Thermal Mass Flow meter Working Principle

The thermal mass meter measures gas flow based on the concept of convective heat transfer. The flow meters are available in either inline flow bodies or insertion-style. In either case, the meter’s probe inserts into a gas stream of a pipe, stack or duct. Toward the tip of the meter’s probe are two sensors. These sensors are resistance temperature detectors (RTDs) or resistance thermometers and measure temperature. The RTDs consist of durable reference-grade platinum windings clad in a protective 316 SS or Hastelloy C sheath. One of the RTDs is heated by an integrated circuit and functions as the flow sensor, while a second RTD acts as the reference sensor, and determines the gas temperature. The  proprietary circuitry maintains a continuous overheat between the flow sensor and reference sensor. As gas flows by the heated RTD, flowing gas molecules transport heat away from it, and as a result, the sensor cools, and the energy is lost. The circuit balance disrupts, and the temperature difference (ΔT) between the heated RTD and the reference RTD changes. Within a second, the circuit restores the lost energy by heating the flow sensor, to adjust the overheat temperature. The electrical power required to sustain this overheat denotes the mass flow signal.

Thermal flowmeters can be divided into the following two categories:

  • Flowmeters that measure the rise in temperature of the fluid after a known amount of heat has been added to it. They can be called heat transfer flowmeters.
  • Flowmeters that measure the effect of the flowing fluid on a hot body. These instruments are sometimes called hot-wire probes or heated-thermopile flow meter.

Both types of flowmeters can be used to measure flow rates in terms of mass, which is a very desirable measurement, especially on gas service.

Range :-  From 0.5 sccm to 40,000 lbm/h (10,000 kg/h)

Design Pressure : Up to 1000 PSIG (69 bars)

Rangeability :- 10:1 to 100:1

Process Fluid :- Air, gas, liquids, and slurries

Design Temperature: – Up to 350 ° F (176 ° C), higher with special designs

Price :- Thermal mass flowmeter costs around  $500

Installation : Easy to installed

Thermal mass flow meter advantages and disadvantages

Advantage  thermal mass flow meter:-

  • Thermal flow meters have no moving parts, which reduces maintenance and permits the use in demanding application areas, including saturated gas.
  • Gas mass meters calculate mass flow rather than volumetric flow and do not require temperature or pressure correction, which means there is no additional expense for the purchase and installation of other equipment.
  • Thermal flowmeters provide excellent accuracy and repeatability over a wide range of flow rates.
  • Thermal flow meters can measure flow in large pipes.

Disadvantage of thermal mass flow meter :-  

  • It is used for gas measurements only.
  • It requires inlet and outlet sections.
  • Condensation of moisture in saturated gases on temperature detector will cause thermometer to read low. Moreover, coating as well as material build up on sensor will inhibit heat transfer. This causes meter to read low.
  • The other sources of error in meter readings include variation in specific heat caused due to changes in composition of gas.




How to install and calibrate a Thermal mass flow meter

How to calibrate your mass flow meter

Calibrating a thermal mass flow meter requires using the factor to correct the output for each gas. By default, thermal mass flow meters are calibrated for nitrogen/air. Thus, the flow meter range listed is valid only for nitrogen/air. Higher end models will have the option to choose the gas and the output will be corrected accordingly.

 

Calibration is typically done using the actual fluid. By comparing the flow rate to the recommended range listed, you can calibrate the mass flow meter until the actual flow rate matches the recommended range.

Here are the steps you need to follow when installing a mass flow meter

  1. Make sure the piping system is clean before installation
  2. Install a filter upstream to ensure a moisture and oil-free gas stream and a downstream filter if back flow is a concern
  3. Make sure the piping diameter is adequate for the flow rate and that there are no abrupt angles or objects in the flow path which can cause turbulence. Recommended pipe diameter is 10 times the distance between an angle and the inlet of the flow instrument
  4. Mount your mass flow meter in a horizontal position and avoid installing near heat sources or mechanical vibrations
  5. Check your system for leaks before applying fluid pressure




Installation of Inline Flow Meters

The inline style flow meter assembly includes the flow sensing element, temperature sensing element, bridge amplifier/signal output board, microprocessor circuit board, transmitter enclosure, and flow section. Depending on the flow section size and/or other requirements, the flow section may include flow conditioning plates.

The flow section is typically specified to match the user’s flow conduit and is plumbed directly in the flow line. This design has the sensing elements mounted directly in the flow section for exposure to the process gas. Inline mounting styles are available through EPI™ for line sizes 1/4″ pipe and larger. Consult our factory for flow section end mounting options. Inline flow meters are calibrated with the flow sensing element mounted in place within the flow section. The sensor should not be removed as the accuracy of the flow signal will be affected. Should it become necessary to remove the sensing element for any reason, the element must be replaced in the same alignment as it was originally positioned.

Installation of Insertion Flow Meters

The insertion style flow meters include the flow sensing element, temperature sensing element, bridge amplifier/signal output board, microprocessor circuit board, transmitter enclosure, and the probe assembly which supports the sensing elements. This design requires the probe assembly to be inserted into the process gas flow conduit. Insertion styles are available with 1/2″, 3/4″ or 1″ OD probes. Insertion style flow meters may be installed with properly sized bored-through tube fittings to mount them in place. Tube fittings, with or without mounting flange, are available from the factory as an option. Installing the tube fitting consists of preparing the flow conduit to accept the fitting by first drilling a clearance hole for the transmitter probe assembly, welding it in place, or threading it into the proper size half coupling which has been welded to the flow conduit. The tube length will be determined by EPI™ based upon the installation specifications. Optional ball valve assemblies are available through EPI™ which allow the removal of the insertion style averaging tubes for service, calibration, cleaning, etc. The valve provides a means to seal off leaks of the process gas at the point of insertion after the probe assembly has been removed. Installation requires fitting the flow section to which the insertion probe assembly will be inserted with a threaded half coupling of the proper size to accommodate the ball valve retractor. In some instances, this requires direct threading together (or with a reducing bushing) of the retractor assembly. In other cases, it requires welding the half coupling in place and drilling a clearance hole through for the probe assembly. If the flow section is under pressure, a hot tap drill rig (not available through EPI) may be required.




Thermal Mass flow meter Manufacturer

Thermal Mass Flow meter Manufacturers / VendorsCountryWebsitePhone No.
ABB AGGERMANYwww.abb.de+49 62143810
ABB LIMITEDUKwww.abb.com+44 1925741111
BOPP & REUTHER MESSTECHNIK GMBHGERMANYwww.burmt.de+49 6232657508
EMERSONUKwww.frco.com+44 1162822822
ENDRESS & HAUSERGERMANYwww.endress.com+49 71562090
SIEMENSGERMANYwww.powergeneration.siemens.co+49 9131180
TOKYO KEISO COJAPANwww.tokyokeiso.co.jp+81 334311625

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