Supply Air Rate with Recirculation Calculator

1. What is a Supply Air Rate with Recirculation Calculator?

Definition: This calculator computes the total supply air rate ( $Q_{s}$ ) in an HVAC system as the sum of the recirculation rate and outdoor air rate, used in systems with filtration to reduce outdoor air requirements.

Purpose: It is used in HVAC systems to determine the total air supply needed to maintain indoor air quality while incorporating recirculation with filtration, optimizing energy efficiency and system performance.

2. How Does the Calculator Work?

The calculator uses the following formula for the supply air rate:

Supply Air Rate: $Q_{s} = Q_{r} + Q_{o}$

Where:

$Q_{s}$ : Supply air rate (cfm/person, m³/s/person, L/s/person)
$Q_{r}$ : Recirculation rate (cfm/person, m³/s/person, L/s/person)
$Q_{o}$ : Outdoor air rate (cfm/person, m³/s/person, L/s/person)

Unit Conversions:

Air Flow Rates ( $Q_{s}$ , $Q_{r}$ , $Q_{o}$ ): cfm/person, m³/s/person (1 cfm = 0.000471947 m³/s), L/s/person (1 cfm = 0.471947 L/s)

Steps:

Enter the recirculation rate ( $Q_{r}$ ) and outdoor air rate ( $Q_{o}$ ), and select their units.
Convert air flow rates to cfm/person.
Calculate the supply air rate using the formula (result in cfm/person).
Convert the result to the selected unit (cfm/person, m³/s/person, or L/s/person).
Display the result with 5 decimal places, or in scientific notation if the value is greater than 10,000 or less than 0.00001.

3. Importance of Supply Air Rate with Recirculation Calculation

Calculating the supply air rate with recirculation is crucial for:

Indoor Air Quality: Ensures sufficient air supply to maintain air quality by combining recirculated and outdoor air, often with filtration to remove pollutants.
Energy Efficiency: Reduces outdoor air requirements through recirculation, minimizing heating and cooling loads while maintaining acceptable air quality.
HVAC Design: Helps design systems that balance fresh air intake with recirculation, optimizing system performance and occupant comfort.

4. Using the Calculator

Examples:

Example 1: For $Q_{r} = 60 cfm/person$ , $Q_{o} = 15 cfm/person$ , supply air rate in cfm/person:
- $Q_{s} = 60 + 15 = 75.00000$
- Since 75.00000 < 10000 and > 0.00001, display with 5 decimal places: $75.00000$
Example 2: For $Q_{r} = 0.05 m³/s/person$ , $Q_{o} = 0.01 m³/s/person$ , supply air rate in m³/s/person:
- Convert: $Q_{r} = 0.05 \times 2118.88 = 105.944 cfm/person$ , $Q_{o} = 0.01 \times 2118.88 = 21.1888 cfm/person$
- $Q_{s} = 105.944 + 21.1888 = 127.13280 cfm/person$
- Convert to m³/s/person: $127.13280 \times 0.000471947 = 0.06000$
- Since 0.06000 < 10000 and > 0.00001, display with 5 decimal places: $0.06000$
Example 3: For $Q_{r} = 0.1 L/s/person$ , $Q_{o} = 0.05 L/s/person$ , supply air rate in L/s/person:
- Convert: $Q_{r} = 0.1 \times 2.11888 = 0.211888 cfm/person$ , $Q_{o} = 0.05 \times 2.11888 = 0.105944 cfm/person$
- $Q_{s} = 0.211888 + 0.105944 = 0.317832 cfm/person$
- Convert to L/s/person: $0.317832 \times 0.471947 = 0.15000$
- Since 0.15000 < 10000 and > 0.00001, display with 5 decimal places: $0.15000$

5. Frequently Asked Questions (FAQ)

Q: What is the supply air rate in HVAC systems?
A: The supply air rate ( $Q_{s}$ ) in HVAC systems is the total rate of air delivered to a space, consisting of both outdoor air and recirculated air, to maintain indoor air quality and thermal comfort.

Q: Why is recirculation used in HVAC systems?
A: Recirculation is used to reduce the amount of outdoor air needed, which minimizes energy consumption for heating or cooling while maintaining air quality through filtration, especially in systems with high filtration efficiency.

Q: How can I determine the recirculation and outdoor air rates for my system?
A: Recirculation rate ( $Q_{r}$ ) and outdoor air rate ( $Q_{o}$ ) can be determined based on HVAC design standards (e.g., ASHRAE guidelines), occupant density, and air quality requirements, often calculated per person for a given space.