Air Handling Unit (AHU) and Duct Sizing Computation for Multi-Story Buildings: A Comprehensive Guide -Architectural Cooling Load Calculation

Key Factors in AHU and Duct Sizing for Architectural Cooling Load Calculation

Air Handling Unit (AHU) and duct sizing are critical aspects of HVAC system design, directly impacting Architectural Cooling Load Calculation:

• Building thermal comfort

• Energy efficiency

• System performance

• Operational costs

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Key Factors in AHU and Duct Sizing for Architectural Cooling Load Calculation

1. Building Characteristics Influencing Sizing

   
   

   1. Factors to Consider:

      
      

      1. Total floor area

      2. Number of floors

      3. Building occupancy

      4. Geographic location

      5. Climate conditions

      6. Usage type (residential, commercial, mixed-use)

         
         

2. Airflow Rate

   
   

   1. Refer to the tables below for the Air Velocity according to building types.

      
      

      Residential Spaces

Space Type	Ventilation Rate	Remarks
Living Room	7-8 L/s per person	Typical family living areas
Bedroom	5-7 L/s per person	Personal sleeping spaces
Kitchen	15-20 L/s per person	Cooking and odor removal
Bathroom	10-15 L/s per person	Humidity and odor control

Commercial Spaces

Space Type	Ventilation Rate	Remarks
Office (Open Plan)	8-10 L/s per person	Standard workspace
Private Office	5-7 L/s per person	Personal sleeping spaces
Private Office	10-12 L/s per person	Meeting and discussion areas
Reception Area	6-8 L/s per person	Low-activity zone

Educational Facilities

Space Type	Ventilation Rate	Remarks
Classroom	9-12 L/s per person	Learning environment
Laboratory	12-15 L/s per person	Higher ventilation due to equipment
Library	7-9 L/s per person	Quiet study areas
Gymnasium	15-20 L/s per person	High-activity space

Healthcare Facilities

Space Type	Ventilation Rate	Remarks
Patient Room	10-12 L/s per person	10-12 L/s per person
Operating Room	120-25 L/s per person	Sterile environment
Waiting Area	7-9 L/s per person	Low-activity zone
Gymnasium	15-20 L/s per person	Special medical requirements

Retail and Commercial

Space Type	Ventilation Rate	Remarks
Retail Store	6-8 L/s per person	General shopping areas
Supermarket	8-10 L/s per person	Food handling areas
Restaurant Dining Area	10-12 L/s per person	Food service spaces
Kitchen (Commercial)	20-25 L/s per person	High heat and odor generation

Industrial and Special Spaces

Space Type	Ventilation Rate	Remarks
Warehouse	5-7 L/s per person	Low-activity industrial space
Manufacturing Area	10-15 L/s per person	Production environments
Server Room	15-20 L/s per square meter	Heat and equipment cooling
Art Studio	9-12 L/s per person	Creative workspace

Performance Spaces

Space Type	Ventilation Rate	Remarks
Theater	10-12 L/s per person	Audience areas
Concert Hall	8-10 L/s per person	Performance venues
Cinema	7-9 L/s per person	Enclosed entertainment space

Sources:

1. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)

2. World Health Organization (WHO) Indoor Air Quality Guidelines

3. U.S. Green Building Council (LEED Certification Guidelines)

4. Building Research Establishment (BRE)

5. National Institute of Building Sciences (NIBS)

6. Indoor Air Quality Scientific Database

   
   

   Recommended Verification Methods

   
   

   1. Cross-reference multiple sources

   2. Consult local building codes

   3. Consider specific building use cases

   4. Perform site-specific measurements

      
      

   Disclaimer

   
   

   • Rates are guidelines

   • Actual requirements may vary

   • Local regulations take precedence

   • Professional engineering assessment recommended

     
     

   Note: The data compilation involves:

   
   

   • Systematic review of multiple authoritative sources

   • Averaging of recommended rates

   • Consideration of global best practices

   • Alignment with current (2024) standards

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AHU Sizing Computation

• Sample Calculation for a 5-Story Office Building

• Given:

