Thermal mass flow meter

Thermal mass flow meters are designed to accurately monitor and measure mass flow (as opposed to measuring volumetric flow) of clean gases, a parameter that is not temperature dependent.

Therefore, the thermal mass flow meter does not require a correction for changes in gas temperature, pressure, viscosity and density.

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

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

Make sure the piping system is clean before installation
Install a filter upstream to ensure a moisture and oil-free gas stream and a downstream filter if back flow is a concern
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
Mount your mass flow meter in a horizontal position and avoid installing near heat sources or mechanical vibrations
Check your system for leaks before applying fluid pressure

What is Thermal Flow Meter ?

Wind chill is a phenomenon common to anyone who has ever lived in a cold environment. When the ambient air temperature is substantially colder than the temperature of your body, heat will transfer from your body to the surrounding air. If there is no breeze to move air past your body, the air molecules immediately surrounding your body will begin to warm up as they absorb heat from your body, which will then decrease the rate of heat loss.

However, if there is even a slight breeze of air moving past your body, your body will come into contact with more cool (unheated) air molecules than it would otherwise, causing a greater rate of heat loss. Thus, your perception of the surrounding temperature will be cooler than if there were no breeze.

We may exploit this principle to measure mass flow rate, by placing a heated object in the midst of a fluid flowstream, and measuring how much heat the flowing fluid convects away from the heated object. The “wind chill” experienced by that heated object is a function of true mass flow rate (and not just volumetric flow rate) because the mechanism of heat loss is the rate at which fluid molecules contact the heated object, with each of those molecules having a definite mass.

The simplest form of thermal mass flowmeter is the hot-wire anemometer, used to measure air speed. This flowmeter consists of a metal wire through which an electric current passes to heat it up.

An electric circuit monitors the resistance of this wire (which is directly proportional to wire temperature because most metals have a definite temperature coefficient of resistance). If air speed past the wire increases, more heat will be drawn away from the wire and cause its temperature to drop.

The circuit senses this temperature change and compensates by increasing current through the wire to bring its temperature back up to setpoint. The amount of electrical power required to maintain the hot wire at a constant elevated temperature is a direct function of mass air flow rate past the wire.

Most mass air flow sensors used in automotive engine control applications employ this principle. It is important for engine control computers to measure mass air flow and not just volumetric air flow because it is important to maintain proper air/fuel ratio even if the air density changes due to changes in altitude.

In other words, the computer needs to know how many air molecules are entering the engine per second in order to properly meter the correct amount of fuel into the engine for complete and efficient combustion. The “hot wire” mass air flow sensor is simple and inexpensive to produce in quantity, which is why it finds common use in automotive applications.

Industrial thermal mass flowmeters usually consist of a specially designed “flowtube” with two temperature sensors inside: one that is heated and one that is unheated. The heated sensor acts as the mass flow sensor (cooling down as flow rate increases) while the unheated sensor serves to compensate for the “ambient” temperature of the process fluid.

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.

Thermal Flow Meter Applications

Thermal mass flow meters are suitable for a variety of processes requiring mass flow measurement and are frequently used for the regulation of low gas flows.

Some common gas flow applications for thermal mass flow meters include:

Compressed air flow and distribution
Natural gas consumption eg for burner and boiler feed control
Monitoring and control of stack or flue gas (where composition known)
Landfill gas recovery
Flare gas measurement
Gas flow mixing & blending
Gas leak testing and detection

Thermal mass flow meter working principle and Applications