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To keep you oriented as we discuss pressure, the basic concepts are treated in this sequence: generation, transmission, storage, and utilization of compressed air in a pneumatic system. This section covers air compressors, the devices that generate air pressure. Pressure Generation: Compression The pressure exerted by a confined gas results from rapid and repeated bombardment of the container walls by the enormous number of gas molecules present. The pressure can be increased by increasing the number or force of the collisions. Increasing the temperature does this by speeding up the molecules (Charles' Law). Another way is to increase the average number of molecules in a given volume. This is compression. It can be done by either decreasing the volume (Boyle's Law) or increasing the amount of gas. Liquids and solids can be compressed only with difficulty. But gases are easily compressed because their molecules are relatively far apart and move freely and randomly within a confined space. Compression decreases the volume available to each molecule. This means that each particle has a shorter distance to travel before colliding with another particle or the wall. Thus, proportionately more collisions occur in a given span of time, resulting in a higher pressure. Compression Work Requirements An air compressor does most of its work during the compression stroke. This adds energy to the air by increasing its pressure. Compression also generates heat, however, and the amount of work required to compress a quantity of air to a given pressure depends on how fast this heat is removed. The compression work done will lie between the theoretical work requirements of two processes: •Adiabatic-a process having no cooling; the heat remains in the air, caus- ing a pressure rise that increases compression work requirements to a maximum value. •Isothermal-a process that provides perfect cooling; thus, there is no change in air temperature and the work required for compression is held to a minImum. The difference in the amount of work required to compress air to psi by these two processes is about percent. Most industrial air compressors are near adiabatic, since the process is too fast to allow much heat to escape through the compressor casing. Section II Air Compressors: Basic Operation An air compressor operates by converting mechanical energy into pneumatic energy via compression. The input energy could come from a drive motor, gasoline engine, or power takeoff. The ordinary hand bellows used by early smelters and blacksmiths was a simple type of air compressor. It admitted air through large holes as it expanded. As the bellows were compressed, it expelled air through a small nozzle, thus increasing the pressure inside the bellows and the velocity of the expelled air. Modern compressors use pistons, vanes, and other pumping mechanisms to draw air from the atmosphere, compress it, and discharge it into a receiver or pressure system. Table2 summarizes the capabilities of various compressor types. The most basic types of air compressors are designated as .positive displacement" and "nonpositive displacement" (sometimes called "dynamic"). The characteristic action of a positive displacement compressor is thus a distinct volumetric change-a literal displacement action by which successive volumes of air are confined within a closed chamber of fixed volume and the pressure is gradually increased by reducing the volume of the space. The forces are static-that is, the pumping rate is essentially constant, given a fixed operating speed. The principle is the same as the action of a piston/cylinder assembly in a simple hand pump. Positive Displacement Compressors Positive displacement compressors generally provide the most economic solution for systems requiring relatively high pressures. Their chief disadvantage is that the displacing mechanism provides lower mass flow rates than nonpositive displacement compressors (see pages-). Pressure Characteristics - A compressor with a positive displacement pumping mechanism has these important pressure characteristics: • The pressure against which the compressor works rises to high
