Extruders — Dry Cooking or Wet Cooking

• Dry Cooking Extruders

Dry cooking extruders are designed to run without adding any moisture to the ingredients. They are used in processing low moisture, highly expanded feed.

In dry extrusion there is no preconditioning of the ingredients with steam or moisture before entering the extruder barrel. All the heat produced by the dry extrusion process is generated by the mechanical energy applied to the ingredients via the screws, steamlocks and the barrel wall.

These simple dry extruders have since been transformed into more complex pieces of equipment with increasing product flexibility. Longer barrels can now provide increased capacity and cook. Increased selections of screws, steam locks and barrel liners allow high, medium or low shear configurations. Precision metering pumps for water and steam injection to the extruder barrel or to the conditioner provide additional capabilities, such as increased product cook and better shaped extruded products.

Aquafeed Extruder

• Wet Cooking Extruders

Wet cooking extruders are more complex equipment with precision metering systems for steam or water injection into either the conditioner and/or the extruder barrel. Preconditioning is a very important part of the extrusion process. The single most important aspect of the preconditioning system is the potential for additional mixing and the retention time, which is necessary for chemical or physical reactions to take place.

Conditioner types are also varied with a single or double mixing shaft, and may include differential diameter or differential speed conditioning cylinder systems. Preconditioning of ingredients prior to entry into the extruder barrel increases the degree of cook and reduces the wear on internal extruder parts. To increase control of the cooking process, the extruder barrel of a wet extruder is also jacketed for steam or water injection for additional heating or cooling.

Like the dry extruder, the wet extruder has single and/or double flightscrews, 

shearlocks, and heads. However, the wet extrusion system has a greater variety of single and double flight screws. It also has a more complex array of single or double, cut-flight screws and cone head screws, shearlocks and heads for more intricate extruder configurations.

Resource: http://www.aquafeed.com