The present invention mainly relates to a device for the separation of particulated solids from a pressurized fluid, in particular from a highly viscous mass of a thermoplastic material, to be implemented on machines for the extrusion of said materials.
It is well known that the filters provided on said devices are affected by gradual clogging due to the deposition of fouling particulated solids within the filters. Thus, after reaching a certain clogging level, it becomes necessary to clean said filters by means of the so-called "return pressure" principle.
Several patents have been published relating to this subject. German Patent No. 2 502 669 is to be considered as the most meaningful among them. Said patent teaches a device comprising a filtration assembly consisting of a circular foraminated throughput block in the interior of which the filter cloth is housed, said filter cloth being divided into a plurality of circular sectors, each of said circular sectors forming a throughput chamber that is tightly sealed against the remaining sectors. A rotary gate valve having a shape almost corresponding with that of a sector and the pivotal axis of which passes through the center-point of the circular throughput block is placed, in rest position, on a circular sector of the throughput block having no throughput holes. When the gate valve, suitably operated, turns a full 360.degree. starting from the rest position and returning to the rest position, it passes in front of all the circular sectors of the block and tightly occludes them on the fluid inlet side for a very short time, thus causing the inlet-side pressure to drop. Since in the internal wall of the gate valve there is a conduit communicating with the outside, as the gate valve occludes a circular sector the amount of non filtered fluid that is in the depressurization chamber thus created between the gate valve and the throughput block is forcefully caused to flow outside through said outlet conduit, due to the pressure of the filtered fluid that is on the discharge side of the filtration block.
In this way, every time the gate valve passes in front of the circular sectors forming the filtration block the fouling particulated solids that, due to the delivery pressure, have accumulated on the wall of said block are eliminated with no substantial prejudice to the filtration flow.
Nevertheless, the devices that are based upon the above-mentioned German Patent show the following drawbacks:
The tight-seal gate valve, in the closing phase of every circular sector, can meet fouling particles, like iron, wood or other particles, that hinder a perfect closing due to the interposition of said particles between the wall of the stationary sector and the rotary wall of the gate valve; this results in a jamming of the sector. PA0 The device necessarily operates with a high pressure on the discharge side of the filter, due to the following reasons: PA0 The flow capacity of the device according to the above-mentioned Patent is seriously affected by the diameter of the throughput - filtration block. In fact, since the discharge outlet in the internal wall of the gate valve is located nearby the center of the block and is therefore in an opposite position relative to the circumferential periphery of the filtration block, excessively enlarging its diameter for the purpose of increasing its flow capacity would lead to having an excessive distance between the circumferential periphery and the discharge point, with serious deviations of the outlet flow and alteration of the homogeneity of discharge. PA0 It makes it possible to increase to the maximum level the flow capacity of the filter with a double filtration area. PA0 It provides a "return pressure" purification system that acts simultaneously on the whole area of the filtration element by conveying the discharge material without deviation of the outlet flow. PA0 It provides a minimal operating pressure on the discharge side of the fiter by pressurizing-depressurizing the whole filter simultaneously during the whole time of a few seconds corresponding to the purging cycle and causing the return fluid during discharge to undergo a minimal head loss thanks to a suitable configuration of the conduit.
(1) Since the gate valve is moving, the time it closes a circular sector is practically infinitesimal, therefore it is necessary to compensate the quite short depressurization by increasing the pressure on the discharge side of the filter, also taking into account the delay of return response of the fluid, due to its elasticity. PA1 (2) The pressure on the discharge side of the filter must also be kept high to compensate for the high pressure drop the fluid undergoes because it passes through an outlet conduit that is necessarily long and tortuous. This high pressure on the discharge side of the filter not only causes an expensive increase in the energy consumption, but in certain cases an over-pressure is not even allowed due to the nature of the fluid to be filtered and, in other cases, typically in the field of thermoplastic materials, it is incompatible with other situations of the system that demand the process to be carried out below a certain critical pressure. A typical example is offered by extrusion with degassing where, by exceeding such a critical pressure, the fluid is caused to flow back and come out of the degassing valve.