Pumps

Chapter: Pharmaceutical Engineering: Fluid Flow

Pumps can be divided into positive displacement pumps, which may be reciprocating or rotary, and impeller pumps.

PUMPS

Equations (2.8) and (2.9) examined the power requirement for driving a liquid through a system against an opposing head. This energy is normally added by a pump. In different processes, the quantities to be delivered, the opposing head, and the nature of the fluid vary widely, and many pumps are made to meet these differing requirements. Basically, however, pumps can be divided into positive displacement pumps, which may be reciprocating or rotary, and impeller pumps. Positive displacement pumps displace a fixed volume of fluid with each stroke or revolution. Impeller pumps, on the other hand, impart high kinetic energy to the fluid, which is subsequently converted to pressure energy. The volume discharged depends on the opposing head.

Equipment for pumping gases and liquids is essentially similar. Machines delivering gases are commonly called compressors or blowers. Compressors discharge at relatively high pressures, and blowers, at relatively low pressures. The lower density and viscosity of gases lead to the use of higher operating speeds and, to minimize leakage, smaller clearance between moving parts.

FIGURE 2.13 Positive displacement pumps (A) Reciprocating Piston; (B) Double Acting Piston and (C) Diaphragm pumps.

Positive Displacement Pumps

Positive displacement pumps are most commonly used for the discharge of relatively small quantities of fluid against relatively large heads. The small clearance between moving parts precludes the pumping of abrasive slurries.

The single-acting piston pump in Figure 2.13A exemplifies the recip-rocating pump. The fluid is drawn into a cylinder through an inlet valve by movement of the piston to the right. The stroke in the opposite direction drives fluid through the outlet valve. Leakage past the piston may be prevented by rings or packing. Cessation of pumping on the return stroke is overcome in the double-acting piston pump by utilizing the volume on both sides of the piston. Fluid is drawn in on one side by a stroke that delivers the fluid on the other (Fig. 2.13B). In both pumps, delivery fluctuates. Operation, however, is simple, and both are efficient under widely varying conditions. The principle is widely used in gas compressors. In pumping liquids, no priming is necessary because the pump will effectively discharge air present in the pump or feed lines.

A modification, known as the diaphragm pump, is constructed so that reciprocating parts do not contact the pumped liquid (Fig. 2.13C). A flexible disk, fixed at the periphery, expands and contracts the pumping chamber, drawing in and discharging liquid through valves.

Rotary positive displacement pumps operate, presenting an expanding chamber to the fluid that is then sealed and conveyed to the outlet. Both liquids and gases are discharged so that priming is not necessary. The principle is illustrated in Figure 2.14, which describes a gear pump, a lobe pump, and a vane pump. In the gear pump, the liquid is conveyed in the spaces formed between a case and the consecutive teeth of two gears that intermesh at the center of the pump to prevent the return of the liquid to the inlet. The lobe pump, widely used as a liquid pump and as a blower, operates in a similar manner. Each impeller carries two or three lobes that interact with very small clearance to convey fluid from inlet to outlet.

FIGURE 2.14 Rotary Pumps: (A) Gear; (B) Lobe; and (C) Vane.

Sliding vanes, mounted in the surface of an off-center rotor but maintained in contact with the case by centrifugal force or spring loading, provide the pumping action of the vane pump. Fluid is drawn into the chamber created by two vanes at the inlet. The fluid is rotated and expelled by contraction at the outlet. Besides liquid pumping, the principle of the vane pump is used in blowers and, by evacuating at the inlet and discharging to atmosphere at the outlet, in vacuum pumps.

The Mono pump consists of a stator in the form of a double internal helix and a single helical rotor. The latter maintains a constant seal across the stator, and this seal travels continuously through the pump. The pump is suitable for viscous and nonviscous liquids. The stator is commonly made of a rubber or similar material, so slurries are effectively delivered. Discharge is nonpulsating and can be made against very high pressures. The pump is commonly used to drive clarifying and cake filters.

Centrifugal Impeller Pumps

The centrifugal impeller pump is the type most widely used in the chemical industry. The impeller consists of a number of vanes, usually curved backward from the direction of rotation. The vanes may be open or, more commonly, closed between one or two supporting plates. This reduces swirl and increases efficiency. The impeller is rotated at high speeds, imparting radial and tangential momenta to a liquid that is fed axially to the center and that spirals through the impeller. In the simple volute pump (Fig. 2.15A), the liquid is received into a volute chamber. The cross section increases toward the tangential outlet. The liquid, therefore, decelerates, allowing a conversion of kinetic energy to pressure energy. In

FIGURE 2.15 Centrifugal Impeller Pumps: (A) Volute and (B) Diffuser.

the diffuser pump, correctly aligned blades of a diffusing ring over which the fluid velocity decreases smoothly receive the liquid from the impeller, and the pressure rises. Flow through a diffuser pump is described in Figure 2.15B.

Because of the less precise control of the direction of the liquid leaving the impeller, the volute pump is less efficient than the diffuser pump. However, it is more easily fabricated in corrosion-resistant materials and is more commonly used. The pump, which is compact and without valves, may be used to pump slurries and corrosive liquid, steadily delivering large volumes against moder-ately large heads. For large heads, pumps are used in series. Unlike positive displacement pumps, impeller pumps continue to operate if the delivery line is closed, the kinetic energy of the liquid being degraded to heat.

A disadvantage of the centrifugal pump is that the conditions under which a pump of given size will operate with high efficiency are limited. The relation between the quantity discharged and the opposing head for a volute pump operating at a given speed is shown in Figure 2.16. As the head increases, the

FIGURE 2.16 Performance curve of a volute pump running at fixed speed.

quantity discharged decreases. The mechanical efficiency of the pump is the ratio of the power acquired by the liquid, given by equation (2.9), to the power input. A maximum value is shown in Figure 2.16, indicating optimal operating conditions. The effect on the efficiency when the pump operates at other con-ditions can be seen from the figure, and to achieve reasonable operating effi-ciency for a given discharge and opposing head, a pump of suitable size and operating speed must be used.

A second disadvantage of the centrifugal pump lies in priming. If the pump contains air alone, the low kinetic energy imparted by the impeller creates a very small pressure increase across the pump, and liquid is neither drawn into the pump nor discharged. To begin pumping, the impeller must be primed with the liquid to be pumped. Where possible, the pump is placed below the level of the supply. Alternatively, a nonreturn valve could be placed on the suction side of the pump to prevent draining when rotation ceases.

The same principle is employed in centrifugal fans and blowers used to displace large quantities of air and other gases. The gas enters the impeller axially and is moved outward into a scroll. The opposing static head is usually small, and energy appears mainly as the kinetic energy of the moving gas stream.

Other Impeller Pumps

The propeller pump, exemplified by a domestic fan, is used to deliver large quantities of fluids against low heads. These conditions are common in recir-culation systems. The principle is also employed in fans used for ventilation, the supply of air for drying, and other similar operations.

Example 1

In the figure, what is the energy loss due to pressure?