1C31189G03 Thermal Conductivity Gas Analyzer Overview? Technology information automation
There are three ways of heat transfer: heat convection, heat radiation and heat conduction. Thermal conductivity is a measure of the heat conduction capacity of a substance. Objects with large thermal conductivity are excellent thermal conductors. The small thermal conductivity is a poor conductor of heat or a thermal insulator.
3.1 Relative heat conduction coefficient
1C31189G03 The excellent value of the gas thermal conductivity is very small, and basically within the same order of magnitude, and the difference between each other is not very wide, so the concept of “relative thermal conductivity” is usually used in engineering. The so-called relative heat transfer coefficient refers to the ratio of the heat transfer coefficient of various gases to the heat transfer coefficient of air under the same conditions.
1C31189G03 It can be seen from the table that the thermal conductivity of H2 is particularly large, which is more than 7 times that of general gases. When measuring, the following two conditions must be met. First, the thermal conductivity of the component to be measured has a large difference with the thermal conductivity of other components in the gas mixture, and the larger the more sensitive; The other is to require the thermal conductivity of the other components to be equal or very close. In this way, the thermal conductivity of the gas mixture changes with the volume content of the measured component, so as long as the thermal conductivity of the gas mixture is measured, the content of the measured component can be known. The hydrogen analyzer commonly used in fertilizer companies adopts this principle.
3.2 Basic principle of thermal conductivity gas analyzer
1C31189G03 Thermal conductivity gas analyzer is a kind of physical gas analysis instrument. According to the principle that different gases have different thermal conductivity, the content of some components is calculated by measuring the thermal conductivity of mixed gases. This kind of analysis instrument is simple and reliable, suitable for many kinds of gases, and is a basic analysis instrument. The thermal conductivity gas analyzer has a wide range of applications, in addition to the usual analysis of hydrogen, ammonia, carbon dioxide, sulfur dioxide and low concentration flammable gas content, but also as a detector in the chromatographic analyzer to analyze other components. The PA200-ROD thermal conductivity gas analyzer of Chongqing Chuanyi No. 9 Factory is used in this project.
1C31189G03 Because the thermal conductivity of a gas is small, its variation is even smaller, so it is difficult to accurately measure it by direct methods. In the industry, indirect methods are often used, that is, through the thermal conductivity detector (also known as the thermal conductivity cell), and in practical applications, the change in the number of gas thermal conductivity is often converted into a change in resistance, and then measured by bridge. As shown in the following picture
Schematic diagram of heat conduction cell
A resistance wire with large resistivity and large temperature coefficient is tensioned and suspended in the center of a cylindrical metal shell with good thermal conductivity. There are gas inlet and outlet at both ends of the shell. 1C31189G03 The cylinder is filled with gas to be measured, and the resistance wire is heated with a constant current. Since the current through the resistance wire is constant, the heat generated per unit time on the resistance is also fixed. When the sample gas to be measured passes through the cell at a slow speed, the heat on the resistance wire will be transferred by the gas to the cell wall in the form of thermal conduction. When the heat transfer rate of the gas is equal to the heat rate of the current on the resistance wire (this state is called thermal equilibrium), the temperature of the resistance wire will be stable at a certain value, and the equilibrium temperature determines the resistance value of the resistance wire. If the concentration of the components to be measured in the gas mixture changes, the thermal conductivity of the gas mixture also changes, the thermal conductivity rate of the gas and the equilibrium temperature of the resistance wire will also change, and eventually the resistance value of the resistance wire will change accordingly, thus achieving the conversion of the change between the thermal conductivity of the gas and the resistance value of the resistance wire.