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The operation principle and design of extruders

For more than 60 - years existence of extrusion technologies a wide variety of machine designs for their realization has appeared.

Extrusion – is short-time high-temperature process which is successfully applied both in food and feed mill industries.

The operation principle of extruders is determined by the technological essence of an extrusion process.

Extrusion is a complex physico-chemical process which goes under the influence of mechanical force, moisture and high-temperature.

The processed product is heated up due to the transformation of mechanical energy into heat which is escaped at overcoming an internal friction and plastic deformation of the product (an autogenous opreating regime) or due to external heating (a polythrone operating regime).

The changeable parameters of extrusion proccessing are raw material content, its nature and humidity.

During an extrusion changes of temperature, pressure, duration and intensity of the influence on raw material are possible.

There are three basic extrusion techniques: cold forming, thermal processing and forming and so-called "hot" extrusion.

Recently the method of "hot" extrusion which is carried out at high-rates, pressure and significant transition of mechanical power in thermal has become widesperad.

For the realization of extrusion process screw extruder in which the basic driven element is a screw of a special construction rotating in the cylindrical housing is applied. The forming matrix is installed on housing end.

Distinctive design feature of the extruder driven part is that chambers and screws are built according to the definite technological tasks. Extruders come standart with the loading chamber into which raw material and different compounds are charged, enclosed chambers with openings for measuring temperature and pressure and also for liquid components and sampling. Different elements can be installed on the screws which enables to create an additional resistance to conveyance of the product and to mix it during this conveyance.

Regarding processing stages the driven part of the extruder can be conditionally divided into three zones: I - raw material reception zone; II - plasticization and squeeze zone; III - product pressing off zone.

There are various driven parts of screw extruders.

1. Single-screw, including.:

- the cylindrical screw with fixed lead;

- the cylindrical screw with variable lead;

- with helix in the screw casing;

- the taper screw;

- the taper screw with the lead that decreases to extruder end.

2. Double-screw that do not mesh including:

- screws rotating in one direction;

- screws rotating in different directions.

3. Double-screws that mesh including.:

- self-cleaning screws rotating in one direction;

- partly self-cleaning screws rotating in different directions.

The operation principle of extruders will be the following: an internal pressure of placed into an enclosed chamber grain particles increases at heating due to moisture evaporation The instant depressurization of the chamber results in the expansion of air-steam mixture and causes increase in volume of grain particles.

In feed mill industry single-screw extruders with the cylindrical screw with fixed lead are basically applied .

The driven part of the above mentioned extruders consists of a set of screws between which retaining clouts are placed in the definite sequence, the set of screws and clouts is fastened on the stud with the help of a bolt with a conic head. Longitudinal slots on internal surface of the driven part housing eliminate product rotation during its conveyance. A conic nut with a bore is usually installed on housing exit. The temperature of the processed product is regulated by the change of spacing between tapers of the hold-down screw bolts and an output nut, or the change of diameter of a discharge point.

The extruder models differ by the design of the elements of the driven part, the amount of working chambers, the presence of additional systems expanding technological opportunity of extruders, feeders design, drives, etc.

The cylindrical form of the driven part is the most adjusted to production and therefore has lower cost of manufacture.

Ttwo basic extruder models for a feed mills industry were previously manufactured by the domestic industry. They are: PEK-125х8 with productivity up to 600 kg / h and KMZ-2М with productivity up to 500 kg / h. Extruder PEK-125h8 has also been equipped with an electric heater of the driven part.

Modern extruder designs implement as the technology of dry extrusion named so because extrusion ccurs by means of heat escaping during product passage through the driven part of the extruder and an opportunity to use steam during extrusion. For this purpose an extruder is evaluated with the chamber of steam preprocessing of raw material (air conditioner). Steam using doubles productivity and reduces the wear of extruder driven parts.

Dry extrusion process occupies less than 30 seconds. For this time the raw material goes through refinement, commixing, thermal processing, disinfecting, dehydration, stabilization and increase in volume.

In the design of extruders manufactured by Joint-Stock Company "CherkassyElevatorMash" the technology of dry extrusion is implemented.

Now the company manufaxtures 4 models: E-150, E-250, E-500, E-1000, with productivity accordingly up to 150, 250, 500, 1000 kg / h.

E-150 is intended for grain and peas extrusion;

E-250, E-500 and E-1000 are intended correspondingly - for grain and peas extrusion, for soya extrusion and universal.

Manufacturing of such modelling line of extruders has allowed to expand a circle of equipment users from small farms to mixed fodder plants.

Design of the driven part of the above mentioned extruders – single-screw with the cylindrical screw and with fixed lead. Each model has its individual “configuration" of the driven part. Each models is supplied with processed products temperature detectors. And in extruder E-1000 the temperature is controlled in two zones - in commixing zone in compression zone.

The extruders of E-model are designed with ease of maintenance being a high priority. So driven parts are to allow clearing and replacement of all elements during the shortest time. Cilindrical part barrels have built-in driven sleeves that raises maintainability of the extruder. Wearing details of the driven part are fabricated from special wear-resistant cast iron.

The extruders E-500 and E-1000 are shipped complete with water supply system that simplifies extruder starting during grain and peas processing, and also improves oprating regime, in patricular during crushed grain extrusion.

All models are shipped complete with special devices for extruded articles crushing.

While readjusting the extruders E-250, E-500 on soya extrusion the "configuration" of the driven part is changed and screw speed due to the replacement of the driving pulley of a V-belt transmission.is enhanced.

In comparison with the models E-150,250,500 in which the feeder is fabticated as the vibrating chute, extruder E-1000 is equipped with the screw feeder which electric drive has frequency regulation of rotation speed.

All extruders, except for E-150, are certificated.

Almost 6-years of extrusion equipment experience has shown that:

- extruders were basically used for processing not crushed, offgrade grain unpurified from mineral admixtures. Hence there is an increased wear of the dtiven part components;

- a lot of users had a low level of extrusion technology knowledge and hence misused the equipment;

- operating conditions do not meet the stated demands of power supply;

- users frequently do not account the productive time of equipment operation.

All this makes difficult to react to remarks on the equipment upgrading objectively and in time. At the same time the selection of new materials for wearing components of the driven part, the development of a design connected with increasing of equipment reliability are constantly carried on.

In the nearest future it is supposed to design soya extruded articles cooler with productivity 500 kg / h and 1000 kg / h.

An experimental model of the barrel-type cooler with productivity of 1000 kg / h has already been manufactured.

Solving the problem of cooling soya extruded articles will give complex solution of soay processing problem.

Besides extruder E-1000 is projected with the air conditioner that will in fact allow to double its productivity.

There is also a question of waste products of poultry keeping, cattle breeding and food-industry processing into forage with the help of manufactured equipment

One of main tasks of this conference – is to establish relations and to find mutually advantageous forms of cooperation between developers of extrusion technologies, equipment manufacturers and a user. It will allow to solve many problems on further updating of the equipment, broadening its technological abilities.

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