2.3. Parameters affecting screening performance

2.3.1. Classes of parameters

The parameters affecting the screening result (i.e. screening parameters) can be divided into three main classes (Yu et al. 1994a): design parameters, operating parameters and furnish parameters. The screening result — i.e. the quality difference obtained between the accepted and rejected pulp in terms of the amount of debris, average fibre length, freeness etc. — is a compromise between capacity (accept production rate) and screening efficiency, and it arises from the combined effect of these parameters.

2.3.2. Design parameters

Design parameters consist of the general configuration of the screening device, the rotor design and the geometry of the screen plate.

The feed to the screen can be axial or tangential, but this has not been observed to have any significant influence on screening efficiency or capacity in general (Niinimäki et al. 1996a). Too small a feed chamber in terms of volume (i.e. too short) may nevertheless lead to a significant decrease in the capacity of an axially fed pressure screen (Ämmälä et al. 1999a). Due to internal back flow phenomena (Ämmälä et al. 1999b), some of the pulp circulates back into the feed chamber from the screen basket, and if the feed chamber is short, the pulp has no time to accelerate to its full tangential speed, which causes it to flocculate and thicken in the front of the screen basket due to insufficient turbulence, resulting in a lower capacity.

Niinimäki et al. (1996b), having investigated the effect of the rotor body (see Fig. 2), suggest that a semi-open construction will yield a more even axial flow and pressure distribution within the screen basket than an open construction, which may have a beneficial effect on screening efficiency (Hautala et al. 1999). Wakelin & Corson (1995), when testing different rotor types (a foil and two types of bump rotor), found differences in their reject thickening behaviour. It has also been suggested that the aerodynamics of the rotor affect the energy consumption in screening (Hacker & Presley 1995). A variety of rotor types are illustrated in Fig. 3.

The capacity and runnability of a screen, and also screening efficiency to some extent, can be controlled by means of pulsation. There are several designs of pulsation elements, but the foil type is probably the most common one nowadays. The aim is to obtain adequate pulsation by adjusting the number of foils and their shape, width, clearance, incident angle and tip speed. An even flow through the screen plate without any reciprocating pulp flow (Ora et al. 1993) and with a low pulsation level (Kleinhappel et al. 1995), i.e. only a small difference between the peaks of the positive and negative pulses, is advantageous for screening efficiency. To prevent blinding of the screen plate, however, the negative pulses should be strong enough to ensure that the suction flow can remix the flocs, fibres and contaminants resting on the edge of the screen opening (Yu & DeFoe 1994a).

Reject thickening is related to the ratio of the duration of the positive pulse to that of the negative pulse in each pulsation cycle. The greater this ratio is, the more probable it is that the reject will tend to thicken (Yu 1994). This explains most of the differences observed between the types of rotor. In the case of the foil type, the width of the foil can be used to control reject thickening. In low-consistency screening is possible to use narrow foils, because the reject pulp can be allowed to thicken to a reasonable extent, but in the case of high-consistency screening, long, powerful suction pulses brought about by very wide foils are needed to avert reject thickening and screen plugging (McDonald 1993, Fredriksson 1995).

A decrease in the clearance between the foil and screen plate has been reported to lead to improved capacity but reduced screening efficiency (Levis 1991), although this effect can be considered minor compared with that of increasing the angle of incidence (Niinimäki et al. 1998b). Pulsation has been observed to be independent of the properties of the fluid, i.e. it will be the same for water and for a fibre suspension having a consistency of 2% (Yu 1994).

Screen plate design is more crucial to screening than the general configuration of the screen device. Slotted screen plates have been found to have superior shive removal efficiency to holed screen plates (Hooper 1987), but the capacity of the latter is usually better, because they have a larger open area. The invention of contoured screen plates has increased the capacity of slotted screens, however. Moreover, slotted screens manufactured from wedged wires (Fig. 4) may give as much as a 100% greater open surface area than screen plates having machined slots (Ahnger & Hautala 1994). Increased aperture size and contouring will improve capacity but lower the screening efficiency (Bliss & Vittori 1992). An increase in contouring has been observed to give a significant increase in the yield of long fibres, whereas slot width has no effect (Goosney 1993). Corresponding observations have been presented by Niinimäki et al. (1998a). The geometry of the screen plate affects capacity and the tendency for plugging. Improper positioning of the apertures in relation to the profiling may even reduce capacity by 50–80% (Yu & DeFoe 1994b). Also, if the apertures are too close to each other this may cause stapling of the long fibres and again reduce capacity (Gooding & Craig 1992). The specific energy consumption in screening is lower for contoured screen plates than for smooth ones (Vitori & Phillippe 1989).