  
  

  • Occupants per floor: 10 people

  • Total occupants=10 x 5 = 50 people

    
    

• Ventilation Requirement:

  
  

  • 10 L/s per person :Refer to the table for the Office (Open Plan)

  • Total airflow: 50 people x 10 L/s = 500 L/s

  • Convert L/s to m³/hr : 500 * 3.6 = 1,800 m³/hr

  • AHU Size =1,800 m³/hr

    
    

• AHU Sizing Factors:

  
  

  • Add 20-30% for system inefficiencies

  • Final AHU capacity: 2,340 m³/hr

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Duct Sizing Computation

• Duct Sizing Principles

1. Maintain optimal air velocity

2. Minimize pressure drop

3. Ensure uniform air distribution

   
   

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Practical Duct Sizing Example

• HVAC Design Objective: Provide adequate ventilation and air distribution for a 5-story Office Building

• Total ventilation Rate requirement:

• Ventilation Rate for an Office (Open Plan) =10 L/s per person

• Occupants per floor= 10 people x 5 floors = 50 people

• Total ventilation requirement:

  • 50 people x 10 L/s = 500 L/s

  • Convert L/s to m³/hr: 500 x 3.6= 1,800 m³/hr

  • Main supply duct:  1,800 m³/hr

  • Recommended velocity: 8-12 m/s

  • Selected velocity: 10 m/s for balanced performance

    
    

• Duct Size Calculation

• Main Duct Size Calculation

  
  

  • Convert  1,800 m³/hr to m³/s

    • 1,800 ÷ 3600 = 0.50  m³/s

  • Selected velocity: 10 m/s

  • Cross-sectional Area = Airflow ÷ Velocity

    • 0.50 m³/s ÷ 10 m/s = 0.05 m²

    • A = 0.05 m²

      
      

  • Dimensions

• Typical aspect ratio: 1:2

• Get the Square Root for the Width = √(0.05 * 0.5)

• Width ≈ 0.158 m or 158 mm

  
  

• Height = 0.05 m² ÷ 0.158 m

• Height ≈ 0.316 m or 316 mm

• Dimensions: 158 mm x 316 mm

•  Branch Duct Sizing

  
  

  • Per floor distribution

    • Total airflow: 0.50 m³/hr ÷ 5 floors

    • Per floor: 0.10 m³/hr

    • Branch duct velocity: 6-8 m/s

    • Selected velocity: 7 m/s

      
      

  • Branch Duct Calculation

    • Airflow for 5 floors: 0.10 m³/hr x 5 floors = 0.50 m³/hr

    • Cross-sectional Area = 0.50 ÷ 7 m/s

    • A = 0.07 m²

      
      

      • Rectangular Duct Dimensions

      • Typical aspect ratio: 1:2

      • Get the Square Root for the Width = √(0.07 * 0.5)

      • Width =0.187 m (187mm)

    
    

    • Height=0.07 ÷ 0.187

    • Height=0.374

      
      

    • Dimensions: 187 mm x 374 mm

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Considerations for Multi-Story Buildings

1. Vertical Duct Routing

   
   

   • Minimize horizontal duct length

   • Use central shaft for efficient distribution

   • Consider floor-to-floor height

     
     

2. Pressure Equalization

   
   

   • Install balancing dampers

   • Use static pressure reset controls

   • Implement zone-based control

     
     

3. Multi-Zone AHU Configuration

   
   

   • Separate AHUs for different building zones

   • Consider thermal diversity

   • Implement variable air volume (VAV) systems

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Best Practices

1. Conduct detailed load calculations

2. Use computational fluid dynamics (CFD) simulations

3. Consider future flexibility

4. Implement energy-efficient designs

5. Regular system commissioning

   
   

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Conclusion

Accurate AHU and duct sizing require a holistic approach, combining:

• Precise calculations

• Understanding building dynamics

• Energy efficiency considerations

  
  

Continuous learning and adaptation are key to successful HVAC system design.

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