Written by SIMA Intelligent Buildings
Introduction
The main purpose of a cooling tower is to cool the warm condenser water coming from the chiller so that it returns colder to the condenser.
In this part, we will explore the factors that drive the cooling effect in a cooling tower. In Part 2, we will discuss the difference between cooling tower thermodynamical efficiency and electrical efficiency (kW/ton).
Cooling Tower Heat Rejection Mechanism
In a cooling tower, approximately 85–90% of rejected heat is transferred through evaporation. Only 10–15% is transferred through sensible cooling of air. Because of this, we will focus on the evaporation effect and evaporation rate.
Cooling towers are mass-transfer devices, where heat rejection depends on how effectively water can evaporate into the air stream. If evaporation is not understood, cooling tower efficiency cannot be understood.
Evaporation Rate Explained
General Evaporation Rate Formula (Vapor-Pressure Driven)
Where:
- = evaporation mass rate
- = overall mass-transfer coefficient (water quality related)
- = effective air–water contact area (tower fill media wet area)
- = vapor pressure of water at the liquid surface (water temperature)
- = partial pressure of water vapor in the air (air wet-bulb / humidity)
The following image explains water vapor pressure in the air for equal-temperature water.
The following image explains vapor pressure of water at the liquid surface.
Evaporation Driving Force
All other variables remaining equal, the most important conclusions are:
- Warm water in contact with dry air = maximum evaporation rate
- Cold water in contact with humid air = minimum evaporation rate
When water evaporates at the surface, a large amount of energy is removed from the remaining water, reducing its temperature.
When air absorbs water trough evaporation its vapor pressure (wetbulb temperature) increases thus reducing its capacity to keep evaporating more water.
Cooling Tower Design – Evaporation Rate Maximization
Fill Media
HVAC cooling towers are commonly referred to as thin-film cooling towers.
This means the cooling tower is designed to produce a continuous thin film of water across the entire fill media area.
Evaporation occurs at the water surface where water interacts with air. The primary purpose of fill media is to maximize the total wet surface area.
Nozzles
The purpose of nozzles is to spray water in a pattern that promotes a uniform and continuous thin film over the entire fill media so that wet area is maximized.
From the above, we can see that the larger the wet area, the greater the evaporation potential.
Water Quality
Water quality affects both the mass-transfer coefficient (K) and the stability of the thin water film.
Higher mineral-content water can reduce evaporation rate, clog nozzles, and produce dry areas over the fill.
Factors That Affect the Wet Area
- Low water flow can produce dry areas, reducing total wet area
- Dirty or scaled fill can break the water film and produce preferential channeling instead of a thin film
- Clogged nozzles degrade spray patterns and produce both dry areas and water channeling
- Excessively high water flow can produce a thick water film that acts as thermal mass, reducing evaporation rate, and can also create smaller droplets that increase drift (water carried out through the fan)
L/G Ratio (Definition Only)
L/G ratio is the ratio of liquid water flow (L) to airflow (G) in a cooling tower.
It is a fundamental parameter that links water flow, airflow, and evaporation potential.
📌 We will explore how L/G ratio affects evaporation rate, cooling tower effectiveness, and kW/ton in the next article.
These are the main (though not the only) variables affecting cooling tower evaporation rate and, therefore, cooling tower efficiency.
In the next article, we will explore the difference between cooling tower thermodynamical efficiency and electrical efficiency (kW/ton). This will naturally lead into chiller plant efficiency, which we will cover in Articles 5 and 6.
If you have questions or want to share real-world examples, please leave a comment. Where possible, we will integrate them into the next article or discuss them directly in the comments.tiple interacting curves is better suited for AI-driven optimization than for static, rule-based control logic.
Written by SIMA Intelligent Buildings
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