Variable pitch

Many manufacturers have experimented with variable conveyor pitches. These have been used to enhance dryness in effluent dewatering. The pitch starts wide at the centrate discharge end. narrowing progressively along the 

A patent by KHD (now Bird Humboldt) specifies a standard pitch in the cylindrical section of the bowl with a narrower pitch on the beach [15]. This obviates the need for a baffle at the foot of the beach as the narrow pitch produces a deeper cake forming the seal. The narrow pitch provides a back pressure on which the standard pitch can work. 

An alternative use for the variable pitch is to improve centrate quality. For this the pitch of the flight adjacent to the front hub is increased, as much as double that for the rest of the conveyor. This design is particularly useful for applications in which the decanter can become heavily loaded with solids. The increased pitch tends to keep the centrate discharge region of the bowl relatively free of solids, thus reducing re-enlrainment of solids as the centrate streams towards the weir. 

---

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Fig. 10. Mass flow screw feeder designs, (a) Combined tapered shaft and variable pitch screw feeder where A represents a conical shaft and constant pitch (feed section) B, constant shaft and increasing pitch (feed section) and C, constant shaft and constant pitch (conveying section), (b) Stepped shaft screw feeder where A represents a stepped diameter shaft and constant pitch (feed section) and B, constant shaft and constant pitch (conveying section).

Recent developments in cooling-tower fan energy management also include automatic variable-pitch propeller-type fans and inverter-type devices to permit variable fan speeds. These schemes involve tracking the loaa at a constant leaving-water temperature. 

The variable-pitch arrangement at constant motor speed changes the pitch of the olades through a pneumatic signal from the leaving-water temperature. As the thermal load and/or the ambient wet-bulb temperature decreases, the blade pitch reduces air flow and less fan energy is required. 

Screw feeders are also used to assist in bin unloading and in producing uniform feed. Of importance here is the need for a variable-pitch screw to produce a uniform draw of material across the entire hopper opening (Fig. 21-27). For uniform flow to occur, the screw-feeder opening-to-diameter ratio should not exceed 6. 

Temperature control of the process fluid may be accomplished easily through the use of shutters, variable pitch fan blades, variable speed drives, or, in multiple fan installations, by shutting off fans as required. 

A variable pitch screw that transports the cake radially towards the discharge point. 

Axial-flow fan A fan positioned in a cylindrical casing in which the air enters and leaves the impeller in a direction parallel to the casing axis. The fan may have fixed-pitch blades or variable-pitch blades. 

Sperry applied for his first patent—a dynamo-electric machine— when he was twenty. His final patent application—for a variable pitch propeller— was submitted in 1930, shortly after his death. In total, 355 of his over 400 patent applications matured as actual granted patents. 

Axial or propeller blade fatis are eithet Irelt or gear driven. Some drivers are variable speed tnoiors. and some fatis have variable pitch blades. Iti s iecial circumstances, steam turbine, gas or gasolitie etigine drivers are used. Geais should be carefully specilied to avoid overload and should be special ) sealed to prevent moisture entering the case. 

The blades are usually fixed pitch up to 48-in. diameter with applications for adjustable pitch above this size. Fixed pitch is used up to 60-in. diameter with aluminum fen blades when direct-connected to a motor shaft. Variable pitch is used with belts, gears, etc., between the fen shaft and the driver to allow for the possibilities of slight unbalance between blades due to pitch angle variation. Aluminum blades are used up to 300°F, and plastic is limited to about 160°-180°F air stream temperature. 

The effect of cold weather on the freezing of tube-side fluids and increasing horsepower due to increased air density can not be overlooked. Usual practice is to reduce fan output by using a two-speed motor, louvers on variable-pitch fans, or drivers. ... 

When only one fan and/or exchanger exists per process service, it may be advisable to control with an automatic variable pitch fan, unless a single- or two-speed drive is considered adequate. If the process service consists of several exchanger sections or tube bundles per cell (groups of bundles) and multiple fans are used, see Figure 10-193. If single fans are used per cell, see Figure 10-194. If several... 

Figure 10-191. Temperature control and horsepower savings with automatic variable pitch fans. (Used by permission Hudson Products Corporation.)...

Rubin, F. L., Power Requirements Are Lower for Air-Cooled Heat Exchangers with Variable-Pitch Fan Blades, Oil and Gas Jour, Oct. 11, (1982). 

Most worms are cylindrical in shape with a uniform pitch diameter. However, a variable pitch diameter is used in the double-enveloping worm. This configuration is used when increased load capacity is required. 

The laboratory batch crystallizer used in this study is shown in Figure 2. It consisted of a cylindricd vessel (ID = 155 mm, height = 250 mm) of three litre working capacity agitated by a variable speed 4-bladed (variable-pitch) impeUer. For temperature control, the crystallizer was immersed in a constant temperature water-bath. 

Practically therefore a primary break of variable pitch does not seem to be needed, a ring mt cury teeak of fixed pitdi ( f[Pg.1]

Fan motors should be sized at 25% above the brake horsepower. Fans should be belt driven with variable pitch drives up to and including 25 FIP and fixed pitch drives at about 25 FIP. Fans must be radial and they must be balanced (static and dynamic) and mounted on anti-vibration supports. The connection of the fan outlet to the inside panels must be done by means of an internal flexible connector. 

Fans are belt drive with variable-pitch drives for 25 FIP or less and hxed-pitch drives for drives above 25 HP. 

Various types of screws are used for modern conveyors. These include short-pitch, variable-pitch, cut flights, ribbon, and paddle screws. The Procedure given above also applies to these screws. 

---