Machinery's Handbook, 31st Edition
Pressure and Flow
2755
Pressure and Flow Fluids are often delivered at a much higher pressure than needed. This pressure can be measured through a variety of means and brought down to a set level using a pressure regulator. When delivery pressure fluctuates, the installation of a regulator set at a lower pressure can stabilize pressure downstream for the fluid power components in the circuit. Pressure Generation.— Pressure is raised in fluids in a variety of ways, the most com - mon being pumps, compressors, and diffusers. Diffusers are sometimes contained within pumps, and are commonly part of HVAC systems. Compressors, Fans, Blowers, and Pumps: Liquids have their pressures increased by pumps, while gases have their pressures increased by compressors, fans, and blowers. Compressors and some types of fans and blowers are similar to pumps, but the compres sion of gases is subject to the laws of thermodynamics. More information about gas com pressors, fans, and blowers can be found in the section Pneumatics starting on page 2810. Diffusers: Diffusers use a gradual enlargement of a pipe to convert velocity energy into pressure. Diffusers are used both to raise pressure and to decrease velocity of a fluid. They are used in pipelines to reduce the energy loss when the pipe goes through an enlargement. More on the energy loss of sudden versus gradual enlargement can be found later in this text. The pressure increase generated by a diffuser can be calculated using the following equation: ∆ p p o p i – γ v i 2 v o 2 – ( ) 2 g ----------- h l – = = where p o and v o are pressure and velocity at the outlet, p i and v i are pressure and velocity at the inlet, and h l is the energy loss through the diffuser. Pressure Measurement.— Pressure is measured in the United States in terms of pounds per square inch (psi). Where pressures are quite low, inches of water gauge (inH 2 O, or in.w.g.) is used. In SI units, pressure is measured in pascals (Pa), kilopascals (kPa), and Megapascals (MPa). Other units of pressure are bars and mmH 2 O. Pressure is usually expressed relative to atmospheric pressure (gauge pressure), as opposed to absolute pres- sure. Negative gauge pressure is a vacuum. Some common pressure unit conversions are given in the following table. Pressure Conversions US SI 1 inH 2 O = 0.036 psi 1 mmH 2 O = 9.81 Pa 1 psi = 6895 Pa 1 Pa = 0.000145 Psi 1 psi = 144 psf 1 Pa = 1 N/m2 1 atm = 14.7 psi 1 bar = 100,000 Pa 1 atm = 1.01 bar 1 bar = 0.99 atm 1 inHg = 0.49 psi 1 mmHg = 133 Pa Manometers: Manometers are devices which expose a static column of relatively dense fluid (mercury and oils are commonly used) to a differential pressure. These are used for liquid pressure measurement. In many cases, one end is exposed to a pressurized fluid while the other is left open to atmospheric pressure. The change in elevation of the column of fluid can be used to calculate the pressure of the fluid being measured by using the rela tionship between pressure and elevation. Commercial manometers are calibrated and can be purchased in a variety of ranges. Piezometer tubes are types of manometers used for relatively low pressures. They consist of a vertical glass tube inserted into the wall of a pipe or pressure vessel. Its end is open to
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