What ACH (Air Changes Per Hour) Does My Plant Need? – A Practical Industrial Guide

 

What ACH (Air Changes Per Hour) Does My Plant Need? – A Practical Industrial Guide


Introduction

One of the most common questions plant owners and EHS teams ask is:
“What ACH (Air Changes Per Hour) does my plant need?”

ACH is a critical ventilation parameter that determines how many times the air inside your plant is replaced with fresh air every hour. Too little ACH leads to poor air quality, heat buildup, and safety risks. Too much ACH increases energy consumption unnecessarily.

This blog explains how to determine the right ACH for your plant, using practical industrial logic—not guesswork.

What Is ACH (Air Changes Per Hour)?

ACH represents the number of times the total air volume of a space is replaced in one hour.

Formula:
ACH = (Total air supplied or exhausted per hour) ÷ (Room volume)

Where:

  • Airflow is in m³/hr

  • Room volume = floor area × height

ACH applies to:

  • Factories

  • Warehouses

  • Workshops

  • Food plants

  • Chemical and process plants

Why ACH Is So Important in Industrial Plants

Correct ACH helps to:

  • Remove heat generated by machines

  • Dilute fumes, dust, vapors, and odors

  • Maintain oxygen levels

  • Improve worker comfort and productivity

  • Meet safety and audit requirements

ACH is not just about comfort—it is a safety and compliance parameter.

Typical ACH Requirements for Different Types of Plants

General Industrial Guidelines

Plant / Area TypeRecommended ACH Range
Light assembly / electronics6 – 10 ACH
Mechanical workshops8 – 12 ACH
Food processing (non-cooking)10 – 15 ACH
Food processing (cooking / frying)15 – 25 ACH
Plastic molding / rubber12 – 20 ACH
Welding / fabrication15 – 30 ACH
Chemical processing20 – 40 ACH
Battery / solvent handling25 – 50 ACH
Warehouses (non-conditioned)4 – 6 ACH

⚠️ These are starting ranges. Final ACH must be based on actual process conditions.

Key Factors That Decide How Much ACH Your Plant Needs

1. Heat Load from Machinery

High machine heat = higher ACH required to remove heat and prevent heat stress.

Examples:

  • Injection molding machines

  • Furnaces and ovens

  • Compressors and motors

2. Fumes, Dust & Vapors

Processes involving:

  • Chemicals

  • Solvents

  • Welding fumes

  • Oil mist

require higher ACH to dilute contaminants to safe levels.

3. Number of Workers

More people = higher CO₂ and oxygen demand.

Crowded workspaces require increased fresh air to avoid fatigue and discomfort.

4. Ceiling Height & Volume

Higher ceilings increase room volume, which affects airflow calculation.

High-bay factories may need large airflow but moderate ACH.

5. Local Exhaust Availability

If you have effective local exhaust ventilation (LEV):

  • Overall ACH can be lower

  • Ventilation becomes more efficient

Without LEV, general ACH must be higher to compensate.

ACH vs Exhaust-Only Ventilation (Critical Insight)

Many plants use only exhaust fans without fresh air supply.

This causes:

  • Strong negative pressure

  • Uncontrolled air infiltration

  • Reduced exhaust effectiveness

  • Dust and humidity ingress

👉 Correct ACH must include BOTH exhaust and make-up (fresh) air.

How to Estimate ACH for Your Plant (Simple Method)

Step 1: Calculate Plant Volume

Example:

  • Area = 10,000 sq. ft (930 m²)

  • Height = 8 m

Volume = 930 × 8 = 7,440 m³

Step 2: Decide Required ACH

Assume plant type needs 12 ACH

Step 3: Calculate Airflow

Required airflow = 7,440 × 12
= 89,280 m³/hr

This airflow must be:

  • Supplied as fresh air

  • Exhausted in a controlled manner

Should ACH Be Same Across Entire Plant?

❌ No.

Best Practice: Zoning

  • High ACH at process zones

  • Moderate ACH in assembly areas

  • Low ACH in storage areas

Zoning reduces energy consumption while maintaining safety.

ACH vs Comfort Cooling (Important Difference)

ACH is about air quality and safety
AC tonnage is about temperature control

You can have:

  • Correct ACH but poor cooling

  • Good cooling but insufficient ACH

Both must be designed together for optimal performance.

Common Mistakes When Selecting ACH

  • Using office ACH values for factories

  • Ignoring machine heat

  • No fresh air balancing

  • Over-ventilating entire plant uniformly

  • Not considering future expansion

These mistakes increase cost and reduce effectiveness.

When Higher ACH Is Mandatory

You must use higher ACH when:

  • Toxic or flammable substances are present

  • Regulatory or safety audits demand it

  • Workers report breathing discomfort

  • Heat stress is observed

  • Odors linger despite exhaust

Conclusion

There is no single ACH value that fits all plants. The right ACH depends on process heat, contaminants, occupancy, and safety requirements—not just floor area.

A properly designed ACH:

  • Protects worker health

  • Improves productivity

  • Enhances safety

  • Reduces HVAC energy waste

👉 ACH is a design decision, not a thumb rule.

For More Information Visit Our Website: www.wcsipl.com // www.wcsipl.net

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