Wet Bulb Calculator
The Wet Bulb Calculator determines wet bulb temperature from dry bulb temperature and relative humidity. Wet bulb temperature is crucial for HVAC design, cooling tower performance, evaporative cooling systems, and human comfort assessment in hot climates.
Understanding Wet Bulb Temperature
Wet bulb temperature is the lowest temperature that can be reached by evaporative cooling. It represents the temperature of a wet thermometer bulb exposed to moving air, where evaporation cools the bulb until equilibrium is reached between heat and mass transfer.
Key Concepts
Physical Meaning
Wet bulb temperature indicates:
- The cooling potential of evaporation
- The minimum temperature achievable by evaporative cooling
- The adiabatic saturation temperature of air
- A measure of the combined effect of temperature and humidity
Relationship to Other Parameters
- Always ≤ Dry Bulb Temperature: Wet bulb is never higher than dry bulb
- Equals Dry Bulb at 100% RH: When air is saturated
- Lower with Lower Humidity: Greater difference at lower RH
- Independent of Pressure: Minimally affected by atmospheric pressure
Calculation Methods
Iterative Psychrometric Method
The most accurate method involves iterative solution of psychrometric equations:
- Assume initial wet bulb temperature
- Calculate saturation vapor pressure at wet bulb temperature
- Apply psychrometric constant and energy balance
- Iterate until convergence
Approximation Formula (Stull 2011)
For quick estimates at standard pressure:
Tw = T × atan[0.151977 × (RH% + 8.313659)^0.5] + atan(T + RH%) - atan(RH% - 1.676331) + 0.00391838 × (RH%)^1.5 × atan(0.023101 × RH%) - 4.686035
Applications
HVAC and Cooling Systems
- Evaporative Cooling: Determine cooling potential and efficiency
- Cooling Towers: Calculate approach temperature and performance
- Air Conditioning: Size equipment for latent and sensible loads
- Natural Ventilation: Assess passive cooling strategies
Industrial Processes
- Drying Operations: Optimize drying conditions
- Textile Industry: Control humidity in manufacturing
- Food Processing: Maintain product quality during processing
- Chemical Processes: Control reaction environments
Human Comfort and Safety
- Heat Stress Assessment: Evaluate dangerous conditions
- Workplace Safety: Monitor industrial environments
- Sports and Recreation: Assess outdoor activity safety
- Building Design: Optimize natural cooling strategies
Wet Bulb Globe Temperature (WBGT)
Related to wet bulb temperature, WBGT is used for heat stress assessment:
- Outdoor WBGT: 0.7 × Tw + 0.2 × Tg + 0.1 × Ta
- Indoor WBGT: 0.7 × Tw + 0.3 × Tg
- Where Tw = wet bulb, Tg = globe temperature, Ta = dry bulb
Critical Wet Bulb Temperatures
| Wet Bulb (°C) | Wet Bulb (°F) | Condition | Impact |
|---|---|---|---|
| < 15 | < 59 | Comfortable | No heat stress concerns |
| 15-20 | 59-68 | Caution | Monitor activity levels |
| 20-25 | 68-77 | Warning | Limit strenuous activity |
| 25-30 | 77-86 | Danger | Heat exhaustion likely |
| > 30 | > 86 | Extreme Danger | Heat stroke imminent |
Measurement Techniques
Sling Psychrometer
- Traditional method using wet and dry bulb thermometers
- Requires manual operation and air movement
- Accurate when properly used
Aspirated Psychrometer
- Motorized air movement over thermometers
- More consistent air velocity
- Reduced operator error
Electronic Instruments
- Digital psychrometers with automatic calculation
- Continuous monitoring capability
- Data logging and remote monitoring
Factors Affecting Accuracy
Measurement Conditions
- Air Velocity: Minimum 2.5 m/s required for accurate readings
- Radiation Shield: Protect from direct sunlight and heat sources
- Wick Condition: Clean, properly wetted wick essential
- Water Quality: Use distilled water to prevent contamination
Environmental Factors
- Atmospheric Pressure: Affects evaporation rate slightly
- Air Composition: Pollutants can affect readings
- Temperature Gradients: Ensure representative sampling
Design Applications
Evaporative Cooling Systems
- Direct evaporative coolers: approach wet bulb by 2-5°C
- Indirect evaporative coolers: approach wet bulb by 5-8°C
- Cooling towers: approach wet bulb by 3-6°C
Natural Ventilation
- Night flush cooling effectiveness
- Passive downdraft cooling potential
- Courtyard and atrium cooling strategies
Note: Wet bulb temperature calculations are most accurate when using iterative psychrometric methods. Simple approximations may have errors of ±1-2°C under extreme conditions.