How to calculate treated fresh air capacity? Calculating Treated Fresh Air (TFA) capacity involves determining the amount of conditioned outdoor air needed to maintain good indoor air quality (IAQ) within a space. Here’s a breakdown of the steps involved:
Factors Affecting TFA Calculation:
- Occupancy: The number of people using the space significantly impacts the TFA requirement. More occupants generate more pollutants, necessitating a higher fresh air supply.
- Ventilation Rate per Person: Building codes and standards (like ASHRAE) specify ventilation rates per person (often in CFM – cubic feet per minute) based on occupancy type (office, classroom, gym). This determines the minimum amount of outdoor air needed per person for good IAQ.
- Infiltration Rate: Buildings experience unintentional air leakage called infiltration. While infiltration can contribute to fresh air supply, it’s often uncontrollable. Building codes or ASHRAE standards provide infiltration rates (percentage of conditioned space volume per hour) for different building types.
Calculation Steps:
- Identify Occupancy: Determine the number of occupants in the space you’re calculating TFA for. Refer to building codes or occupancy standards for appropriate values.
- Ventilation Rate per Person: Based on the occupancy type, consult building codes or ASHRAE standards to find the recommended ventilation rate per person (CFM).
- Total Required Outdoor Airflow (CFM): Multiply the ventilation rate per person (from step 2) by the number of occupants (from step 1). This gives you the total amount of outdoor air required in CFM.
Formula:
Total Required Outdoor Airflow (CFM) = Ventilation Rate per Person (CFM/Person) x Number of Occupants
Account for Infiltration (Optional):
Infiltration can contribute to fresh air supply. The approach of including or excluding infiltration depends on the specific standards or design guidelines you’re following.
To consider infiltration, calculate the airflow it provides:
a. Multiply the infiltration rate (as a percentage) by the volume of the conditioned space (in cubic feet).
b. Divide the product by 60 (minutes per hour) to convert it to CFM.
Formula:
Airflow from Infiltration (CFM) = Infiltration Rate (%) x Volume of Conditioned Space (ft³) / 60 (min/hour)
Treated Fresh Air Capacity (TFA):
There are two approaches depending on how you handle infiltration:
A. Subtracting Infiltration (if applicable):
Subtract the airflow from infiltration (step 4) from the total required outdoor airflow (step 3). This represents the TFA capacity needed by the space after accounting for some fresh air contribution from infiltration.
Formula:
Treated Fresh Air Capacity (TFA) (CFM) = Total Required Outdoor Airflow (CFM) – Airflow from Infiltration (CFM)
B. Ignoring Infiltration (if applicable):
Some calculations might disregard infiltration entirely. In this case, the total required outdoor airflow (step 3) is considered the TFA capacity.
Important Note:
Always consult the relevant codes or design recommendations for your project to determine the appropriate way to handle infiltration in your TFA calculation. Some codes may require a minimum TFA even if infiltration seems to meet the total required outdoor airflow.
Example:
Let’s say you’re calculating TFA for an office space with 15 occupants:
- Step 1: Number of Occupants = 15
- Step 2: Ventilation Rate per Person = 20 CFM/Person (obtained from building codes)
- Step 3: Total Required Outdoor Airflow (CFM) = 20 CFM/Person x 15 People = 300 CFM
- Step 4 (Optional): Let’s say the infiltration rate is 1% per hour and the conditioned space volume is 7,500 cubic feet.
- Airflow from Infiltration (CFM) = (1% x 7,500 ft³) / 60 min/hour = 12.5 CFM (approximately)
Optional Step 5 (Subtracting Infiltration):
Treated Fresh Air Capacity (TFA) (CFM) = 300 CFM – 12.5 CFM = 287.5 CFM (approximately)
OR
(Ignoring Infiltration):
Treated Fresh Air Capacity (TFA) (CFM) = Total Required Outdoor Airflow (CFM) = 300 CFM
Remember, the chosen approach (subtracting or ignoring infiltration) depends on the specific standards or design guidelines you’re following. Always consult the relevant codes for your project.
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