The application of thermal mass flowmeter in chemical industry

Thermal type gas mass flowmeter is dedicated to a single component or a fixed proportion of mixed gas measurement, has been widely used in petroleum, chemical, semiconductor, medical instrument, biological engineering, combustion control, distribution, environmental monitoring, precision instruments, scientific research, measurement, food, metallurgy, aerospace and other fields. Amount of thermal mass flowmeter used for precise measurement and automatic control the quality of the gas flow. The standard input and output signal, can realize central control computer. Application in daqing petrochemical company has a variety of forms, such as the hydrogen flow meter FT - polypropylene device 121 a/B using heat flow meter is BROOKS quantity, range of 1. 45 kg/H and 9. 5Kg/H。 Compared with the traditional flowmeter, it does not need to configure the temperature, pressure transmitter, without for temperature and pressure compensation can directly measure the mass flow rate ( In the standard condition, 0 ℃, 101. 325 kpa) 。 When the gas as the control variables in the process of production, Such as combustion, chemical reactions, ventilation exhaust, product drying, etc. ) , using mass flow controller directly measured, can control the number of molar gas. If you want to keep the quantitative gas mixture as mixing or ingredients, or to optimize the chemical reaction process, so far there are no better than using a mass flow controller technology. Mass flow controller continuously adjustable control flow, through the display, also accumulated flow rate can be obtained. Thermal mass flowmeter or piping system, valves sealing detection better instruments, it directly shows the air leakage. Mass flowmeter price performance ratio is high, easy to install, easy to operate, use the mass flowmeter and mass flow controller products is one of the most reasonable choice. Due to this kind of sensor is hot type principle of mass flowmeter, if not dry gas, will affect the heat transfer efficiency, thus affecting the sensor output signal and the accuracy of measurement. Second, the basic principle of thermal mass flowmeter on the basis of the heat source and the different methods of temperature measurement can be divided into two kinds of contact type and non-contact. Contact thermal mass flowmeter of this kind of mass flowmeter heating element and temperature measuring element in be current of the pipe body, and direct contact with fluid is often referred to as Thomas flowmeter, suitable for measuring the larger mass flow of gas. The structure principle is shown in figure 1. Because of heating and measuring element and the current direct contact with the body, so the components susceptible to fluid corrosion and wear and tear, that affect the instrument sensitivity and service life. When measuring high velocity, corrosive fluid unfavorable choose, this is the shortcoming of contact. Figure 1: the type of contact thermal mass flowmeter structure principle, 3 l & ndash; Thermocouple. 2 a heater; Non-contact thermal mass flowmeter this flowmeter heating and temperature measuring elements are placed in the fluid pipe, do not contact directly with the current body, overcomes the drawback of contact. Thermal decay flow type ( Is the contact of the mass flowmeter typical structure) As shown in figure 2. Instrument measuring catheter for thin-wall small nickel pipe, nickel tube on both sides of the external wire winding platinum resistance 3, 5 as temperature measuring coil, and serves as a measuring bridge of R1, R2, two arms. The middle of the two temperature measuring coil wrapped around 4, manganese copper wire heating coil as the instrument of the heater. When the fluid static, due to temperature measuring coil installed symmetrically on both sides of heater and equal value ( The 100 or so) , so the measuring bridge in a state of balance. But when the fluid flows through temperature measurement in the nickel pipe resistance, and destroys the heater temperature, two temperature measuring coil in different temperature field, and thus lead to resistance to change. Two temperature measuring coil resistance, destroy the balance of the bridge. According to the principle of balance of bridge, by the galvanometer 8 the change of the measured resistance value, M mass flow can be obtained. Non-contact thermal mass flowmeter 1 & ndash; Measure the catheter; 2 - Isothermal shell; 3 - Temperature measuring coil; 4 - The heating coil 7 & ndash; Zero resistance; 8 - Galvanometer thermal decay flowmeter is suitable for measuring tiny mass flow rate of liquid and gas. 0 - can be obtained - Tiny liquid flow and l0L 100 cubic cm/h/h around tiny gas flow. In order to make the structure is simplified, and some products are cancelled the heater, using only two measuring resistance, both as a heating element and as the temperature measuring element. This design, because of thermal inertia, meter reflects the speed is small, have lower sensitivity; By measurement of body temperature changes affect the accuracy of the instrument. Measuring flow in this way, the general is the use of control of the outer wall of tube heater heat to maintain a constant boundary temperature difference between inside and outside, and then according to the heat source reflect the mass flow rate measurement. Thermal mass flowmeter is now developing rapidly: hotline mass flowmeter, the boundary layer thermal capillary mass flowmeter and mass flowmeter, the shunt type IC substrate technology of thermal mass flowmeter, etc. Three, the practical application and the maintenance factory for control in the resin molecular weight polyolefin plant need to adjust the injection rate of hydrogen, oxygen, and so on, and adopted the four thermal decay mass flowmeter. According to the actual situation of thermal decay mass flowmeter to run over the past few years, to make the following analysis: (its reliability, accuracy A) 1 reliability. The average time between failures ( MTBF） Using the average time between failures ( MTBF） To measure the reliability of the equipment.

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