Control parameters for the secondary loop of the steam heater
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This post was last edited by HEJIYUER on 2020-10-2 at 20:16. For heating liquids such as water with steam, cascade control using TIC-FIC is commonly employed, as shown in the figure. . Question: Why isn’t a method of controlling the steam inlet pressure, such as TIC-PIC, used for the secondary loop? . Is there a difference in their applications?
That’s what I think; whatever method is used, as long as it achieves the goal, it’s fine. It’s just that using flow rate and temperature in combination makes it more stable.
The P2 behind the valve = shell-side pressure = the pressure corresponding to the condensation temperature; the heat transfer rate is directly related to the temperature difference. Isn’t it more straightforward to use a PIC to control the pressure?
Relatively speaking, the sensitivity to temperature is much higher than that to pressure, don’t you think?
The control valve changes the steam flow, and then the shell-side pressure changes. In other words, the steam flow changes first, and then the pressure on the shell side changes. The shell-side pressure event constant is often high. There is a principle in cascade control design: the time constant of the primary plant (the temperature plant) should differ significantly from that of the secondary plant (the flow rate plant or pressure plant), so that the secondary loop can exhibit the ability to quickly overcome disturbances.
This post was last edited by HEJIYUER on 2020-10-4 at 19:43. Is the change in shell-side pressure, such as a decrease, caused by a reduction in steam flow, or is it driven by the load on the cold side? The time constant of the condensation pressure with respect to temperature will not be very large, and P is also suitable as a secondary loop.
Shell-side pressure can also be used as a secondary loop; both approaches are feasible. The shell-side pressure decreases, possibly due to a reduced steam flow or an increased degree of steam condensation (for example, an increased material flow). In terms of the control scheme, steam flow rate serves as a secondary controlled parameter, with the secondary loop accounting for the disturbances in flow rate caused by changes in steam pressure. It is insensitive to disturbances in material flow rate. Shell-side pressure is selected as the secondary controlled parameter; the secondary loop takes into account not only changes in steam pressure and flow rate but also changes in material flow rate. In terms of the time constant, with the shell-side pressure as a secondary parameter, the time constant is slightly larger, but not by much.
I agree with your view on the immunity of the secondary circuit. However, changes in the cold-side load are not disturbances. For example, if the process requirements dictate an increase in load, that is a rigid requirement of the process, and the control system must \"follow and match\" it. Analyzing the subsequent process after increasing the load based on the characteristics of the heat exchange process: First, the pressure on the shell side decreases, the flow rate through the FV increases, the command valve of the FIC closes; after some time, the temperature drops, and then the TIC commands the FV to open wider in order to match the new load. So I think, isn’t it more direct to use PIC?
I think that although in normal situations pressure is generated by the flow rate, here there is steam condensation in the shell side, so this conventional relationship no longer applies. When the cold flow rate is high, the hot flow rate is also high, and thus more condensation occurs. However, the pressure in the shell side might remain the same; perhaps because the flow velocity of the cold fluid in the tube side is higher, the heat transfer coefficient of the tubes increases, resulting in greater heat exchange, and the pressure could even decrease: lol.
Personally, I think using traffic makes calculations easier and more intuitive.