Shaft Design

When considering centrifugal machines it is important to base the motor rating on the end of curve condition of the driven machine, because in practice the machine may need to run at this extreme condition for a reasonably long period of time. This condition is generally defined as 125% of the capacity of the machine at the maximum working efficiency point on the head-flow curve for the designed shaft speed. 

The largest ISF in operation is at Hachinohe where the latest expansion was carried out in 1998. Information about the furnace and condenser before and after the expansion has been provided by Oshita (9) and is given in Table IV. The furnace now has a shaft area of 27.3 m and produced 114,000 tonnes of ISF zinc in 1999 compared with its original design shaft area of 17.2 m and capacity of 54,000 t/y zinc. The latest changes made at Hachinohe have actually decreased the blast intensity in the furnace so it might be expected that some further increase in output could be achieved, if sufficient feed were available. 

Both turbines are initially motored at 25% of design shaft speed to preclude bearing damage using energy from solar collectors and/or batteries. 

A twin-screw extmder is used to reduce residual monomers from ca 50 to 0.6%, at 170°C and 3 kPa with a residence time of 2 min (94). In another design, a heated casing encloses the vented devolatilization chamber, which encloses a rotating shaft with specially designed blades (99,100). These continuously regenerate a large surface area to faciUtate the efficient vaporization of monomers. The devolatilization equipment used for the production of polystyrene and ABS is generally suitable for SAN production. 

A widely used type of pump—mixer—settler, developed by IsraeH Mining Industries (IMI) (115), is shown in Figure 13a. A unit having capacity 8.3 m /min (2000 gal /min) has been used in phosphoric acid plants (116). The unique feature of this design is that the pumping device is not required to act as the mixer, and the two phases are dispersed by a separate impeller mounted on a shaft miming coaxially with the drive to the pump. 

The Oldshue-Rushton column (Eig. 15d) was developed (162) in the early 1950s and has been widely used in the chemical industry. It consists essentially of a number of compartments separated by horizontal stator-ring baffles, each fitted with vertical baffles and a turbine-type impeller mounted on a central shaft. Columns up to 2.74 m in diameter have been reported in service (162—167). Scale-up is reported to be reliably predictable (168) although only limited performance data are available (169). A detailed description and review of design criteria are available (170). 

Near top speed, a fan may operate at a speed that is near or above the natural frequency of the wheel and shaft. Under such conditions, the fan can vibrate badly even when the wheel is clean and properly balanced. Whereas manufacturers often do not check the natural frequency of the wheel and shaft ia standard designs, many have suitable computer programs for such calculations. Frequency calculations should be made on large high speed fans. The first critical wheel and shaft speed of a fan that is subject to wheel deposits or out-of-balance wear should be about 25—50% above the normal operating speed. 

An obvious method of increasing the filtration area in the vessel is to stack several plates on top of each other the plates are operated in parallel. One design, known as the plate filter, uses circular plates and a stack that can be removed as one assembly. This allows the stack to be replaced after the filtration period with a clean stack, and the filter can be put back into operation quickly. The filter consists of dimpled plates supporting perforated plates on which filter cloth or paper is placed. The space between the dimpled plates and the cloth is coimected to the filtrate outlet, which is either into the hoUow shaft or into the vessel, the other being used for the feed. When the feed is into the vessel, a scavenger plate may have to be fitted because the vessel will be full of unfiltered slurry at the end of the filtration period. This type of filter is available with filtration areas up to 25 m and cakes up to 50 mm thick. 

Cup anemometers have shaped cups mounted on the spokes of a wheel. The cups, under the action of the fluid forces, spin in a horizontal plane about a vertical shaft mounted in bearings. Vane or propeller types use a multibladed rotor, the axis of which is parallel to the flow direction as the rotating member. Both designs are commonly used for wind speed measurement or similar appHcations such as the velocity in ventilation ducts. Because of inertia, anemometers are most accurate under steady conditions. Velocity fluctuations cause readings that are too high. 

The Stuckofen or old high bloomery appeared in Germany in ca 1300 AD. This type of furnace was 3—5 m high and enclosed a tapered vertical shaft that was 1—1.2 m in diameter. Small openings near the bottom were provided for no22les (tuyeres, pronounced tweers) that permitted air, suppHed by bellows, to be blown into the furnace. Modem blast furnaces have essentially the same fundamental design. 

A combination of tapered shaft diameter and increasing pitch is shown in Figure 10a. This allows a length-to-diameter ratio of about 6 1 instead of 3 1. A half pitch screw is used over the tapered diameter. This approach results in an exceUent mass flow pattern provided that the hopper to which it attaches is also designed for mass flow. 

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).

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