Transient temperatures in a dryer around a web break are very complicated to predict but a new model helps papermakers get back online quickly
September 2008
By Kenneth C. Hill, President and David Vijh, Senior Process Control Engineer, Kadant Johnson Systems, Knoxville, TN
Dryer temperature is a critical variable in determining how easily and quickly a paper machine recovers from a sheet break. A high-speed, lightweight machine can have four to six sheet breaks per day and recovery times can range from 10 minutes to an hour. During that time, the condensing load and heat flow of the dryers changes rapidly, from the highly loaded “sheet on” condition, to the lightly loaded “sheet off” condition. The rapid change in load causes a corresponding change in the dryer surface temperature. A drying model is needed to accurately and consistently predict the ideal dryer temperature for recovery during sheet breaks.
The thermal response of a dryer and the drying rate of the paper are directly related to thermal resistance between the steam inside the dryer and the moisture held by the paper fibers. Condensate, scale on the inside and the outside of the dryer shell, the dryer shell itself, air that is trapped in the interface between the paper and the dryer surface, and the outer layers of fibers in the paper create thermal resistance between the steam and the moisture in the paper.
During a sheet break, the heat flow drops to levels that are typically 15% to 20% of the condensing load in the dryers. If the saturated steam temperature is held constant, by not changing the dryer pressure, the shell temperature will increase during the break. A higher shell temperature during the break will make it difficult to thread the tail. When widened, the sheet will be below the moisture target until the dryer drops in temperature to the steady state, sheet-on temperature.
Lowering the saturated steam temperature in the dryers by lowering the pressure can compensate for the reduction in heat flow and increase in heat transfer coefficient. The difficulty is in determining how much to lower the pressures. The amount of pressure letdown required is dependent on operating conditions. A “one size fits all” pressure letdown of the dryers is insufficient to produce consistent results. A drying model can simulate the drying conditions and predict the dryer shell temperature with the sheet on and off the dryers. Such a model must compensate for changes in condensing load, dryer pressure, operating speed, etc.
Rapid Sheet Break Recovery
Tail threading, a key factor in recovering from a sheet break, is significantly affected by dryer temperatures. Ideally, the sheet break dryer surface temperature should be the same as the steady state, sheet-on sheet temperature. Sheet shrinkage and dryer diameters will then be similar during the tail threading process. Common problems during tail threading are tail “snap-offs” and the tail sticking to the dryer surfaces. Dryer temperatures that are too hot, or cold, can cause both problems.
Many operators prefer a “wet” tail when threading a lightweight paper machine, meaning the tail is 2% to 4% above the normal moisture level for tail threading. This requires that the dryer surface temperatures be reduced below the normal operating temperature. Very low dryer pressures can sometimes be required to produce these conditions, placing a high demand on the turndown capabilities of the dryer drainage system.
An “active dryer sheet break control strategy” can use a drying model to predict the ideal tail threading dryer temperature, while allowing for manual adjustments by experienced operators. Three criteria are used to evaluate the effectiveness of a dryer sheet break recovery strategy:
• The tail must “thread” through the dryers easily.
• The machine must return to first quality moisture quickly following a break.
• There should be no “break backs” once the sheet has been reestablished. This is a common occurrence when the sheet moisture following a break is outside an acceptable range.
An active sheet break control strategy calculates the ideal moisture response dryer temperature. This is the temperature that minimizes the time required to get the dryers back to steady state temperature once the sheet has been widened. The optimum temperature varies with dryer operating conditions. A drying model predicts the dryer temperatures with the sheet on and off the dryers and calculates the ideal temperature.
When the load rapidly changes during the sheet widening process, it takes time for the dryers to come back to steady state temperature. The sheet moisture deviates from target during this transient period. A key requirement is to minimize the time that it takes to get back to steady state. The ideal temperature for recovery to first quality moisture is not the same as the ideal tail threading temperature.
On heavier weight machines, the speeds are slow enough and the tail is strong enough that if the dryers are not at the ideal tail threading temperature, the machine still tails quickly. However, recovery back to first quality moisture can represent a serious loss of production. If the dryer pressures (temperatures) are turned down too much, then the sheet will be at high moisture compared with target. If the dryer pressures are not turned down enough, the sheet will be too dry compared with target. The amount of deviation from the target moisture determines how rapidly steady state is reached.
