Solid heat storage steam generators use their thermal "bricks" to provide a heat source, instantly vaporizing water to produce steam. During this process, the control system must not only ensure stable and standardized steam pressure but also consider steam dryness (quality) and temperature. Steam dryness refers to the proportion of the gas phase in the steam. A high dryness indicates low water content and high quality steam. Steam temperature, on the other hand, determines the steam's enthalpy and applicable process range. Pressure, dryness, and temperature are interrelated: pressure determines saturation temperature, dryness affects steam latent heat content, and temperature can increase dryness through superheating. Therefore, a coordinated control strategy must be adopted to balance these three parameters to produce "good steam"—steam with stable pressure, high dryness, and appropriate temperature—to meet user process requirements.
Industrial field experience shows that controlling only pressure while ignoring dryness results in excessive water content (wet steam) and insufficient heat content in the output steam. Conversely, excessive pursuit of dryness can cause pressure fluctuations or overheating. Therefore, the control system requires comprehensive adjustments to ensure the steam has the correct pressure, a sufficiently high dryness, and a stable temperature. This is particularly important for solid thermal storage generators, as their heat source has inherent hysteresis and evaporation dynamics are rapid. Even the slightest mismatch can affect steam quality. Below, Henan Rentai will discuss specific strategies for pressure control, dryness compensation, and temperature regulation, as well as how they can be decoupled and coordinated.
Steam Pressure Closed-Loop Control
Steam pressure is the most fundamental control metric for thermal storage steam generation. Pressure stabilization typically employs classic PID closed-loop control: A pressure transmitter feeds the steam outlet pressure back to the controller, where it is compared with the setpoint. The PID algorithm then outputs a control signal to adjust the actuator. The choice of actuator depends on the system design and generally includes:
Feedwater Flow Control: For once-through evaporators, adjusting the feedwater pump frequency converter or the feedwater valve opening changes the water inlet flow, thereby affecting the instantaneous evaporation rate. When pressure drops (high steam consumption), the controller reduces the feedwater flow to reduce steam generation, thereby mitigating the pressure drop. Conversely, when pressure rises, the controller increases the water inlet flow, allowing more water to absorb heat and convert into steam, mitigating the pressure increase. This solution is counterintuitive but effective because, given a given heat supply, reducing the feedwater rate can increase steam temperature and dryness, thereby raising pressure, while increasing the feedwater rate temporarily reduces steam parameters to stabilize pressure.
Heating power regulation: Some systems maintain pressure by controlling heating power. When a pressure drop is detected, the controller increases the electric heating input (for example, by activating more heating elements or increasing the SCR firing angle) to increase steam production and replenish pressure. If pressure is too high, the heating power is reduced. Because solid thermal energy storage provides an energy buffer, power regulation has a limited impact on immediate pressure. However, it can be used as a long-term regulation in conjunction with flow control to achieve coarse pressure regulation.
Steam outlet valve throttling: If the system has an automatically regulating steam outlet valve, the steam outflow rate can be varied by narrowing or widening the outlet valve, thereby affecting pressure. The steam outlet valve is similar to backpressure control: when pressure is low, slightly narrowing the valve to reduce steam output can temporarily prevent further pressure drops. When pressure is too high, widening the valve to increase steam release and reduce pressure. This method responds quickly but is limited to short-term regulation and is typically used in combination with the previous two methods.
The pressure control loop must respond quickly and stably. Cascade control is often used to improve quality: the primary control loop uses steam pressure as the controlled variable, while the secondary control loop controls feedwater flow or valve position. This allows pressure deviations to be converted to the setpoint of the inner loop, minimizing overshoot. For solid thermal storage evaporation systems, pressure fluctuations are not drastic due to their large thermal inertia. However, rapid response is still required to fluctuate steam loads (such as sudden decreases or increases in steam demand). Reliable pressure control ensures a stable steam supply from the steam generator, which is the foundation of coordinated control.
