World Aquaculture 69 (Continued from page 66) There are many possible adjustments on an extruder that allow density control. Most require stopping the equipment for mechanical changes. Balancing the extruder barrel design with the use of back pressure valves (BPV) and/or what is referred to as the mid-barrel valves (MBV) allows for adjustments during extruder operation to overcome variable parameters. Extruders have always had methods of disrupting the flow of material to increase the shear or energy inputs. The two inbarrel valves allow for adjustment easily and quickly by the operator to provide pellets of the density and internal cell structure control needed by the aquatic industry today. They simply are great aids in the operation and reduction of down time caused by ingredient fluctuations or equipment wear. The effect of the MBV when operated at 50 percent closed achieves a wet density of 400 gm/L with 28 kWh/t into the feed. Simply by closing the valve to 92 percent, the density goes down to 279 gm/L and 55 kWh/t imparted into the feed. The BPV is presently the valve of choice because of the numbers working in the field and the quick response seen as it is mounted directly behind the die plate. The BPV photo shows a negative pneumatic hood and the BPV hinged off a support structure for ease in use. Another technology is the mid barrel atmospheric vent, which can be used in conjunction with a product retaining screw and possible vacuum assist. This technology has been selectively used on feed pellets for some tuna feeds because of the diameter, oil levels and reduced fines generation. The external density management system (EDMS), as shown in Figure 6, is a device that also mounts on the end of an extruder barrel and assists in controlling the density of sinking feeds. In the photo, the extruder barrel feeds a back-pressure valve with a pressure vessel mounted above a discharge airlock. There are a few designs available, but the one discussed utilizes this combination of BPV technology mounted on the extruder barrel before the pressure vessel. The BPV allows independent adjustment of the energy input and cook level before passing through the actual die enclosed inside a small pressure device with a rotary vane discharge and controls to maintain a selected pressure. The basis of this design is the fact that water does not boil at elevated pressures. Expansion occurs when the product with superheated water exits the extruder die. The water immediately changes phase from a liquid to a gas inside the pellet causing expansion. The increase in pressure around the die keeps the water from changing into steam and, thus, expansion is controlled and the pellet is formed and set before exit from this area. The result is a higher capacity sinking feed with lower moisture levels out of the extrusion cooker than what occur without this device. The technology is used mainly for pellets such shrimp feeds that require long water stability. Additional technology for even higher production levels of extruded shrimp feed exists and approaches 10 to 15 t per hour. This requires the use of a PDU or product densification unit. Methods to control density and specific mechanical energy inputs also result in cell structure development as well as cell size control. The comparison of the four photographs seen in Figure 7 shows the effect of increasing the specific mechanical energy in a feed formulation. Notice that as the energy input increases as shown on a percentage increase basis from 0 to 40 percent, the size of the cells themselves decrease and cell numbers increase. The use of twin-screw extruders with increased power and torque capabilities are becoming popular with aquatic feed producers for the range in diameters, ingredient flexibility, increased rpm and horsepower inputs that can lead to significant increases in production capacity. The production range with four extruder sizes start at 300-7,500 kg/hr and the same models have expanded the range to achieve 1,700-22,000 kg/ hr. This allows the cost for use of twin-screw extruders to come down inasmuch as a smaller model can be used to provide the production of what larger models formerly produced. Additions to extruders as described, combined with balanced extrusion systems in terms of preconditioning and extruder barrel designs, have greatly reduced the change over time in multiple use feed productions facilities. These factors also can greatly increase the production capabilities of single product production facilities. Various methods exist to perform many of the same functions as outlined above. Matching the production method consumer requirements results in greater tonnages produced more efficiently with lower waste over a broader product variety. Notes 1Skretting Australasian Aquaculture Conference, Adelaide, South Australia, 2006 2Wenger Manufacturing, Inc., Sabetha Kansas, USA References Hardy, R. W. 1999. Alternative Protein Sources for Aquatic Feeds. Feed Management Magazine 50(1):25-28. Gulf of Mexico plan delayed The Gulf of Mexico Fishery Management Council’s plan for development of a management plan for aquaculture in the Gulf of Mexico still awaits approval from the National Oceanic and Atmospheric Administration (NOAA), a situation that has gone on for several months. The effort may be delayed further now that opponents to the plan have enticed several Congressmen to write NOAA expressing their concerns that approval of the plan will lead to degradation of the environment in the Gulf.
RkJQdWJsaXNoZXIy MjExNDY=