Engineering Fundamentals and Quantitative Problem Solving
This domain tests quantitative problem solving across HVAC, electrical, plumbing, and mechanics. You need clean unit conversions, correct formulas, and the judgment to know when a calculated answer is physically plausible.
Core formulas
- Sensible heat: Btu/h = 1.08 × CFM × ΔT for standard air.
- Total heat: Btu/h = 4.5 × CFM × Δh.
- Fan and pump laws: flow ∝ speed; head ∝ speed²; power ∝ speed³.
- Pump brake horsepower: BHP = GPM × head (ft) × specific gravity ÷ (3,960 × efficiency).
- Three-phase power: kW = √3 × volts × amps × power factor ÷ 1,000.
- Refrigeration: 1 ton = 12,000 Btu/h; 1 boiler hp = 33,475 Btu/h.
- Water heat: Btu/h = 500 × GPM × ΔT.
Worked example
A pump delivers 600 GPM at 90 ft of head, specific gravity 1.0, efficiency 72%. BHP = (600 × 90 × 1.0) ÷ (3,960 × 0.72) = 54,000 ÷ 2,851 = 18.9 bhp. A 20 hp motor with a 1.15 service factor can carry about 23 hp, so it is workable if the actual duty point is confirmed; many specifications still choose 25 hp for margin. Check NPSH and the system curve before accepting the selection.
Traps that cost marks
- Using 1.08 with non-standard air or mixing CFM with L/s.
- Forgetting power factor in AC power calculations.
- Using total pressure where static pressure is required.
- Rounding intermediate results too early.
- Treating nominal tonnage as delivered capacity.