Pressure drop during fluid flow Group 6: Lee Yi Ren 3S4 Yuan Xin 3S4 Kenneth Loh 3S2
Dec 29, 2015
Pressure drop during fluid flow
Group 6:Lee Yi Ren 3S4Yuan Xin 3S4Kenneth Loh 3S2
PressureForce applied uniformly over a surface,
measured as force per unit of area.
Where:p is the pressureF is the normal forceA 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 DesignBaffles 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 numberTo determine the fluid (liquid or gas) drop along a pipe
or pipe component
Where:Re = Reynolds Number = Velocity of FlowD = Diameter of PipeV = 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 pipesPressure drop in circular pipes:
Where: = Pressure Drop = Pipe Friction CoefficientL = Length of PipeD = Pipe Diameterp = 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 Coefficientp = 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 vapourCompressible 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 incomingT1 = Temperature incomingp2 = Pressure leavingT2 = Temperature leaving
Pressure Drop Online-Calculatorhttp://www.pressure-drop.com/Online-C
alculator/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 byvertical difference of pipechanges 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 sizeThe 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 dropwhich we cannot explain, please visit
http://www-unix.ecs.umass.edu/~rlaurenc/Courses/che333/Reference/exchanger.pdf
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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&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=be8f37adea38b6f5589f7d2adafa5009
www.mechengcalculations.com/jmm/pipe004.html www.jlcusa.com/datasheets/GL%20flow/Pressure%20Drop
%20All%20Types.pdf