Efficient Calculation and Application of Percent Outdoor Air

In manufacturing and industrial environments, maintaining optimal indoor air quality is vital for worker health and productivity. Industrial hygienists and safety managers play a key role in ensuring that facilities meet air quality standards. A crucial aspect of this is calculating the Percent Outdoor Air (%OA). TSI’s IAQ instruments are designed to measure %OA, aiding in effective air quality management. It's important what %OA is, why it is important in industrial settings, and how to use these instruments for its calculation.
 

Understanding Percent Outdoor Air

Percent Outdoor Air (%OA) refers to the proportion of fresh outdoor air in the total volume of air supplied to a space. This metric is essential because fresh outdoor air helps dilute indoor contaminants, improving air quality. In industrial settings, pollutants such as CO₂, chemical vapors, and particulate matter can accumulate. Prolonged exposure to these pollutants can lead to fatigue, nausea, and decreased productivity among workers.
 

Methods for Calculating Fresh Outdoor Air Volume

Traditionally, calculating the volume of fresh outdoor air entering a system involved conducting a duct traverse of the outdoor air intake. However, this method is often impractical in industrial settings due to complex ductwork. An alternative method involves the equation:

Volume of Fresh Outdoor Air = Volume of delivered air x %Outdoor Air of delivered air

Here, the total volume of delivered air includes both outdoor and return air. The %OA can be determined using various methods, with CO₂ concentration and temperature being the most common indicators.
 

Calculating Percent Outdoor Air

To determine %OA, you need readings from three locations: outdoor air, supply air, and return air. Using CO2 concentrations, the formula is:

%OA = (X_R​−X_O)​ / (X_R​−X_S​​) x 100

Where:

• X_R = Return air CO₂ concentration OR temperature
• X_S = Supply air CO₂ concentration OR temperature
• X_O = Outdoor air CO₂ concentration OR temperature 

Example Calculation Using CO₂ Levels

Consider an example with the following CO₂ readings:

• Outdoor air: X_O ​= 400 ppm
• Supply air: X_S = 645 ppm
• Return air: X_R = 823 ppm

The %OA is calculated as:
%OA = (X_R−X_S) / (X_R−X_O) x 100% = %OA = (823−645) / (823−400) x 100% = 42.1%

 

Example Calculation Using Temperature

Similarly, temperature measurements can be used:

• Outdoor air: X_O =35∘X_O = 35°F
• Supply air: X_S =55∘X_S = 55°F
• Return air: X_R =70∘X_R = 70°F

The %OA calculation would be: %OA = (70−55) / (70−35) x 100 = 42.9%
 

Measurement Locations

Accurate %OA readings require proper placement of sensors:

• Outdoor air: Away from exhaust vents.
• Return air (CO₂): Inside the return duct, before mixing with outdoor air.
• Supply air (CO₂): Inside the supply duct.
• Return air (Temperature): Close to the air handling unit before mixing with outdoor air.
• Supply air (Temperature): Before air conditioning, ideally between the filter and the chilled water coil.

Illustration: Air Handling Unit Schematic

Avoiding Calculation Errors

Negative or above 100% values indicate errors. Retake measurements if supply air values do not fall between return and outdoor air values. Common issues include:

• Conditioned air affecting temperature readings.
• Mixing of supply air with room air.
• Insufficient mixing of return air.
• Close outdoor and indoor temperatures.

 

Choosing Between CO₂ and Temperature

Use CO₂ measurements in buildings with many occupants, ensuring significant differences between outdoor and return air CO₂ levels. Use temperature measurements when there is a substantial difference between outdoor and indoor temperatures, particularly during extreme weather conditions.
 

Determining Acceptable %OA Values

The ideal %OA varies based on airflow and occupancy. The ASHRAE recommendation of 20 CFM/person can guide you. For instance, an industrial space with 1,000 CFM airflow and 10 workers requires 20% OA to meet the standard: (1,000CFM x 20%OA) / 10 people = 20CFM/person 

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