Machinery's Handbook, 31st Edition
2810
PNEUMATICS Pneumatics
Pneumatic systems use compressed and flowing gas, usually air, to perform work. Many pneumatic components, such as valves, actuators, and compressors share similarities with components used with liquids. In many cases, pneumatic components and calcula- tions can be found in earlier sections. Gas Consumption Calculation and Measurement.— It is common practice to exhaust air used in pneumatic circuits to atmosphere through silencers, or to an exhaust manifold and/or filter. Since that air is lost, and compression of air costs money, air consumption should be evaluated and limited as much as possible when designing a pneumatic circuit. Silencers are essential to limit noise produced by exhausting air. In US units, the volumetric flow rate of gases is normally expressed in cubic feet per minute (cfm). Velocity in US units is normally expressed in feet per minute. SI units for gas volumetric flow are cubic meters per second and meters per second for velocity. Liters per second is used for volumetric flows in low ranges. Gas Consumption: Compressors and devices using compressed air are often rated in terms of free air delivery (f.a.d.), which is the quantity of air used per unit time assuming the air is at standard atmospheric pressure and temperature. These values are 14.7 psia (0.101 MPa) and 60°F (15°C). When conditions differ from standard, the following equa tion can be used: Q a Q s p s T a p atm p a + ( ) T s = ----------------- where Q a is actual volumetric flow rate, Q s is flow rate at standard conditions, p s is standard absolute atmospheric pressure, p atm is actual absolute atmospheric pressure, p a is actual gauge pressure, T a and T s are actual and standard absolute temperature (°R or K). This equation is often used in another form to calculate the flow of free air required to supply a given compressed air flow. When the free air is at standard temperature, this takes the form of the following equation: Q s Q a p atm p a + ( ) p s = ------------------ where Q s is the free air flow at standard temperature, Q a is the compressed air flow, p a is actual gage pressure, and p s is the absolute standard pressure. The ratio of absolute pres sure to absolute standard pressure is known as the compression ratio . In US units, the free flow is normally given in units of SCFM and compressed air flow is in units of CFM. Fans and Blowers.— Fans and blowers add energy to air. When static pressure is very low, fans are typically used. Blowers are appropriate when static pressure is higher. Types of Fans and Blowers: Axial propeller fans have a number of blades arranged around a spinning shaft. They move air axially through themselves with very low pres- sure and velocity increase. Axial fans are normally used to move large quantities of air against static pressures below 0.4 psig (3kPa). A diffuser can be added after a fan to in- crease static pressure output. Axial propeller fans are the least expensive of the fan types, but are not as efficient as centrifugal fans. Vane-axial blowers are similar to propeller fans, but have highly optimized casing geometry that creates much higher pressures than propeller fans. Centrifugal fans and blowers use a rotating impeller to accelerate air radially outward from a central intake. Significant velocity energy is added, which is then converted to pressure energy when the air is slowed using a volute. The inlet and volute geometry is usually organized so that air enters and exits the centrifugal device axially. Scroll casings are arranged such that the air exiting the device is radial. Centrifugal fans are appropriate up to a static pressure of approximately 2 psi (15 kPa). Forward-curved centrifugal fans
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