Steam inlet heat supply valve

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. 

The basic reason for using different control-valve trims is to keep the stability of the control loop fairly constant over a wide range of flows. Linear-trim valves are used, for example, when the pressure drop over the control valve is fairly constant and a linear relationship exists between the controlled variable and the flow rate of the manipulated variable. Consider the flow of steam from a constant-pressure supply header. The steam flows into the shell side of a heat exchanger. A process liquid stream flows through the tube side and is heated by the steam. There is a linear relationship between the process outlet temperature and steam flow (with constant process flow rate and inlet temperature) since every pound of steam provides a certain amount of heat. 

Where Q = Heat-Transfer Rate Fs = Steam Mass Flow AHs = Latent Heat of Vaporization F = Feed Rate Cp = Heat Capacity of Feed T0 = Steam Supply Temperature Pt = Steam Supply Pressure P2 = Steam Valve Outlet Pressure Ps = Condensing Pressure Tj = Inlet Temperature T2 = Outlet Temperature A Tm = Log Mean Temperature Difference Ts = Condensing Steam Temperature... 

---