Steam Dryness Compensation Control
Steam dryness is a key indicator of steam quality and directly impacts the heating efficiency of downstream processes. Excessively low dryness indicates excessive moisture (wet steam), resulting in insufficient latent heat and the tendency to form condensate in pipelines. A dryness close to 100% is dry, saturated steam or slightly superheated, which is ideal. Due to the high instantaneous evaporation rate of regenerative steam generators, improper control can result in wet steam output. Therefore, a specialized dryness control strategy is required, typically implemented through feedforward-feedback compensation. While dryness is difficult to measure directly, information can be obtained through two methods: First, the dryness can be estimated using the difference between steam temperature and pressure (superheat). If the measured steam temperature is higher than the saturation temperature at the corresponding pressure, the steam is dry and superheated. Otherwise, if the temperature is equal to the saturation temperature, the dryness is determined by the water content. Second, the steam moisture content can be measured using a dryness sensor or a sampling condensation method. In some steam injection boilers, dryness measurement devices are installed to generate real-time dryness signals. The control system uses this dryness signal to indirectly control the main loop: rather than directly adjusting the dryness, it adjusts related process variables to influence dryness.
Specific compensation strategies include:
Feedforward water regulation: When load changes cause a change in steam dryness trend, the control output is preemptively corrected by using the feedwater flow signal. For example, if a sudden decrease in steam demand (a rising pressure trend) is detected, and the system predicts an increase in steam dryness (even overheating), the feedwater flow rate is increased to "wet" the steam. Conversely, if steam demand increases, the feedwater flow rate is reduced to prevent the dryness from falling too low. This feedforward approach uses the direct impact of water flow on dryness to provide rapid corrections. Reheating/Separation: Some designs incorporate a dryness increaser (such as a small superheater or water separator) at the steam outlet. The control system can adjust the heat input or separation efficiency of the dryness increaser based on dryness deviations. For example, if the dryness is too low, the superheater's electrical heating power can be increased to reheat some of the wet steam to increase the dryness; or the water separator can be activated to discharge some of the water phase. The controller controls the start and stop of the dryness increaser and its power output based on dryness feedback in a closed-loop manner.
Temperature Correction: Dryness control is achieved indirectly by controlling the steam outlet temperature. When the dryness requirement is high, the control system can slightly raise the steam outlet temperature to a slightly superheated state to ensure sufficient water vaporization and to allow for sufficient superheat margin. This is achieved by adjusting the heat storage release rate and the feed-water ratio. For example, a steam outlet superheat target (a few degrees Celsius) can be set. Once the pressure circuit meets this target, the temperature control valve or electrical heating can be adjusted to achieve the steam superheat target, thereby ensuring a dryness close to 100%.
An intelligent dryness control system employing this strategy can maintain high dryness output across a wide load range. For example, a new steam generator uses variable frequency control to maintain a high, stable steam quality across the load range of 20% to 100%, unaffected by pressure fluctuations. Quality control is often coupled with pressure control: increasing or decreasing feedwater to maintain pressure also affects quality, requiring a coordinated approach. Therefore, many systems employ fuzzy control or model predictive control, treating quality as an additional output. While pressure is adjusted, feedwater and heating are fine-tuned to maintain quality. This multivariable control improves steam quality consistency, ensuring consistently dry, high-quality steam output.
Steam Outlet Temperature Control and Superheat Control
Although steam temperature and pressure correspond precisely under saturated conditions, actual systems may require steam temperature control. Certain steam applications (such as sterilization and process heating) require a specific temperature. Furthermore, slight overheating can help improve quality and prevent condensation during transport. Therefore, solid thermal storage steam generators typically include a temperature control mechanism.
Temperature control can be achieved through the following means:
Reservoir temperature management: The higher the temperature of the thermal storage element itself, the higher the generated steam temperature, even to the point of becoming superheated. If superheated steam is desired, the control system can allow the heat storage temperature to exceed the saturation temperature for the corresponding pressure. This will further heat the steam during evaporation, resulting in superheat. The steam outlet temperature can be increased by adjusting the end-of-charge temperature or adding a superheat storage stage. The controller manages this: when a higher temperature is required, the heat storage temperature is increased or the heating time is extended; otherwise, the heat storage temperature is kept slightly above saturation.
Secondary Heater: A small electric heater (superheater) is installed at the steam outlet. The control system controls its power based on outlet temperature feedback to ensure the steam reaches the target temperature. This electric superheater offers fast response and precise regulation. When the outlet temperature is detected to be below the set point, the controller increases the superheating power until the target temperature is reached; if the temperature exceeds the set point, the power is reduced. This is similar to the superheat stage control in traditional boilers, but in an electric steam generator, it can be smaller and dynamically independent.
