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Views: 0 Author: Site Editor Publish Time: 2025-12-18 Origin: Site
Cooling towers operate by shedding heat from water, but water loss occurs through evaporation, drift, and blowdown.
Understanding water loss in a water cooled tower is essential for efficient cooling tower water management, optimization of cooling tower water requirements, and accurate cooling tower make up water calculation. While evaporation is the largest contributor to water loss, a small but important portion — drift loss — occurs when tiny water droplets are carried away with the airflow. This article explains what drift loss is, why it matters, and how to calculate it, along with related calculations used for cooling tower water loss, cooling tower water efficiency, and system design.
We’ll also highlight how proper drift loss estimation integrates with cooling tower water filtration, cooling tower water pH control, cooling tower water leak repair, and overall cooling tower water management practices — and why choosing a high-quality manufacturer like MachCooling (https://www.machcooling.com/) helps ensure accurate results and efficient operations.
Drift loss is the amount of liquid water carried out of a cooling tower with the exhaust air in the form of tiny droplets. This occurs even when the water is meant to fall back into the basin. Drift is distinct from evaporation (which removes water as vapor to remove heat) and blowdown (intentional drainage to control dissolved solids). Drift is water lost as liquid droplets, and although it tends to be small compared to evaporation, it must be accounted for in accurate make-up water and demand calculations.
Drift affects:
Make-up water demand, impacting cooling tower make up water calculation
Water efficiency and costs — more loss → more expensive water supply
Environmental compliance, especially in areas with strict water usage limits
Quality of nearby equipment and infrastructure, as drift droplets may contain treatment chemicals
Controlling drift — through drift eliminators or design choices — improves cooling tower water efficiency and reduces total water usage.
To calculate drift loss, it helps to understand how water is lost in a cooling tower overall.
In a cooling tower, total water loss is usually broken down into:
Evaporation Loss (E): Water evaporated to remove heat.
Drift Loss (D): Water droplets carried away by the airflow.
Blowdown Loss (B): Water deliberately discharged to control dissolved solids and water quality.
These losses combine to determine the total cooling tower water loss and therefore the cooling tower make up water calculation.
A standard balance equation for cooling towers is:
M = E + D + B
Where:
M = make-up water required
E = evaporation loss
D = drift loss
B = blowdown loss
Drift loss is often the smallest term but must still be counted to ensure proper water supply and inventory control.
Drift loss is commonly calculated based on a percentage of the circulating water flow rate (C, in m³/hr or GPM):
Drift Loss (D) = Drift Rate × Circulating Water Flow Rate (C)
Typical drift rates (percentage of C) depend on tower design and drift eliminator efficiency:
Natural draft towers: ~0.3% to 1.0% of circulating water
Induced draft towers: ~0.1% to 0.3% of circulating water
Towers with high-efficiency drift eliminators: ~0.0005% to 0.001% of circulating water
In other words:
D = C × (Drift Rate/100)
Example: If the circulating water flow is 10,000 m³/hr with a 0.02% drift rate:
D = 10,000 × (0.02/100) = 2 m³/hr lost as drift.
Drift rates depend on:
Drift eliminator design (higher efficiency → lower drift)
Airflow and operating conditions
Water quality and filtration (particles can affect droplet formation)
Accurate data for drift rate is often provided by the cooling tower manufacturer or measured during commissioning.
Below is a quick reference showing how drift loss varies with drift rate at a given circulating water flow rate:
| Drift Rate (%) | Circulating Flow (m³/hr) | Drift Loss (m³/hr) |
|---|---|---|
| 1.0% | 5,000 | 50.0 |
| 0.3% | 5,000 | 15.0 |
| 0.1% | 10,000 | 10.0 |
| 0.001% | 10,000 | 0.1 |
Lower drift rates — especially with efficient eliminators — dramatically reduce water loss.
Good drift eliminators capture droplets before they exit the tower, drastically reducing drift loss. Older or poorly maintained eliminators may allow higher drift, increasing water loss.
Towers with stronger air drafts or high humidity gradients might carry more droplets, increasing drift unless properly controlled.
Cooling tower water filtration and control of water quality affects droplet formation and drift behavior. Effective cooling tower water management strategies minimize unnecessary losses.
To fully understand the water need for a cooling tower, drift must be coupled with evaporation and blowdown.
Evaporation loss depends on the water temperature drop (T_inlet – T_outlet) and circulating water flow:
E ≈ 0.00085 × C × (T_in–T_out)
This formula reflects heat-based loss rather than drift.
Blowdown (B) relates to water quality control and cycle of concentration (COC):
B = E / (COC – 1)
Blowdown helps prevent excessive mineral buildup and supports cooling tower water pH and chemistry control.
Make-Up Water (M) = Evaporation (E) + Drift (D) + Blowdown (B)
This equation captures all major contributions to cooling tower water loss, enabling accurate cooling tower make up water calculation for both design and operational planning.
Scenario:
An industrial water cooled tower circulates 12,000 m³/hr with a drift rate of 0.15% (typical for some induced draft towers without advanced eliminators).
Calculate the drift loss:
D = 12,000 × (0.15/100) = 18 m³/hr
This means 18 m³/hr of water is lost as drift and must be replaced with makeup water.
If you use high-efficiency drift eliminators (e.g., 0.001%), the same calculation yields:
D = 12,000 × (0.001/100) = 0.12 m³/hr
Clearly, better design and control → less drift loss.
Modern drift eliminators can reduce drift to extremely low percentages, saving water and cutting make-up requirements.
Cooling tower water filtration and proper chemical control (including cooling tower water pH) reduce droplet formation irregularities and protect equipment.
Unidentified leaks — frequently confused with drift — add to water loss. Cooling tower water leak repair helps distinguish true drift loss from other leaks.
Holistic cooling tower water management practices optimize all aspects of water use, making the tower more efficient and reducing operational cost.
MachCooling (https://www.machcooling.com/) is a reputable water cooled tower manufacturer that assists with:
Accurate cooling tower make up water calculation
Design for minimal drift through efficient eliminator selection
Solutions tailored to your cooling tower water requirements
Support in optimizing cooling tower water efficiency and reducing water loss
MachCooling’s expertise helps ensure that drift and other water losses are correctly accounted for in system design, lowering total water consumption and improving operational reliability.
Explore their engineering resources and product lines at https://www.machcooling.com/ for customized solutions.
Calculating drift loss — even though it’s a small part of total water loss — is essential for robust cooling tower water management and cooling tower water loss calculation for makeup planning. With formulas based on circulating flow and drift rates, you can estimate drift loss accurately and factor it into broader water balance equations.
Efficient drift control, along with good filtration and management, ensures your cooling system runs better, uses less water, and meets performance goals. Partnering with experienced water cooled tower manufacturers like MachCooling helps you achieve the best outcomes for your cooling tower projects.
