US2276631A - Discharge element of flat screens - Google Patents

Description

March 17, 1942. w. G. E. SMITH DISCHARGE ELEMENT 0F FLAT SCREENS Filed April 18, 1939 Fig. I

m m s r. mm a n n W w mm a 4 r w Patented Mar. 17, 1942 UNITED STATES PATENT OFFICE 2,276,631 DISCHARGE ELEMENT or FLAT SCREENS Walter G. E. Smith, Portland, Oreg.

, Application April 18, 1939, Serial No. 268,515

2 Claims.

My invention relates to so-called fiat or diaphragm screens used in screening wood pulp in the making of paper.

In the usual construction the flat screens are secured in place in the upper part of a vat or trough the bottom of which is closed by a diaphragm, thus providing an inclosed space between screen plates and diaphragm, which space I shall term diaphragm chamber. The pulp stock to be screened is caused to fiow over the screen plates, which are usually slightly inclined, and the fine fibers and water pass through the slots of the screen into thediaphragm chamber, and from there through a controlled outlet known as a flow box," and thence to a discharge flume. The diaphragm is oscillated vertically to promote the passing of the fine fibers through the slots of the screens. The up-strokes of the diaphragm are assumed to cause the stock and air in the diaphragm chamber to surge up repeatedly through the screen plates, thus momentarily lifting the mat of fibers from the screen plates and preventing the clogging of their slots; and the down-strokes of the diaphragm induce a suction and discharge of stock through the screen plates into the diaphragm chamber. The flow box parallels the diaphragm chamber and the flow through the flow box into the flume is controlled by a wall or dam.

When screening with a low dam in the flow box, the potential production is at its maximum. But the stock holds more or less air, and there is a tendency for part of this air to escape and accumulate in the diaphragm chamber; and this accumulation increases in proportion to the space between the surface of. the stock in the diaphragm chamber and the screen plates. The air so accumulated may increase to such deegree that a condition of what I term air-binding results; That is, the level of the stock in the diaphragm chamber drops so low as to permit a stratum of air of considerable depth to form between the screen plates and the surface of the stock in thediaphragm chamber. This stratum of air then acts as a cushion, and prevents the up-strokes m the vibration of the diaphragm, producing the required cleaning efiect on the slots of the screen plates. Production then practically ceases.

Accumulation of air in the diaphragm chamber is futhermore attributable to the manner in which the diaphragm is installed in the screening apparatus. The diaphragm is firmly held at its perimeter, and maximum oscillation can be imparted only to its freely movable central section, which is spaced some considerable distance the diaphragm fades to nothing in the marginal area of the diaphragm, and the movement of the currents produced in the stock held in the diaphragm chamber over said freely oscillatable section is quite different than that produced over the substantially static sections.

Furthermore, the consistency and the volume of stock flowing over and through the screen plates constantly changes. High consistency stock does not flow freely. For example, in one instance, the stock being screened may flow at a very high rate through the screen'plates into the diaphragm chamber, and at another instance diaphragm chamber empties faster than it can fill, and in consequence leaves a larger space in the diaphragm chamber for air to collect and form the cushion as above-mentioned, unless the stock in the diaphragm chamber is held at the proper level.

Furthermore, the diiference in displacement between those portions of the stock overlying the freely oscillatable sections and those lying over the substantially static sections of the diaphrgm also causes the release of some-air in the diaphragm chamber during the receding movement of the diaphragm, after each upstroke.

To prevent the air-binding condition, the level of the dam has heretofore been raised, thereby to bring the level of the stock in the diaphragm chamber more closely to the screen plates, and thus reduce the space in which air may be entrapped. The high level dam, however, while reducing the tendency toair bind, also decreases potential production; for the volume of stock so held inthe diaphragm chamber may be so great in depth that the forces of the up-strokes of the vibrations of diaphragm will be absorbed to such extent as to cause a sluggish removal of stock over the dam of the flow box; and, further, tov

dissipate said forces to such degree as to have little effect in keeping the slots of the screen plates open. Such conditions reduce production to inefliciency.

In short, a greatervolume of stock is produced when a low dam is used, on account of the head developed in the diaphragm chamber, but such strokes of the diaphragm.

condition induces air-binding, due to the low level of stock held in diaphragm chamber, which air binding is prevented if a high dam be used. But a high level reduces production. In

' short, it was my observation that a low dam and a high dam has each its own desirable and detrimental effect. I therefore concluded that a construction utilizing the desirable features of both a low dam and high dam would be desirable, and I conceived an improvement upon flat screen apparatus by which the combined eflects induced by both a low dam and a high dam are obtainable.

I discovered thatthe customary flow box in flat-screening apparatus is inefllcient and must be supplemented by a siphoning means connected to and enclosing the outlet of the-diaphragm chamber and controlling and assisting the discharge from said chamber. The siphoning means .must be, arranged so that the top-bend thereof constitutes a dam, the heightof which must be arranged relatively to the depth of the diaphragm chamber, and the character and force of the deair which may become entrapped, or for admitting air to relieve a partial vacuum pulled by the siphoning means, in case of faulty arrangement of the siphoning means.

