Steam inlet heat supply valve opening

Opening the steam or hot-oil inlet heat supply valve does not seem to get more heat into the tower. 

It is better not to use a steam inlet control valve when using low-pressure steam. The channel head pressure will then always equal the steam header supply pressure. The flow of steam to the reboiler can then be controlled only by raising or lowering the water level in the channel head, as shown in Fig. 8.5. This sort of control scheme will work perfectly well until the water level drops to the bottom of the channel head. If the condensate drain control valve then opens further, in an attempt to increase steam flow into the reboiler, the condensate seal is blown, and the reboiler heat duty drops. 

---