外文翻译---化工工业离心泵-化学工程(编辑修改稿)

外文翻译---化工工业离心泵-化学工程(编辑修改稿)内容摘要：

pump liquids with very wide ranging properties and suspensions with a high solids content including ,for example ,cement slurries ,and may be constructed from a very wide rang of corrosion resistant materials .The whole pump casing may be constructed from plastic such as polypropylene or it may be fitted with a corrosionresistant lining .Because it operates at high speed ,it may be directly coupled to an electric motor and it will give a high flow rate for its size . In this type of pump ,the fluid is fed to the centre of a rotating impeller and is thrown outward by centrifugal action .As a result of the high speed of rotation the liquid acquires a high kiic energy and the pressure difference between the suction and delivery sides arises from the conversion of kiic energy into pressure energy . The impeller consists of a series of curved vanes so shaped that the flow within the pump is as smooth as possible .The greater the number of vanes on the impeller ,the greater is the 12 control over the direction of the liquid and hence the smaller are the losses due to turbulence and circulation between the vanes .In the open impeller ,the vanes are fixed to a central hub ,whereas in the closed type the vanes are held between two supporting plates and leakage across the impeller is reduced .As will be seen later ,the angle of the tips of the blades very largely determines the operating characteristics of the pump . The liquid enters the casing of the pump， normally in an axial direction， and is picked up by the vanes of the impeller． In the simple type of centrifugal pump， the liquid discharges into a volute， a chamber of gradually increasing cross—section with a tangential outlet． A volute type of pump is shown in Fig.(a)． In the turbine pump[Fig． (b)]the liquid flows from the moving vanes of the impeller through a series of fixed vanes forming a diffusion ring． This gives a more gradual change in direction to the fluid and more efficient conversion of kiic energy into pressure energy than is obtained with the volute type． The angle of the leading edge of the fixed vanes should be such that the fluid is received without shock． The liquids flows along the surface of the impeller vane with a certain velocity whilst the tip of the vane is moving relative to the casing of the pump． The direction of motion of the liquid relative to the pump casingand the required angle of the fixed vanes—is found by pounding these two velocities． In Fig． c， c. vu is the velocity of the liquid relative to the vane and tu is the tangential velocity of the tip of the vane； pounding these two velocities gives the resultant velocity 2uof the liquid． It is apparent， therefore， that the required vane angle in the diffuser is dependent on the throughput， the speed of rotation， and the angle of the impeller blades． The pump will 13 therefore operate at maximum efficiency only over a narrow range of conditions． Virtual head of a centrifugal pump The maximum pressure is developed when the whole of the excess kiic energy of the fluid is converted into pressure energy. As indicated below． the head is proportional to the square of the radius and to the speed， and is of the order of 60m for a single—stage centrifugal pump； for higher pressures， multistage pumps must be used． Consider the liquid which is rotating at a distance of between r and r+dr from the centre of the pump(Fig． d)． d The mass of this element of fluid dm is given by 2πrdrdρ， where ρ is the density of the fluid and 6 is the width of the element of fluid。 If the fluid is traveling with a velocity u and at an angle θ to the tangential direction． The angular momentum of this mass of fluid = dM (urcosθ) The torque acting on the fluid dτ is equal to the rate of change of angular momentum with time， as it goes through the pump Dτ = dM α/αt(urcosθ)=2πrbρdrα/αt(urcosθ) The volumetric rate of flow of liquid through the pump： Q=2πrbα/αt Dr =Q ρ d(urcosθ) The total torque acting on the liquid in the pump is therefore obtained integrating dτ between the limits denoted by suffix 1 and suffix 2， where suffix 1 refers to the conditions at the inlet to the pump and suffix 2 refers to the condition at the discharge． Thus， τ=Q ρ(2u 2rcos 2 1u 1r cos 1 ) The advantages and disadvantages of the centrifugal pump The main advantages are： (1) It is simple in construction and can， therefore， be made in a wide range of materials (2)There is a plete absence of valves． 14 (3)It operates at high speed(up to 100 Hz)and， therefore， can be coupled directly to an electric motor. In general， the higher the speed the smaller the pump and motor for a give n duty． (4)It gives a steady delivery． (5)Maintenance costs are lower than for any other type of pump． (6)No damage is done to the pump if the delivery line bees blocked， provided it is not run in this condition for a prolonged period． (7)It is much smaller than other pumps of equal capacity． It can， therefore， be made into a sealed unit with the driving motor and immersed in the suction tank． (8)Liquids containing high proportions of suspended solids are readily handled． The main disadvantages are： (1)The single—stage pump will not develop a high pressure． Multistage pumps will develop greater heads bat they are very much more expensive and cannot readily be made in corrosion—resistant material because of their greater plexity． It is generally better to use very high speeds in order to reduce the number of stages required． (2)It operates at a high efficiency over only a limited range of conditions。 this applies especially to turbine pumps． (3)It is not usually selfpriming.。