Due to the deviation in steam flow on the steam side inside the tubes of the superheater tube loop, or due to uneven heating of each tube in the superheater tube loop, there is a phenomenon where the enthalpy increase in individual parallel tubes deviates from the average enthalpy increase of the tube loop. This phenomenon is called thermal deviation. Excessive thermal deviation can cause metal overheating and damage, seriously affecting the safe operation of the boiler. The causes of thermal deviation are as follows:
(1) Uneven heating of superheater tubes
① In large boilers, the flue outlet of the combustion chamber is generally 7-8 meters high and 8-20 meters wide. In such a large flue cross-sectional area, it is difficult for the flue gas temperature distribution to be uniform. Therefore, vertical superheaters may be affected by uneven flue gas temperatures across the width of the flue, resulting in uneven heating;
② Uneven flue gas velocity distribution causes uneven heating of the superheater;
③ Local ash accumulation on the water-cooled walls or slag prevention tubes of the combustion chamber can lead to uneven heating of the superheater;
④ Due to significant changes in flue gas temperature along the depth direction, if the plane of the serpentine tubes is not parallel to the front wall, it will cause uneven heating.
(2) Uneven steam flow inside the superheater tubes
① If the pressure difference at both ends of the parallel tubes is the same but the resistance of each tube is unequal, then the tube with higher resistance will have less steam flow;
② Different connection methods of inlet and outlet pipes in the header result in unequal pressure differences at the inlet and outlet ends of the parallel tubes, causing unequal steam flow.
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**What is thermal deviation? What are the causes of thermal deviation in superheaters?**
Thermal deviation refers to the phenomenon where the enthalpy increase in individual parallel tubes deviates from the average enthalpy increase of the tube loop due to deviations in steam flow on the steam side inside the tubes of the superheater tube loop or uneven heating of each tube in the superheater tube loop. Causes include uneven heating of the superheater tubes due to factors such as uneven flue gas temperature distribution, uneven flue gas velocity distribution, local ash accumulation, and improper arrangement of serpentine tubes. Additionally, uneven steam flow inside the superheater tubes caused by differences in resistance or header connection methods also contributes to thermal deviation.
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*Edited and organized by Beijing Hermann Wall-Hung Boiler After-Sales Service Center.*
*Hermann Wall-Hung Boiler Repair, Beijing Hermann Wall-Hung Boiler After-Sales Service Phone Number - Interpretation of the concept of thermal deviation.*