By Jerry Speetjens
The purpose of steam flaking feed grains is to gelatinize starches, in order to make them more water soluble and therefore more digestible for the animal.
Over recent years, there has been a trend towards higher quality flakes with increased gelatinization levels, mainly achieved through conditioning at higher moisture levels and allowing more retention time prior to flaking. These higher added-moisture levels require more moisture removal during the drying-cooling process to ensure shelf life in commercial feed operations.
The steam flaking process can be broken down into conditioning, flaking and drying-cooling.
Conditioning consists of three variables that influence gelatinization: adding moisture, adding heat and allowing retention.
Adding moisture well ahead of the steam chest is necessary to allow water to penetrate the grain and reach the starches. In some parts of the world, it is customary to add water in a tempering bin and pre-soak the grain for up to 24 hours.
Moisture is also added in the steam chest by live steam injection, but for maximum gelatinization levels, the steam chest should be preceded by a tempering bin. This should result in presenting the grain to the flaker at around 21% to 22% moisture content. Surfactants are often used to increase moisture penetration into the grain.
Heat is added in the steam chest by live steam injection. Typically the grain is heated to 100°C inside the atmospheric steam chest and kept at that temperature until it reaches the flaker. Even higher temperatures would be better but would require using a pressurized steam chest.
Once the grain is inside the steam chest, it is essential to provide even heat distribution and then allow it retention time. For best results, most modern lines are sized to have at least one-hour retention time in the steam chest.
Flaking is the squeezing of the grain kernel between two counter-rotating rolls.
This mechanical deformation is the fourth factor that influences gelatinization, by fracturing the starch matrix and rupturing the cell walls. Reducing the thickness of the flake also increases solubility and rate of digestion of the grain by the animal.
Good conditioning is essential to deliver a softer grain to the mills, increasing flake quality by reducing fines. Softer grain also results in increased flaker capacity, reduced motor amp draw and reduced wear on the rolls.
DRYING AND COOLING
Depending on the type of grain and the required shelf life, different methods of drying and cooling are used to reduce moisture.
Ambient air cooler. Removing moisture from the product is essentially the evaporation of water. When using only an ambient air cooler the energy to evaporate moisture is derived from the heat within the grain kernel. The limitation of drying is then simply the amount of heat available within the kernel. This type of moisture removal cools down the product and is referred to as evaporative cooling.
At the same time, the air that is used to carry away this evaporated moisture is heated by passing through the product. This is referred to as convective cooling. In ambient air drying and cooling, the moisture removal stops once the product has reached ambient temperature because there is no more heat available for evaporation. In this situation, the only way to have good control of the final moisture is by regulating the amount of moisture added prior to the flaker.
Experience has shown that an ambient air cooler can remove up to 3% to 4% moisture. To reach the higher levels of gelatinization that are becoming customary, the grain needs to be flaked at moistures around 21% or 22%. An ambient air cooler cannot bring the moisture down to the required level, so external heat becomes necessary.
Dryer-cooler with external heat. The dryer-cooler used in such conditions consists of two separate sections, the first (top) drying section and a secondary cooling section. As Figure 1 shows, there is one recirculation fan pulling air though two separate decks. The air pulled though the lower deck consists of ambient air and takes care of cooling. The air drawn through the upper deck or drying deck is a mixture of the (warm) cooling air and the air heated in an external heat source, like a gas heater or steam exchanger. A majority of the drying air is recirculated back into the heater, which maximizes energy efficiency. A second fan and automatic air damper regulate the amount of air to be exhausted. This amount is automatically the cooling air volume since the cooling air is the only make up air into the system. The percentage of air that can be recirculated therefore depends on how much cooling air is needed and how much moisture needs to be carried out of the system without risk of condensation.
Counterflow machines are rapidly becoming the norm for drying and cooling of steam flaked grains. The counterflow principle provides the greatest efficiency since the product flows gradually downward in a layer, while the air flows in the opposite, upward direction through the layer. This way the warmest product meets the warmest air in the top of the layer while the almost cool product meets the coolest ambient air right at the bottom of the layer. Counterflow uses less air than other types of coolers.
A specially designed rotating grid allows for processing of very moist, thin flakes without the risk of bridging inside the machine or damage to the product. Figure 2 shows such a grid design. Other reasons for its popularity are the low maintenance, due to few moving parts, small footprint and low investment cost.
Control of the drying process
In a system with an external heat source, it is possible to flake at the optimum moisture level and subsequently remove all the excess moisture before storage. To prevent overdrying or underdrying, it is necessary to control the amount of energy added to the upper drying deck by controlling the air volume and the heater air temperature.
The air volumes on the drying section and on the cooling section are independently and automatically volume controlled by butterfly valves in the ducting. The heater air temperature is kept at the operators’ setpoint by a single setpoint controller. A controlled air volume and a controlled heater for air temperature result in the ability to add more or less energy simply by adjusting the heater setpoint, or the air volume setpoint.
Requirements for counterflow dryers-coolers for flaked grains include:
• A wide inlet rotary valve that allows as short as possible transition from the flaker to the dryer-cooler and also prevents bridging due to the products’ poor flow characteristics;
• A rotating spreading device that can distribute the flakes over the whole surface of the dryer-cooler;
• Tall bin walls to accommodate enough retention time for the low bulk density flakes;
• A discharge grid that prevents blockages inside the drying-cooling bin and handles the product very gently so as not to create unnecessary fines;
• Separate sections for drying and cooling ensure better control over both processes;
• Automatic air volume control to allow for constant air volume through the system, to prevent products being drawn into the exhaust system during startup and shutdown, and allow for controlled, repeatable drying and cooling;
• External heat source with automatic drying air temperature control;
• Recirculation of air to maximize efficiency; and
• Stainless steel ducting and cyclone to prevent corrosion caused by high moisture levels carried in the exhaust air.
--- Jerry Speetjens is the Area Manager North America for Geelen Counterflow. E-mail firstname.lastname@example.org.