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Specific Work done by Turbo Machines - Pumps, Compressors and Fans

Specific work from pumps, fans, compressors and turbines

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Specific work is work per unit weight. Specific work in turbo machines as fans, pumps, compressors or turbines has the SI-units

  • Nm/kg = J/kg = m2/s2

Specific Work of a Pump or Fan

Specific work of a pump or fan working with an incompressible fluid can be expressed as:

w = (p2 - p1) / ρ                                   (1)

where

w = specific work (Nm/kg, J/kg, m2/s2)

p = pressure (N/m2)

ρ = density (kg/m3)

Specific Work of a Turbine

Specific work of a turbine with an incompressible fluid can be expressed as:

w = (p1 - p2) / ρ                                      (2)

Specific Work of a Compressor

A compressor works with compressible fluids and the specific work for an isentropic compressor process can be expressed with the help of

p1 v1κ = p2 v2κ                                        (3)

where

v = volume (m3)

κ = cp / cv - ratio of specific heats (J/kg K)

Specific work:

w = κ / (κ -1) R T1 [( p2 / p1)((κ-1)/κ) - 1]                                     (4)

where

R = individual gas constant (J/kg K)

T = absolute temperature (K)

Specific Work of a Gas Turbine

A gas turbine expands a compressible fluid and the specific work can be expressed as

w = κ / (κ -1) R T1 [1 - ( p2 / p1)((κ-1)/κ)]                                     (5)

Head in Turbomachines

The specific work can on basis of the energy equation be expressed with the head as:

w = g h                                         (6)

where

h head (m)

g = acceleration of gravity (m/s2)

Transformed to express head:

h = w / g                                         (7)

Example - Specific Work of a Water Pump

A water pump works between 1 bar (105 N/m2) and 10 bar (10 105 N/m2). The specific work can be calculated with (1):

w = (p2 - p1) / ρ

    = ((10 105 N/m2) - (105 N/m2)) / (1000 kg/m3)

    = 900 Nm/kg

Dividing by acceleration of gravity the head can be calculated using (7):

hwater = (900 Nm/kg) / (9,81 kg/s2)

    = 91,74 (m) water column

Example - Specific Work of an Air Compressor

An air compressor works with air at 20 oC compressing the air from 1 bar absolute (105 N/m2) to 10 bar (10 105 N/m2). The specific work can be expressed with (4):

w = κ / (κ -1) R T1 [( p2 / p1)((κ-1)/κ) - 1]

    = ((1.4 J/kg K) / ((1.4 J/kg K) - 1 )) (286.9 J/kg K) ((273 K) + (20 K)) [((10 105 N/m2) / (105 N/m2))(((1.4 J/kg K) - 1)/(1.4 J/kg.K)) - 1 ]

    = 273826 Nm/kg

where

κair = 1.4 (J/kg K) - ratio of specific heat air

Rair = 286.9 (J/kg K) - individual gas constant air

Dividing by acceleration of gravity the head can be calculated using (7):

hair = (274200 N m/kg) / (9.81 kg/s2)

    = 27951 (m) air column

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