Further details of my invention are hereinafter fully described with referenceto the accompanying drawing.

. The accompanying drawing represents diagrammatically, but illustratively merely, two

types of syphon attachments which may be used. Figure 1 shows a vertical section of a common type of flat-screen apparatus, as used for screening paper-making pulp, with my invention applied thereto; the section being taken through vice producing the vibrations of the diaphragm.

The height of the dam so formed by the top-bend of the siphoning means must also be'such that the intake-side of the siphoning means will be adapted to hold an amount of stock of suflicient volume to tendto impose some resistance to the effect of sudden-changes in pressure'exerted on the stock in said intake side by the up-movement of the diaphragm-bottom of the diaphragm chamber tending to push stock over said dam.

Such resistance will cause part of the force of the up-movements of the diaphragm: to-be directed upwardly, to keeping the slots of the screen plate open and thus obtaining the maximum produc tion. Without such resistance, the action of the diaphragm would have little, if any, cleaning effect upon the slots of the screen plates, because the forces of the up-movements of the diaphragm would be expended at the point of least resistance, which would be the phragm chamber, and would be dissipated, preventing eifective action being exerted upon the slots of the screen plates. These slots would then tend to become sealed, and it requires only a few seconds in operation for the slots to bedischarge port of the dia' tain stockto the come sealed. .On the other hand, such resistance must notbe so great as to tion by the diaphragm to force the stock over the dam.- The latter result is prevented by providing siphoning means which assists the moving of stock from the diaphragm chamber over the dam. Furthermore, the length of the siphon leg must be such that the combined forces of the downstrokes of the diaphragm and the pull of the siphoning means on the stock in the diaphragm chamber will be less than the forces of the up- All these features are imperative to assure that a part of the force of the upstrokes of the diaphragm will be directed upward to eilect reverse, cleaning surges of the material in the diaphragm chamber through the slots in the screen plate, thus keeping the slots open, as indispensable for obtaining maximum production. The discharge end of the siphoning means must be provided with an air seal, to prevent exterior air entering the siphoning means. Such seal is conveniently obtained by providing the discharge end of the siphoning means with an up-bend extending to a level arranged relatively to therequired pull of the siphoning means.

It is convenient also to have a valved-vent in the require a pumping acthe diaphragm chamber in the direction of, and

through the usual stock-discharging outlet thereconventionally indicated by c. The inclosed diaphragm chamber c has the usual stock-discharging outlet f. Instead of the usual flow box" for receiving the discharge from the outlet of the diaphragm chamber, I provide a siphoning means 9 for controlling andaiding said discharge; the inlet side of said siphoning means being connected to'and enclosing said outlet. Let it be assumed that the depth of the space between screen plate b and diaphragm c is three and one-half inches, and that the distance between the screen plate b and the top of the outlet f is one inch; and that the highest point i in the. lower walla of the bend :i of the siphoning means 0 is one-half inch below the screen plate b. This highest point i in efiect constitutes a dam. Thus the intake side k of the siphoning means 9 will always relevel 1, thereby sealing the outlet of the diaphragm chamber against the admission therethrough of air from the exterior.

The pressure of the volume of stock held in the intake side 7c of the siphoning means 'g must be suflicient to resist to some; degree the pushing of the stock over the dam provided by top-bend i of the siphoning means in order to cause-the up-movements of the diaphragm e to be effectively directed upward for clearing the slots of the screen plates.

To provide the required resistance the pressure of the volume of stock retained in the intake side of the siphoning mean must balance approximately, the pressure of the volume of stock held by the diaphragm chamber e, as above mentioned. The force of the up-strokes of the diaphragm 0 will then be divided; a part of such force will be effectively applied tocause the air,

and stock in the diaphragm chamber to be directed against, and to surge through the slots of the screen plates b with each upstroke of the diaphragm and thus clear the slots; and the The length of the syphon leg 9 must be adjusted so as not to cause the syphon to exert too great a pull on the stock in the diaphragm chamber, which would supplement unduly the return the upper bend s of the siphoning means is 10- cated half way between the'screen plate 0 and diaphragm p. This construction, also, accomplishes the ideal arrangement of having the.

volume of stock in the diaphragm chamber and in said intake side approximately in balance.

The up-bend u in the lower or discharge end of the syphon leg, in either construction described, is provided to obtain a secondary seal in the siphoning means and cause the same to be started by the sudden discharge of the stock accumulating from time to time in the siphon leg. Without this lower end up-bend and secondary seal u, the stock overflowing the top- -bend of the siphoning means would flow out, air could enter the discharge end, and no syphon action would be produced.

Instead of providing an up-turn bend as u, the

flow-box is used, which is, in effect, the same thing as the up-turn bend u already mentioned. The discharge end of the long siphon legmust not be submerged to too great a depth, since such may develop conditions which would prevent any flow from the diaphragm chamber; because the forces applied by the diaphragm are fixed, therefore can evacuate the air accumulating in the diaphragm chamber only through a certain depth of stock in which the discharge end of the siphon leg is submerged. If this depth is too great the force applied by the diaphragm may not be sufficient to force the accumulated air through the volume of stock enclosing said discharge end; in that case air binding may be developed, and no efllcient flow through my siphoning element be produced. I therefore consider the described construction more practical.

