FLOW MEASUREMENT


Flow Meter Goes Down the Pipe


Thermal mass flow measurement technology helps in compressed air system loss management.


M


onitoring compressed air usage within industrial applications has become


increasingly important


as the associated costs continue to rise dramatically. With compressed air systems often being the largest excess user of electricity for a business due to air leaks or poorly performing components, the results can be a significant amount of wasted energy. Whilst this may have been an easy loss source to overlook in the past, energy efficiency has become more of a focus in recent years and it is therefore more important for all businesses to identify and rectify any potential energy wastage. According to flow specialist company, Bell Flow Systems, problem areas in compressed air systems can be identified by fitting thermal mass flow meters positioned to calculate outflow generated air with downstream usage.


FLOW MEASUREMENT TECHNIQUES There are several ways of measuring compressed air flow in a system using orifice plates and turbine or vortex meters for example, which will measure the uncompensated volumetric flow of air. However, it is often more important to have a corrected flow rate linked to a particular pressure and temperature in normal conditions. This requires a multi-variable system which can be expensive to buy and maintain and may also increase the chance of errors considering the extra sensors required. The alternative is to use thermal mass flow meters, which measure the mass flow of the air directly, removing any need to monitor the pressure and temperature separately. Compressed air thermal mass flow meters can detect leakages much faster than a pressure sensor, they can be used for cost allocation and they can be used for efficiency monitoring and maintenance management.


10 /// DAQ, Sensors & Instrumentation Vol 2 No. 2


❱ ❱ Flow is measured through calculating temperature differences along the pipe from a known heat source


❱ ❱ Thermal mass flow meters have different mounting options for a range of pipe sizes for measuring compressed air flow and reduce losses


THERMAL MASS A MEMS (CMOS) type Thermal Mass flow meter senses flow via the convective heat transfer flow measurement principle, in which a micro-heater is placed in the middle of two temperature sensors symmetrically positioned at upstream and downstream points. The cavity below the micro-heater and sensors plays the role of thermal isolation as it is filled with the gas. When the flow passes through the micro-heater, heat is carried away by the flowing medium, with the temperature differences measured by the up and down-stream temperature sensors being directly proportional to the mass flow of the compressed air or gas being measured. These types of sensor feature zero moving parts in a nearly unobstructed straight through flow path, which facilitates extremely low pressure drop measurement. Maintenance costs are low and this type of meter is very sensitive at low flow rates, which allows for a high turn-down ratio whilst remaining accurate over a wide


measuring range. Insertion style models can also be used in larger pipe sizes, with the option to install via a Hot-tap kit under full system pressure. Bell Flow Systems recommends placing thermal mass flow meters in dry air downstream of all air treatment filters to prevent sensor degradation. If temperature changes rapidly, this can affect the accuracy of the flow meter and can shorten its service lifetime as well. Large temperature variations, which can be found downstream of malfunctioning desiccant dryers, should be avoided


FLOW METERS A number of thermal mass flow


meters are available for different bore diameters and with flanged or insertion style mounting. Digital and analogue outputs are available with optional communication outputs. Bell Flow Systems has made Bluetooth connectivity available with a mobile phone app allowing the user to check and set meter parameters on a mobile device.


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