Screw conveyors power requirement

Power to Operate a Screw Conveyor. The power required to operate a screw conveyor is dependent, to a large extent, on the handling characteristics of the material to be transported. Formulas for calculating power use empirically derived factors to account for the conveying characteristics of specific materials, the configuration of the screw, and the beating friction. These formulas have been developed by CEMA and can be found ia the hterature (24,25) and ia engineering handbooks. It is assumed that the total power is equal to the sum of the power required to overcome friction and the power required to transport the material. 

Table 21-5 indicates screw-conveyor performance on the basis of material classifications as listed in Table 21-4 and defined in Table 21-3. Table 21-6 gives a wide range of capacities and power requirements for various sizes of screws handling 801 kg/m (50 lb/ fU) of material of average conveyabihty. Within reasonable limits, values from Tables 21-5 and 21-6 can be interpolated for preliminary estimates and designs. 

Compute the required power input to the conveyor. Use the relation hp = 10 6(ALN + CWLF), where hp is hp input to the screw conveyor head shaft A is size factor from step 3 L is conveyor length, in ft N is conveyor r/min C is quantity of material handled, in ft3/h W is density of material, in lb/ft3 F is material factor from step 2. For this conveyor, using the data listed above hp = 10 6(54 x 100 x 60 + 1500 x 40 x 100 x 4.0) = 24.3hp(18.1kW).Witha90percent motor efficiency, the required motor rating would be 24.3/0.90 = 27 hp (20.1 kW). A 30-hp (22.4-kW) motor would be chosen to drive this conveyor. Since this is not an excessive power input, the 6-in (152.4-mm) conveyor is suitable for this application. 

The consumption of power by screw conveyors is relatively high, but they are usually short—a series of conveyors discharging from one to the other being used if the distance the material has to be handled is more appreciable—and have quite limited capacity, so the power consideration is not of great importance. The additional power required to lift the load should the conveyor be installed at an inclination is consequently not so marked. The inclination is limited to 10 or 15 deg. with the horizontal. A convenient formula for ascertaining power requirements of screw conveyors follows. 

The conditions for commencing flow in a confined situation are completely different from those at which flow then continues. Power requirements are also sensitive to apparently minor features of the design. The specification of equipment and the selection of a suitable drive unit must, therefore, be carefully matched. Only in the most straightforward applications, such as screw conveyors, are well-proven formulas published relative to a wide range of duties and different bulk products handled. 

The power required to drive screw conveyors depends on the length of the conveying path, the handling rate (throughput), the bulk density of the material, the pitch of the helix, and the resistances due to the nature of the material to be handled. The values listed in Table 19 are based on a pitch s = 0.75 D, loading (p = 0.2, bulk density 5 = 1.5t/m and circumferential velocity of the helix V = 1. Om/second. The figures under Pmotor indicate the power consumption for various horizontal conveying distances of up to 30m. 

Manually adjusted screw or ratchet take-ups that adjust the position of the tail pulley to control belt tension can be used on relatively short, light duty conveyors. Automatic take-ups are used on conveyors over about 25 to 30 m long. The most common is the weighted automatic gravity take-up (see Fig. la). Other types of automatic take-ups have hydrauHc or pneumatic powered devices to adjust a snub pulley position and maintain a constant belt tension. The requited take-up movement varies according to the characteristics of the belt constmction and the belt length. Typically, take-up movements for pHed belts are 2% to 3% of the center distance between head and tail pulley, and about 0.5% for steel cable belts. The take-up movements requited for soHd woven belts are usually shorter because of the lower elastic stretch. Take-up requirements for a particular situation should be confirmed by the belt manufacturer. 

Required power is made up of two components, that necessary to drive the screw empty and that necessary to move the material. The first component is a function of conveyor length, speed of rotation, and friction in the conveyor bearings. The second is a function of the total weight of material conveyed per unit of time, conveyed length, and depth to which the trough is loaded. The latter power item is in turn a function of the internaf friction and friciion on metal of the conveyed material. 

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