想请问板上的各位 Determination of specific surface involves the measurement of the solid surface area as well as that of solid volume and presents no problem for uniform geometric shapes. For irregular solid, especially for porous materials, however, surface area determination becomes involved. The surface of porous materials is necessarily full of holes and projections. Different surface areas are usually defined in connection with porous materials, viz., total surface area, external visible surface area, geometric surface area, etc. A simple rule of thumb is you should consider using a (PD) pump whenever you might be operating a centrifugal at other than at the BEP. Of course the further away from the BEP you get the more likely a PD pump will be a better choice. This can typically happen at low flow conditions, modest to high head conditions, or any type of elevated viscosity. As you can see from the efficiency curve it takes more horsepower to operate a centrifugal outside of its BEP. This horsepower has a cost, the initial cost of the larger motor plus a higher life cycle cost in energy consumed. Many times the PD pump will have a lower initial cost as well as a lower operating cost. 此两篇文字叙述虽然能够之後每个字的意思 但是却难以把整段解释的很好 (英文程度不够 ORZ) 所以不知道有哪位可以帮忙的吗? 另外问有没有人 有单操的一些历届考题的解答?? PS 我不是故意要打"单"这个音的.... --

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