Mechanical Pumps

 Mechanical Pumps.

A device that expends energy in order to raise, transport, or compress fluids. The earliest pumps were devices for raising water, such as the Persian and Roman waterwheels and the more                 sophisticated Archimedes screw.

Classification of pumps

Pumps are classified according to the way in which energy is imparted to the fluid. The basic methods are (1) Possitive displacement, (2) addition of kinetic energy, and (3) use of electromagnetic force.
A fluid can be displaced either mechanically or by the use of another fluid. Kinetic energy may be added to a fluid either by rotating it at high speed or by providing an impulse in the direction of flow. In order to use electromagnetic force, the fluid being pumped must be a good electrical conductor. Pumps used to transport or pressurize gases are called compressors, blowers, or fans. Pumps in which displacement is accomplished mechanically are called positive displacement pumps. Kinetic pumps impart kinetic energy to the fluid by means of a rapidly rotating impeller.

Positive displacement pumps.

Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, Reciprocating and Rotary.

Reciprocating pumps include piston, plunger, and diaphragm types;                                                                                  images of pumps respectively
Rotary pumps include gear, lobe, screw, and vane pumps.
 Gear pumps can be divided into two types which are external gear and internal gear pumps

• External gear pumps are designed such that One of the gears is driven and the other runs free. A partial vacuum, created by the unmeshing of the rotating gears, draws fluid into the pump. This fluid is then transferred to the other side of the pump between the rotating gear teeth and the fixed casing. As the rotating gears mesh together, they generate an increase in pressure that forces the fluid into the outlet line. A gear pump can discharge fluid in either direction, depending on the direction of the gear rotation.
• Internal gear pumps are designed such that The driven gear is a rotor with internally cut teeth, which mesh with the teeth of an externally cut idler gear, set off-center from the rotor. The crescent part of the fixed casing divides the fluid flow between the idler gear and the rotor. Gear pumps can pump liquids containing vapors or gases. Since they depend on the liquid pumped to lubricate the internal moving parts, they are not suitable for pumping gases. They deliver a constant output with negligible pulsations for a given rotor speed. Erosion and corrosion lead to an increase in the amount of liquid slipping back through the pump. Since gear pumps are subject to clogging, they are not suitable for pumping liquids containing solid particles. Since they do not need check valves, however, they can be used to pump very viscous liquids. 

Lobe pumps are similar in design and operation with external gear pumps  in that fluid flows around the interior of the casing. Unlike external gear pumps, however, the lobes do not make contact. Lobe contact is prevented by external timing gears located in the gearbox. Pump shaft support bearings are located in the gearbox, and since the bearings are out of the pumped liquid, pressure is limited by bearing location and shaft deflection.

1. As the lobes come out of mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the lobes as they rotate.
2. Liquid travels around the interior of the casing in the pockets between the lobes and the casing—it does not pass between the lobes.
3. Finally, the meshing of the lobes forces liquid through the outlet port under pressure.

Screw pumps  is designed such that it uses one or several screws to move fluids or solids along the screw(s) axis. The simplest form of it is the (Archimedes' screw pump) where a single screw rotates in a cylindrical cavity, thereby moving the material along the screw's spindle. This ancient construction is still used in many low-tech applications, such as irrigation systems and in agricultural machinery for transporting grain and other solids.

Vane (sliding vane) pump is illustrated where the rotor is mounted off-center, the rectangular vanes are positioned at regular intervals around the curved surface of the rotor. Each vane is free to move in a slot. The centrifugal force from rotation throws the vanes outward to form a seal against the fixed

casing. As the rotor revolves, a partial vacuum is created at the suction side of the pump, drawing in fluid. This fluid is then transferred to the other side of the pump in the space between the rotor and the fixed casing. At the discharge side, the available volume is decreased, and the resultant increase in pressure forces the fluid into the outlet line; the pumping rate can be varied by changing the degree of eccentricity of the rotor. Vane pumps do not need inlet and outlet check valves; they can pump liquids containing vapors or gases but are not suitable for pumping liquids containing solid particles. Vane-type compressors are used to pump gases.
Vane pumps deliver a constant output with negligible pulsations for a given rotor speed. They are robust, and their vanes, easily replaced, are self-compensating for wear. Pumping capacity is not affected until the vanes are badly worn.

Kinetic pumps.

Kinetic pumps can be divided into two classes, centrifugal and regenerative pumps. In this type of pumps velocity is imparted to the fluid where most of this velocity head is then converted to pressure head. Even though the first centrifugal pump was introduced about 16th century, kinetic pumps were little used until the 20th century.

Centrifugal pumps include radial, axial, and mixed flow units. A radial flow pump is commonly referred to as a straight centrifugal pump; the most common type is the volute pump, Fluid enters the pump near the axis of an impeller rotating at high speed. The fluid is thrown radially outward into the pump casing. A partial vacuum is created that continuously draws more fluid into the pump.
Volute centrifugal pumps are robust and relatively inexpensive, quiet, and dependable, and their

performance is relatively unaffected by corrosion and erosion. They are compact, simple in construction, and do not require inlet and outlet check valves.
Volute centrifugal pumps can pump liquids containing solid particles, but, when pumping liquids containing more than a small amount of vapor, their suction is broken by cavitation. Volute centrifugal pumps operate best when pumping relatively nonviscous liquids, and their capacity is greatly reduced when used to pump viscous liquids.
Another type of radial flow centrifugal pump is the diffuser pump, in which, after the fluid has left the impeller, it is passed through a ring of fixed vanes that diffuse the liquid, providing a more controlled flow and a more efficient conversion of velocity head into pressure head.

axial flow centrifugal pumps are pumps that carries the rotor as a propeller. Fluid flows parallel to

the axis as illustrated as in the diagram. Diffusion vanes are located in the discharge port of the pump to eliminate the rotational velocity of the fluid imparted by the propeller. Axial flow compressors are also used to pump gases. In mixed flow pumps, fluid is discharged both radially and axially into a volute-type casing.





Regenerative pumps is also called a turbine, or peripheral, pump. The impeller has vanes on both sides of the rim that rotate in a ringlike channel in the pump's casing. The fluid does not discharge freely from the tip of the impeller but is recirculated back to a lower point on the impeller diameter. This
recirculation, or regeneration, increases the head developed. Because of close clearances, regenerative pumps cannot be used to pump liquids containing solid particles. They can pump liquids containing vapours and gases, and in fact they can pump gases provided that they contain sufficient liquid to seal the close clearances. Regenerative pumps are suitable only for pumping mobile liquids.

Vacuum pumps are simply compressors that take in gas at a pressure lower than atmospheric pressure, compress it, and discharge the gas at atmospheric pressure. Since gas at low pressures has a large volume, vacuum pumps tend to be bulky. Steam jet ejectors are extensively used industrially for creating vacuum. Reciprocating piston and rotary-vane pumps are also widely used for producing vacuum.