ENGR 260 Sections 7'107'12

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ENGR 260Sections 7.10-7.12
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Reversible Steady Flow Work

• For quasi-equilibrium, steady flow
• (Reversible work output)
• If incompressible
• If no work (Bernoulli!!)

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Reversible Steady Flow Work

• For reversible steady state with no potential or
  kinetic
• Note dependency on specific volume.
• To minimize work in, keep v as low as possible
• To maximize work out, have v as large as possible
• Steam plants pump handles liquid/turbine
  handles gas

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Example 7-12

• Determine the compressor work input required to
  compress steam isentropically from 100 kPa to 1
  MPa, assuming that the steam exists as (a)
  saturated liquid and (b) saturated vapor at the
  inlet state.

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Isentropic Efficiency

• All real processes have some irreversibilites.
• Irreversibilities always decrease performance
• Can we measure degree of degradation?
• With Carnot cycle we established theoretical
  limits by comparing real to ideal and measuring
  efficiency

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Isentropic Efficiency Turbines
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Example 7-14

• Steam enters an adiabatic turbine steadily at 3
  MPa and 400oC and leaves at 50 kPa and 100oC. If
  the power output of the turbine is 2 MW,
  determine (a) the isentropic efficiency of the
  turbine and (b) the mass flow rate of the steam
  flowing through the turbine.

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Isentropic Efficiency Compressors/Pumps
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Example 7-15

• Air is compressed by an adiabatic compressor from
  100 kPa and 12oC to a pressure of 800 kPa at a
  steady rate of 0.2 kg/s. If the isentropic
  efficiency of the compressor is 80, determine
  (a) the exit temperature of air and (b) the
  required power input to the compressor.

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Isentropic Efficiency - Nozzles
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Example 7-16

• Air at 200 kPa and 950 K enters an adiabatic
  nozzle at low velocity and is discharged at a
  pressure of 80 kPa. If the isentropic efficiency
  of the nozzle is 92, determine (a) the maximum
  possible exit velocity, (b) the exit temperature,
  and (c) the actual exit velocity of the air.
  Assume constant specific heats for air.

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