How to Calculate Blowdown in Cooling Tower

Introduction

In cooling tower systems, blowdown (also called bleed-off) is essential for maintaining water quality and preventing scaling and corrosion. The accuracy of the blowdown calculation directly affects the operational efficiency, energy consumption, and maintenance cost of the cooling tower.

To help users manage make-up water and blowdown rate more effectively, this guide explains formulas, influencing factors, examples, diagrams, and engineering recommendations—combined with real-world experience from MACH Cooling (https://www.machcooling.com/).

 Importance of Blowdown in Cooling Towers

As a cooling tower operates, heat is removed through water evaporation, while dissolved minerals continuously accumulate in the circulating water.
Without proper blowdown, several issues will occur:

• Rapid increase of TDS (total dissolved solids)

• Higher risk of scaling and fouling

• Accelerated corrosion

• Higher chemical treatment cost

A proper blowdown rate maintains a stable Cycles of Concentration (COC), typically 3–7 cycles in most industrial systems.

Basic Principle of Blowdown Calculation

Blowdown (BD) depends mainly on:

• Evaporation loss (E)

• Drift loss (D)

• Cycles of concentration (COC)

The widely used blowdown equation is as follows.

 Blowdown Formula

Standard Formula (Most Common)

Where:

• BD: Blowdown rate (m³/h)

• E: Evaporation loss (m³/h)

• COC: Cycles of Concentration

Evaporation Estimation Formula

A common estimation method is:

Where:

• Q: Heat load (kcal/h or kW)

Or based on refrigeration tons (RT):

 Additional Losses

Drift loss:

Make-up water:

Calculation Example (with Table)

Assume a cooling tower system with:

Step 1: Evaporation loss

Step 2: Blowdown rate

Step 3: Drift loss

Step 4: Make-up water

Recommended Diagrams for the Article

Image Example 1: Cooling Tower Blowdown Flow Diagram

Title: Cooling Tower Blowdown Flow Diagram
Description: A process flow diagram showing make-up water → circulating water → evaporation loss → drift loss → blowdown discharge.

Image Example 2: Blowdown Calculation Formula Diagram

Title: Blowdown Calculation Formula
Description: A block diagram illustrating inputs (E, D, COC) and output BD.

(If you want, I can generate both images immediately.)

 Factors Affecting Blowdown Requirements

 1. Water Quality Requirements

Different applications tolerate different TDS and hardness levels:

• Precision manufacturing → low TDS

• Power plants / chemical facilities → higher COC acceptable

2. Chemical Treatment Efficiency

With scale inhibitors and corrosion inhibitors, COC can be raised:

Higher COC → lower blowdown → lower water usage.

3. Cooling Tower Efficiency and Drift Loss

A high-performance cooling tower reduces evaporation and drift losses.

MACH Cooling towers use:

• High-efficiency PVC or PP fill material

• Drift eliminators with less than 0.001% drift rate

• Intelligent fan control

These improvements can reduce blowdown demand by 10–15%.

How to Optimize Blowdown: Engineering Solutions

 Solution 1: Increase COC

• Improve make-up water quality

• Use corrosion & scale inhibitors

• Install MACH Cooling side-stream filtration units

 Solution 2: Reduce Drift Loss

Using high-efficiency drift eliminators (available from MACH Cooling) lowers TDS concentration increase, reducing blowdown.

Solution 3: Automatic Blowdown Control

Automatic bleed control ensures consistent COC and prevents:

• Over-blowdown → water waste

• Under-blowdown → scaling and corrosion

Conclusion

Blowdown calculation in cooling towers relies on accurate evaporation estimation, COC control, and system monitoring. This article provides formulas, examples, diagrams, and optimization strategies.

To further enhance performance, MACH Cooling (https://www.machcooling.com/) offers high-efficiency cooling towers and water treatment solutions that significantly reduce:

• Make-up water consumption

• Blowdown loss

• Operating cost