Positive Displacement Machines:

Reciprocating pump

A reciprocating pump is a positive plunger pump. It is often used where relatively small quantity of liquid is to be handled and where delivery pressure is quite large.

Reciprocating pumps can be classified based on

1. Sides in contact with water

a) Single acting Reciprocating pump

b) Double acting reciprocating pump

2. Numbers of cylinder used

a) Single cylinder pump

b) Two cylinder pumps

c) Multi-cylinder pumps

Indicator Diagram:

A pressure volume diagram (or PV diagram, or volume-pressure loop) is used to describe corresponding changes in volume and pressure in a system. They are commonly used in thermodynamics, cardiovascular physiology, and respiratory physiology.

PV diagrams, originally called indicator diagrams, were developed for understanding the efficiency of steam engines.

A PV diagram plots the change in pressure P with respect to volume V for some process or processes. Typically in thermodynamics, the set of processes forms a cycle, so that upon completion of the cycle there has been no net change in state of the system; i.e. the device returns to the starting pressure and volume.

The figure shows the features of a typical PV diagram. A series of numbered states (1 through 4) are noted. The path between each state consists of some process (A through D) which alters the pressure or volume of the system (or both).

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Fig: PV Diagram

A key feature of the diagram is that the amount of energy expended or received by the system as work can be estimated as the area under the curve on the chart. For a cyclic diagram, the net work is that enclosed by the curve. In the example given in the figure, the processes 1-2-3 produce a work output, but processes from 3-4-1 require a smaller energy input to return to the starting position / state; thus the net work is the difference between the two.

Note that this figure is highly idealized, and a diagram showing the processes in a real device would tend to depict a more complex shape of the PV curve.

Rotary Pumps:

A rotary vane pump is a positive-displacement pump that consists of vanes mounted to a rotor that rotates inside of a cavity. In some cases these vanes can be variable length and/or tensioned to maintain contact with the walls as the pump rotates. 

Working Principle of Rotary Pumps

This article is in continuation of an article that describes the characteristics of rotary pumps. In this article we will discuss the construction and working of various types of rotary pumps.

· In the previous article we learned about the various characteristics, main parts and special features of the rotary pumps. In this article we will learn about the working of various types of rotary pumps, i.e. screw, lobe, gear and vane pump. We will also learn how the working of screw pump is different from that of other types of pumps.

· Working of Gear, Vane or Lobe Pump

Though the construction, shape and design of gear, vane and globe pumps are a bit different, they fall under the same category of rotary pumps. The working of all these pumps is based on the same principle, i.e pumping of the liquid with the help of rotating elements. The rotating elements can be gears, screws or vanes. We will understand the basic principle of all these pumps by taking the working system of gear pumps into consideration.

A gear pumps rotating assembly consists of two gears, a driving gear and a driven gear. Pumping in a gear pump takes place when the driving gear meshes with the driven gear to form cavities that moves the fluid. The cavities are the clearances between the gears or lobes and the body of the pump. The mating surfaces of the gears’ mesh provide continuous sealing between the inlet and outlet ports.

As the rotation of the pump continues, the movement of the cavities progresses, thus moving the fluid along the way. After some time, a point is reached where the seal between the captured fluid and the outlet part of the pump is broken. At this point of time the vanes, gears or lobes, forces the volume of the captured fluid out of the pump. A continuous cycle takes place due to the simultaneously opening of new cavities at the inlet port. This leads to a continuous flow of liquid though the suction and discharge ports.

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