In order to release air which may have become entrapped in the top-bend of the siphoning means, or to admit air to relieve a partial vacuum pulled'by said siphoning means, I provide a "vent indicated by '0 which may be of any suitable construction.

When the design for operating conditions of a flat screen are properly coordinated, that is, the stock, the speed of pulsations, depth of diaphragm chamber and size ofdischarge spout are correctly proportioned, no vent in the siphoning element of my invention was found. But if my invention is applied to an old-style flat screen, an unbalanced action may occur, which may have to be relieved by providing a vent in my siphon element as mentioned.

Operation.-When screening process is started, stock flowing over and through screen plates will soon fill up diaphragm chamber to the level determined by the high point or dam i in Fig. 1, or y t in Fig. 2. Further stock received by diaphragm chamber will overflow said high point and collect in the lower end it of the siphoning means until the volume therein accumulated causes a discharge producing a suflicient vacuum therein to cause the siphoning means to act. In that way the level of stock in diaphragm chamber and the seal of its outlet against admission of exterior air is constantly maintained; also the up-stroke forces exerted by' the oscillatable sec-' tion of the diaphragm upon stock in diaphragm chamber are applied directly to the service of clearing the screen plate slots, and not dissipated by a forcible discharge of the stock over the dam.

The best results are obtained by keeping the input in the diaphragm chamber and the discharge therefrom approximately in balance. Therefore the capacity of the siphoning means for aiding the discharge of stock from the diaphragm chamber must approximately equal the rate at which the stock passes through the screen slots into the diaphragm chamber. To accomplish this the speed and the character of the vibration of the diaphragm-vibrating devices, the size of the screen slots, and the consistency and flow of the stock have to be taken into consideration. -If the discharge rate is not sufficiently rapid to balance the intake into the diaphragm chamber, it may be that air has collected in the top-bend of the siphoning means. To relieve such condition I have provided the up-bend of the siphoning means with a valved-vent indicated by 1: through which to let out the entrapped air. Similarly. ii. the siphoning means tends to discharge thestock more rapidly than received in the diaphragm chamber, as may be caused by the siphoning-means pulling a partial vacuum, the opening of said vent will permit air to enter and thus reduce the partial vacuum.

If the top-bend of the siphoning means be located too high the consequent level ofstock will cause the siphoning means to be slow to start, and require the application of greater force by the diaphragm to push the stock over said topbend of the siphoning means. Similarly, if the top-bend is lower than required to produce the desired required level, the siphoning means will tend to pull down the level of the stock in the diaphragm chamber too low for effective application of the force of the up-movements of the diaphragm to keeping the screen slots open.

In short, the siphoning means must be designed relatively to the character of the action of the diaphragm; that is, whether producing long strokes, or short pulsations; and in either case must be adapted to impose a slight resistance to the discharge of the stock over the dam constituted by the up-bend of the siphoning means, as stated.

But this resistance must not be so great that the greater part of the force of the up-movements of the diaphragm will be consumed in discharging stock from the diaphragm chamber. If the said balance is approximately maintained, the power requirements for vibrating the diaphragm are greatly reduced.

The variables in the construction of the cooperating elements of a flat-screen apparatus and, the variables in the volume, consistency and flow of the stock to be screened make it impossible to lay down any exact construction suiting all conditions. But any skilled mechanic can readily apply my invention to a diaphragm screening apparatus so as to operate on the principle above described, and thus obtain a continuous operation and maximum production.

The intermittent discharge apparent in ordinary flow-box construction is entirely eliminated.

I claim: I

1. In a device of the kind described, having a fiat screen and a diaphragm chamber, an outlet from the chamber, and a siphon having a short leg extending up connected to said outlet and a long leg extending down from the upper end of the short leg in a substantially U-bend, the lower wall of said U-bend forming a dam against the outflow of material from said chamber with the top of the dam being on a plane below the plane of the screen and above the plane of the top of the outlet the height of the dam being sufficient to normally maintain the level of the liquid in the chamber at a height below the bottom of the screen and at least about as high as a plane passing half-way between the top and bottom of the chamber.

2. In a device of the kind described, having a flat screen and a diaphragm-chamber, an outlet from the chamber, a siphon having a short leg extending up connected to said outlet and a long leg extending down from the upper end of the short leg in a substantially U-bend, the lower wall of said U-bend forming a dam against the "outflow of material from said chamber with the top of the dam being on a. plane below the plane of the screen and above the plane of the top of the outlet the height 01' the dam being suflicient to normally maintain the level or the liquid in the chamber at a height below the bottom of the screen and at least about as high as a plane passing half-way between the top and bottom of the chamber, and an upwardly opening U-bend forming an air trap at the bottom of said long leg.

WALTER G. E. SMITH.

Images