Mixing of Air Streams: Psychrometric Mixing Process and AHU Design

Introduction

The mixing of air streams is a fundamental process in modern HVAC systems, especially within Air Handling Units (AHUs), where it’s critical to achieve desired environmental conditions for indoor spaces. Combining air streams with different temperatures and humidity levels to create a conditioned, comfortable mixed air stream involves a detailed understanding of psychrometric principles, airflow dynamics, and equipment design. This article provides a comprehensive examination of the psychrometric mixing process, the engineering design of AHU mixing sections, and practical insights into optimizing performance, efficiency, and cost-effectiveness.

Whether you are designing a new AHU, retrofitting an existing system, or troubleshooting operational issues, grasping the concepts discussed here—including fundamental equations, worked examples, and best practices—will empower you to create HVAC solutions that meet rigorous environmental standards and maximize occupant comfort.

For foundational concepts in psychrometrics, please visit our HVAC Psychrometrics Fundamentals page.

Technical Background

Overview of Psychrometric Properties

Psychrometrics is the study of the thermodynamic properties of moist air. Key parameters essential to understanding air mixing include:

• Dry-bulb temperature (Tdb) - the actual air temperature measured by a thermometer.
• Wet-bulb temperature (Twb) - temperature measured by a thermometer covered with a wet wick; useful in determining humidity and enthalpy.
• Relative humidity (RH) - percentage measure of moisture present relative to the maximum possible at a given temperature.
• Humidity ratio (W) - mass of water vapor per unit mass of dry air, typically expressed kg water/kg dry air.
• Enthalpy (h) - total heat content of the moist air, combination of sensible and latent heat.

Psychrometric Mixing Process

When two air streams—commonly outside air (OA) and return air (RA)—with different conditions mix, the resulting mixed air lies on a straight line between the two points on a psychrometric chart. This line represents the weighted average properties of the two streams based on their mass flow rates.

Mass and humidity balance equations govern the mixing process:

m_o * W_o + m_r * W_r = m_m * W_m       (1)
m_o * h_o + m_r * h_r = m_m * h_m       (2)
m_o + m_r = m_m                         (3)

Where:

• m = mass flow rate [kg/s]
• W = humidity ratio [kg water/kg dry air]
• h = enthalpy of moist air [kJ/kg dry air]
• Subscripts o, r, and m refer to outside air, return air, and mixed air, respectively.

Given the total mixed air flow (m_m) and the outside airflow (m_o), the return airflow is:

m_r = m_m - m_o

Using equations (1) and (2), mixed air humidity ratio and enthalpy can be calculated, after which the dry-bulb temperature (T_db,m) can be derived.

Calculation of Mixed Air Properties

Variable	Symbol	Unit	Description
Outside Air Mass Flow Rate	mo	kg/s	Mass flow rate of outside (fresh) air
Return Air Mass Flow Rate	mr	kg/s	Mass flow rate of return air
Total Mixed Air Mass Flow Rate	mm	kg/s	Sum of outside and return air flow rates
Humidity Ratio	W	kg/kg dry air	Moisture content of air
Enthalpy	h	kJ/kg dry air	Total heat content
Dry-bulb Temperature	Tdb	°C or °F	Measured air temperature

Example: Basic Psychrometric Mixing Calculation

Given:

• Outside Air (OA): T_db,o = 10°C, RH = 60%, m_o = 1.5 kg/s
• Return Air (RA): T_db,r = 24°C, RH = 40%, m_m (total) = 10 kg/s

Step 1: Determine W and h for both air streams:

Using psychrometric charts or ASHRAE Handbook equations:

Parameter	Outside Air	Return Air
Humidity Ratio (W)	0.0065 kg/kg	0.0085 kg/kg
Enthalpy (h)	32 kJ/kg	50 kJ/kg

Step 2: Calculate return air flow, m_r = 10 - 1.5 = 8.5 kg/s

Step 3: Calculate mixed air humidity ratio:

W_m = (1.5 * 0.0065 + 8.5 * 0.0085) / 10 = (0.00975 + 0.07225)/10 = 0.0082 kg/kg

Step 4: Calculate mixed air enthalpy:

h_m = (1.5 * 32 + 8.5 * 50)/10 = (48 + 425)/10 = 47.3 kJ/kg

Step 5: Using psychrometric relations, find T_db,m ≈ 22.6°C (from enthalpy and humidity ratio)

This result indicates that mixing cooler outside air with warmer return air results in a slightly cooler mixed air temperature with intermediate humidity content.

