Air Change Rate (ACH) Calculator
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Air Change Rate (ACH) Calculator
ACH: N/A
Understanding Air Change Rate (ACH)
The Air Change Rate (ACH), also known as Air Changes per Hour, is a measure of how many times the entire volume of air within a specific space (like a room, building, or chamber) is replaced by fresh or conditioned air within one hour. It's a critical metric in various fields, including HVAC (Heating, Ventilation, and Air Conditioning), indoor air quality management, industrial processes, and environmental engineering.
Why is ACH Important?
- Indoor Air Quality (IAQ): Higher ACH generally leads to better IAQ by diluting and removing indoor pollutants such as CO2, VOCs (Volatile Organic Compounds), odors, and particulate matter.
- HVAC System Design: ACH is fundamental for sizing ventilation systems, ensuring that buildings receive adequate fresh air to meet comfort and health standards.
- Energy Efficiency: While ventilation is necessary, excessive ventilation (very high ACH) can lead to energy waste as heated or cooled air is expelled and replaced by outside air.
- Contaminant Control: In sensitive environments like hospitals, laboratories, or cleanrooms, maintaining specific ACH levels is crucial for controlling the spread of airborne contaminants or maintaining sterile conditions.
- Building Performance: ACH can indicate the airtightness of a building. Drafty buildings often have higher, uncontrolled ACH.
The Calculation Formula
The Air Change Rate (ACH) is calculated using the following formula:
ACH = (Airflow Rate in CFM × 60 minutes/hour) / Room Volume in cubic feet
Where:
- Airflow Rate (CFM): This is the volume of air being moved by a ventilation system (like an exhaust fan or air handler) per minute. It is typically measured in Cubic Feet per Minute (CFM).
- 60 minutes/hour: This conversion factor is used to express the rate on an hourly basis, as ACH is conventionally measured per hour.
- Room Volume (cubic feet): This is the total interior volume of the space being considered, calculated as Length × Width × Height.
The calculator above simplifies this slightly by allowing you to input the desired time period in minutes and then calculates the ACH based on that specific exchange of air. If you want to calculate the direct ACH, you would typically use a 60-minute period.
Example Calculation
Let's say you have a room with the following specifications:
- Room Volume: 1200 cubic feet
- Airflow Rate from your ventilation system: 300 CFM
- Time Period: 60 minutes (1 hour)
Using the calculator or the formula:
ACH = (300 CFM × 60 minutes/hour) / 1200 cubic feet
ACH = 18000 / 1200
ACH = 15
This means the air in the room is replaced 15 times every hour.
Interpreting ACH Values
Recommended ACH values vary greatly depending on the application:
- Residential Buildings: Often range from 0.35 to 1.0 ACH for basic ventilation.
- Commercial Offices: Typically require 3-5 ACH.
- Hospitals (Operating Rooms): May require 20+ ACH to maintain sterility.
- Cleanrooms: Can range from 50 ACH up to 1000+ ACH depending on the class of cleanroom.
It's crucial to consult relevant building codes, industry standards (like ASHRAE), or professional guidelines for specific ACH requirements for your situation.
function calculateACH() {
var roomVolume = parseFloat(document.getElementById("roomVolume").value);
var airflowRate = parseFloat(document.getElementById("airflowRate").value);
var timePeriod = parseFloat(document.getElementById("timePeriod").value);
var resultElement = document.getElementById("result");
// Validate inputs
if (isNaN(roomVolume) || roomVolume <= 0) {
resultElement.innerHTML = "Please enter a valid Room Volume.";
return;
}
if (isNaN(airflowRate) || airflowRate <= 0) {
resultElement.innerHTML = "Please enter a valid Airflow Rate (CFM).";
return;
}
if (isNaN(timePeriod) || timePeriod <= 0) {
resultElement.innerHTML = "Please enter a valid Time Period (minutes).";
return;
}
// Calculate total air exchanged in the given time period
var totalAirExchanged = airflowRate * timePeriod;
// Calculate ACH based on the volume and the total air exchanged in the given time period
var ach = totalAirExchanged / roomVolume;
// Convert to ACH (per hour) if the time period was not 60 minutes
// The formula: (CFM * timePeriod) / RoomVolume gives air changes per 'timePeriod' duration.
// To get ACH, we need to scale this to an hour.
// If timePeriod is 60, ach already represents ACH.
// If timePeriod is not 60, we need to adjust:
// ACH = (Total Air Exchanged / Room Volume) * (60 / timePeriod)
// Which simplifies to:
// ACH = (Airflow Rate * 60) / Room Volume
// Our current 'ach' variable is actually (Airflow Rate * timePeriod) / Room Volume
// So, to get ACH, we multiply our current 'ach' by (60 / timePeriod)
var actualACH = ach * (60 / timePeriod);
resultElement.innerHTML = "ACH:
" + actualACH.toFixed(2) + "";
}