The CADR of your air purifier depends on two factors.
- The volume of your room
- The ACH value (Air changes per hour)
Here is the CADR calculator that can help you determine the air purifier you need to purchase for your home.
Step 1: Enter the length, width, and ceiling height in feet.
Step 2: Select the ACH value. It should be 2 ACH for standard air purification. If you have asthma and allergic patients in your home, you should select 5 ACH.
If the air is heavily contaminated with cigarette smoke, you can select the ACH value as 8. The logic is that the higher the ACH, the faster is the air cleaning efficiency in the room. Hospitals, beauty parlors, industrial boiler rooms, commercial places like malls and theaters can have higher ACH.
Here is the formula
CADR = [(ACH x L x W x H) / 60] cfm
CADR = [(ACH x L x W x H) / 60) x 1.699] m3h
- The dimensions of your room are 15 feet X 12 feet X 10 feet = 1800 cubic feet.
- ACH value depends on the air quality in the room.
This table should help you calculate the CADR.
|ACH = 2 Normal Room||ACH = 5 Asthmatic patients||ACH = 8 Cigarette smoke-filled||ACH = 10 Highly polluted like industries|
|The volume of the room||1800 cubic feet||1800 cubic feet||1800 cubic feet||1800 cubic feet|
|Air to be changed per hour||1800 X 2 = 3600 cubic feet per hour||1800 X 5 = 9000 cubic feet per hour||1800 X 8 = 14400 cubic feet per hour||1800 X 10 = 18000 cubic feet per hour|
|Convert to cubic meter per hour (1 cubic feet per hour = 0.028 cubic meter per hour)||3600 X 0.028 = 100.80||9000 X 0.028 = 252||14400 X 0.028 = 403.20||18000 X 0.028 = 504|
|CADR for room size 180 square feet||100||252||400||500|
You should know that there are three CADR values for an air purifier.
If you live in industrial areas or near the main roads where there can be heavy traffic, it is better to concentrate more on the Smoke CADR value. Similarly, if there are heavy cigarette smokers in the room, the Smoke CADR is crucial. The highest possible smoke CADR is 450. Industrial pollutants can require higher CADR.
Dust particles are common in almost every home. If you have allergy issues or asthma patients and children in the house, the Dust CADR assumes significance. The maximum Dust CADR is 400.
The Pollen CADR comes into play when you have pollen contaminants or animal dander in your homes. The highest Pollen CADR is 450.
The ACH Concept
ACH signifies Air Changes per hour. It measures the number of times your air purifier filters the air in the room in one hour. Thus, if your air purifier has an ACH of 4, it means that the machine filters the air inside the room four times an hour.
The ideal ACH depends on the air quality inside your room. If the air quality is good enough, an ACH of 2 should suffice. If you have elder parents, asthmatic patients, or suffering from allergic issues, the ideal ACH is 5. On the other hand, rooms heavily contaminated with tobacco smoke or other smoke like industrial and vehicular emissions require an ACH of 8.
Generally, the air purifier manufacturers do not mention the ACH in their product manuals. Hence, you might have to calculate the ACH on your own.
However, this calculation can help you determine the ACH.
You will need the following information.
Generally, air purifiers in India come with CADR mentioned in cubic meters per hour. You have to convert this figure into cubic feet per minute. The conversion equation is to multiply the CADR cubic meters/hour figure by 0.588. (1 cubic meter/hour is equal to 0.588 cubic feet per minute.
The Coverage Area
The coverage area is usually mentioned in square feet. If the area is given in square meters, you can multiply the figure by 10.76 to get the corresponding value in square feet.
The Ceiling height
Generally, Indian homes have ceiling height up to nine or ten feet. You will require this value at the final step of the calculation.
Step 1: Get the CADR in cubic feet per minute. Generally, you have this figure readily available. If your air purifier manufacturer provides the airflow rate, multiply the value by 2/3 (The AHAM 2/3 formula to determine CADR).
Step 2: Air changed in one hour = CADR in cubic feet per minute X 60
Step 3: Find the room value by multiplying the area of the room by the ceiling height.
Step 4: The ACH Rating = air changed in one hour divided by the Room Volume
This example should clarify things better for you.
- Look for the CADR value provided by the manufacturer. At times you will find the airflow value in CFM instead of the CADR. It makes your job easy. For example, assume that the CADR value provided by the manufacturer is 300 cubic meters per hour.
- Convert this value into cubic feet per minute. 300 x 0.588 = 176.4 cubic feet per minute
- Calculate your room area. Area = Length x width. Let us assume it to be 250 square feet.
- Air changed in one hour = CADR cfm x 60 = 176.4 X 60 = 10,584
- Assume the room height to be 9 feet.
- Therefore, the room volume amounts to 250 X 9 = 2250 cubic feet
- ACH Rating = Air changed in one hour / Room volume = 10,584 / 2250 = 4.70
These calculators should have made the concept of CADR and ACH clear.
Dr. V. Suvidha has Ph.D in Electrical Engineering. She is an Expert in Power Systems, Electrical Machines, Micro Grids, and Renewable Energy Systems.