Air Changes Per Hour|CFM|Pharma

Air changes per hour is the common and most widely used term in pharmaceutical industries and other fields where HVAC systems are installed.Air changes per hour are also called ACPH. Most people and even pharmaceutical professionals are not aware of the method  to calculate ACPH.In this article, we will discuss

  • What is ACPH?
  • Importance of ACPH
  • How to Calculate ACPH?
  • What is CFM?
  • Recommended ACPH
  • Queries to Solve Questions

What is ACPH?

Air changes per hour is the volume of air supplied and removed from an area in one hour using an HVAC system.

ACPH is the volume of air per hour divided by the volume of area or room.

As the name indicates air changes mean the replacement of air or changing the old air with new air in one hour.

Importance of ACPH

Air changes per hour are important because of the following.

  • To ensure proper ventilation.
  • To remove dust, smoke fumes from an area,room or building.
  • To remove carbon dioxide and other gases from the area.
  • To maintain clean room status by removing non-viable particles.
  • To maintain clean room status by removing viable count adhere to the non-viable particles.

Also Read: 

GMP vs cGMP and 21 CFR.

Recommended Air changes

  • In the pharmaceutical industry different recommended ranges of Air Changes Per Hour are given.
  • ACPH is given in a range which means that a fixed value is not given.
  • The range has the flexibility to adopt any value which lies between the range.
  • For lower range,air movement or velocity is low and for higher range, air velocity or movement is high.
  • For some general areas, ACPH are given as follow,









For Clean rooms recommended ACPH are as follow,

ISO 146144-1










If we select a lower number of ACPH  in pharmaceuticals it meets regulatory requirements but the level of cleanliness is different at lower and upper ranges.

Selection of lower range ACPH reduces the cost of operation due to less energy cost.


ISO Class 8 recommended range is 5-48 ACPH.

If we select lower range e.g 5 ACPH it means that 5 times in an hour air in the room will be replaced.

If we select upper range e.g ACPH 48 then it means air replacement is high that is 48 times in an hour.

As we increase the ACPH from low to high it changes the status of the area from less clean to more clean due to the increased swiping action of air.

Also Read: 

Pharmaceutical Questions and Answers.

How to Calculate ACPH?

To calculate ACPH the following formula is used.

ACPH=T×60/Volume of the Room

 ACPH=Air changes Per Hour

T=Total air volume supplied in the room

Step 1

Total Air Volume

In the first step, we will calculate T or the total air volume supplied in the room by using following formula,



V=Velocity of air from HEPA filter in the room

           A=Area of HEPA filter

From the above formula we can see that to calculate ACPH,first we have to calculate velocity of air and volume of the room.


How to Calculate Velocity of Air?

  • The velocity of air is the speed at which air comes in the room from a HEPA filter.
  • It is taken in feet/minute or Meter/Second.
  • To determine the air velocity anemometer is used.

We take 5 values of air velocity from the filter face,4 values are taken from each corner and the 5th value is taken from the centre of the  HEPA filter.

Average velocity is calculated as below

Air Velocity V= V1+V2+V3+V4+V5 /5


Suppose values of air velocity from V1 to V5 are as follow in feet/minute

  • V1=240
  • V2=280
  • V3=200
  • V4=250
  • V5=230

Put values in above formula as follow,

Air Velocity V= V1+V2+V3+V4+V5 /5

V= 240+280+200+250+230/5


                    V=240 feet/minute


So the average velocity of air in the room is 240 feet/minute.

How to Calculate Area of Filter?

A is Area of HEPA filter and is determined by the multiplying length and width of the HEPA filter as follows. 

A=Length of HEPA× Width of HEPA


Calculate area of HEPA filter having 2.5 feet length and 2 feet width.

As we know

A=Length of HEPA× Width of HEPA


                      A= L×w

Put values as 

                     A=2.5×2= 5 square feet


Total air vol is calculated by following formula



Calculate Total air volume if air velocity is 240 feet/minute and area of filter is 5 square feet.

Put values in following formula as

T=Velocity of Air× Area of filter

        T =240×5


Also Read: 

100 MCQs to revise your pharmaceutical knowledge.

Step 2

Calculation of Room Volume.

In step two we will find the volume of the area or room where the air is supplied. volume of room or area  is measured by multiplying the length, width and height of the room as follow

            Volume of Room = l×w×h

  • l= Length of the Room
  • w=Width of Room
  • h=Height of Room


Calculate the volume of room having 

  • Length=10 feet
  • Width=  10 feet
  • Height= 12 feet

Put values in the formula as

Volume of Room = l×w×h

Vol of room =10×10×12

                     =1200 cubic feet

What is CFM?

  • CFM is cubic feet per minute.
  • It is used to measure the air volume.
  • Volume of air supplied to the room is called CFM.
  • CFM indicates capacity of a fan or blower.
  • If the total volume of air in a room is 500ft³ then this room will require a 500 CFM blower to empty the room from air in one minute.
  • CFM is calculated by measuring the air velocity and multiplying it with the area of the filter.

CFM=Velocity of air×Area of filter

As we know 

            T= Velocity of air×Area of filter



We can write following formula 

ACPH=T×60/Volume of the Room


ACPH=CFM×60/Volume of the Room

If a room has more than one inlet filter then for each filter CFM is calculated and results are combined.

Query 1

Calculate ACPH of a room having following parameters

  • Length=10
  • width=10
  • Height=12
  • CFM = 800

Use following formula

ACPH=CFM×60/Volume of Room

First we will calculate volume of room by using following formula

          Volume of Room=l×w×h

Put values in the above formula

    Vol of room = 10×10×12 = 1200 ft³

Now calculate the ACPH using following formula

ACPH=CFM×60/Volume of Room

Put values in the above formula

         ACPH= 800×60/1200=40


Air changes Per Hour for the above mentioned room is 40.

Query 2 

In query 2 we will calculate the CFM from given ACPH or cross check our answer of query 1 as follows.

Calculate CFM of a AHU supplying air to a room having 

  • Length=10
  • Width=10
  • Height=12
  • ACPH =40


First we will calculate volume of room using following formula

Volume of Room=l×w×h

Put values in the above formula

          Volume of room = 10×10×12 

                                      = 1200 ft³

Now calculate the CFM using following formula

ACPH=CFM×60/Volume of Room

We will rearrange the above formula as

ACPH×Volume of Room/60=CFM


CFM=ACPH×Volume of Room /60

Put values in above formula as

            CFM= 40×1200/60



As we can see that CFM are the same as we use in the first query.


We can find ACPH of any area if CFM is given and in the same way we can find out the CFM from the given ACPH.

Also Read: 

Lux level for Different areas in pharma.

Query 3

In query 3 we will learn to calculate the ACPH when air velocity and area of filter is given but CFM is not given.

Calculate the Air changes for a room when

  • Velocity of air = 240
  • Area of filter =     5 
  • Room volume= 1200 cubic feet.


First we will calculate Total air volume by using following formula


Put values in above formula as



Now calculate ACPH using  following formula

ACPH=T×60/Volume of Room

Put values in above formula as

ACPH=1200×60/1200  = 60

Popular Posts:

  1. Types of airlocks in pharmaceutical industries.
  2. Types and functions of HEPA filters.
  3. Cleanroom classification in pharmaceutical industries.
  4. HVAC or Heating Ventilation & Airconditioning System.
  5. Potency calculation of Active pharmaceutical Ingredient.
  6. Easy Steps to calculate Double cone blender capacity.
  7. Line Clearance in pharmaceuticals