Fluid flow pumps

N.P. Cheremisinoff, Fluid Flow Pumps, Pipes and Channels, Ann Arbor Science, Ann Arbor, MI, 1981. 

Volume lA hird Edition, which covers process planning, scheduling, and flowsheet design, fluid, flow, pumping of liquids, mechanical separations, mixing of liquids, ejector and vacuum systems, and pressure-relieving devices. 

Both of these pressure difference terms are not a lost energy because the energy is recovered in other parts of the fluid flow circuit if the circuit is a closed one. In an open flow circuit, the circulating pump or fan must work against these pressure differences or drops. 

Vapor Pressure. The Shiley Infusaid implantable infusion pump utilizes energy stored in a two-phase fluorinated hydrocarbon fluid. The pump consists of a refillable chamber that holds the dmg and a chamber that holds the fluid. The equiUbrium vapor pressure of the fluid, a constant 60 kPa (450 mm Hg), compresses the bellows, pumping the dmg through a bacterial filter, a capillary flow restrictor, and an infusion cannula to the target body site (56,116). 

Figure 10-32 shows the schematic of a pump, moving a fluid from tank A to tank B, both of which are at the same level. Tne only force that the pump has to overcome in this case is the pipe function, variation of which with fluid flow rate is also shown in the figure. On the other for the use shown in Figure 10-33, the pump in addition to pipe friction should overcome head due to difference in elevation between tanks A and B. In this case, elevation head is constant, whereas the head required to overcome fric tiou depends on the flow rate. Figure 10-34 shows the pump performance requirement of a valve opening and closing. 

At design head, on the other hand, capacity does not change markedly with speed, so that once the design point has been passed the pump-turbine acts as a restriction in the hue. Since most or these units operate on a relatively fixed pressure differential, they then tend to act Eke an orifice to limit flow, and little or no benefit can be realized from any overcapacity in terms of fluid flow available to the unit in the actual iustaUatiou. 

The above considerations give us a technique for estimating the required jet momentum and outlet flow rates. Other important parameters are the heights of the inlet and outlet apertures. The choice of these parameters will not, in general, have a significant effect on the overall fluid flow pattern and the resulting distribution of the contaminant, and these should be chosen to optimize the performance of the inlet and exhaust pumps. 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

Pumps, compressors, turbines, drivers, and auxiliary machinery should be designed to provide reliable, rugged performance. Pump selection and performance depend on the capacity required and tlie nature of Uie fluids involved. Remotely controlled power switches and shutoff valves are necessary to control fluid flow during an emergency. The inlets for air compressors should be strategically located to prevent the intake of hazardous materials. 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Note when used for pump system balance, this Zhf must be used as a negative number ( — 0.1863) because it is a pressure loss associated with the fluid flowing. For pipe line sizing, the pressure head on the tank of 5 psig and any elevation difference between tank outlet nozzle and pump suction centerline do not enter into the calculations. 

The friction losses for fluid flow in pipe valves and fittings are determined as presented in Chapter 2. Entrance and exit losses must be considered in these determinations, but are not to be determined for the pump entrance or discharge connections into the casing. 

Suppose you have determined that two cylinders have the same volume and that one cylinder is twice as long as the other. In this case, the cross-sectional area of the longer tube will be half of the cross-sectional area of the other tube. If fluid is pumped into each cylinder at the same rate, both pistons will reach their full travel at the same time. However, the piston in the smaller cylinder must travel twice as fast because it has twice as far to go. There are two ways of controlling the speed of the piston, (1) by varying the size of the cylinder and (2) by varying the volume of flow (gpm) to the cylinders. 

A similar action takes place in a fluid power system in which the fluid takes the place of the projectile. For example, the pump in a hydraulic system imparts energy to the fluid, which overcomes the inertia of the fluid at rest and causes it to flow through the lines. The fluid flows against some type of actuator that is at rest. The fluid tends to continue flowing, overcomes the inertia of the actuator, and moves the actuator to do work. Friction uses up a portion of the energy as the fluid flows through the lines and components. 

System components, such as pumps, valves and gauges, create both turbulent flow and high friction components. Pressure drop, or a loss of pressure, is created by a combination of turbulent flow and friction as the fluid flows through the unit. System components that are designed to provide minimum interruption of flow and pressure should be selected for the system. 

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