Variable chamber press

Fig. 21. Membrane plate press, ie, recessed plate with variable chamber press.

These media are described as two-sided, with an upper slick surface for filtration being produced by a sateen weave. This surface is fiuther modified by calendering. Another important feature of this design is that ultrafines are not retained by the cloth, being released downstream by the underside fiumel-shaped pores. A published industrial report [Technical Textiles International, 1992] refers to success with this type of cloth (monofilament polyester, calendered to screen size 30 pm) in high- pressure variable chamber pressing of dyestuff hydroxides, stearates, etc. 

Hongxiaiig, 1991] refer to a cake moisture reduction to less than 25% as being necessary for effective cake discharge. Low moistures in variable chamber pressing of dyestuffs, metallic hydroxides and stearates followed the use of monofilament polyester cloth calendered to reduce the screen size to 30 pm [Reid, 1974], as mentioned above. Carleton and Heywood [1983] have discussed the fundamental aspects of filter cake properties and cake discharge mechanism. The effectiveness of discharge will dq>end on ... 

Grotq> C Variable-chamber presses containing means of squeezing the solids after... 

Continuous belt presses. Figure 11.44, compete widi solid-bowl centrifuges and variable-chamber presses in ndge dewatering applicatioits. Table 11.11 presents ical productivities of such units it can be seen that each of these units has advarrtages and disadvantages. Usually, a clear optimum choice of filter is difiScuh to identify. 

Excluding variable chamber presses, which rely on mechanical squeezing of the cake and which are dealt with in the following section, presssure filters may be grouped into two categories, plate-and-frame filter presses and pressure vessels containing filter elements. 

Cylindrica.1 Presses. Another group of filters that utili2e the variable chamber principle are those with a cylindrical filter surface. There are two designs in this category, both of which originate from the United Kingdom. 

Continuous Compression Filters. The variable chamber principle appHed to batch filtration, as described before, can also be used continuously in belt presses and screw presses. 

Comparisons are available on the relative performance and costs for dewatering municipal sludges (2). The relative performance of different filters and conditioners on waste sludges is shown in Table 3. The same sludge was treated on two belt-filter presses, two different centrifuges, and rotary vacuum filter (75). In another study, a variable chamber filter press, fixed-volume filter press, continuous belt-filter press, and rotary vacuum filter were compared for performance, capacity, and capital and operating costs (69). 

The variable Chamber filter has recaitly [Young, 1991] be demonstrated as the economic option in idudge dewatering, en conq>ared with c trifiiges and belt presses. Cake solids above the 24% DS autothermic incineration limit can be achieved, albeit at hi er flocculation doses than normally used in press operation. 

Cylindrical Variable-Chamber Filters. Cylindrical filtering sur ces are inhorent in the tube press. Figure 11.42, and the VC fibo . Figure 11.43. hr the latter, the filter is mounted on a... 

Significant improvements were made in the 1980s and early 1990s in high capacity, automated variable volume filters that incorporate automatic pressure filtration, expression, washing, and air displacement. Some of the large plate-and-frame automatic presses can operate at up to 2 MPa (ca 285 psig), with up to 100 chambers (25,26). 

Since the maximum fluid velocity obtainable in a converging nozzle is speed of sound, a nozzle of this kind can deliver a constant flow rate into a regi of variable pressure. Suppose a compressible fluid enters a converging nozzle pressure Pi and discharges from the nozzle into a chamber of variable press P2. If this discharge pressure is P)t the flow is zero. As P2 decreases below the flow rate and velocity increase. Ultimately, the pressure ratio P2/Pi reach a critical value at which the velocity in the throat is sonic. Further reduction i P2 has no effect on the conditions in the nozzle. The flow remains constant, ah the velocity in the throat is that given by Eq. (7.21), regardless of the value P2/P , provided it is always less than the critical value. For steam, the criti value of this ratio is about 0.55 at moderate temperatures and pressures. 

The typical sequence of operations in horizontal diaphragm and filter presses is shown schematically in Figure 6.3 (see also Sections 1.4.2.5 and 1.4.2.3). Suspension is fed into the chambers of the press, with either a positive displacement or centrifugal pump to initiate respectively constant rate/variable pressure or variable rate/variable pressure filtration. Cakes are usually formed simultaneously on the two opposing sides of each chamber. In plate... 

At the end of cake formation with the pump the two cakes in each individual half chamber of a recessed plate and a plate and frame filter press join to form a single cake. With the horizontal and vertical, double sided diaphragm presses single cakes also form when each chamber of the press is entirely filled with cake using the pump. In these diaphragm presses and the vertical, single sided press, however, constant pressure filtration may follow the variable pressme filtration period. When the chambers are not completely filled by cake and some unfiltered suspension remains, additional cake formation... 

Spin-Flash Drying. This process is in essence fluidized-bed drying. Filter cake is introduced into the chamber by a screw feeder and drops to the bottom. A stirrer breaks up the particles until they are small enough to be carried up by hot air to a cyclone via a central constriction of variable diameter which acts as a classifier. The dried powder does not have the good flow properties required for automatic pressing. 

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