass flow meters from opposite sides of a duct (see above diagram) and average their output signals. By using standard thermal mass flow meters for accurate and reliable mass flow measurement, you can realize a lower installed cost, as well as fewer maintenance headaches in the future.
Thermal mass flow meters operate on the basis of heat transfer and measure mass flow by detecting heat dissipation from a heated surface. Flow meters using this technology measure flow rate in units of mass flow (SCFM or NM3/hr), rather than flow at operating conditions. Because temperature and pressure variations will influence gas density, mass flow measurement provides optimum measurement accuracy despite temperature and pressure variations. Because thermal mass flow meters function without moving parts, they require little to no maintenance and cause fewer on-line errors.
Magnetrol offers a white paper on the subject of mass flow measurement, which discusses mass flow measurement in larger pipes and ducts, as well as other characteristics of thermal mass flow meters. Download it now.