The interaction with the moisture control loop can extend the recovery time to first quality moisture. Dryer temperatures are transient around a break. Dryers have a slow response time. If the moisture controller is brought into service too quickly, it attempts to make a correction during this transient period. This will cause the moisture control loop to cycle. The moisture controller will attempt to correct the moisture by changing dryer pressures in the opposite direction to the transient change in dryer temperature.
Some moisture control loops have been observed to cycle for over 30 minutes around sheet breaks. The problems can be eliminated if the sheet is close to first quality moisture when the moisture control is activated.
Dryer Temperature Response
Understanding the transient dryer temperature response helps produce consistent results for recovery from sheet breaks. The response of the dryers to a step load change depends on the heat flow, the overall heat transfer coefficient, the thermal capacity of the dryer, and the efficiency of the condensate removal system.
Under simulated best-case conditions, dryer pressures are low and stationary syphons with dryer bars are used. The combination of low pressures and efficient heat transfer produce a rapid temperature response, improving response time to as little as three minutes. Such drying conditions would make it easy to predict tail threading conditions. The dryers would be at steady state conditions quickly and tail threading would not take place while dryer temperatures are transient.
Sheet Break Strategy
A dryer response model was developed by Kadant Johnson to predict dryer temperature response during a sheet break. This model is used to determine the ideal sheet break dryer temperatures and pressure turndowns. Inputs such as pressure, condensing load, speed, shell thickness, syphon clearance, and heat transfer coefficient are used in the model. For purposes of modeling, the sheet break was simulated by reducing the condensate load from a normal load to a radiation load. The heat transfer coefficient was simultaneously increased from normal sheet-on coefficients to sheet break coefficients.
Undershooting of dryer temperatures is common on machines that have a sheet break strategy that is focused on optimizing dryer temperatures for tail threading. These machines require that the dryer pressures be let down below the ideal level for moisture recovery. In this case, the sheet break strategy should anticipate the undershoot of dryer temperature and overshoot the pressure ramp up following the break to compensate. The ideal temperature for moisture recovery is one that quickly decreases to the steady state, sheet-on temperature with no undershoot of temperature.
The ideal sheet break dryer temperature will depend on the primary objective of the sheet break recovery. If the primary objective is tail threading, then the ideal temperature will match the sheet break dryer surface temperatures to the steady state, sheet-on conditions. If the primary objective is to achieve sheet moisture targets, then the ideal temperature is one that returns the dryer temperature to steady state quickly with no undershoot of temperature. In either case, a drying model is required to calculate the ideal temperature.
The best method for implementing the sheet break control strategy is to do an “on-line” calculation. A dryer model takes inputs from the control and gauging system to continuously calculate the operating dryer surface temperature and the sheet break dryer surface temperature. The ideal sheet break temperature is calculated based on whether the need is tail threading or recovery to first quality moisture. An operator offset is used to provide an experience factor in the strategy and an adjustment from the ideal temperature.
Once the dryer temperature response is known, the strategy can be adjusted. If the tail threading temperature target is below the ideal for moisture recovery, an overshoot can be programmed into the pressure ramp up. This strategy compensates for the anticipated undershoot of temperature. The hand-off to the moisture controller can be delayed until the dryer temperature is close to the anticipated steady state target. This minimizes the possibility of cycling created by the interaction with the moisture controller.
The on-line model allows operators to input different tailing moisture targets and bring the tail across the machine at a target different than the steady state moisture. A “wet” tail or one that is below the steady state moisture target can improve tailing on some machines. The program automatically calculates the dryer temperatures required to meet this “tailing moisture” target. This feature has proven to be particularly useful when tailing a coater or size press.
Conclusions
Dryer surface temperature response during sheet breaks is difficult to predict. The magnitude of the temperature change and the transient response time can vary considerably depending on heat flow, heat transfer efficiency, speed, and operating pressure. A drying model is required to accurately predict the dryer temperatures during normal sheet on conditions and during sheet breaks. The model should be self-adjusting to allow for changes on the machine. Active modeling and control of dryers during sheet breaks helps to reduce the sheet break time and cull losses.
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