Lifts, power requirements

Power, P, defiaed as the rate at which work is performed, is expressed ia terms of energy divided by time and is most commonly given in units of horsepower, as for the power suppHed by mechanical devices such as diesel engines, or in the SI units of watts, especially when measuring electrical power. One horsepower is equivalent to the amount of power needed to lift 33,000 pounds (14,982 kg) one foot (30.5 cm) in one minute. One watt is equivalent to the power required to perform one joule of work per second. In a simple direct-current circuit where potential is represented by E ... 

Appendix Rules of ihumb for every day use 12/323 Power requirements for pumps 12/323 Power requirements for lifts 12/323 Power requirements for fans 12/323... 

In terms of a minute, one horsepower is the power required to lift 33,000 pounds one foot. A unit of power, then, is equal to a rate of 33,000 foot-pounds of work per minute, or 550 foot-pounds per second. ... 

A pneumatic transfer line has 300 ft of straight pipe, two long radius elbows, and a lift of 50 ft. A two-stage cyclone is at the receiving end. Solid with a density of 125 lb/cuft is at the rate of 10 tons/hr and the free air is at 5000ft/min. Inlet condition is 27psia and 100°F. Investigate the relation btween line diameter and power requirement. 

Power is always expressed in energy per unit of time. One horsepower is equal to the amount of power required to lift 33,000 pounds a distance of 1 foot in 1 minute. One watt equals the power needed to do 1 joule of work per second. There are 746 watts in 1 horsepower. 

The liquid is put into the absorption tower by means of a nozzle at the top of the tower. The pressure just before the nozzle is 35 psig. Assume the pump for the liquid must supply power to lift the liquid to the top of the tower and compress the liquid to 35 psig. Use a 10 percent safety factor on the above pumping-power requirements to take care of the friction in the lines and other minor losses. 

Bucket elevators. The bucket elevators lift catalyst, at 900-1000°F., about 200 ft. Each elevator consists of two chains with a continuous line of alloy-steel buckets loosely supported between them (217). The upper shaft is fixed in position but the bottom shaft can move to offset thermal expansion and wear of the chain. Chain speeds are 90 to 125 ft./minute at catalyst-circulation rates of 100 to 150 tons/hour. Power requirement is 60 to 80 horsepower (241). Differential band brakes protect against reverse rotation in case of a power failure. Sprockets with renewable teeth were used at first (217), but a traction-wheel drive was later substituted because wear is more evenly distributed and life is longer (241). Repair techniques have been developed for extending chain life (316). Graphite is used to lubricate the chains (239). The upper shaft and main bearings are water-cooled and the bearings are lubricated with oil. 

Power.—Power requirements of overhead cranes for hoisting, trolley and bridge motors depend upon speed of travel (minimum) and the load lifted (maximum capacity). The power of the motors are thus subject to selection to a certain extent and is governed by the Formulas (5a), (56) and (5c) ... 

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. 

Bucket-elevator equipment being balanced when not loaded, the power requirements consist in that required to lift the load, overcome the frictional resistance of the apparatus and that required to drag the buckets through the load collected in the elevator boot. The latter consumption of power is not great if the elevator is started before the feed to the boot, as the material is caught in the buckets as they rise in the boot. 

What is the minimum power required for a heat engine to lift a 80 kg mass 5 m in 20 s if it releases 1000 J of heat energy from its exhaust each second ... 

Wakeling, J. M. and Ellington, C. P. Dragonfly flight. III. Lift and power requirements. Journal of Experimental Biology 200 583-600 1997. 

As can be seen in Fig. 6.2, the energy requirement for gas circulation can be ignored. In fact, most monolith loop layouts do not require a compressor. Then, apart from the power requirement of a gas-liquid disperser, we need energy (1) to force the gas through the column, (2) to force the liquid through the column and (3) to lift the liquid in the liquid recycle line from the bottom of the column to the top of the column, which yields... 

Students often confuse the concept of work with power. Power represents how fast you want to do the work. It is the time rate of doing work, or said another way, it is the work done divided by the time that it took to petfisrm it. For instance, in Example 10.16, if we want to lift the box in 3 seconds, then the power required is... 

Note that 1 J/s is called 1 watt fW). If you wanted to lift the box in 1.5 seconds, then the required power to do this task is 150 J/1.5 s or 100 W Thus, twice as much power is required. It is important to note that the work done in each case is the same however, the power requirement is different. Remember if you want to do work in a shorter time, then you are going to need more power. The shorter the time, the more power required to do that work. We will discuss power and its units in much more detail in Chapter 13. 

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