Cooling towers are critical to HVAC and process cooling plants. But how we control the speed of those massive fans can make the difference between an efficient system—or wasted energy. Because fan power scales roughly with the cube of speed, small reductions in rpm can produce outsized kW savings. This article breaks down your control options—constant speed, two-speed, dual-motor arrangements, and variable frequency drives (VFDs)—then compares efficiency when staging multiple towers or cells.
The Physics in One Minute (Fan Affinity Laws)
For axial fans used on most towers:
- Airflow (Q) ≈ speed (N)
- Static pressure (ΔP) ≈ N²
- Fan power (P) ≈ N³
So, if you run a fan at 50% speed, airflow drops to ~50%, but power drops to ~12.5% (0.5³).
Control Options
A. Constant Speed (On/Off)
How it works: These fans are either on or off. It’s the simplest and lowest-cost method, but it comes with drawbacks—coarse temperature control, higher average power, and more wear from frequent starts and stops.
Motor runs at synchronous slip speed via across-the-line starter or soft-starter. Capacity is controlled by cycling the fan on and off and/or using basin bypass or waterflow modulation.
Pros
- Lowest first cost, simple controls
- Robust and familiar
Cons
- Coarse control, temperature “hunting”
- Highest average kW to meet setpoint
- Frequent starts increase mechanical/electrical stress (unless mitigated with a soft-starter)
- Noise fluctuates during cycling
When to use
- Small towers with permissive temperature deadbands
- Facilities with tight budget and low run hours
B. Two-Speed Motor (Pole-Changing, e.g., 1200/600 rpm)
How it works: One motor with two synchronous speeds via pole switching (two windings or Dahlander). Control steps: OFF → LOW → HIGH.
Pros
- Low/medium first cost
- Meaningful energy reduction at LOW (power ≈ (N_low/N_high)³)
- Fewer starts than pure on/off
Cons
- Only two capacity steps; still coarse
- Requires interlocks and proper sequencing to avoid switching under load
- Less precise approach control than VFD
When to use
- Moderate load variability where three steps suffice
- Retrofit where VFDs are impractical
C. Dual Motors / Dual Fans per Cell
How it works: One tower cell with two smaller fan-motor assemblies instead of one large unit (or two cells run in parallel). Control by staging motors: 0, 1, or 2 fans (and possibly with two-speed/VFD on each).
Pros
- Redundancy: one fan can be down while the other maintains partial capacity
- Finer staging than single constant-speed fan
- Can combine with VFDs for very fine turndown
Cons
- Higher mechanical complexity
- More drives/starters and controls
- Slightly higher static/system effects at multiple inlets/outlets depending on geometry
When to use
- Mission-critical plants (data centers, hospitals)
- Plants needing N+1 redundancy at the cell level
D. Variable Frequency Drive (VFD)
How it works: Electronic speed control with continuous rpm modulation based on condenser-water (CW) leaving temperature or approach to wet-bulb.
Pros
- Best energy performance (precisely exploit the cube law)
- Smooth ramping: reduced inrush, less mechanical stress
- Tight temperature control and quieter operation at part load
- Supports advanced strategies (low approach, plume control, nighttime setbacks)
Cons
- Higher first cost (drive + filters/harmonic mitigation as needed)
- Requires attention to motor insulation (inverter duty), cable length, and minimum speed limits for gear/motor cooling
- Potential for VFD harmonics—consider line reactors/filters and coordination with the utility
When to use
- Almost always the lifecycle-cost winner for medium/large towers with variable loads
- Facilities with demand charges, long operating hours, or noise constraints
Efficiency Comparison: One Fan at Full vs. Two Fans at Half
A simple illustration using the cube law:
- Assume each fan at 100% speed draws 50 kW.
- Option 1: One fan at 100%, the other OFF → Total 50 kW.
- Option 2: Two fans at 50% speed each → Power per fan = 50 × (0.5³) = 6.25 kW → Total 12.5 kW.
For roughly the same net airflow (0.5 + 0.5 = 1.0 “unit”), two at half speed can use ~75% less power than one at full speed.
Why it works in towers (often even better than in ducts):
- Distributing water over more fill area at a lower air velocity often improves heat transfer effectiveness (more contact time, better wetting), so you may achieve the same or better leaving CW temperature at even lower fan speeds.
- Noise drops dramatically at lower speeds.
- Caveats: confirm minimum motor/gear speeds, bearing lubrication needs, and avoid water maldistribution at very low air velocities.
Rule of thumb for multi-cell towers:
Run the maximum number of cells you can at the lowest possible fan speed to meet setpoint, subject to water distribution limits, freeze/plume management, and pump energy trade-offs.
