Thermal Mass Flow Meter dedicated to a single component of gas or a fixed proportion of mixed gas measurement, has been widely used in petroleum, chemical, semiconductor, medical instrumentation, bioengineering, combustion control, gas distribution, environmental monitoring, precision instruments, scientific research, metrology, food, metallurgy, aerospace and other fields.
Calorimetry mass flowmeter is used to measure and control the mass flow of gas. The standard input and output signal can be used to realize centralized computer control. In Daqing Petrochemical Company there are many forms of application, such as the hydrogen flow meter of polypropylene unit ft-121a/b using a Brooks calorimeter, the range is 1.45kg/h and 9.5kg/h.
Compared with the traditional flowmeter, it does not need to configure the temperature, pressure transmitter, without the temperature, pressure compensation can directly measure the mass flow (in the standard state, 0 ℃, $number. 325KPa).
When the gas is used as a control variable in the production process (e.g. combustion, chemical reaction, ventilation and venting, product drying, etc.), the quantity of the gram molecule of the gas can be controlled directly by using the mass flow controller.
There is no better technique than using a mass flow controller to maintain a quantitative mix of gases as mixtures or ingredients, or to optimize the chemical reaction process so far. The mass flow controller can control the flow continuously, and the cumulative flow can be obtained through the display instrument.
Thermal Mass Flow Meter or pipeline system, valve tightness detection of better instruments, it directly shows the leakage volume. Quality Flowmeter performance price ratio, easy to install, easy to operate, the use of mass flowmeter and quality flow controller products is one of the most reasonable choice.
Because this kind of mass flowmeter's sensor is the thermal principle, if the gas is not the dry gas, will affect the heat transfer efficiency, thus affects the sensor output signal and the measurement precision.
The thermal mass flowmeter can be divided into two types: contact type and non-contact type according to the different heat source and temperature measuring mode.
1. Contact Type Heat mass flowmeter
The heating element and the temperature measuring element of the mass flowmeter are placed in the pipeline of the measured fluid, and the direct contact with the fluid is often referred to as the Thomas Flowmeter, which is suitable for measuring the larger mass flow of the gas. The structure principle is shown in Fig. 1. Because the heating and measuring elements are in direct contact with the measured fluid, the components are susceptible to fluid corrosion and wear, affecting the measurement sensitivity and service life of the instrument. It is not suitable to use when measuring high flow rate and corrosive fluid, which is the disadvantage of contact type.
2. Non-contact Heat mass flowmeter
This flowmeter heating and temperature components are placed outside the fluid pipeline, and the measured fluid is not directly contact, overcome the shortcomings of the contact type. Thermal micro-flow type (the typical structure of non-contact mass flowmeter) is shown in Figure 2. The measuring tube of the instrument is a thin-walled nickel tube, and the outer side of the nickel tube is wound with platinum resistance wire 3 and 5 as the temperature measuring coil, and as the two arm R1 and R2 of the measuring bridge. The middle of the two temperature-measuring coils is wound with manganese copper wire and hot coil 4, as the heater of the instrument. When the fluid is stationary, the measuring bridge is in equilibrium because the temperature measurement coil is symmetrically installed on both sides of the heater and the resistance is equal (around 100). However, when the fluid in the nickel tube through the temperature measurement resistance, it destroys the heater of the thermal field, two temperature measuring coils in different temperatures, resulting in a change in the resistance value. The resistance of the two temperature measuring coils is unequal, which destroys the balance of the bridge. According to the balance principle of the bridge, the mass flow m can be obtained by the change of the resistance value measured by 8.
The thermal micro-flowmeter is suitable for measuring the micro-mass flow of liquids and gases. 0--100cm3/h can be measured by the flow of micro-liquid and l0l/h about the small gas flow.
In order to simplify the structure, some products cancel the heater, using only two measuring resistors, both as heating elements and as temperature components. This design, because of the thermal inertia of the reasons, the instrument reflects a relatively small speed, low sensitivity; The temperature change of the measured fluid affects the accuracy of the instrument indication.
In order to improve the flow measurement range of non-contact heat mass flowmeter, a boundary layer mass flowmeter is designed, which can reflect the flow rate by measuring the heat conduction of the boundary layer near the tube wall. In this way, the measurement of flow, generally using the heater to control the outer wall of the heat given to maintain the boundary layer constant temperature difference, and then according to the heat source measurement reflects the mass flow.
The Thermal Mass Flow Meter has developed rapidly: hot-line mass flowmeter, boundary layer mass flowmeter, shunt-type thermal capillary mass flowmeter and thermal mass flowmeter with IC substrate technology.