Thermowells Working and Types
Thermowells Working and Types
Thermowells are used to aid in temperature measurements of process streams. The temperature measurement instruments such as; thermometer, thermocouple, or resistance temperature detectors cannot be inserted directly in to the process stream because the materials can be invaded by corroding chemicals which may lead to malfunctioning of the measurement devices and a need to replace them frequently.
Even if the temperature measurement devices are exposed to process streams directly, in order to replace them the fluids needs to be emptied which will result in additional downtime of the process. Thermowells are used to prevent this by acting as a fitting such that the temperature measurement devices are inserted in the thermowells and then the thermowell is exposed to the process streams. Using this mechanism the temperature measurement device can be simply removed even if the process is running and be replaced by re-inserting a new temperature measurement device in the same thermowell.
The basic design of thermowell is such that it is a cylindrical pipe with one end open and the other end closed. More of mass lies on the closed end of the thermowell thus its center of gravity is closer towards the closed end. The closed end is exposed to the process stream and the open end is from where the temperature measurement device is inserted inside the thermowell.
The end from where the temperature measurement device measures the temperature is at the closed end of the thermowell. Thus the mechanism with which the temperature is measured is; the process stream heats the mass of the thermowell near the closed end of and then the temperature measurement device will measures the temperature of the mass of the thermowell. It is not a direct measurement.
If the temperature of the mass of the thermowell is same as the temperature of the process stream then the temperature measurement device will provide correct readings but if any transience develops in the system then the responsiveness of the temperature measurement device reduces because of the additional conduction resistances provided by the mass of the thermowell. Also, the accuracy of the temperature measurement device may reduce because the thermowell will conduct heat out of the process through its wall. Hence, at temperature equilibration, the temperature of the process stream and the temperature of the mass of the thermowell will not be same. There will always be some error which corresponds to the heat lost due to conduction by thermowell.
Thermowells are supposed to protect the temperature measurement device from chemical attacks, forces due to flow of fluid, and the pressure effects thus materials need to be selected accordingly:
- Teflon thermowell is used when process temperature and pressure are from low moderate.
- Stainless steel thermowell is used when the process fluid is highly corrosive.
- Ceramic thermowell is used when the temperature is high.
- Ceramic inserted in silicon carbide thermowell is used when temperature is high and pressure differential is small.
- Chromium/molybdenum steel thermowell is used in highly pressurized application.
- Alloyed thermowell, which is a mixture of cobalt, nickel, chromium, and tungsten. These are used when process stream contains carbon, sulphur, or chlorine.