evaporative-cooling

name	evaporative-cooling
description	Evaporative cooling — direct/indirect/two-stage evaporative coolers, wet bulb temperature, cooling effectiveness, water consumption, Mollier chart analysis, ASHRAE 55 comfort, desert climate cooling, data center economizer.
metadata	{"priority":7,"promptSignals":{"phrases":["evaporative cooling","swamp cooler","wet bulb cooling","direct evaporative cooler","indirect evaporative","adiabatic cooling"],"minScore":3}}

Evaporative Cooling — Complete Skill

Psychrometric Fundamentals

Wet-bulb temperature (T_wb): temperature air reaches after adiabatic saturation (all heat converts to latent) Dew point (T_dp): temperature at which air is saturated (relative humidity = 100%) Wet bulb depression (WBD): T_db - T_wb [°C]; measure of cooling potential; larger = drier air = more cooling

Humidity ratio (W): mass of water vapor per mass of dry air [kg/kg or gr/lb] Relative humidity (RH): φ = P_v / P_v_sat × 100% [%]

Relationship: T_wb = T_db - (T_db - T_dp) × [Rh^(1/8)] [Magnus approximation; rough] More accurately from psychrometric chart or equations

Direct Evaporative Cooling (DEC)

Process: air passes through wetted media → water evaporates → air temperature drops; humidity rises Adiabatic process: total enthalpy constant: h = constant along wet-bulb line on psychrometric chart Supply air condition: reaches near T_wb (not fully); limited by media effectiveness

Cooling effectiveness (saturation efficiency): ε = (T_db_in - T_db_out) / (T_db_in - T_wb_in) [dimensionless; 0.85–0.95 for good media]

Exit air temperature: T_out = T_db_in - ε × (T_db_in - T_wb_in) = T_db_in - ε × WBD

Water consumption: ṁ_water = ṁ_air × (W_out - W_in) [kg/s; W from psychrometric chart at T_out, φ ≈ 100%ε + φ_in(1-ε)]

Example: Input: 40°C / 20% RH; T_wb = 25.4°C; WBD = 14.6°C; ε = 0.90 T_out = 40 - 0.9 × 14.6 = 40 - 13.1 = 26.9°C → significant cooling! H: 26.9°C with φ ≈ 95% → still comfortable in many climates

Limitation: outlet air is humid → unacceptable in humid climates (T_wb close to T_db) Best for: desert climates (T_wb < 22°C); T_wb < 18°C → very effective

Indirect Evaporative Cooling (IEC)

Process: secondary water circuit evaporated; heat transferred to primary air stream WITHOUT humidifying it Dry-bulb temperature drops; humidity unchanged Less effective than DEC but no humidity addition → suitable for more humid climates

Dew-point effectiveness: ε_dp = (T_db_in - T_db_out) / (T_db_in - T_dp_in) × 100% [%] Best IEC: ε_dp = 70–90% (Maisotsenko cycle achieves 80–110% using multi-stage)

IEC types: Plate type: thin plastic/aluminum plates; secondary evaporation on wet side; primary dry side Tube type: secondary water runs inside tubes; primary air over outside; no moisture transfer Maisotsenko (M-cycle): unique counterflow approach achieving T_dp approach; can reach T_dp not just T_wb

Two-Stage (Indirect + Direct) System

Stage 1 (indirect): pre-cool air without humidifying (T_db drops, W unchanged) Stage 2 (direct): further cooling by DEC on pre-cooled, less humid air

Advantage: lower supply temperature than DEC alone while maintaining acceptable humidity Example: 40°C/20% RH → indirect pre-cool to 30°C/28% → direct evap to T_supply ≈ 22°C

Cooling capacity calculation: Q_cooling = ṁ_air × c_p_air × (T_in - T_out) + ṁ_air × (W_out - W_in) × h_evap For sensible-only cooling: Q = ṁ × c_p × ΔT [kW; c_p = 1.006 kJ/(kg·K)]

Data Center Evaporative Cooling (Free Cooling)

Adiabatic cooler: spray water into outdoor air before entering air-cooled chiller/condensers Reduces T_wb → lower condensing temperature → higher COP

Water savings vs. energy: Power reduction: 30–60% in summer (desert climates) when adiabatic cooling engaged ASHRAE A1 data center: return air T ≤ 45°C; supply 15–27°C

Economizer hours: Hours where T_wb_outdoor < T_supply_required → free cooling available Hours increase significantly in dry climates

Water consumption (data center): Water Usage Effectiveness (WUE) = annual water volume / IT energy [L/kWh] Evaporative cooling: WUE ≈ 2–5 L/kWh; air-cooled (no evap): WUE < 0.5 L/kWh

Media and Equipment

Evaporative media: Cellulose (paper) pads: lowest cost; most common; ε = 0.85; airflow up to 2 m/s Rigid plastic media (PVC): higher durability; ε = 0.85–0.95; chlorine-resistant Fabric pads (dew point cooler): ε_dp = 75–85%

Sump design: Water recirculation rate: 5–10× evaporation rate (to prevent concentration of dissolved minerals) Bleed rate: 10–20% of recirculated water → maintain TDS < 1000 ppm TDS = Total Dissolved Solids; high TDS → scaling on media → reduced effectiveness

Corrosion and Legionella: Legionella prevention: maintain water T < 20°C or > 60°C; or treat with biocide ASHRAE 188: standard for Legionella prevention in building water systems Regular cleaning: media and sump; weekly visual inspection recommended

Cooling Effectiveness vs. Climate

Climates where evaporative cooling is effective:

Climate	T_wb max	Effective?
Hot desert (Phoenix, Riyadh)	< 22°C	Excellent
Hot dry Mediterranean	< 24°C	Good
Hot-humid tropical	27–30°C	Limited; IEC only
Temperate	< 20°C in summer	Excellent for night flush

Hours below T_wb thresholds (annual): Phoenix: T_wb < 18°C → 4000+ hr/yr; T_wb < 22°C → 7000+ hr/yr

Standards

Standard	Scope
ASHRAE 55	Thermal comfort (temperature and humidity limits for occupants)
ASHRAE 188	Legionella risk management
ASHRAE 90.1	Energy standard; evaporative credit for HVAC
ANSI/AMCA 330	Air performance certification for evaporative products
ISO 9001	Cooling system quality management

Output

Provide: outdoor design conditions (T_db [°C], RH [%], T_wb [°C], T_dp [°C]), cooling type (direct/indirect/two-stage), saturation effectiveness ε [%] or ε_dp [%], supply air T [°C] and RH [%], cooling capacity Q [kW], airflow ṁ_air [kg/s], water consumption rate [kg/hr] and total [L/day], water makeup rate and bleed rate [L/hr], media type and depth [mm], WUE [L/kWh] (if data center), annual free-cooling hours [hr/yr], Legionella risk control (ASHRAE 188 compliance), and applicable standard (ASHRAE 55, ASHRAE 188, ASHRAE 90.1).