Dryers control

Cleaning-related studies going on at the locations mentioned above include development of moisture-sensing and dryer-control Instrumentation and of trash-measuring instruments for use as... 

Increasingly, newer fired process heater installations are adding more fuel-air combustion controls and safety instrumentation systems. However, the decision on the extent of fired heater combustion controls, instrumentation, and safety systems to employ is fundamentally a loss prevention and risk tolerance issue, rather than a fire protection one. The following recommended practices, codes and standards apply to fired heater and dryer controls and instrumentation ... 

In the freeze-dryers of today, the entire sterilization process is carried out automatically, controlled and documented by the freeze-dryer control system. This includes a leak test. Depending on the size of the freeze-dryer, the time needed for CIP cleaning and sterilization is between 8 and 12 hours. 

Equipment commonly employed for the drying of solids is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Dryer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum dryers, furnaces, steam-tube dryers, and rotary calciners. 

Freeze-dryer controls have experienced the most dramatic developments in recent years. The original mercury gauge and thermostatically controlled heaters were initially replaced with analogue vacuum gauges, thermocouples and controls. Eventually, the latter two were superseded by digital devices. 

Control and Instrumentation The purpose of the control and instrumentation system is to provide a system that enables the process to produce the product at the desired moisture target and that meets other quality control targets discussed earlier (density, particle size, color, solubility, etc.). This segment discusses key considerations for dryer control and instrumentation. Additional more detailed information can be found in Sec. 8, Process Control. ... 

Any spray dryer control system can be run either automatically or semiautomatically. A fully automatic control system is recommended when the product quality should meet very stringent requirements and when lower operating costs are essential. A timing device starts up the dryer in a predetermined sequence. In case of failure in the control equipment it is always possible to employ manual control and to continue production without interruption. The shutdown and cleaning operations are programmed and controlled by means of timing equipment. The ACS must include a control system for fire detection, a system for fire or explosion prevention, and a programmable system for countermeasures. 

FIGURE 54.12 Strategy for finding a suitable dryer control. 

FIGURE 54.17 New compensated dryer control system (1) outlet temperature eontroller (2) inlet temperature controller (3) fuel valve (4, 5) temperature sensor (6) linear compensation module and (7) time lag module. 

O. Fadum and G. Shinskey, Saving energy through better control of continuous and batch dryers. Control Engineering, 27(3) 69-72 (March 1980). 

An apparent lack of knowledge of the important role that the dryer control plays in product quality and drying efficiency... 

The requirements and characteristics of any industrial dryer control system are [4]... 

In general, it is difficult to make online measurement of product quality it is often an inferred entity based on experience. The moisture content can be a measured variable if a suitable moisture sensor is used in the process. Analyzers are available for moisture analysis, which can be adapted for automatic, closed-loop dryer control. The most successful of these units rely on infrared, microwave, or capacitance detection [1]. Care must be exercised in the selection to allow for changes in bulk density or for void spaces, which will... 

The relationship between output, manipulated, and load variables constitutes the process control system of the dryer (see Eigure 57.1). In particular, the purpose of a dryer control system is to produce a desired output by changing the manipulated variables so as to compensate for changes in the main load variables (disturbances). Inputs are in the form of commands, which the output is expected to follow, and disturbances, which the automatic control is expected to minimize. 

Direct control and online measurement of the solids moisture content would enable significant improvements in dryer control by providing an immediate measure of the moisture content at the dryer exit and by automatic compensation for factors that distmb the control action. In certain dryers, these are already implanented industrially (e.g., in paper dryers). 

In industrial dryer control applications, the three control actions described above can be used individually or in combined modes proportional (P) controller, proportional-integral (PI) controller, or proportional-integral-derivative... 

Inferential control [11,17,18] is an early model-based approach for process control. This control strategy is useful when the main dryer controlled variable (i.e., product moisture content) cannot be measured directly due to some technical difficulties or due to insufficient economic justification for its measurement. Inferential control uses the values of measured outputs (e.g., product or gas temperature and humidity) together with the process model to infer the value of the unmeasured control variable. These estimates are used to adjust the values of the manipulated variables in order to keep the moisture content at the desired levels (Figure 57.5). This control policy can also be used to counteract the disturbances as it is less expensive to infer these disturbances from other available process measurements rather than by measuring them directly. 

FIGURE 57.7 Microprocessor-based dryer control system. 

Typical automatic batch-dryer control systems use the exhaust-air temperature as the controlled variable to determine when to end the drying process. Shinskey [37] and... 

FIGURE 57.12 A conventional flash-dryer control system. 

FIGURE 57.14 A conventional fluid-bed dryer control system. 

An alternative fluid-bed dryer control system has been developed based on the temperature-difference technique [45]. This technique is based upon the fact that the temperature difference between the material and the wet bulb is a function of the moisture content of the material at the instant of temperature measuraneuL... 

FIGURE 57.15 Cross-flow grain dryer control system. 

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