Pumps kinetic head

Volume 1 explains that pumps ean be classified as either positive-displacement or kinetie. The same is true for compressors. In a positive displacement compressor the gas is transported from low pressure to high pressure in a device that reduces its volume and thus inereases its pressure. The most common type of positive displacement eompressors are reeiprocating and rotary (serew or vane) just as was the ease for pumps. Kinetic compressors impart a veloeity head to the gas, which is then converted to a pressure head in accordance with Bernoulli s Law as the gas is slowed down to the velocity in the discharge line. Just as was the case with pumps, centrifugal compressors are the only form of kinetic compressor commonly used. 

In Equation 9.40, the only operating variable is Fr, which is the ratio of the hydrostatic to kinetic head. According to this equation, the induction rate is solely decided by this ratio (Fr ) that is independent of the liquid-phase density. Continuing with this expression for f/, Zundelevich introduced the impeller performance parameter in terms of the pumping capacity (=K D ) and power input through the impeller (P=Nj,p N rP). The dispersion density, p, for use in estimation of power input was... 

Total Head the pressure available at the discharge of a pump as a result of the change of mechanical input energy into kinetic and potential energy. This represents the total energy given to the liquid by the pump. Head, previously known as total dynamic head, is expressed as feet of fluid being pumped. 

The impeller is the working part of a centrifugal pump. The function of the impeller is to increase the velocity or kinetic energy of the liquid. The liquid flows into the impeller, and leaves the impeller, at the same pressure. The black dot shown at the top of the impeller in Fig. 23.6 is called the vane tip. The pressure at the vane tip is the same as the pump s suction pressure. However, as the high-velocity liquid escapes from the impeller and flows into the volute, its velocity decreases. The volute (which is also called the diffuser) is shaped like a cone. It widens out in the manner illustrated in Fig. 23.7. As the liquid flows into the wider section of the volute, its velocity is reduced, and the lost velocity is converted—well, not into pressure, but into feet of head. 

The conversion of the pump s suction pressure to velocity in the eye of the impeller is called the required net positive suction head (NPSH). As the flow-control valve on the discharge of the pump shown in Fig. 25.1 is opened, the velocity of liquid in the eye of the impeller goes up. More of the pump s suction pressure, or feet of head, is converted to velocity, or kinetic energy. This means that the required NPSH of a pump increases as the volumetric flow through the pump increases. 

Maximum head is attained at zero flow, when the maximum kinetic energy gained by the fluid within the impeller is converted into pressure. As the flow through the pump is increased, the fluid leaving the outlet takes with it kinetic energy. Therefore, less of the kinetic energy gained... 

The head (or pressure difference) produced by the pump is a function of the kinetic energy developed at the point of release from the impeller. The head developed by an ideal pump, therefore, should be directly proportional to the square of the impeller speed ... 

A representative example of a flexible, single-parameter analyser with final transfer is the Vitatron Akes, depicted In Fig. 8.5. In the aspiration position, the sample meets the reagent or diluent stream and the reaction mixture is subsequently transferred to the measuring cuvette, from which it is flushed by the aspiration system after detection, the cuvette being suitably washed. The instrument includes a linear sample train, sample turntable, sampling head, dilutor, reagent dispenser, data-input keyboard, photometric detection system, computer, printer, evacuation pump and wash solution doser. It is prepared for kinetic measurements. 

KINETIC ENERGY - The ability of an object to do work by virtue of its motion. (Water moving in a pipe has kinetic energy.) The energy terms that are usually used to describe the operation of a pump are "pressure" and "head". In classical mechanics, equal to one half of the body s mass times the square of its speed. 

When a pump is idle, the velocity in the suction line is zero. When we start the pump, velocity must increase. We must accelerate the velocity of the fluid in the suction line to several ft/sec. The energy to provide this acceleration cannot come from the pump it must come from the liquid itself. The only source of energy available to the liquid in the suction line is gravity (i.e., the static head of liquid in the suction line). The available NPSH to the pump is partially converted to increasing the kinetic energy in the suction line while the operator opens the discharge valve. 

---