Humidification

Humidification operations transfer water between air and water. Learn humidity, relative humidity, dew point, wet-bulb temperature, psychrometric charts, humid heat and enthalpy, cooling towers (range, approach, evaporation, make-up, blowdown) and air conditioning processes.

Cooling towers reject heat from power plants and refineries by evaporating a small fraction of the water; air conditioning adds or removes moisture; dryers depend on air's capacity to hold water. All rely on psychrometry — the thermodynamics of moist air.

Evaporating 1 kg of water removes about 2.4 MJ — which is why cooling towers are so effective.

An analogy

Sweating on a breezy day: evaporation cools your skin. A cooling tower is a giant sweating machine for plant water.

Moist air basics

Air holds more water when warm. Relative humidity compares the actual vapour pressure with saturation; dew point is where cooling makes water condense; wet-bulb temperature is what an evaporating wet surface reaches.

Psychrometric chart

The chart plots humidity vs dry-bulb temperature with lines of RH, wet-bulb, enthalpy and specific volume. Processes — heating, cooling, humidifying, dehumidifying, mixing — are lines on the chart.

Cooling towers

Hot water is sprayed over fill while air flows through; water cools toward the wet-bulb temperature. Range = inlet − outlet water temperature; approach = outlet − wet bulb (typically 3–6 K). Evaporation losses must be replaced (make-up), and blowdown controls dissolved-solid build-up (cycles of concentration).

Design and water treatment

Merkel's equation integrates enthalpy driving force for tower sizing (KaV/L). Drift eliminators, fan control, and water treatment (scale, corrosion, biofouling, Legionella) are key. Hybrid and dry coolers save water in arid regions.

Formulas

• Humidity: H = 0.622 p_v / (P − p_v)
• Relative humidity: RH = p_v / p_sat(T)
• Moist-air enthalpy: h ≈ 1.006 T + H (2501 + 1.86 T) kJ/kg dry air
• Cooling tower evaporation: ṁ_evap ≈ Q / λ (≈ 0.15–0.2 % of flow per K range)
• Blowdown: B = E/(CoC − 1)

Worked example: Humidity of air

Given: 30 °C, 60 % RH, 101.3 kPa; p_sat(30 °C) = 4.246 kPa

Find: Absolute humidity.

• p_v = 0.6 × 4.246 = 2.548 kPa
• H = 0.622 × 2.548/(101.325 − 2.548) = 0.0160 kg/kg

Answer: ≈ 16 g water/kg dry air

Worked example: Cooling tower evaporation

Given: 1000 m³/h water cooled 40 → 30 °C

Find: Heat rejected and evaporation.

• Q = 277.8 kg/s × 4.18 × 10 = 11.6 MW
• Evaporation = 11.6 × 10⁶/2.43 × 10⁶ = 4.8 kg/s = 17.2 t/h (1.7 %)

Answer: 11.6 MW, ≈ 17 t/h evaporated

Common mistakes

• Wrong: Expecting a tower to cool below the wet bulb. — Right: Wet-bulb is the theoretical limit; approach is typically 3–6 K.
• Wrong: Ignoring blowdown. — Right: Without blowdown, dissolved solids build up and scale.

Applications

• Power & refineries: Cooling towers reject condenser and process heat.
• HVAC: Humidifiers, dehumidifiers and evaporative coolers.
• Industrial drying & paper: Air humidity control in dryers and paper machines.

Key takeaways

• H = 0.622 p_v/(P − p_v); RH = p_v/p_sat.
• Wet bulb is the cooling limit.
• Evaporation ≈ Q/λ; blowdown = E/(CoC − 1).
• Psychrometric charts map air processes.

Going further

• Merkel number: KaV/L integrated from enthalpy driving force sizes tower fill.
• Legionella control: Biocides, monitoring and design to prevent outbreaks.
• Hybrid coolers: Combine dry and wet operation to save water.