Water Spray Desuperheating: If the steam temperature is too high (especially at part load, where superheating is more likely), water spray desuperheating can be used. The control system opens the desuperheating water valve and injects a small amount of softened water directly into the steam stream. The water rapidly evaporates, absorbing heat and lowering the steam temperature to a safe level. This desuperheating is controlled by the outlet temperature PID to ensure the temperature remains stable below the set upper limit. This method is generally not commonly used in thermal storage steam generators when dryness control is good, but it can be configured as an overtemperature protection measure.
The balance between steam temperature and dryness is crucial: excessively increasing the temperature increases superheat. While dryness is guaranteed, it may cause overheating damage to certain processes. Outputting at a temperature strictly equal to saturation temperature may result in slightly lower dryness. Control systems typically choose a compromise, such as outputting a slight 5-10°C superheat to balance dryness and temperature requirements. This superheat is maintained constant by coordinating the thermal storage temperature and secondary heating power.
The temperature control loop interacts with both pressure and dryness control and is a subordinate loop. A typical strategy is to prioritize pressure control, supplemented by dryness control, with temperature fine-tuned to ensure the satisfaction of the first two. The control system makes decisions based on priority. For example, if pressure is insufficient, it prioritizes maintaining pressure at the expense of temperature; if dryness is insufficient, it slightly increases temperature to improve it; and if both are normal, it maintains temperature within the target range. Through this coordinated control logic, the three parameters are optimally balanced, ensuring that all steam output indicators meet requirements.
Control Coupling and Decoupling Strategies
As mentioned above, the three control parameters of pressure, dryness, and temperature are interconnected, forming a multiple-input, multiple-output (MIMO) coupled system. To achieve stable control, decoupling and coordination measures are required:
Priority Control: Control priorities are set, generally with pressure being the highest, followed by dryness, and finally temperature. The controller program first calculates the required pressure regulation actions, then corrects for the impact of these actions on dryness, and finally corrects for temperature errors. For example, if pressure requires an increase in feedwater, which would reduce dryness, the system will simultaneously increase the heat storage temperature or reduce the water addition rate to prevent the dryness from falling outside the acceptable range.
Feedforward + Feedback Combination: Measurable disturbances, such as load changes, are introduced into pressure and dryness control through the feedforward channel. For example, a sudden increase in steam flow affects both pressure and dryness. The controller uses the flow signal as a feedforward, immediately increasing heating and water supply. Simultaneously, it uses an empirical model to estimate changes in dryness and preemptively adjust superheat power. This feedforward compensation reduces the interplay between various circuits.
Decoupling control algorithms employ linear decoupling or model predictive control (MPC) strategies to build dynamic models of pressure and dryness, and online calculate a set of control actions to minimize the impact of adjusting one variable on the other outputs. For example, MPC can predict the dual effects of increasing feedwater on pressure and dryness, determining the optimal adjustment that balances both. These advanced algorithms significantly improve the coordination of three-parameter control.
Step-by-step control: Different control priorities are applied under different load conditions. For example, at high loads, steam is prone to moisture, so the control system prioritizes dryness control while tolerating minor pressure fluctuations. At low loads, steam is prone to overheating, so the system prioritizes temperature control to prevent overheating. By defining operating ranges and employing different PID parameters or control schemes, adaptive coordination is achieved.
Conclusion
In short, the "three-in-one" coordinated control of the solid thermal storage steam generator embodies the art of automatic control strategy: pressure control forms the foundational framework, dryness compensation ensures quality, and temperature fine-tuning provides the icing on the cake. Through proper decoupling, these three elements complement each other rather than hindering each other. This output delivers steam with stable pressure, dry quality, and optimal temperature, truly converting heat from the "hot brick" into high-quality steam, meeting the most demanding user requirements.
Henan Rentai Electrical Equipment Co., Ltd. specializes in industrial automation control system solutions, providing one-stop services including PLC+HMI, DCS, and remote monitoring. For inquiries, please call 0371-56520104 / 13526433367 or email info@hnrentai.com.