Introduction
In modern HVAC systems, air handling units (AHUs) are the core components responsible for conditioning and distributing air. Within AHUs, different air streams interact in carefully engineered pathways. Understanding the behavior of these air streams—supply, return, fresh (outdoor), mixed, and exhaust—is vital for HVAC engineers aiming to design efficient and compliant systems. This blog dives into the function, configuration, and control of air streams in AHUs. We’ll cover the purpose of each stream, how they interact, design best practices, common challenges, and examples aligned with ASHRAE/ISHRAE standards.
Types of Air Streams in AHUs
Fresh Air (Outdoor Air) Stream
The AHU introduces fresh air from outside to maintain indoor air quality. According to ASHRAE Standard 62.1, minimum ventilation rates are determined based on occupancy and space usage. Proper damper control and filtration are essential to prevent the infiltration of unconditioned air.
Return Air Stream
Return air flows from conditioned zones back to the AHU, carrying both sensible and latent heat loads. As a result, it directly affects coil selection and overall system performance. In variable air volume (VAV) systems, return airflow fluctuates based on zone demand, so it must be carefully considered during fan sizing and control design.
Mixed Air Stream
The fresh and return air combine in the mixing chamber. Mixed air temperature is calculated as:
Where:
- TMA= Mixed air temperature (°C or °F)
- TRA = Return air temperature
- TOA= Outdoor (fresh) air temperature
- VRA = Return air volume (CFM or m³/h)
- VOA= Outdoor air volume (CFM or m³/h)
If you’re using air percentages instead of volumes:
Where:
- %RA = Percentage of return air (e.g., 70% = 0.70)
- %OA = Percentage of outdoor air (e.g., 30% = 0.30)
Note: %RA+%OA=1
Given:
- TRA = 24°C
- TOA = 10°C
- 70% return air, 30% outdoor air
TMA=(24×0.70)+(10×0.30)=16.8+3=19.8 Deg C
Supply Air Stream
After undergoing conditioning through coils and filters, the supply air is distributed to occupied zones. Its parameters, including temperature, humidity, and velocity, play a critical role in ensuring occupant thermal comfort and maintaining indoor air quality.
Exhaust/Relief Air Stream
Exhaust or relief air is discharged from the system to balance indoor pressure and remove contaminants. In many systems, energy recovery ventilators (ERVs) are used to reclaim energy from this airstream, improving overall efficiency.
Design Considerations for Air Stream Management
ASHRAE Ventilation Requirements
Designers must ensure the system meets the latest ASHRAE 62.1 and 90.1 standards. For enhanced energy efficiency, demand-controlled ventilation (DCV) can be implemented to modulate outdoor air intake based on real-time occupancy.
Filtration and Energy Recovery
ASHRAE 170 recommends using MERV 13 or higher filters, particularly in healthcare or high-occupancy spaces. Incorporating energy recovery wheels or plate heat exchangers between exhaust and fresh air streams significantly reduces heating and cooling loads.
Air Mixing and Thermal Comfort
To avoid thermal stratification and ensure even airflow distribution, systems often use mixing plenums or air blenders. These components promote uniformity in temperature and humidity before air passes through downstream coils.
Control Strategies for AHU Air Streams
- Economizer control: Leverages outdoor air for free cooling when conditions allow.
- Modulating dampers: Regulate the ratio of return to outdoor air.
- Static pressure sensors: Adjust fan speeds dynamically to maintain optimal duct pressure.
- CO2 sensors: Monitor indoor air quality and modulate fresh air intake accordingly.
Real-World Example: Commercial Office Building AHU
In a 10,000 sq. ft. office building, the AHU operates with 30% outdoor air and 70% return air. Both air streams pass through MERV 13 filters. An energy recovery ventilator (ERV) with 60% sensible effectiveness reduces the building’s annual cooling load by 15%, all while maintaining compliance with ASHRAE 62.1 standards.
Common Issues and Troubleshooting Air Streams
- Damper leakage: Can disrupt the intended mixing ratio and reduce system efficiency.
- Sensor drift: Impairs the accuracy of CO2-based demand control ventilation.
- Dirty filters: Increase pressure drop, affecting fan performance and airflow balance.
- Air stratification: Leads to uneven coil performance and occupant discomfort.
Summary
- AHUs handle five key air streams: fresh, return, mixed, supply, and exhaust.
- Each stream plays a role in air quality, energy use, and comfort.
- ASHRAE standards guide minimum ventilation and energy recovery.
- Proper controls and maintenance ensure optimal performance and compliance..
References
FAQ Section
- What is the difference between return and exhaust air in AHUs?
Return air is recirculated back to the AHU for reconditioning, while exhaust air is expelled from the system to maintain pressure balance and indoor air quality. - Why is mixed air temperature important?
It determines the thermal load on downstream coils and helps in designing energy-efficient AHUs. - Can AHUs operate without fresh air intake?
No. Fresh air is required to meet ventilation and indoor air quality standards set by ASHRAE. - What sensors are used to control air streams?
CO2 sensors, temperature sensors, static pressure sensors, and damper actuators are commonly used. - How does an ERV help with air streams?
It transfers heat and sometimes moisture between exhaust and fresh air, improving energy efficiency.