Step-by-Step AHU Mixing Section Design Procedures

1. Define Design Inputs

• Determine required supply air flow rate (m_m) based on building load load calculation guidelines.
• Establish outside air fraction (typically 10-20% for ventilation).
• Define OA and RA psychrometric conditions (T_db, RH, barometric pressure).

2. Calculate Mixed Air Conditions

• Use mass and energy balances demonstrated above.
• Verify mixed air does not reach dew point condensation in mixing box.

3. Select Mixing Section Configuration

• Choose between static mixing boxes with dampers or mechanical mixers including fans or baffles.
• Consider space constraints and maintainability.

4. Sizing Dampers and Mixing Equipment

• Size dampers to handle respective airflows without excessive pressure drop.
• Refer to manufacturer performance specs for damper leakage and airflow capacity.

5. Airflow Distribution and Mixing Quality

• Design ductwork and openings to reduce stratification.
• Use flow straighteners or vanes if necessary.

6. Calculate Pressure Drops and Fan Energy Impact

• Account for additional pressure loss from dampers and mixing elements in fan curve analysis.

7. Incorporate Controls and Sensors

• Install temperature and humidity sensors upstream and downstream of mixing section.
• Implement damper position controls tied to building automation systems.

8. Verify and Validate Design via Simulation & Commissioning

• Use computational fluid dynamics (CFD) when necessary for complex layouts.
• Conduct field commissioning to verify airflow and mixed conditions.

Worked Design Example: AHU Mixing Section for Office Building

Project Specs:

• Supply air required: 3000 cfm (approximately 1.4 kg/s assuming 1.2 kg/m³ density).
• Outside air fraction: 20%
• OA conditions: 35°C dry-bulb, 70% RH
• Return air conditions: 24°C dry-bulb, 50% RH

Calculation Steps:

(Density assumed ~1.2 kg/m³, humidity ratios, and enthalpy from psychrometric tables)

Parameter	Outside Air	Return Air	Unit
Dry-Bulb Temperature	35	24	°C
Relative Humidity	70%	50%	%
Humidity Ratio (W)	0.028	0.0105	kg/kg dry air
Enthalpy (h)	73	48	kJ/kg dry air

Step 1: Calculate OA and RA mass flow rates:

m_m = 1.4 kg/s total
m_o = 0.2 * 1.4 = 0.28 kg/s
m_r = 1.4 - 0.28 = 1.12 kg/s

Step 2: Calculate mixed air humidity ratio W_m:

W_m = (0.28 * 0.028 + 1.12 * 0.0105) / 1.4
W_m = (0.00784 + 0.01176)/1.4 = 0.013714 kg/kg dry air

Step 3: Calculate mixed air enthalpy h_m:

h_m = (0.28 * 73 + 1.12 * 48) / 1.4
h_m = (20.44 + 53.76)/ 1.4 = 74.2/1.4 = 53 kJ/kg dry air

Step 4: Using the psychrometric chart and W_m, h_m, estimate the mixed dry-bulb temperature:

• T_db,m ≈ 26°C

Interpretation: Mixing hot, humid outside air with cooler return air produces a mixed air temperature suited for downstream cooling coil operation.

Selection and Sizing Guidance

Mixing Dampers

• Size dampers for flow capacity allowing 10-20% overdesign margin to accommodate operational variability.
• Material: corrosion-resistant aluminum or galvanized steel to endure moisture and mechanical wear.
• Leakage: select dampers with low leakage class (e.g., Class 3 or better) to prevent air infiltration losses.

Mixing Boxes

• Ensure sufficient length (~3 duct diameters) or use internal baffles/mixers for homogeneous air stream.
• Size cross-sectional area to maintain velocity <500 fpm for noise control and low pressure drop.

Fan and Ductwork

• Adjust fan curves to account for additional pressure drops across mixing components.
• Use smooth duct transitions and minimize turbulence to optimize pressure efficiency.

Sensors and Controls

• Install temperature, humidity, and CO₂ sensors to continuously monitor mixed air quality.
• Use electronic controls with modulating damper actuators for precise control over mixed air fractions.

Best Practices

• Early coordination: Coordinate mixing section design with overall AHU and building load calculations to optimize air quality and energy consumption.
• Proper sealing: Ensure all joints around dampers and ducts are airtight