Fluid Machines

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• Fluid Machines

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Fluid machines

• Pumping machines add energy to a fluid
• Examples Pumps (for liquids), fans, blowers and
  compressors (for gases)
• Positive-displacement machines a finite volume
  of liquid is drawn into a chamber and then forced
  out under high pressure.
• Turbomachines Energy is added to the fluid by
  means of a rotating impeller. A common example is
  the centrifugal pump.
• Turbines extract energy from the fluid.

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Positive Displacement Pump

• Works on the principle of letting fluid flow into
  a cavity from a low-pressure source, trapping the
  fluid, and forcing it out to a high-pressure
  receiver by decreasing the volume of the cavity
• Simplest pump that can be found anywhere from
  liquid soap dispensers, to automobile fuel
  injectors, to the human heart.

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Centrifugal Pump

• Based on the concept of increasing the kinetic
  energy via the centrifugal action of the impeller
  and converting this kinetic energy to work
• Used predominantly for high-flow applications,
  less expensive, and less complex thereby
  minimizing maintenance
• Must be pre-charged with liquid or else it wont
  pump at start-up. Positive displacement pumps
  dont have this limitation.

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Centrifugal Pump Curves

• Pumps from manufacturers are typically rated by
  the amount of fluid work that can be achieved as
  a function of fluid flow
• Fluid work in the pump curves is typically
  expressed in head form
• Work decreases with increasing flow due to
  increased losses incurred at higher flow
  velocities

Theoretical pump curve
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Pump Operating Curves
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NPSH Net Positive Suction Head

• In centrifugal pumps, the fluid must be brought
  up to the rotational speed of the impeller
  blades.
• Increasing the fluid velocity would result in a
  decrease in pressure
• This can cause boiling of the fluid or cavitation
  around the eye of the impeller.
• To prevent this, there must be elevation of the
  fluid before the pump.

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NPSH Net Positive Suction Head

• This height is known as the net positive suction
  head (NPSH).
• The NPSH for a pumping system can be found
  theoretically using B.E.

• The velocity at the eye of the impeller is the
  vector sum of the rotational velocity at that
  point and the radial velocity (see Example 10.6)
• In practice the NPSH can be found from the Pump
  Map (NPSHR where R stands for required).

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NPSH Net Positive Suction Head

• The NPSH calculated above was only for the pump,
  it did not take into account friction losses
  prior to the pump.
• The additional NPSH can be calculated as

• The total NPSH can be found by adding these two
  hs together.
• If the actual height, h, is greater than the sum,
  then the pump should run smoothly. Otherwise
  cavitation can occur resulting in poor
  performance and damage to the pump.
• If insufficient height is available, special
  centrifugal pumps with low NPSHR or P.D. pumps
  may be used.