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Thermowells are tubular fittings used to protect temperature sensors installed in industrial processes. A thermowell consists of a tube closed at one end and mounted in the process stream. A temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the tube, which is usually in the open air outside the process piping or vessel and any thermal insulation. The process fluid transfers heat to the thermowell wall, which in turn transfers heat to the sensor. Since more mass is present, the sensor's response to process temperature changes is delayed. If the sensor fails, it can be easily replaced without draining the vessel or piping. To be representative of the average temperature of fluid, the thermowell must extend a few per cent of the inside diameter of the process pipe or vessel (Normally 1/3 rd of the pipe inside diameter)〔H. M. Hashemian (ed), ''Sensor Performance and Reliability'', ISA 2005 ISBN 1-55617-897-2, page 16〕 A thermowell is typically machined from solid bar stock and gun-drilled to ensure a long straight bore that closely approximates the diameter of the installed sensor (ex: a .260-inch bore matching a .250-inch sensor). A thermowell is typically mounted into the process stream by way of a threaded, welded, sanitary cap or flanged process connection. The temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the thermowell and typically spring-loaded to ensure that the outside tip of the temperature sensor is in metal to metal contact with the inside tip of the thermowell. Thermodynamically, the process fluid transfers heat to the thermowell wall, which in turn transfer heat to the sensor. Since more mass is present with a sensor-well assembly than with a probe directly immersed into the process, the sensor's response to process temperature change is slowed by the addition of the well. Since the mass of the thermowell must be heated to the process temperature, and since the walls of the thermowell conduct heat out of the process, sensor accuracy and responsiveness is negatively impacted by the addition of a thermowell. To be representative of the average temperature of fluid, the thermowell must extend a few percent of the inside diameter of the process pipe or vessel.〔H. M. Hashemian (ed), ''Sensor Performance and Reliability'', ISA 2005 ISBN 1-55617-897-2, page 16〕 A rule of thumb that is sufficient for most industrial applications (approximately 1% accuracy) is to ensure that the thermowell projects 5 times its own diameter into the process plus the length of its sensing element. So, for a grounded thermocouple spring-loaded into a thermowell with a 1-inch-diameter shank and a tip thickness of .25 inch, a typical immersion length would equal 5.5 inches (1 inch OD x 5 + .25 inch well tip thickness +.25 inch sensor wall thickness = 5.5 inches).〔J.V. Nichols D.R. White(2nd ed) "Traceable Temperatures", John Wiley & Sons, Ltd. 2001 ISBN 0-471-49291-4, page 136〕 ==Materials and construction== The thermowell protects the instrument from the pressure, flow-induced forces, and chemical effects of the process fluid. Typically a thermowell is made from metal bar stock. The end of the thermowell may be of reduced diameter (as is the case with a tapered or stepped shank thermowell) to improve the speed of response. For low pressures and temperatures, Teflon may be used to make a thermowell; various types of stainless steel are typical, with other metals used for highly corrosive process fluids. Where temperatures are high and the pressure differential is small, a protection tube may be used with a bare thermocouple element. These are often made of alumina or other ceramic material to prevent chemical attack of the platinum or other thermocouple elements. The ceramic protection tube may be inserted into a heavy outer protection tube manufactured from silicon carbide or other material where increased protection is required. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thermowell」の詳細全文を読む スポンサード リンク
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