Pumps, fans, and NPSH

Pumps and fans run through both HVAC & Refrigeration and Thermal & Fluid Systems — air and water distribution, chilled-water loops, cooling towers, fluid systems. A handful of relationships covers most of the questions.

The relationships that matter

• Affinity laws. For a given pump or fan, flow scales with speed, head scales with speed squared, and power scales with speed cubed. The exponents are the whole trick — and the most common place to slip.
• Operating point. The pump curve and the system curve cross at one point — that's where the machine actually runs. Change the system (a valve, more pipe) and the operating point moves along the pump curve.
• NPSH and cavitation. Available NPSH (set by the suction-side conditions) has to exceed required NPSH (a pump property) with margin, or the pump cavitates. Many problems are really an NPSH-margin check in disguise.
• Series vs. parallel. In series, heads add at the same flow; in parallel, flows add at the same head. Mixing these up flips the answer.
• Hydraulic power comes from flow × head × fluid weight; divide by efficiency for brake power.

Where people lose points

• Wrong affinity-law exponent (using squared where it should be cubed, etc.).
• Series/parallel reversed.
• NPSH sign errors — mishandling vapor pressure, static lift, or friction on the suction side.
• Unit slips between head, pressure, and power — especially across SI and USCS on the TFS exam.

How to study it

Memorize the affinity-law exponents cold, and practice sketching the pump-and-system-curve intersection so the operating-point logic is automatic. Then drill NPSH-margin problems until the suction-side bookkeeping is second nature. These are high-frequency, fast points once the relations are reflexive.

Part of the HVAC & Refrigeration and Thermal & Fluid Systems exams. → Start your 7-day free trial — no card.