The method
Before using the result
- Ideal affinity laws: constant geometry, density and efficiency. System static pressure and efficiency changes can invalidate the predicted operating point.
Worked examples
These original training examples illustrate the method for North American Technician Excellence Certification (NATE). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.
1. Worked example
Explore why a modest speed change can have a larger ideal effect on fan power. Compare the affinity-law estimate with the limitations of an actual duct system.
- Original speed (rpm)
- 1,000
- Proposed speed (rpm)
- 800
- Original flow
- 1,800 m³/h
- Original pressure rise
- 400 Pa
- Original shaft power
- 1 kW
- r = 800 / 1000 = 0.8.
- Flow multiplier 0.8; pressure multiplier 0.6400000000000001; power multiplier 0.5120000000000001.
- Speed ratio: 0.8
- Estimated flow: 0.4 m³/s
- Estimated pressure: 256 Pa
- Estimated shaft power: 512 W
2. Comparison example
Compare a different supplied condition using the same method. For Technical Conquer, the important check is that the input meaning, units and method assumptions still match the problem.
- Original speed (rpm)
- 1,000
- Proposed speed (rpm)
- 1,200
- Original flow
- 1,800 m³/h
- Original pressure rise
- 400 Pa
- Original shaft power
- 1 kW
- r = 1200 / 1000 = 1.2.
- Flow multiplier 1.2; pressure multiplier 1.44; power multiplier 1.7279999999999998.
- Speed ratio: 1.2
- Estimated flow: 0.6 m³/s
- Estimated pressure: 576 Pa
- Estimated shaft power: 1,728 W
Check your understanding
1. Which relationship is used?
For speed ratio r: Q₂ = Q₁r; Δp₂ = Δp₁r²; P₂ = P₁r³.
2. Which assumption should you check first?
Ideal affinity laws: constant geometry, density and efficiency. System static pressure and efficiency changes can invalidate the predicted operating point.
3. How should you verify a different unit system?
Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.
Continue with the matching exam topic
This tool connects to North American Technician Excellence Certification (NATE) free practice. Work through a question independently, then use the calculation steps to identify where your method differs. The tool stays free whether or not you choose paid preparation.