Thermodynamics 1 For Mechanical Engineering- Pressure

Thermodynamics 1- Pressure

1. THERMODYNAMICS: PRESSURE

2. Force exerted by molecules in the atmosphere due to gravity and temperature When force is exerted on fluid, this force is transmitted through out the fluid When fluid is at rest or stationary, force is uniformly through out the fluid Fluids exerts a pressure a pressure on its containing walls which in turn exert the same pressure in the fluid What is Pressure?

3. While working in a system, the thermodynamic medium exerts a force on boundaries of the vessel in which it is contained. The vessel may be a container, or an engine cylinder with a piston etc. The exerted force F per unit area A on a surface, which is normal to the force, is called intensity of pressure or simply pressure P. The normal force exerted by the fluid per unit area of the surface. P= F/A What is Pressure?

4. UNITS OF PRESSURE Units of Pressure   English MKS SI (psi) (Pa)  

5. PRESSURE MEASURING DEVICES is commonly used to measure gage pressure The Bourdon pressure gauge works by measuring the amount of change in a coiled or semicircular metal tube by a pressurized fluid inside. This is due to the principle that a flattened tube tends to regain its circular form when pressurized. Bourdon-Tube Gage

6. PRESSURE MEASURING DEVICES U-tube manometer features a vertical or inclined U-tube column that is filled with a reference liquid (mercury) to display the pressure level. When the columns of the device are exposed to the atmosphere, the levels of liquid in the limbs are equal and this indicates the atmospheric pressure. When one of the columns is connected to the pressure vessel, there will be a difference in the level of the liquid in the limbs, which signifies the pressure of the liquid in the vessel. . Manometer (U-Tube Type)

7. PRESSURE MEASURING DEVICES used to measure atmospheric pressure, also called barometric pressure. Barometer

8. GAGE PRESSURE Pressure above or below atmospheric pressure. the difference between the absolute pressure and atmospheric pressure. It is essentially what you would read on a pressure gauge. It is a relative measurement. Open to atmosphere   hg       +      

9. ATMOSPHERIC PRESSURE Is define being equivalent to 760 mm height of mercury at 0 degree celcius . +    

10. ATMOSPHERIC PRESSURE If liquid used in barometer is mercury, atmospheric pressure becomes        

11. PRESSURE CONVERSION 14.7 psia = 101.325 kPaa = 1.03 kg/cm² abs = 760 mmHg abs. 1 Bar = 100 kPa 1 Torr = 1 mmHg 1 Mpa = 1 N/mm² 1 kPa = 1 KN/m²

12. RELATIONSHIP OF VARIOUS PRESSURES

13. RELATIONSHIP OF VARIOUS PRESSURES Absolute pressure when it is greater than atmospheric pressure : Absolute pressure = Atmospheric pressure + Gage pressure   P abs . = P atm. + P gage

14. RELATIONSHIP OF VARIOUS PRESSURES Absolute pressure when it is less than atmospheric pressure or barometric pressure:   Absolute pressure = Atmospheric pressure – vacuum pressure   P abs . = P atm. - P vacuum

15. EXAMPLE PROBLEM 1 A 51 ft3 tank contains 3 lb of a gas at 800F and a vacuum pressure of 24 in Hg. A.) What is the absolute pressure in psia and psfa ? GIVEN: P gage = 24 in Hg Solution : Pabs = P atm + Pgage   Pabs = 29.92 - 24   Pabs = 5.92 in Hg Conversions: ( )  

16. EXAMPLE PROBLEM 2 A 30 meter vertical column of fluid (density 1878 kg/m 3 ) is located where g = 9.65 m/s 2 . Find the pressure at the base of the column. GIVEN: g= 9.65 Solution : 𝑃=𝛾𝑍   𝑃 = 𝑔 ℎ   𝑃 = 543.7 kPa  

17. EXAMPLE PROBLEM 2 The barometer of a mountain hiker reads 30.150in.Hgabs. at the beginning of a hiking trip and 28.607 in.Hgabs.at the end. Neglecting the effect of altitude on local gravitational acceleration, determine the vertical distance climbed. Assume air density was 0.075 lb /cubic foot. GIVEN: P 1 = 30.150 in. Hg P 2 = 28.607 in. Hg Solution: ∆𝑃=𝛾 ∆h     =