Compressor Isentropic Efficiency
Fluid Mechanics · FE Reference Handbook section
Handbook notes for this section
Definitions and conditions exactly as the handbook states them.
- wa ≡ actual compressor work per unit mass
- ws ≡ isentropic compressor work per unit mass
- Tes ≡ isentropic exit temperature
Core formulas for this FE topic
Definitions, applicability, units, assumptions and worked examples for each relation.
Worked exam-style examples
The four ways this section is written on the real exam — thoughts first, then equations, then substitution.
a positive-displacement blower feeding an aeration basin Given volumetric flow (Q) = 11.5000 m^3/s; pressure rise (\Delta p) = 24,800 Pa; efficiency (\eta) = 0.7700, determine the shaft power (W) in W.
Given
Find
shaft power (W), in W
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except W is given, so isolate W symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for W:
Step 3 — List the givens: volumetric flow (Q) = 11.5000 m^3/s, pressure rise (\Delta p) = 24,800 Pa, efficiency (\eta) = 0.7700.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning W = 370,390 W to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 740,779 — kept a factor of two that cancels in the correct rearrangement.
- 185,195 — dropped that same factor in the other direction.
- 407,429 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a centrifugal blower on a dust-collection duct Given shaft power (W) = 104,739 W; pressure rise (\Delta p) = 44,000 Pa; efficiency (\eta) = 0.6800, determine the volumetric flow (Q) in m^3/s.
Given
Find
volumetric flow (Q), in m^3/s
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except Q is given, so isolate Q symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for Q:
Step 3 — List the givens: shaft power (W) = 104,739 W, pressure rise (\Delta p) = 44,000 Pa, efficiency (\eta) = 0.6800.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Q = 1.6187\ \text{m^3/s}Step 6 — Check: returning Q = 1.6187 m^3/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 3.2374 — kept a factor of two that cancels in the correct rearrangement.
- 0.8093 — dropped that same factor in the other direction.
- 1.7806 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a compressor supplying pneumatic construction tools Given shaft power (W) = 83,942 W; volumetric flow (Q) = 0.4000 m^3/s; pressure rise (\Delta p) = 57,700 Pa, determine the efficiency (\eta).
Given
Find
efficiency (\eta)
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except \eta is given, so isolate \eta symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for \eta:
Step 3 — List the givens: shaft power (W) = 83,942 W, volumetric flow (Q) = 0.4000 m^3/s, pressure rise (\Delta p) = 57,700 Pa.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning \eta = 0.2750 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.5499 — kept a factor of two that cancels in the correct rearrangement.
- 0.1375 — dropped that same factor in the other direction.
- 0.3024 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a positive-displacement blower feeding an aeration basin Given volumetric flow (Q) = 8.1000 m^3/s; pressure rise (\Delta p) = 46,700 Pa; efficiency (\eta) = 0.5600, determine the shaft power (W) in W.
Given
Find
shaft power (W), in W
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except W is given, so isolate W symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for W:
Step 3 — List the givens: volumetric flow (Q) = 8.1000 m^3/s, pressure rise (\Delta p) = 46,700 Pa, efficiency (\eta) = 0.5600.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning W = 675,482 W to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 1,350,964 — kept a factor of two that cancels in the correct rearrangement.
- 337,741 — dropped that same factor in the other direction.
- 743,030 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a centrifugal blower on a dust-collection duct Given shaft power (W) = 29,393 W; pressure rise (\Delta p) = 16,300 Pa; efficiency (\eta) = 0.6300, determine the volumetric flow (Q) in m^3/s.
Given
Find
volumetric flow (Q), in m^3/s
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except Q is given, so isolate Q symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for Q:
Step 3 — List the givens: shaft power (W) = 29,393 W, pressure rise (\Delta p) = 16,300 Pa, efficiency (\eta) = 0.6300.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Q = 1.1360\ \text{m^3/s}Step 6 — Check: returning Q = 1.1360 m^3/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 2.2721 — kept a factor of two that cancels in the correct rearrangement.
