3.8. Hydrocyclones enhanced by adjusting back pressureThe back pressure at the sink outlet, which is associated with col- lection approaches of overflow and underflow, has a great influence on the flow within hydrocyclones and the split ratio. For example, Closed methods of collection, such as the conventional hydrocyclones with a closed “grit pot” under apex, inevitably create the back pressure, which alters split ratio in an entirely predictable, although not related manner. Piller et al. examined the sensitivity of DynaWhirlpool hy- drocyclone operation to the applied back-pressure. They found that the optimum range of the back pressure did exit for normal operating of hydrocyclones. The reasons were as follows: (i) if the back pressure was below the lower threshold limit of the aforementioned optimum range, there would appear an abnormal local fluid recirculation phenomenon; (ii) if the back pressure was above the upper aforementioned back-pressure, there would occur two large recirculation regions, which deteriorated the normal separation performance of hydrocyclones.The back pressure can also affect the separation efficiency. For small-diameter hydrocyclone, the back pressure in overflow increases the separation efficiency at the same split ratio, which means the decrease of the back pressure in the underflow relative to the overflow can enhance the separation efficiency of hydrocyclones. That is, the back pressure in underflow decreased the separation efficiency at the same split ratio and measures should be taken to improve the separa- tion efficiency. Consequently, Strasser proposed a novel coupled hydrocyclone-ejector and employed ejector under apex to improve se- paration efficiency (Fig. 19). Sabbagh et al. used a pump in un- derflow to actively control the separation performance of hydro- cyclones either through providing the back pressure or by the pump suction. They found that the pressure ratio, that is, the ratio of absolute underflow pressure to overflow pressure, resulting from underflow pumping has a similar effect, in terms of affecting downstream condi- tions and controlling hydrocyclone performance, as altering the un- derflow pipe diameter by a valve. Nevertheless, unlike the approaches using a valve, either under automatic control or manually, which increased the danger of blocking the apex, the underflow pumping can avoid the apex blocking successfully. Therefore, compared with adjusting the underflow pipe diameter, although with a large increased energy consumption, the underflow pumping is a robust technique for adjusting the back pressure in underflow and controlling the hydro- cyclone separation performance. Recently, Ni et al.in- troduced the reflux device, which made hydrocyclones have the con- tinuous underflow and reflux function, to hydrocyclones and proposed the novel de-foulant hydrocyclone with continuous flow and reflux function (Fig. 20). As a result, with just a little increased energy con- sumption, the reflux device, which also has a similar influence as changing the underflow pipe diameter, enhanced the hydrocyclone separation considerably and avoided the clogging at apex effectively.Therefore, owing to the importance of the back pressure mentioned above, further investigations are required to study the effects of back pressure on various separation parameters of hydrocyclones and to develop novel enhanced-separation hydrocyclone technologies by ad- justing back pressure.