Mass Flow Measurement Techniques
Mass flowmeters are the types of flowmeters which are mainly employed in mass-related processes such as chemical reactions, heat transfer, etc. In all these processes, accurate measurement of flow is the prerequisite. There are numerous types of mass flowmeters available in the industry. However, the most widely used type is the Coriolis meter. Another type available is thermal type mass flowmeters
Coriolis Meter
A Coriolis meter works on Coriolis Effect, hence it is named so. Coriolis meters are considered to be true mass meters since they tend to measure the mass rate of flow directly while other flowmeter technologies measure volumetric flow. Since mass does not change, no adjustments are needed for varying fluid characteristics. Hence, a Coriolis meter operates in linear fashion.These types of meters exist in variety of designs. The most common type of unit includes
- a U-shaped flow tube
- a sensor assembly
- an electronics unit
In this meter unit, the liquid passes through a U-shaped tube which vibrates in an angular harmonic oscillation. Coriolis forces will then deform the tube and a further vibration component gets added to the already oscillating tube. This added vibration element results in a phase shift or twist in few parts of the tubes. This resulting phase shift which is directly proportional to the liquid mass flow rate is measured with the help of sensors. This measured information is further transferred to the electronics unit where it gets transformed to a voltage proportional to mass flow rate. A Coriolis meter is shown in the figure below:
Main Features
A Coriolis meter can be applied for variety of applications ranging from adhesives and coatings to liquid nitrogen.
Typical rangeability of curved tube designed Coriolis meters vary from 100:1 to 200:1 whereas for straight-tube meters its rangeability is limited i.e. about 30:1 to 50:1. Besides, accuracy of straight tube meters is low.
In case of Coriolis meters, no compensation is needed for temperature and pressure variations. Also mass measurement is insensitive to changes in viscosity and density.
They are particularly helpful for handling liquids whose viscosity changes according to velocity while temperature and pressure remains constant.
These are called universal meters, since they are capable enough to measure nearly all liquids, slurries and gases. They can measure all liquid flows such as Newtonian, non-Newtonian and moderately dense gases as well.
They can be employed to measure liquid density also
No Reynolds number restrictions are allied with Coriolis flow meters.
They are also not affected by deformations in velocity profile.
Thermal-type Mass Flowmeters
Thermal-type mass flowmeters were conventionally designed for measurement of gases. However, thermal units for liquid flow measurements are also available. These types of flowmeters are similar to Coriolis meters in numerous ways. Their working is also independent of density, pressure, and viscosity variations just like Coriolis meters. A basic thermal mass type flowmeter consists of two temperature sensors and an electric heater mounted between them. This heater can be either projected into the liquid stream or can be positioned at the periphery of the pipe. This unit operates in two ways:
- Either introduces a fixed quantity of heat into the flowing fluid stream and then finds the related changes in temperature.
- Or keeps a probe at a constant temperature and then detects the energy needed to achieve so.
The temperature sensing element is never in direct contact with the liquid. The whole package of an electronic thermal type mass flowmeter consists of a flow analyzer, a temperature compensator, and a signal conditioner which usually gives a linear output that is directly proportional to mass flow rate.
Main Features
Accuracy of these meters depends on various factors like calibration integrity of the actual process and changes in temperature, pressure, flow rate, heat capacity and viscosity of the liquid being measured.
These types of flowmeters are frequently employed for monitoring and controlling of mass-related processes like chemical reactions. These processes mainly depend on the relative masses of non reacting ingredients.
Major capability of thermal mass type flowmeters is accurate measurement of low gas flowrates or velocities. These meters are capable to detect gas flows much lower than can be detected via any other metering device.
They offer good rangeability in the range of 10:1 to 100:1, only if they are designed to work in constant-temperature-difference mode.
Although thermal mass type flowmeters are most appropriate for very low flow measurements, yet they can be applied in large flow applications also. For example combustion air, natural gas, or the distribution of compressed air.
Limitations
Major problem faced with the use of thermal mass flow devices is the condensation of moisture on the temperature detecting element. This happens in case of saturated gases only. However, this condensation causes the thermometer to give low readings. Also this can result in corrosion of the device.
Applications
Major gas-flow applications of thermal mass type flowmeters consist of
Combustion air measurement in large boilers.
Semiconductor process gas measurement.
Air sampling in nuclear power plants.
Process gas measurements in chemical and petrochemical industries.
Research and Development areas.
Gas chromatography.
Filter and leak testing.
Sources
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