Cyclone collectors

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

Pneumatic-Conveyor Dryers A pneumatic-conveyor dryer consists of a long tube or duct carrying a gas at high velocity, a fan to propel the gas, a suitable feeder for addition and dispersion of particulate solids in the gas stream, and a cyclone collector or other separation equipment for final recoveiy of sohds from the gas. 

Based upon 1200 F air-inlet temperature. With an inlet temperature of 700 F the evaporative capacity would he half that given in the table. Prices are based upon carbon steel construction with cyclone collector (January, 1995). Motors and secondary dust collectors excluded... 

If the product in no way adheres to the dryer parts and simple cyclone collectors are sufficient for gas-sohds separation, batch operation of a spray dryer may be considered. Otherwise, the time and costs for cleaning the large equipment parts make them rarely economical for other than continuous processing of a single material. 

Bone black is sometimes ground veiy fine for paint, ink, or chemical uses. A tube mih often is used, the mill discharging to a fan which blows the material to a series of cyclone collectors in tandem. 

Inertial collectors for mists and droplets are widely used. They include cyclone collectors, baffle systems, and skimmers in ductwork. Inertial devices can be used as primary collection systems, precleaners for other... 

For. 1 given cyclone collector, plot centrifugal force as a function of particle specific grav ity (0.5U-3.00), gas velocity (175-1750 m min ), and radius of curvature (30-250 cm)... 

A very important analytical tool that is overlooked by many sourcetesting personnel is the microscope. Microscopic analysis of a particulate sample can tell a great deal about the type of material collected as well as its size distribution. This analysis is necessary if the sample was collected to aid in the selechon of a piece of control equipment. All of the efficiency curves for particulate control devices are based on fractional sizes. One would not try to remove a submicron-size aerosol with a cyclone collector, but unless a size analysis is made on the sampled material, one is merely guessing at the actual size range. Figure 32-8 is a photomicrograph of material collected during a source test. 

Air Treatment Systems. Fabric filters and cyclone collectors are considered to be mechanical separation systems the treatment code for these systems is A06. The treatment code for wet scrubbers is A03. Information on each air treatment system must be entered individually in Section 7. The cyclone collector and fabric filter on the lead oxide mill exhaust are sequential treatment systems, because they treat the same wastestream in sequence. Therefore, sequential treatment must be indicated for both systems in column D of Section 7. You are required to indicate the influent concentration only to... 

Cyclone collectors are popularly used both for particle removal and for particle sampling (Fig. 13.1). The separation process of a cyclone relies on the centrifugal accelerations that are produced when particle-laden fluid experi-... 

Pressure loss through the cyclone is also a key performance parameter, and this depends mainly on the design of the cyclone. In general, the pressure drop across the cyclone collector is small compared with most other dust collectors, but the higher the collection efficiency required, the larger the pressure drop and hence the energy consumption required. 

The steady, laminar, incompressible fluid flow in cyclone collectors is governed by the Navier-Stokes equations ... 

As a simple and efficient particle separation device, cyclone collectors can be used for anything from dust removal in a fluid stream to material collection in the fluid conveying system. However, the cyclone is not suitable or economical for the separation of extremely small particles (say, less than 1 /xm), which frequently occur in industrial processes. It is recommended that the size of particles to be separated in an industrial ventilation cyclone be in the region of around 10 to 100 p.m. However, for the purpose of aerosol sampling, the size of particles to be separated may be much less than 10 jxm. 

A wet cyclonic scrubber is a cyclone collector with centrally located, coarse water sprays. These water sprays are usually directed radially outward,... 

A venturi scrubber is a venturi-shaped air passage with water introduced just ahead of or into the venturi throat. The liquid-gas contact is at a maximum in the venturi throat. The relative velocity between gas and liquid aerosol droplets is high, with the gas velocities in the range of 50-100 m/s. The particles are conditioned in the throat, and condensation is the important collection mechanism. After the particles in the gas have been deposited on droplets, a comparatively simple device such as a cyclone collector can be used to collect the wetted dust. 

In these systems, the total collection efficiencies of the dry product are 85 per cent for the drying vessel, 90 per cent for the cyclone collector and 98 per cent for the scrubber-condenser. The net efficiency of the system may be as high as 99.97 per cent if the scrubber effluent is considered as product. All the runs are based on 1.25 kg/s product and 0.75 kg/s evaporation at an elevation of 300 m above sea level. The total air flow is measured at the outlet before the stream is split into the recycle and bleed portions and, for such flows, the design of suitable fans is outlined by Jorgensen164 . The calculations outlined here may be confirmed by the use of psychometric charts, and this procedure has been considered in some detail by Cook and Demount165. ... 

A cyclone collector is another type of dry collector and is in the form of an inverted cone, but with no box. The incoming air containing contaminants is passed into the cone where it is spun at high speed. The spinning causes the solids to settle out to the periphery and fall into the apex of the cone for removal. Because of the relative densities, it is most effective with larger coarse particles, and not useful for fine particles. 

A wet centrifugal collector is similar in principle to the dry centrifugal or cyclone collector except that the materials, gas, mist, vapor, or particulate is collected on wet surfaces. 

Improved cyclone collectors and filters were developed and used in gins to eliminate all but the finest of particulates released by cotton during gin processing O, 6, 7, ). 

Because it is particles in the smaller size range, <2.5 /x.m (PM25), that are of greatest interest with respect to health effects, inlet systems are normally used that exclude larger particles. These size exclusion inlets are usually based on filters, cyclone collectors, or impactors, the principle behind which is discussed shortly. Inlet cutoff diameters from 2.2 to 15 /im are achieved in commercial instruments using these techniques (Chow, 1995). 

FIGURE 11.59 Schematic diagram of one type of cyclone collector (adapted from Ayer and Hochstrasser, 1979). 

The dry product is primarily collected in cyclone collectors (a few bag houses still remain), sieved, and finally packaged in moisture barrier containers. The exit air from the dryer often has to be treated to meet local pollution control laws. While many of the older dryers use gas incineration, as energy costs have increased these incineration systems have become quite costly to operate. New dryer installations use scrubbing systems (e.g., aqueous/chemical sprays) to remove entrained solids and gaseous volatile flavors. 

The flow pattern is such that a cocurrent spray dryer must be relatively long and small in diameter (Fig. 2), whereas a countercurrent dryer is shorter and larger in diameter. A third type, sometimes referred to as a mixed-flow dryer (Fig. 3), uses an air pattern similar to a cyclone collector, i.e., the spray is introduced at the upcoming air stream (countercurrent) and the particles transfer to the air sweeping the wall (cocurrent). 

---