- 0.5680 — dropped that same factor in the other direction.
- 1.2497 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a compressor supplying pneumatic construction tools Given shaft power (W) = 138,164 W; volumetric flow (Q) = 5.5000 m^3/s; pressure rise (\Delta p) = 34,000 Pa, determine the efficiency (\eta).
Given
Find
efficiency (\eta)
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except \eta is given, so isolate \eta symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for \eta:
Step 3 — List the givens: shaft power (W) = 138,164 W, volumetric flow (Q) = 5.5000 m^3/s, pressure rise (\Delta p) = 34,000 Pa.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning \eta = 1.3535 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 2.7069 — kept a factor of two that cancels in the correct rearrangement.
- 0.6767 — dropped that same factor in the other direction.
- 1.4888 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a positive-displacement blower feeding an aeration basin Given volumetric flow (Q) = 8.4000 m^3/s; pressure rise (\Delta p) = 26,900 Pa; efficiency (\eta) = 0.6100, determine the shaft power (W) in W.
Given
Find
shaft power (W), in W
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except W is given, so isolate W symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for W:
Step 3 — List the givens: volumetric flow (Q) = 8.4000 m^3/s, pressure rise (\Delta p) = 26,900 Pa, efficiency (\eta) = 0.6100.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning W = 370,426 W to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 740,852 — kept a factor of two that cancels in the correct rearrangement.
- 185,213 — dropped that same factor in the other direction.
- 407,469 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a centrifugal blower on a dust-collection duct Given shaft power (W) = 170,637 W; pressure rise (\Delta p) = 11,500 Pa; efficiency (\eta) = 0.5700, determine the volumetric flow (Q) in m^3/s.
Given
Find
volumetric flow (Q), in m^3/s
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except Q is given, so isolate Q symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for Q:
Step 3 — List the givens: shaft power (W) = 170,637 W, pressure rise (\Delta p) = 11,500 Pa, efficiency (\eta) = 0.5700.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Q = 8.4577\ \text{m^3/s}Step 6 — Check: returning Q = 8.4577 m^3/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 16.9153 — kept a factor of two that cancels in the correct rearrangement.
- 4.2288 — dropped that same factor in the other direction.
- 9.3034 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a compressor supplying pneumatic construction tools Given shaft power (W) = 104,396 W; volumetric flow (Q) = 6.6000 m^3/s; pressure rise (\Delta p) = 50,600 Pa, determine the efficiency (\eta).
Given
Find
efficiency (\eta)
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except \eta is given, so isolate \eta symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for \eta:
Step 3 — List the givens: shaft power (W) = 104,396 W, volumetric flow (Q) = 6.6000 m^3/s, pressure rise (\Delta p) = 50,600 Pa.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning \eta = 3.1990 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 6.3979 — kept a factor of two that cancels in the correct rearrangement.
- 1.5995 — dropped that same factor in the other direction.
- 3.5189 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors
a positive-displacement blower feeding an aeration basin Given volumetric flow (Q) = 7.4000 m^3/s; pressure rise (\Delta p) = 19,800 Pa; efficiency (\eta) = 0.7700, determine the shaft power (W) in W.
Given
Find
shaft power (W), in W
Start with the thinking
- The governing relation printed in this handbook section is Blower / compressor fluid power.
- Everything except W is given, so isolate W symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- A blower raises the pressure of an air stream across an aeration basin diffuser system.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for W:
Step 3 — List the givens: volumetric flow (Q) = 7.4000 m^3/s, pressure rise (\Delta p) = 19,800 Pa, efficiency (\eta) = 0.7700.
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning W = 190,286 W to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 380,571 — kept a factor of two that cancels in the correct rearrangement.
- 95,143 — dropped that same factor in the other direction.
- 209,314 — rounded an intermediate value before the final step.
Reference: FE Handbook — Fans, Blowers, and Compressors