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)
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)
v= volume (m3)
κ=cp / cv- ratio of specific heats (J/kg K)
w=κ / (κ -1) R T1 [( p2 / p1)((κ-1)/κ) - 1] (4)
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
w = g h (6)
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
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
κair= 1.4 (J/kg K) - ratio of specific heat air
Rair= 286.9 (J/kg K) - individual gas constant air
hair= (274200 N m/kg) / (9.81 kg/s2)
= 27951 (m) air column
The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more.
Actual air compressor capacity (ACFM) vs. standard air capacity (SCFM) and inlet air capacity (ICFM).
The overall pump and fan efficiency is the ratio power gained by the fluid to the shaft power supplied.
Fluid flow with constant entropy is also called isentropic flow.
Calculate pumps hydraulic and shaft power.
Horsepower required to pump water.
Characterizing of impeller types in pumps in a unique and coherent manner.
Suction Specific Speed can be used to determine stable and reliable operations for pumps with max efficiency without cavitation.
The difference between pumps, compressors, blowers and fans.
Reciprocating, rotary screw and rotary centrifugal air compressors.
Work done by a force acting on an object.