2.3.3. Operating parameters

The operating parameters are aperture velocity, the tip speed of the rotor, feed consistency and the volumetric reject rate.

The effects of the accept flow rate (i.e. aperture velocity) are linked to the pressure difference across the screen plate. An increase in aperture velocity is reported to reduce screening efficiency (Dulude 1994, Julien Saint Amand & Perrin 1999), especially if the debris is compressible, as is the case with stickies (Heise 1992, Winkler & Kelly 1994). On the other hand, there are also observations pointing to no effect of slot velocity on screening efficiency (Seifert 1993). A higher aperture velocity will increase the accept consistency, leading to an improvement in production rate (Bliss & Vitori 1992).

A higher foil tip speed will increases the capacity and reduce the screening efficiency (McCarthy 1988), an effect that appears to be clearer with profiled screen plates and is based on increasing turbulence and fluidisation of the pulp suspension, which is thought to reduce the flow resistance over the screen plate (Frejborg 1986, 1987). Energy consumption is related almost entirely to rotor frequency (Niinimäki et al. 1998b), the frequency required being dependent on the network strength of the fibre suspension and the desired accept capacity.

Feed consistency is the most widely used control variable in pressure screening. Capacity increases with increasing feed consistency, but then decreases rapidly after a certain threshold consistency has been reached (McCarthy 1988). Increasing the feed consistency is usually believed to improve screening efficiency (Levis 1991, Goosney 1993, Bliss & Vitori 1992, Laine et al. 1995), although it has also been suggested that the latter is independent of feed consistency (Dulude 1994, Julien Saint Amand & Perrin 1999).

The volumetric reject rate is mainly responsible for the operating point of screening, because it is instrumental in determining the reject thickening behaviour, together with the mass reject rate. This latter is not an operating parameter as such, but rather a combined function of operating, design and furnish parameters, but it has been found to be very useful because the screening efficiency responds to the mass reject rate significantly if particle separation is based on probability (Kubat & Steenberg 1955). This can be seen in Fig. 5, where typical screening efficiency curves are presented as a function of the mass reject rate. Any increase in the screening quotient, Q (introduced in Eq. 6 later), indicates increasing selectivity of debris removal. With the value Q = 0 debris is split between the accept and reject flows according to their mass flow, while the theoretical value Q = 1 denotes perfect debris removal to the reject at any mass reject rate, so that the accept will not contain any debris at all.

2.3.4. Furnish parameters

The furnish parameters can be considered to comprise pH, temperature, fluid viscosity and fibre properties, and also the amount of debris and entrained air in the suspension.

Screening efficiency has been reported to decrease and capacity to increase as the pH increases (Levis 1991). This is thought to be due to the lubrication effect of alkalis, which improves the passage of fibres through the screen apertures, but it is more probable that it originates from the effect of swelling on the flexibility of the fibres. An increase in temperature will reduce the screening efficiency, as debris will soften at higher temperatures (Levis 1991, Dulude 1994). McCarthy (1988) attributes the effect of temperature to the change in the viscosity of the fluid, but Wakelin & Paul (2000) suggest that the increase in the passage of fibres at higher temperatures is due to softening of the fibres and not to any alteration in the viscosity. The effect of viscosity was studied by Paul et al. (1999), who found that the screening capacity and long-fibre yield could be improved markedly by increasing the viscosity of the fluid with carboxymethyl cellulose. The effect was assumed to be a result of more single fibres being present near the screen plate, together with a more favourable flow field. The finding can be explained by the postulation of Zhao & Kerekes (1993) that the uniformity of a suspension increases with increasing viscosity of the liquid, because a high viscosity will lower the mobility of the fibres, restraining reflocculation of the dispersed fibres under conditions of decaying turbulence. An additional explanation may arise because of the drag force of the fluid affecting the fibres, an effect that becomes stronger the higher is the viscosity of the liquid, so that a strong drag force can impel fibres to pass through the apertures of the screen plate with the flow of fluid.

Fibre dimensions, especially length, have been found to have a powerful influence on the passing probability of particles (Kumar 1991). An increase in the freeness of the feed pulp will lead to higher mass reject rates and reject thickening if screening conditions remain unchanged (Wakelin et al. 1994). It has been suggested that the amount of debris in the accept pulp may correlate with the amount in the feed pulp (Sealey & Miller 1981). This debris may be wood-based, e.g. shives or bark, or it may be of artificial origin, e.g. plastics, stickies or ink (Bliss 1990).

A low or moderate air content will usually have no effect on the functioning of pressure screening under typical industrial conditions (Ämmälä et al. 2000b). If the volumetric air content is high, however, and the feed pressure and aperture velocity are low, fractionation of the pulp may decrease considerably. This may also lead to blinding of the screen plate if a screen is working close to its maximum capacity.