Pressure drop during fluid flow Group 6: Lee Yi Ren 3S4 Yuan Xin 3S4 Kenneth Loh 3S2

Pressure Force applied uniformly over a surface, measured as force per unit of area. Where: p is the pressure F is the normal force A is the area.

What is pressure drop? The decrease in pressure from one point of the tube to another downstream Usually the result of friction of the fluid against the tube Tube convergence, divergence, turns and other physical properties will affect the pressure drop.

Small VS large tubes High flow rates in small tubes give larger pressure drop. Low flow rates in large tubes give lower pressure drop.

Baffle Design Baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection.

Criteria to build In order to select an appropriate heat exchanger, the system designers (or equipment vendors) would firstly consider the design limitations for each heat exchanger type. Although cost is often the first criterion evaluated, there are several other important selection criteria which include: High/ Low pressure limits Thermal Performance Temperature ranges Product Mix (liquid/liquid, particulates or high-solids liquid) Pressure Drops across the exchanger Fluid flow capacity Cleanability, maintenance and repair Materials required for construction Ability and ease of future expansion

Causes of pressure drop Friction Changes of kinetic energy Vertical pipe difference or elevation Calculation of pressure drop caused by friction in circular pipes

Reynold’s number To determine the fluid (liquid or gas) drop along a pipe or pipe component Where: Re = Reynolds Number = Velocity of Flow D = Diameter of Pipe V = Viscosity

Reynold’s number If the Reynolds number < 2320, than you have laminar flow.  Laminar flow is characterized by the gliding of concentric cylindrical layers past one another in orderly fashion. The velocity of the fluid is at its maximum at the pipe axis and decreases sharply to zero at the wall. The pressure drop caused by friction of laminar flow does not depend of the roughness of pipe.

Reynold’s number If the Reynolds number > 4000, you have turbulent flow. There is an irregular motion of fluid particles in directions transverse to the direction of the main flow. The velocity distribution of turbulent flow is more uniform across the pipe diameter than in laminar flow. The pressure drop caused by friction of turbulent flow depends on the roughness of pipe.

Pressure drop in circular pipes Where: = Pressure Drop = Pipe Friction Coefficient L = Length of Pipe D = Pipe Diameter p = Density = Flow Velocity

Resistance coefficients If you have valves, elbows and other elements along your pipe then you calculate the pressure drop with resistance coefficients specifically for the element. Resistance coefficients are in most cases found through practical tests and through vendor specification documents. If the resistance coefficient is known, than we can calculate the pressure drop for the element.

Pressure drop for the element   Where: = Pressure Drop = Resistance Coefficient p = Density = Flow Velocity

Pressure drop by vertical gravity or vertical elevation Where: = Pressure Drop p = Density g = Acceleration of Gravity = Vertical Elevation or Drop

Pressure drop of gasses and vapour Compressible fluids expands caused by pressure drops (friction) and the velocity will increase. Therefore is the pressure drop along the pipe not constant. Where: p1 = Pressure incoming T1 = Temperature incoming p2 = Pressure leaving T2 = Temperature leaving

Too Many Formulas? Need help?

Pressure Drop Online-Calculator http://www.pressure-drop.com/Online- Calculator/index.html http://www.kaeser.com/Online_Services/ Toolbox/Pressure_drop/default.asp

SF Pressure Drop 6.0 calculates pressure drops of flowing liquids and gases in pipes (laminar and turbulent flow) calculate pressure changes caused by vertical difference of pipe changes of kinetic energy calculates pressure drops in pipe elements (example: changes of direction) in diverse fittings (valves, bellows etc.)

Pressure drops permitted by the system affect heat exchanger size The highest allowable pressure drop will result in substantial savings in heat exchanger size.

Maximum use of available pressure drop ensures the smallest heat exchanger design by maximizing velocities for higher heat transfer coefficients. There are some applications, however, where pressure drop can be costly and can limit throughput. In these cases a reduction in pressure drop is desirable.

To learn more about pressure drop which we cannot explain, please visit http://www- unix.ecs.umass.edu/~rlaurenc/Courses/ch e333/Reference/exchanger.pdf Page 6

Acknowledgements http://www.engineersedge.com/fluid_flow/pressure_drop/pressur e_drop.htm http://en.wikipedia.org/wiki/Pressure_drop http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V 3H-4HG69XM- 3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_a cct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b e8f37adea38b6f5589f7d2adafa5009 www.mechengcalculations.com/jmm/pipe004.html www.jlcusa.com/datasheets/GL%20flow/Pressure%20Drop%20All %20Types.pdf

Thank you for your kind attention