Archimedean Screw

In this type of equipment, an Archimedean screw is used to pressurize material in a plastic state and force it through a suitable die. This principle is most widely used in molding and forming operations, such as with plastics [22] and structural clay products [9]. These applications are highly specialized and will not be discussed here since they are usually considered to be outside normal size enlargement techniques. 

Almost exclusively used for extrusion of PVC-U into pipe profile and sheet, extrusion equipment consists of a cylindrical barrel containing two close fitting, intermeshed, Archimedean screws. PVC dry blend is fed in via a hopper at the start of the screws and is conveyed forward, softened, sheared and finally gelled by rotation of the screws and the heat of the barrel. The resulting homogeneous viscous melt is forced through a heated die fitted to the exit end of the barrel. Thereafter the downstream equipment sizes or calibrates, cools and pulls off the product which is also cut to desired lengths. 

The basic hydrodynamic equilibrium (the two liquid phases are evenly distributed from one end of the coil, called the head, and any excess of either phase is accumulated at the other end, called the tail). Here, the tail-head relationship of the rotating coil is defined by the direction of the Archimedean screw force which drives all objects toward the head of the coil. 

In general, the retention of the stationary phase in the coil rotated in the unit gravity field entirely relies on relatively weak Archimedean screw force. In this situation, application of a high flow rate of the mobile phase would cause a depletion of the stationary phase from the column. This problem can be solved by the utilization of synchronous planetary centrifuges, free of rotary seals, which enable one to increase the rotational speed and, consequently, enhance the Archimedean screw force. The seal-free principle can be applied to various types of synchronous planetary motion. In all cases, the holder revolves around the centrifuge axis and simultaneously rotates about its own axis at the same angular velocity w. 

At a first approximation, the hydrodynamic phenomenon observed also may be explained by the interplay between two force components acting on the fluid. At the distal portion of the coil, both the strong radial force field and the reduced relative flow of the two phases establish a clear and stable interface between the two liquid phases. At the proximal portion of the spiral column, where the strength of the radial-force component is minimized, the effect of the Archimedean screw force becomes visualized as agitation at the interface caused by the relative movement of two liquid layers [2]. 

The basic hydrodynamic equilibrium system (Fig. 1, right) uses a rotating coil which generates an Archimedean screw effect where all objects in different density present in the coil are driven toward one end, conventionally called the head. The mobile phase introduced through the head of the coil is mixed with the stationary phase to establish a hydrodynamic equilibrium, where a portion of the stationary phase is retained in each turn of the coil. This process continues until the mobile phase elutes from the tail of the coil. After the hydrodynamic equilibrium is established throughout the coil, the mobile phase displaces only the same phase, leaving the other phase stationary in the coil. Consequently, solutes introduced locally at the head of the coil is subjected to an efficient partition process between the two phases and separated according their partition coefficients. 

Retention of the stationary phase in the coiled column of the HSCCC system totally relies on the Archimedean screw force generated by the planetary motion of the column. However, this force is often too weak to retain the stationary phase for some polar solvent systems, such as aqueous-aqueous polymer phase systems, resulting in carryover of the stationary phase from the column. 

The paste thus obtained is transferred from the mixing house to the cartridge huts, in which it is made into cartridges by an Archimedean screw machine of the type shown in Fig. 37 the cylinder of paste issuing from a nozzle of required diameter is cut into the lengths appropriate for each size of cartridge. 

A is a square tank, and B is a vertical shaft which moves in footstep and collar as in Fig. 2, the latter being fastened on a plate 17. D is an Archimedean screw surrounded by a pipe G, which is fastened to the bottom of the tank the bottom of it is perforated so that the screw can draw the liquor from the bottom of the tank, pump it up in the direction of the arrows,... 

Fir. 3.—Sectional elevation of vessel for mixing liquids by means of an Archimedean screw. 

There are several extruders that do not use an Archimedean screw for transportation of materials, but still belong to the category of continuous extruders. These machines are sometimes referred to as screw-less extruders. Usually they employ a disk or a drum. Most disk extruders are based on viscous drag transport principle. To this category belong stepped disk extruders [Westover, 1962 Raleigh, 1879], drum extruders, spiral extruders [Ingen Housz, 1975], diskpack extruder [Tadmor, 1979, 1980 Tadmor et al, 1979, 1983 Hold et al., 1979 Valsamis, 1983], and many others. 

There are a number of machines that do not utilize Archimedean screw for transport of the material, but still fall within the category of continuous extruders. Sometimes these machines are referred to as screwless extruders or elastic melt extruder. These machines employ disks or drums to melt, mix, pump, and extrude the material and are based on viscous drag transport principle. They are not widely used in industry as forming machines, and will not be discussed here. 

The spread plate technique may be automated by the use of a piece of apparatus known as the spiral plater. An agar plate is rotated on an Archimedean spiral whilst being inoculated. The volume of the sample decreases (and is therefore effectively diluted) as the spiral moves towards the outer edge of the plate. A specialised counting grid relating the area of plate to the sample volume enables colonies in the appropriate sector to be counted. An electronic colony counter travelling in the same Archimedean screw may be used. 

Addison, H. Experiments on an Archimedean screw. Selected Engineering Papers, No. 75 (Institution of Civil Engineers). 

Screen bowl horizontal continuous archimedean screw 2000- 3000 as above with bar screen beadi... 

All hydrodynamic CCC systems utilize an Archimedean screw force that is created by coil rotation in the force field some use low-speed rotation of the coil in vmit gravity while others subject the coil to a high-speed planetary motion. An Archimedean force drives all objects in the coil toward one end of the coil, which is called the head the other end is called the tail. 

Three steps required for multistage solvent extraction, i.e., phase mixing, phase settling, and transfer of the mobile phase, are defined clearly in the discontinuous countercurrent distribution process using the Craig apparatus. These basic requirements are essentially fulfilled by the use of a coiled tube in a continuous fashion. Solvent extraction using a coiled column is most efficiently performed with a horizontally laid coil that rotates about its own axis. In this horizontal coil orientation, the rotation induces the well known Archimedean screw force, which can be utilized for performing countercurrent solvent extraction. 

This Archimedean screw effect on the hydrodynamic distribution of the solvent phases is illustrated in Figure 1, where each coil, consisting of five helical turns, is placed horizontally and rotated slowly around its axis. In Figure lA, air bubbles and glass beads are introduced in the coil, previously filled with water, and both ends of the coil are sealed. Then, rotation of the coil in the indicated direction induces an Archimedean screw force that drives all the suspended objects (lighter or heavier than the... 

Rgure 1 Archimedean screw effect in a rotating coil. (A) Motion of air bubbles and glass beads suspended in water (B) Motion of droplets of one phase of an equilibrated two-phase solvent system suspended in the other phase (C) Hydrodynamic equilibrium of two immiscible solvent phases in a slowly rotating coil. 

The multilayer coil assembly described above utilizes the Archimedean screw effect produced by the unit gravity. Thus, the relatively weak gravitational field limits the efficiency of the system. It has been found that the use of a centrifugal force field enhances the... 

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