AIR CHANGES PER HOUR CALCULATOR

Table 1:

Recommended Air Velocity Rates

Class Iso 146144-1 (Standard 209E)	Average Airflow Velocity	Air Changes Per Hour
ISO 8 (Class 100,000)	0.005 - 0.041 m/sec (1 - 8 ft/min)	5 - 48
ISO 7 (Class 10,000)	0.051 - 0.076 m/sec (10 - 15 ft/min)	60 - 90
ISO 6 (Class 1,000)	0.127 - 0.203 m/sec (25 - 40 ft/min)	150 - 240
ISO 5 (Class 100)	0.203 - 0.406 m/sec (40 - 80 ft/min)	240 - 480
ISO 4 (Class 10)	0.254 - 0.457 m/sec (50 - 90 ft/min)	300 - 540
ISO 3 (Class 1)	0.305 - 0.457 m/sec (60 - 90 ft/min)	360 - 540
ISO 1-2	0.305 - 0.508 m/sec (60 - 100 ft/min)	360 - 600

Note: Actual average velocity and air changes required may vary depending
on
the application and floor plan. Source: Institute of Environmental Science

Table 2:

Recommended Ceiling Filter Coverage

Class	Ceiling Coverage
ISO 8 (Class 100,000)	5 - 15%
ISO 7 (Class 10,000)	15 - 20%
ISO 6 (Class 1,000)	25 - 40%
ISO 5 (Class 100)	35 - 70%
ISO 4 (Class 10)	50 - 90%
ISO 3 (Class 1)	60 - 100%*
ISO 1-2	80 - 100%*

Note: Actual average velocity and air changes required may vary
depending on the application and floor plan.
* ULPA filters required in Class 1/10 applications. HEPA filters
required for all others.

AIR CHANGES PER HOUR CALCULATOR

Calculation of air volume The volume of air required to operate a ventilating and cooling system is a very fundamental requirement. Before air volume is estimated or calculated, careful consideration should be given to the following factors. The objectives of the ventilation system Be sure that the decision you make will accomplish the most important objectives or will overcome the most important problems. This should be the overriding consideration in the selection of a system of ventilation. The budget Availability of money to finance the system is a factor that must be considered early in the planning stage. The funds available will affect the objectives you set and will influence the system of ventilation selected to meet the objectives. Alternate methods of calculating the required air volume The method selected for calculating air volume will relate closely to the objectives to be accomplished by the system. Based on the objectives and funds available, you can now consider the alternatives and select the best method to use in calculating air volume requirements. A description of these methods follows. Rate of air change method This is a time-honored approach to a determination of air volume requirements. It is based on the theory that a complete change of air in a room or building should be made at a certain time frequency. The rate selected is frequently an arbitrary decision. It may be based on experience with similar installations or may be established by a health or safety code. Many fan manufacturers have published charts that show recommended rates of air change for typical installations. The formula for calculating the air volume in CFM by the rate of air change method is as follows:				Typical layout of fans for rate of air change method Although the rate of air change method of calculating air volume has been used for many years, American Coolair engineers have found it unsatisfactory except in relatively small buildings or rooms. For jobs that involve personnel comfort. this method is not recommended if the building is over 50,000 cubic feet in content or more than l00' in length. CFM per square foot of floor area method This method of calculation is a modern adaptation of the rate of air change formula. Total air volume (cubic feet per minute) is determined by multiplying the total square feet of floor area by an arbitrary CFM per square foot figure. The figure selected may be as low as 2 CFM or as high as 12 CFM per square foot. Four CFM per square foot has been recom mended as a minimum for summer ventilation of large assembly type operations. This method of calculation, like the rate of air change method, is likely to produce unsatisfactory results in many cases. Failure to control air distribution and air velocity can be a major weakness in the whole concept. Selection of a CFM per square foot figure should be based on experience and a proven method of air distribution. Rate of air velocity method This method of calculating air volume needed for a system is highly recommended by American Coolair. A breeze conditioning system can be highly effective in providing personnel comfort in hot weather if the recommendations outlined below are observed. It has been determined from field experience that an average air velocity of 150 feet per minute (FPM) to 200 FPM is usually sufficient for personnel cooling under summertime conditions. The CFM required to do the job is calculated by multiplying the cross section of an area through which the air is to move by the desired velocity. This is expressed in the following formula. Air velocity method of determining CFM
CFM = (cu. ft./min.)	Area to be cooled (cubic feet)*
	Recommended rate of air change (minutes)
* Area to be cooled (cu. ft.) = Length (ft.) x Width (ft.) x Average Ceiling Height (ft.) Example: A laundry 100' long by 30' wide with a 15' ceiling height requires a complete air change each 1/2 minute. The necessary air volume (fan capacity) is de determined from the above formula as follows:
CFM =	100 x 30 x 15		= 90,000 cu. ft. / min.

How to calculate air change rates

How to calculate air change rates - air

change rate equations in imperial

and SI units

Air Change Rate - Imperial Units

Air change rate per hour can be expressed in imperial Units as

n = 60 Q / V (1)

where

r = air change rate per hour

Q = fresh air flow through the room (Cubic Feet per Minute (cfm))

V = volume of the room (Cubic Feet)

Air Change Rate - SI Units

Air change rate expressed in SI-units:

n = 3600 Q / V (2)

where

r = air change rate per hour

Q = fresh air flow through the room (m³/s)

V = volume of the room (m³)

Example - Air Change Rate SI Units

With an air flow of 3 m³/s in a 20000 m³ room the air flow rate can

be calculated as

n = 3600 3 (m³/s) / 20000 (m³)

= 0.54