Why did the main steam temperature of once-through boiler increase at the initial start?

Wrong thinking: opening the bypass to increase the steam flow will play a cooling role, and the steam temperature will drop. (actual observation: the bypass is turned up and the steam temperature is raised). Correct thinking: open the bypass, the steam pressure will decrease, the steam flow will decrease, the steam heating time will be prolonged, and the steam temperature will increase. (It can be assumed that the diameter of the pipeline is infinite, and the ideal state that the evaporated steam cannot fill the whole pipeline can be achieved. When the bypass valve is opened to a certain extent, the steam flow rate drops to only slowly drift, so the stay time of steam in the furnace is prolonged. )

There is another special phenomenon. After the bypass is opened, the steam temperature will suddenly rise step by step, which cannot represent the actual situation of steam temperature change. This phenomenon is entirely caused by the characteristics of thermocouple itself. 1. The sensitivity of thermocouple is directly proportional to the potential difference generated by itself. The higher the potential difference, the higher the sensitivity. 2. After opening the bypass door, the high-temperature steam has enough time to heat the thermocouple, which leads to the rapid increase of potential difference, so the reflected steam temperature change curve is stepped. 3. The thermocouple of the superheater is generally arranged in the box at the top of the boiler, and the internal pipeline is not in direct contact with the flue gas. Therefore, opening the bypass door and pressure change are beneficial to the flow of steam in the customs, so as to receive new high-temperature steam in the pipe wall welded by thermocouple.