This article is one of the Flow Measurement blogs, and covers thermal mass flowmeters theory, application, advantages and precautions to be observed in their use. Main keywords for this article are Thermal Mass Flowmeter Installation.Thermal Mass Flowmeter Sizing.Advantages thermal mass flowmeters.Disadvantages Thermal Mass Flowmeter Design Requirement.
Thermal Mass Flowmeter References
Reference is made in this standard to the following documents. Flow Measurement (PIP PCCFL001).
Thermal Mass Flowmeter Design Requirement
- Thermal mass flowmeters are true mass flow instruments for accurately measuring gas flows. Most flow meters are volumetric, and infer mass flow through calculations derived by utilizing other physical process measurements, for example absolute pressure, differential pressure, temperature and viscosity. Because of this method of obtaining a mass flow output, they are known as inferential mass flowmeters as opposed to direct mass flow meters. Mass flow can be derived from the rise in temperature of a process gas stream, subject to a heat flux. Heat absorption depends on the heat capacity and mass flow rate of the gas to be measured and is relatively independent of ambient temperature, thermal conductivity, and viscosity of the gas.
- There are essentially three basic types of thermal mass flowmeters; constant temperature, constant power and constant energy which are based on the different methods used to excite the sensor.
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Constant Temperature Thermal Mass Flowmeters
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Constant temperature mass flowmeters are based on heat loss. Constant temperature thermal mass flowmeters require two active sensors which are typically platinum RTDs that are operated in a balanced mode. One acts as a temperature sensor reference while the other is the active heated sensor. Heat loss produced by the flowing medium tends to unbalance the heated flow sensor, which is forced back into balance by the electronic controller.
With this method of operating the constant temperature sensor, only the skin temperature is affected by the fluid flow heat loss. This allows the sensor core temperature to be maintained, and produces a very fast response to fluid velocity and temperature changes. Additionally, because the power is applied as needed, the system has a wide operating range of flow and temperature. The heated sensor maintains a reference of overheat above the environmental temperature sensed by the unheated element. The effects of variations in density are virtually eliminated by molecular heat transfer and sensor temperature corrections. These meters typically have a very high turn down ratio when appropriately sized to the order of 50:1.
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Constant Power Thermal Mass Flowmeters
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These mass flowmeters require three active elements. A constant current heating element is connected to an RTD. This heated RTD acts as a heat loss flow sensor while a second RTD operates as an environmental temperature sensor. When the fluid is at rest the heat loss is at a minimum.
Heat loss increases with increasing fluid velocity. In this mode of operation the mass of the sensor has to change its temperature, making it slow to respond to fluid velocity changes. In addition, this mode of operation has a limited useful temperature range due to the constant current applied. The dynamic temperature range may be extended by applying more power (current) to the heater, but this can result in excessive heat applied to the heater under no flow conditions. The effects of variations in density are virtually eliminated by molecular heat transfer and sensor temperature corrections. These meters typically have a turn down ratio of 100:1.
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Calorimetric or Energy Balance Thermal Mass Flowmeters
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These mass flowmeters require one heating element and two temperature sensors. Although many design variations exist, they all have a similar operating method. Typically the heater is attached to the middle of a flow tube with a constant heat input. Two matched RTD’s or thermocouples are attached equidistant upstream and downstream of the heater. The temperature differential at flowing conditions is sensed, producing an output signal. Because both temperature sensors experience the same temperature and pressure conditions, the design is inherently unaffected by density changes and the result will be a true mass flow output. Limitations of this type of mass flowmeter design include a maximum flow rate of approximately 200 liters per minute, non-industrial packaging, and a tendency to clog in dirty fluids. These meters typically have a turn down ratio of 10:1 and are not widely used.
- Thermal flow meters are normally used for low flow rates of clean gases. They are widely used in the semiconductor industry and are available with integral control valves for use as complete self-contained flow controllers.
Advantages thermal mass flowmeters are:
a. No temperature or pressure compensation required
b. Linear output (as temperature differential is proportional to mass flow)
c. DC voltage or 4 to 20 mA dc outputs available
d. Accuracy plus or minus 1 percent
e. Range greater than 100:1 is possible
f. Repeatability plus or minus 0.2 percent of full scale
g. Can be used on corrosive process streams if proper materials are specified
Disadvantages thermal mass flowmeter are:
a. Practical for gas flows only
b. Subject to blockage by foreign particles or precipitated deposits due to small
openings in flow meter
c. Power requirements excessive for heated stream type in larger pipe sizes
d. Has to taken out of process line for servicing
e. Accurate field calibration is difficult
Thermal Mass Flowmeter Sizing
In-line sizes from 1 / 8 of an inch to 8 inches are available. Manufacturer shall be consulted for sizing, which is dependent on the properties and flow conditions of the process gas.
Thermal Mass Flowmeter Installation
Thermal meters are installed directly in the process line. They shall be mounted horizontally, since thermally induced currents will cause errors at low flow rates if mounted vertically. Upstream and downstream pipe runs are similar to those required for orifice plates.
Thermal Mass Flowmeter Calibration
Calibration is usually done by the manufacturer, using a gas similar or identical to the process gas to be measured.
Field Checkout and Startup
- The thermal flowmeter shall not be put into service until the process lines have been cleaned.
- The operation and maintenance manual supplied with the instrument shall be followed for checkout and startup.