Thermal mass flowmeter
What are Mass flowmeters?
A Mass flowmeter measures the amount of fluid pass through the pipe. Mass flow measurement gives a more accurate account of fluids and is not affected by density, pressure and temperature (unlike volumetric measurements).
Although most meters can infer mass flow rate from volumetric flow measurements, there are a number of ways to measure mass flow directly.
Thermal Mass Flowmeters:
Thermal mass flowmeter work on the principle of loss of temperature when a fluid stream passes across it.
The two main types of thermal mass flow measuring devices are:
- Thermal Anemometer
- Temperature rise flowmeter
Thermal anemometer:
The thermal anemometer works by measuring the heat dissipation from a probe inserted in the line. The amount of heat taken from the probe is dependent on the fluid velocity and density but is also a direct measure of the mass flow rate. The temperature is also measured by the calculation. They are also referred to as ‘Hot wire probes’.
The probe can either be constant current or constant temperature.
In the constant current type, a fixed current is passed through the probe which causes heating in the probe. As the flow rate varies, so does the amount of heat taken from the probe and hence the temperature changes. The temperature is measured to derive the flow.
For the constant temperature type, a feedback loop is required to maintain a constant temperature. As the change of flow affects the temperature, the current needs to be regulated to maintain probe temperature. The flow rate is determined by the power required to heat the probe. The constant temperature devices have a faster response to flow changes.
The temperature probe must protrude into the flow stream, and therefore may be easily damaged by corrosion and erosion. In addition, the robustness of the system is compromised by the protrusions into the fluid stream, increasing the chances of leakage.
However thermal anemometer has fast response times, < 0.5milliseconds.
Temperature rise flowmeter:
Temperature rise flowmeters work on the principle of heating the flow stream. By heating the flow stream at one point, the temperature can be measured both upstream and downstream of the heating point. Calculating the difference between the temperatures gives information about the flow rate.
This method requires the measurement of actually heating the process fluid. It is therefore limited to gas applications at low flow rates.
As with the hot wire probe, the temperature sensors and the heater must protrude into the flow stream, and therefore may be easily damaged by corrosion and erosion. Also, the robustness of the system is compromised by the protrusions into the fluid stream, increasing the chances of leakage.
There are external fixing types of temperature rise flowmeter for large pipelines:
External fixing provides Non-contact, non-intrusive sensing No obstruction to flow and Reduced maintenance.
Advantages:
- Thermal flowmeters are used to measure gas with low pressure
- Used measure low flow
- These flowmeters are frequently employed for monitoring and controlling of mass-related processes like chemical reactions.
- They offer good rangeability
Disadvantages:
- Practically for gas only.
- Power requirement execessive in large pipelines.
- Accurate field calibration is difficult.
