Let's Talk (Dryer) Control, Part 2
Continuing my discussion of the variables that can be altered to achieve a specification or control dryer operation, I'll look at control parameters than can control operation.
Controlling on Exhaust Temperature. The customary method of controlling a dryer is by modulating the inlet temperature (source of energy) or feed rate against fluctuations in the exhaust temperature. The exhaust temperature is monitored by a temperature element (thermocouple, RTD or thermistor) that (with or without an integral transmitter) sends an analog signal to a controller (discreet stand-alone or PLC). The controller's output will modulate the energy or feed input to the process.
This method of supervision permits controlled startup and shutdown of the dryer, which is a significant benefit. If there is no load on the dryer to absorb the energy, the system will control to the setpoint, reducing the risk of damage to associated systems such as dust collection.
Controlling on Humidity. The fundamental mode for controlling on humidity is to modulate the feed rate to control the humidity in the exhaust stream. A moisture-measuring sensor measures the humidity (amount of water vapor in the exhaust air). This approach assumes, reasonably, that the humidity in the ambient air is relatively consistent and the moisture in the exhaust gas stream is being evolved from the feed. Hence, the increase and decrease in the humidity is directly related to the feed, which creates the moisture as the product is dried. Altering the inlet temperature to compensate for fluctuations in the exhaust humidity would be pointless because the quantity of water evaporated from the feed is independent of this control parameter.
Most continuous humidity meters require a constant airflow for accuracy. If the system is not controlled on the exhaust temperature, the volume of air will fluctuate in response to temperature changes in the system. Ultimately, this will reduce the accuracy and effectiveness of the control. This limitation can be overcome by using mass flow as opposed to volumetric flow.
For this method to work effectively, a secondary control loop must be integrated into the overall control logic. Startup, shutdown and slow feed control issues also must be addressed. Controlling on humidity is an effective method of control drying operations. It is, however, an evolving technology. Industry must gain confidence in the reliability and accuracy of the available instrumentation and the control algorithms being developed.
Measuring the air humidity to control the air bleed in recycle systems is extremely beneficial. This enables the system to maximize the use of energy without saturating the process air.
Controlling on Final Moisture Content. Controlling a dryer on the final moisture content of the product can be implemented by modulating either the inlet temperature or the feed rate. The sensor reads the moisture contained in the product and outputs an analog signal to a controller, which will adjust the energy or feed input into the process. Until recent times, the technology to achieve this has been cumbersome. Controlling on final moisture is the only positive continuous method of achieving the desired moisture content in the final product.
It is essential that the product be presented to the sensor in a uniform, continuous manner. Obviously, depending on the type of dryer, the placement and handling required to achieve this must be considered carefully. Also with this method, startup, shutdown and slow feed control issues must be addressed. Theoretically, the discharge temperature will remain relatively constant if the final moisture is being controlled. However, if a source of external moisture or a product inconsistency is presented, problems may result. Again, industry must gain confidence in the reliability and accuracy of the available instrumentation and control algorithms.
Controlling on Product Temperature. Product temperature is measured using invasive thermocouples or RTDs or by noninvasive infrared thermometers or optical pyrometers. Aspects relating to controlling on product temperature are similar to controlling on discharge moisture content. The fundamental difference is that the moisture from temperature is a theoretically derived value whereas that from a moisture sensor is an absolute value.
Drying as an industry is more evolutionary than revolutionary. Any real revolutions that will impact the industry in the short-term most certainly will be from a control standpoint. As technology evolves, one will be able to control systems in real-time by performing a heat and mass balance across the system.
Personally, I have reservations about the true benefit of overinstrumentation and complex control, leaning more toward enhanced traditional control methods. Using cascading controls that switch from one control parameter to another as the operation reaches steady-state conditions is preferable.
Most importantly, the drying system has to have control. Many existing installations have primitive controls, and upgrading or adding a control system to an existing machine will realize enormous benefits. I know this because I have seen it. You out there -- and you know who you are -- need to address this.