Fluid Dynamics and Turbomachines

A: That's the most common mistake — stopping at hydraulic power and forgetting how fast efficiency erodes at this scale. Start from first principles: ρgQH gives about 98 kW hydraulic. Single-stage cooling water pumps rarely beat 70% wire-to-water once you roll in pump and motor. That pushes you into the 140 kW neighborhood. The 50 kW answer feels tidy but drops losses. The big 400 kW guess imports cavitation thinking into power sizing, and velocity head doesn't make static head disappear.

A: That's the most common mistake — forcing pressure ratio logic onto a flow-dominated problem. At 120 kg/s and PR 1.25, axial machines live in their comfort zone: high flow, small ΔP per stage, good efficiency, and a slimmer envelope. Centrifugals can be coerced there, but impeller size and tip speed spiral. Screws fall apart on flow. Two stages plus intercooling fixes temperature, not the basic mismatch.

A: That's the most common mistake — treating a datasheet point like it’s a law of nature. Rated head assumes stated density, temperature, and impeller trim. FAT loops often run cold water and sometimes a different trim to protect margin. Suction conditions shift NPSH, not developed head. Wear doesn’t show up brand-new. Read the fine print before you write an NCR.

A: That's the most common mistake — mixing formulas across unit systems. Metric specific speed uses rpm, m³/s, and meters straight up. Plugging in gives Ns ≈ 1480√0.12 / 35^(3/4), landing near 120. Converting rpm to rad/s or head to pressure double-counts constants. A value near 35 feels neat but comes from dropping the exponent on head.