High Density Cleaner
The pulp slurry from the dump chest is fed to three parallel high density cleaners at a consistency of 3.5 percent. The accepted pulp from the high density cleaners flows on directly to the primary coarse screen. The rejects such as staples and paperclips, are trapped in the reject chambers where water is injected to separate fiber from rejects to improve system yield. Timers periodically isolate the reject chambers and open dump valves to purge the reject chamber of rejects. The end of the time cycle closes the dump valves and opens the chamber to the cleaner for reject collection once again. The purpose of high density cleaners is to protect the pressure screens from large, heavy debris not removed by the detrashing system. Feed pressure to the cleaners is fixed by the feed pump. The pressure drop across the cleaner is fixed. The pressure of the accepts pipe is automatically controlled to maintain feed pressure to the primary coarse screens.

Cleaner Diameter
The volume of feed slurry generally dictates the size of the cleaner selected for a particular application. High-density cleaners typically range from 6 to 30 inches in diameter. However, the diameter of the cleaner has a direct effect on the centrifugal force imparted on the heavy reject particles. Cleaners with smaller diameters generate significantly higher centrifugal forces than larger-diameter cleaners. Consequently, smaller-diameter cleaners clean more effectively and remove smaller heavy reject particles.

At lower feed consistencies where the viscosity is low, the difference in cleaning efficiency between small-and large-diameter cleaners is quite small. However, at higher feed consistencies in the 4.0% area, the difference in cleaning efficiency can be quite dramatic. In these cases, more small-diameter units can be used in place of a single large-diameter unit. Multiple high-density cleaners can feed off a single inline manifold.

Pressure drop
Pressure drop is generally measured as the difference in pressure between the inlet side and the accepts side of the cleaner. The pressure drop measures the amount of energy used to push a certain volume of feed slurry through the cleaner. The higher the pressure drop or more energy consumed, the faster the slurry is rotating in the cleaner and the higher the centrifugal force imparted on the high-specific-gravity reject particles. Therefore, higher pressure drops generally result in higher cleaning efficiency.

Typically, pressure drops for high-density cleaners range from 6 to 30 psi, with standard operating pressure drops in the 10-to 20-psi range. Although higher pressure drops result in superior cleaning efficiency, they also result in higher horsepower consumption by the pump and greater wear on both the pump and the internal liners of the cleaner.

The accepts of the cleaner are generally discharged against a back pressure, since this flow is typically discharging to pressurized downstream equipment. Typical back pressures on the high-density cleaner discharge range from 5 to 60 psi, depending on the downstream pressure requirements. Use of high-density cleaners. however. should be limited to systems where the accepts are discharging against a maximum back pressure of 35 psi. Higher back pressures on the cleaner overflow can cause serious problems with the automatic reject discharge system on the heavy reject collection chamber.

Automatic Reject Discharge System
The automatic reject discharge system consists of an upper valve and a lower valve that control the discharge of high-specific-gravity rejects from the rejects collection chamber. At high overflow back pressures, the gate in the upper isolation valve is subject to deformation and deflection. which can cause the gate to open slowly or not at all. If this happens, the unit can become plugged.

Internal orifice size
Although the capacity and cleaning efficiency of a high-density cleaner are usually determined by the diameter of the unit, the size of the inlet opening into the cleaner and the size of the vortex finder extending down from the top of the cleaner are also important. In general, larger feed inlet openings and large-diameter vortex finders result in larger capacities at a given pressure drop but reduce cleaning efficiency. Smaller feed inlet openings and smaller-diameter vortex finders reduce the capacity of the cleaner at a given pressure drop but generally improve cleaning efficiencies.

Reject collection chamber and automatic reject system. Design of the automatic reject discharge system and the reject collection chamber is critical to the operation and maintenance of a high-density cleaner. Inadequate design of valves and the collection chamber can cause poor cleaning efficiency, plugging in the lower end of the cleaner, severe wear on the upper and lower isolation valves, severe wear in the reject collection chamber, and tremendous loss of fiber when discharging heavy rejects from the chamber.

Depending on the quantity and type of heavy rejects to be handled, a high-density cleaner can have a number of suitable reject collection chamber designs. A relatively clean, medium-to-good-quality waste fiber generally requires a conical grit pot with 4-in. or 6-in. automatic valves to control the dump cycle. On the other hand, an extremely dirty, low-quality secondary fiber requires a cylindrical or expanded cylindrical reject collection chamber with 10-in. automatic valves to control the dump cycle.

Under conditions with extremely dirty waste fibers and large amounts of high reject material, good design and correct materials of construction are necessary to counteract severe abrasion. Reject collection chambers should be built of thick-walled, unlined, mild or stainless steel, or should be designed with replaceable AR steel plate liners.

Rotation breakers should be used in all reject collection chambers to prevent swirling of the high reject material, which can cause increased wear. Premium-quality valves with replaceable upper and lower metal or urethane seats should be used. These valves should include flush ports to keep staples and small pieces of metal wire from getting in between the replaceable seats and wearing out the valve bodies.

The reject collection chamber design are having a port of elutriation water to flush fiber out of the chamber The amount of elutriation water and the pressure of the elutriation water line depend on the specific application but are generally in the 15-to 25-gpm range, with a line pressure approximately 25 to 30 psi higher than the maximum operating pressure of the cleaner.

A flowmeter and pressure gauge should be used on the elutriation line to control the amount and pressure of the water entering the reject collection chamber. If too much elutriation water is used, heavy reject material can wash out of the collection chamber and be forced into the cleaner's accepts. If too little elutriation water is used, fiber will accumulate and pack in the reject collection chamber, resulting in high fiber loss or in plugging of the chamber during the dump cycle. Some reject collection chambers are constructed of clear plastic or have a sight glass on the chamber to make it easy to regulate the elutriation water or to determine when to dump the reject.

The dump cycle of the automatic reject discharge system is controlled by an electric timer that determines how often the chamber is dumped and the length of the dump. During normal operation, the upper valve in the automatic discharge system is open and the lower valve is closed. During the dump cycle, the upper valve closes and the lower valve opens.