4) Multi-Tower / Multi-Cell Control Strategies
A. Common Sequencing Priorities
- Meet LWT setpoint (e.g., 85°F / 29.4°C) with a small deadband.
- Maximize active cells, then modulate all fans down together (with VFDs).
- Respect minimum fan speed (e.g., 20–25%) for motor/gear cooling and to maintain water distribution.
- If you hit minimum speed on all active cells and are still below load → deactivate one cell (to keep others above their minimum and maintain water distribution quality).
- In cold/wet conditions, include plume and icing logic (bypass, basin heaters, intermittent reverse jog if manufacturer allows).
B. Pump & System Interactions
- If pumps are constant speed and head doesn’t change much when enabling extra cells, the fan-energy benefit typically dominates.
- If enabling more cells adds significant hydraulic head (uncommon), re-evaluate the fan vs. pump energy trade-off.
- With variable-flow condenser pumps, coordinate VFD setpoints: unnecessary high waterflow can offset fan savings.
C. Practical Limits
- Minimum waterflow per cell: stay within manufacturer’s turndown for proper fill wetting.
- Freezing risk: winter operation may require cycling fans off, bypassing fill, or minimum speeds to prevent ice.
- Water treatment/plume: more cells at low speed can increase plume risk in certain ambient conditions—use plume abatement strategies if required.
Option-by-Option Energy & Control Summary
| Option | Energy at Part Load | Control Resolution | Reliability/Stress | First Cost | Best Use Case |
| Constant Speed | Poor (cycling) | Coarse (on/off) | More starts; simple | Low | Small/simple towers, low run hours |
| Two-Speed | Fair | Medium (low/high) | Fewer starts; still stepped | Low–Medium | Moderate variability; simple upgrades |
| Dual Motors/Fans | Good (with staging) | Medium–High (0/1/2 fans) | Redundancy; more components | Medium–High | Mission-critical; N+1 needs |
| VFD | Excellent (∝ N³) | High (continuous) | Soft starts; least wear | Medium–High | Most variable-load plants |
6) Control Set Points & Tuning Tips
- Primary loop variable: Leaving CW temperature (or approach to ambient wet-bulb).
- Setpoint strategy: Fixed setpoint (e.g., 85°F) or reset based on chiller efficiency (some chillers prefer warmer CW at light loads to reduce lift; always coordinate tower and chiller curves).
- PID tuning: With VFDs, use slow integral action to avoid oscillation; apply a small deadband (e.g., ±0.5–1.0°F).
- Starts per hour: Enforce maximum starts if any staged (non-VFD) fans remain.
- Minimum speed: Honor manufacturer minimum (often 20–30%) for gear/motor cooling and ensure adequate airflow through the motor.
- Safety interlocks: High/low basin level, vibration switch, gear oil pressure/temp (if applicable), fan contactor/VFD status, freeze protection, and motor space heaters.
7) Reliability, Maintenance, and Noise
- VFD benefits: Soft starts reduce mechanical shock on gears, couplings, and blades; lower average speed reduces wear and noise.
- Two-speed motors: Check contactor and interlock sequencing; avoid switching between speeds under load.
- Dual-fan cells: Plan for access, vibration monitoring per fan, and balanced staging to equalize wear.
- Noise: Since acoustic power falls sharply with rpm, low-speed multi-cell operation is typically the quietest strategy.
8) Quick Worked Example (Energy)
Goal: Deliver “1.0 unit” of airflow.
- One fan at 100%: 1.0 airflow → 1.0³ = 1.0 power unit (e.g., 50 kW).
- Two fans at 50% each: 0.5 + 0.5 = 1.0 airflow → 2 × (0.5³) = 0.25 power units (e.g., 12.5 kW).
Savings: 75% fan power reduction, often with better heat transfer due to more wetted fill area at lower face velocity.
9) Commissioning Checklist (Field-Ready)
- Verify rotation, tip clearance, blade pitch, and vibration cutouts.
- Confirm minimum VFD speed and motor/gear cooling requirements.
- Calibrate LWT sensor; confirm wet-bulb source if using approach control.
- Test multi-cell sequence: enable extra cells before increasing speed, and shed cells last.
- Validate freeze protection logic (bypass, heaters, reverse-jog if specified).
- Trend fan kW, LWT, ambient WB; verify stable control and expected cube-law savings.
10) Bottom Line
- If you can only pick one upgrade, choose VFDs—they offer the largest, most controllable energy savings and better temperature stability.
- In multi-cell towers, operate more cells at lower speeds rather than one cell at full speed, subject to manufacturer turndown, plume, and freeze constraints.
- For critical facilities, consider dual-fan/cell redundancy, ideally each on a VFD, to combine reliability with ultra-low kW/ton of heat rejection.
