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Pressure and Pressure Scales
Pressure “A pressure is the ratio of a force to the area on which the force acts. ” Pressure = Force Area Pound force, Kilogram force Newton, dyne Square Inches, Square feet Square Centimeters, Square Meters Common units include: Pounds per Square Inch (psi) Kilo-Pascals (k. Pa)
Pressure • An important operating parameter that is relevant in many applications • Pressure applied over a given area can be used for useful work. e. g – Steam reforming, Water pressure(energy) • Pressures can be measured to monitor the condition of other process parameters. – Flow,
Pressure units SI system: Pascal N/m 2 (Pa), AES: psi • One Newton (1 N) of force applied to an area of one square meter (1 m 2). • 1 Pa is equal to 1 N/m 2 • small unit, often use kilopascal 1 k. Pa=1000 Pa. • Force of one pound (1 lbf) applied to an area of one square inch (1 in 2). Metric System : Bar • One bar (1 bar) is equal to 100 k. Pa or 14. 5 psi.
More Pressure Scales PSI and k. Pa are the most common pressure scales but there a few more: • Inches* of water /mm Hg • Inches* of mercury/ ft of H 2 O • Bar 100 k. Pa=1 bar • Atmosphere (atm) • Torr (vacuum) • dynes/cm 2 , * or millimeters when using metric
Pressure Scales Applied process pressure is 27. 6806 “H 20 2. 03602 “Hg 0. 068947 0. 068046 Bar Atmos 1 psi or 6. 89 k. Pa The choice of scales will depend on • the amount of pressure being measured (high pressure = psi/k. Pa, low pressure = inches H 20) • The type of application ( flow = inches H 20, blood pressure = inches of Hg. )
Pressure Conversion Chart Pressure Units psi k. Pa inches of Hg inches of H 20 atmospheres bar psi 1 6. 894 2. 036 27. 681 . 06895 k. Pa 0. 1450 1 . 2953 4. 0147 . 009669 . 01 inches of Hg 0. 4912 3. 3864 1 13. 595 . 03342 . 03386 inches of H 2 O 0. 03613 . 2491 . 07355 1 . 002458 . 002491 atmospheres 14. 696 101. 33 29. 92 406. 8 1 1. 0133 bar 14. 504 100 29. 53 401. 86 . 9869 1 1 psi = 6. 89 k. Pa 1 inch Hg = 0. 49 psi 100 inch H 20 = 3. 61 psi 1 Bar = 14. 5 psi = 100 k. Pa
Conversion Factors • 1 cubic foot of water that weighs 62. 4 lbs acting over an area of 144 in 2 produces a pressure of 0. 433 pound per square inch (psi) • The same volume of water weighs 28. 3 Kilograms over an area of 929 cm 2, therefore the pressure is 0. 03 kilograms per square centimeter.
Fluid Pressure and Pressure Head • Pressure is exerted on the top of the cylinder of the water by the atmosphere and on the bottom of the cylinder itself by the water. • The Pressure at bottom of the static (nonmoving )column of the water exerted on the sealing plate is P = pgh + Po P=Pressure at bottom of the column Po=Pressure at the top of the column
• • Area= 1 cm 2 Height 50 cm Sp gr at 20 C and density of Hg is 13. 55 g/cm 3. Then force exerting by Hg on the bottom plate with 1 cm 2 area is – 6. 64 N In AES (psi)
Pressures caused by a fluid: Fluid pressure • If a fluid is flowing through a horizontal pipe and a leak develops, a force must be applied over the area of the hole that causes the leak. • The fluid pressure may be defined as the ratio F/ A, where F is the minimum force that would have to be exerted on a frictionless plug in the hole to keep the fluid from emerging. Fluid Pressure on the base of a tank
Pressures caused by a fluid: Hydrostatic Pressure (HP) • The pressure at the base of a vertical column of fluid with density and height is called the hydrostatic pressure. • The mass of the fluid will exert a force on the base of the container i. e. HP. • F thus equals the force on the top surface plus weight of fluid. • P=Po + pgh • Caused by the mass of a fluid. Area not matter, so applicable to every system Pressure at base of fluid column (HP)
Pressure Head • A pressure can also be • The height of a hypothetical expressed as a vertical column would exert the height of a column of liquid given pressure at its base if • The relationship between the pressure at the top the pressure at the base of were zero. a column of fluid of height h and the pressure at the top • The equivalence between a pressure P (force/area) and is 'particularly simple if the corresponding head Ph these pressures are (height of a fluid) is given expressed as heads of the given fluid: if the column is mercury, for example, then
Absolute and Relative Pressure Scales • Expressed as Absolute or relative. • Depends upon nature of measuring device to make measurements Open End would measure Relative pressure Ref is Atm P Close End No Pressure = Vacuum Absolute pressure Precise value, unchange AT Abs 0= Perfect Vaccum Relative 0=Atmospheric P
Types of Pressure • Atmospheric Pressure • Is the pressure caused by the weight of the earth’s atmosphere. Often called Barometric Pressure. • Absolute Pressure is the Total Pressure. An absolute pressure of zero is a perfect vaccum. Absolute Pressure must be used in all calculations unless a pressure difference is used. • Gauge Pressure, is the Pressure Relative to atmospheric pressure. • Vaccum Pressure, is a guage pressure i. e pressure below atmospheric pressure.
Suffix ‘a’ and ‘g’ • • • Psi and atm often carry Indicate whether Pressure is absolute or guage psig: Guage Pressure in psia: Absolute pressure in Psi atma: Absolute pressure in atmg: atmospheric pressure in guage
Standard Atmosphere • Pressure equivalent to 760 mm. Hg at sea level and at 0 C. • Unit is atm • Pressure equivalent to standard atmosphere are