Thermodynamics & Industrial Cooling

What is Wet-Bulb Temperature?

It is the lowest theoretical temperature limit to which air can be cooled adiabatically (without heat exchange) solely by the evaporation of water. It serves as a critical reference parameter in industrial cooling towers, HVAC engineering, and meteorology.

Adiabatic Saturation Principle

When water evaporates, it absorbs latent heat from the air to change phase, lowering the air's sensible heat. In this constant-enthalpy process, the temperature indicated by the thermometer at the exact moment the air reaches 100% relative humidity is the wet-bulb temperature.

Dry Air: High evaporation rate; the gap between wet-bulb and dry-bulb temperatures widens significantly.
Saturated Air (100% RH): Evaporation ceases; Dry-Bulb = Wet-Bulb = Dew Point.

Psychrometric & Physiological Limits

Wet-bulb temperature is represented by constant-slope lines on psychrometric enthalpy charts. Because the human body also regulates heat via perspiration (evaporation), the wet-bulb value is the primary indicator of physiological heat stress.

Critical Physiological Threshold (31°C - 35°C WB): When the wet-bulb temperature reaches 35°C (e.g., 38°C air temperature at 80% relative humidity), the human body can no longer dissipate heat, creating a severe risk of fatal hyperthermia.

Industrial System Design

Thermodynamic Limits in Cooling Towers

Minimum Theoretical Cooling Limit = WB

Cooling towers operate on evaporative principles. Regardless of atmospheric conditions, the temperature of the water entering the tower can never drop below the ambient wet-bulb temperature of that geographical location. Design efficiency is evaluated by two key parameters:

Approach Value

The temperature difference between the cold water leaving the tower and the ambient wet-bulb temperature:
Approach = Cold Water Temperature - Wet-Bulb Temperature

Cost Relationship: When the approach value drops below 2°C, the required fill media surface area and fan CAPEX increase exponentially. The industrial standard generally ranges between 3°C and 5°C.

Cooling Range

The temperature difference between the hot water entering the tower and the cold water leaving it:
Range = Hot Water Temperature - Cold Water Temperature

Efficiency: The range value is directly proportional to the thermal load carried by the system. As the regional wet-bulb value drops, the tower operates with lower airflow and energy consumption for the same cooling capacity.

Engineering Warning: When sizing cooling tower capacity, the reference must not be the annual average wet-bulb temperature of the region, but rather the Summer 1% or 0.4% Design (Extreme) Wet-Bulb Temperature. Otherwise, the facility will face critical thermal bottlenecks during summer peak loads.

Summer Design Wet-Bulb Temperatures for Turkish Provinces

Critical summer conditions (1% exceedance frequency) for industrial cooling tower and HVAC design, compliant with ASHRAE and TS 825 standards.

Design Standard: Summer Peak (1%)

Province / Region	Climate Character	Dry-Bulb Temp (DB)	Wet-Bulb Temp (WB)	Recommended Tower Outlet Water (Min.)
Adana	High Humidity / Coastal	36.5 °C	27.8 °C	31.0 °C - 32.0 °C
Antalya	Extreme Humidity / Coastal	38.0 °C	28.2 °C	31.5 °C - 32.5 °C
İzmir	Hot / Moderate Humidity	36.8 °C	25.4 °C	28.5 °C - 29.5 °C
İstanbul (Marmara)	Temperate / Humid	33.2 °C	24.5 °C	28.0 °C - 29.0 °C
Bursa	Transitional Climate	34.8 °C	24.8 °C	28.0 °C - 29.0 °C
Samsun	High Relative Humidity	31.5 °C	25.1 °C	28.5 °C - 29.5 °C
Ankara	Dry Continental (High Efficiency)	34.6 °C	20.8 °C	24.0 °C - 25.0 °C
Gaziantep	Hot & Dry Continental	38.5 °C	22.4 °C	25.5 °C - 26.5 °C
Konya	Dry Continental	33.8 °C	19.8 °C	23.0 °C - 24.0 °C

* The "Recommended Tower Outlet Water" values in the table are calculated based on an average approach value of 3.0°C - 3.5°C, accepted as the industrial standard. In dry climates (Ankara, Konya, Gaziantep), evaporative cooling efficiency is significantly higher compared to coastal regions.

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