Introduction

The need to channel water flow and other fluids must have originated in the early civilizations, and one of the better-known inventions, the Archimedes screw, dates back to the third century b.c.e. The Archimedes screw, or screwpump, shown in Fig. 11.1, was used to transfer water from lower reservoirs into higher irrigation ditches.

Over the years, many inventions focused on developing various machines either for pumping fluids or using fluid energy to drive other machinery (e.g., turbines). These machines may be classified as positive-displacement or continuous-flow machines. Some mechanical solutions using these two types of hardware are shown schematically in Fig. 11.2.

For example, the most basic configuration is a piston sliding inside a cylinder, as shown in Fig. 11.2(a). If this schematic is considered a pump, then while the piston is moving to the left it is pushing the fluid out of the cylinder through an open valve. The pumping operation of fresh fluid can continue when the piston is moving backward, closing this (exhaust) valve and opening the intake valve, creating a reciprocating cycle. This type of machine is called a positive-displacement machine because a fixed volume of fluid is captured in the cylinder and then transferred across the pump. Another example is the rotating-gear pump, shown in Fig. 11.2(b). This is also a positive-displacement machine, because there are fixed volumes of fluid between the outer wall and the gears.