Does the Wet-Bulb Temperature Change?
Yes, the wet-bulb temperature is not constant.
At any location, it varies depending on:
- Seasons
- Months
- Days
- Even day and night cycles
Main Factors Affecting Wet-Bulb Temperature
Air Temperature:
As temperature increases → wet-bulb temperature generally increases
Because higher heat means a higher energy level.
Humidity (Most Critical Factor):
- High humidity → evaporation is difficult → wet-bulb increases
- Low humidity → evaporation is easier → wet-bulb decreases
Airflow (Wind):
- Wind → increases evaporation → lowers wet-bulb
- Still air → moisture accumulates → increases wet-bulb
Atmospheric Pressure:
- Lower pressure (high altitude) → easier evaporation
→ slightly lower wet-bulb temperature
Solar Radiation:
- Direct sunlight may increase measurement values
→ Measurements should be taken in shaded conditions
Long-Term Changes
- Global Temperature Increase:
As global temperatures rise, wet-bulb temperature also increases. - Changesin Humidity Levels:
Affected by:
- Dams
- Irrigation systems
- Drying lakes
- River changes
- Climate Change:
- Hotter summers
- Higher humidity levels
- Extreme weather events
- Urbanization (Heat Island Effect):
- Concrete and asphalt retain heat
- Reduced green areas
→ Both temperature and perceived humidity increase
- Air Circulation:
- Wind lowers wet-bulb
- Still air increases it
PSYCHROMETRIC CHART USAGE
Chart Structure:
- X-axis → Dry-bulb temperature
- Y-axis → Humidity ratio
- Inclined lines → Wet-bulb temperature
- Curves → Relative humidity
Determining a Point:
Two parameters are sufficient:
- Dry-bulb + Relative humidity
- Dry-bulb + Wet-bulb
- Dry-bulb + Humidity ratio
Example:
- Dry-bulb: 30°C
- Wet-bulb: 22°C
Steps:
- Locate 30°C on X-axis
- Move vertically
- Find 22°C wet-bulb line
- Intersection = air condition
From this point:
- Relative humidity
- Enthalpy
- Humidity ratio
- Dew point
- Specific volume can be determined
COOLING TOWER APPLICATION
Key Principle:
Wet-bulb temperature = minimum achievable temperature
- Cooling water cannot be cooled below this value
Performance Parameter:
Approach = Cold water – Wet-bulb
Example:
- Cold water: 28°C
- Wet-bulb: 24°C
→ Approach = 4°C
Optimum Values:
- ≈ 3°C → optimum
- 3°C → smaller tower, lower cost
- <3°C → exponentially increasing size and cost
Critical Limit:
At ≈ 35°C wet-bulb,
the human body can no longer cool itself → life-threatening condition