Depending on the moisture content and the volume of grain to be processed, grain drying can be carried out using different drying techniques and equipment.
Grain is dried before storage to inhibit microbial and pest growth (in a continuous-flow or batch drying process). Moisture can also be controlled at the storage silo through
in-storage dryer or aeration drying.
Continuous-flow dryers are used before storage and can be categorised according to the direction in which the air flows in relation to the grain. The main types of continuous-flow dryer are: cross-flow (e.g. screen dryers), mixed-flow (e.g. rack dryers) and concurrent-flow /counter-flow (e.g. tower/column dryers).
In-storage drying or aeration drying, such as bin drying, is a process whereby grain is dried while it is stored in a silo.
The specific control system used may differ in each case but the principles and objectives are the same, i.e. to maintain moisture content at a desired level.
Control of moisture content can be performed manually or automatically. Manual control is the simplest and most inexpensive method, while automatic (feedback) control is less labour-intensive and more accurate.
Advanced process controls, such as feed-forward controllers, can help to minimise dryer energy consumption by controlling energy inputs more precisely to meet moisture requirements.
In more stable operations, such as in-storage dryers and aeration drying, variable speed drives (VSDs) are used to adjust the air flow in aeration fans. The controller varies the speed of the fans according to grain humidity and temperature levels, providing better control and reducing energy consumption.
Benefits of automation
The moisture content of wet grain can sometimes vary significantly over a 24-hour period. This is due mostly to differences in harvest-procedure preferences, soil types and weather conditions. Moisture content differences of 10 to 15 percent can sometimes be seen in the dryer feed.
Regardless of the moisture content at the dryer input, all the grain in a batch must be dried to approximately the same average moisture content. In the case of continuous-flow dryers, disturbances in the feed are a challenge to operators (and automatic controllers), who need to adjust the speed of the grain feeder (e.g. unload auger) and/or amount of hot air to control the moisture content of the dried product. This is why manual control in operations with large variations in moisture often leads to significant overdrying or underdrying and are highly inefficient with respect to energy-saving.
Automatic feedback control of continuous-flow dryers is usually designed to minimise such occurrences. The controller compares the moisture content measured at dryer output to the desired set point and adjusts the energy input accordingly.
Figure 1 compares drying systems that use closed-loop feedback control with those that rely on manual control.
Energy savings that can be made by advanced control systems range from five to 20 percent.
Advanced systems, such as feed-forward control, also measure the input moisture content in order to account for disturbances not addressed by feedback systems. They also measure the feed rate and inlet/outlet temperatures to enable calculation of the material and energy balance, which in turn allows these systems to estimate the quantity of water that will be evaporated and the fuel quantity required to dry grain to that point, making it a much more energy-efficient method of control.
The measurement of feed rate, inlet/outlet moisture content and temperature requires extra sensors, which increases the cost of the system.
More advanced types of drying controls, such as ‘fuzzy logic’ (expert) controllers, use complex mathematical models of the grain drying process and are very expensive. Typically, they are used in applications in which moisture control is critical.
Sensors commonly used in controlling the grain drying process include thermocouples and resistance thermometers (for controlling air temperature); infra-red pyrometers (for controlling product surface temperatures); and wet-bulb and dry-bulb thermometers, resistance sensors and absorption capacitive sensors (for de-humidifying the air).
Control retrofit options are available for existing dryers. Depending on the type of control required, controllers may require additional sensors, particularly for determining levels of grain moisture and measuring air temperature.
Return on investment
Installation costs for automatic systems are generally three to four times that of manual control systems, and automatically controlled drying systems typically have higher maintenance costs, too.
Minimising energy consumption is normally a secondary objective in feedback control, however: its primary benefits flow from its capacity to preserve grain quality.
Key factors when evaluating quotes
When selecting a control system, accuracy and responsiveness are key parameters, as the cost of the system increases with more precise control of moisture. It is important to understand your process requirements – how critical is moisture content to product quality, for instance? – as well as the complexity of the operation (e.g. with respect to variability in feed conditions) if you’re to choose a control that’s appropriate to your application.
In the case of continuous-flow dryers, specific performance metrics (such as the ratio of standard deviation of moisture with and without control) can be used to compare different control options. Other key parameters commonly required by anyone requesting quotes for grain-drying include:
- The type of control required: Does your operation need a control system that is closed-loop, open-loop, PID, feed-forward or employs fuzzy logic?
- Sensor requirements (type and accuracy): Is drying equipment needed to control moisture content, temperature, grain flow?
- Actuator type required: Do you need a drying air-control valve or VSD (fan), auger speed?
- Controller type required: Would a PLC or process computer (model-based controls) suit your needs?
- Accessories needed: What might you require for data logging, real-time monitoring and the like?
Centre for the Analysis and Dissemination of Demonstrated Energy Technologies, 1992. Computer Control Systems for Continuous and Semi-Continuous Grain Dryers. [Online]
Dufour, P., 2009. 'Control Engineering in Drying Technology: Review and Trends'. Drying Technology, pp. 889-904.
Farm Energy Efficiency Resource Center, 2007. Strategies for Managing Energy-Related Grain Drying Costs. [Online]
Robinson, J., 1992. Improve Dryer Control. Chemical Engineering Progress, December, pp. 28–33.
Srzednicki, G., 1996. 'Control Systems for Aeration and Drying of Grain', ACIAR Proceedings. s.l., s.n., pp. 158–166.