Transmission lines pneumatic system

Selection and care of the hydraulic fluid for a machine will have an important effect on how it performs and on the life of the hydraulic components. During the design of equipment that requires fluid power, many factors are considered in selecting the type of system to be used-hydraulic, pneumatic, or a combination of the two. Some of the factors required are speed and accuracy of operation, surrounding atmospheric conditions, economic conditions, availability of replacement fluid, required pressure level, operating temperature range, contamination possibilities, cost of transmission lines, limitations of the equipment, lubricity, safety to the operators, and expected service life of the equipment. 

Transmission lines in process control systems rarely make any significant contribution to the overall loop characteristics. Where signals are transmitted electrically, there is no detectable signal attenuation for any frequencies characteristic of the process components, and even for pneumatic transmission lines as long as 200 feet there is little loss of signal. Transmission lines have distributed properties, but according to Bradner (B3) who has studied pneumatic transmission lines extensively, they can be approximated as second-order systems. 

For engineers to use pneumatics, they must design a pneumatic system. As we noted in Chapter 2, a system is a group of related components that work together to achieve a process. A typical pneumatic system consists of a compressor, a reservoir, transmission lines, control valves, and actuators. These components are shown in Figure 11-13. 

The compressor is a device that increases the pressure of air and pumps the compressed air into a tank. The compressed air tank is called a reservoir. The compressed air is then routed to the desired location through transmission lines. Transmission lines are tubing through which the compressed air flows. Control valves regulate the flow of gases. The pneumatic system uses a cylinder with a movable piston to convert its fluid power into mechanical power in the form of linear motion. 

The components of a hydraulic system are similar to those of the pneumatic system. Figure 11-14 shows the components of a hydraulic system. The pressure of the hydraulic fluid is provided by the system s pump. The hydraulic pump is a device that creates flow in the system. The reservoir stores the hydraulic fluid before it is pressurized by the pump. Transmission lines carry the pressurized hydraulic fluid to the control valve and actuators. The control valves regulate the flow and pressure of the hydraulic fluid. The actuators transfer the fluid power into mechanical power. 

Process control systems and instrumentation can be described in several ways. Flowsheets show the process equipment, instruments, and control systems, as well as interconnections, such as piping and electrical and pneumatic transmission lines. More detailed flowsheets are referred to as piping and instrumentation diagrams (P IDs), They include additional information, such as valve characteristics, piping details (e.g., pipe sizes and fittings), and miscellaneous information, such as drains, vents, and samphng fines. Both types of diagrams are widely used in the process industries. 

Pneumatic systems, using a compressible fluid, air, exhaust the air when the job is done, so they do not have a return line. Air, even when compressed, is hght, so therefore are the transmission hoses, a characteristic also helped by the fact that pneumatic system pressures are relatively low, at 5-8 bar. 

FIGURE 44.8 Typical prototype designs of total artificial hearts (a) pneumatically powered TAH. The right and left ventricular chambers, inflow and outflow valves, as well as the connector for the pneumatic line are visible in the photograph (b) electrically powered TAH. Shown are the external battery pack, transcutaneous energy transmission system (TETS) primary and secondary coils, implanted electronics, energy converter and the blood pumps, compliance chamber and the subcutaneous access port. (Courtesy of G. Rosenberg, Pennsylvania State University.)... 

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