Design Procedures for Cooling-Only Systems: Detailed Airflow Calculation Methodology

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Technical Deep Dive: Airflow Calculation Methods for Cooling-Only Systems

Following our 8-step methodology for designing cooling-only HVAC systems, this technical supplement provides detailed insights into the critical airflow calculation methods essential for Step 3: Calculate Required Zone and Space Supply Airflow Rates. Understanding these calculation approaches enables engineers to select the most appropriate sizing method for specific project requirements.

Design Procedures for Cooling-Only Systems: An 8-Step Methodology

Method 1: Peak Zone Load with Coincident Space Loads

This approach sizes zone airflow based on maximum zone sensible cooling load, then proportionally distributes this airflow to spaces based on their coincident loads during the zone’s peak hour.

Key Principle: Space airflows are calculated only using loads that occur simultaneously with the zone peak.

Practical Application:
For a zone with a 21,600 BTU/h peak load at August 1600 requiring 1,000 CFM, containing two spaces with coincident loads of 8,000 BTU/h and 13,600 BTU/h:

• Space 1 airflow = 1,000 CFM × (8,000/21,600) = 370 CFM
• Space 2 airflow = 1,000 CFM × (13,600/21,600) = 630 CFM

Zone Airflow Equations:

• With supply temperature criteria: V_z = Q_zc / [ρ_aC_paK (T_zc-T_sc)]
• With supply CFM criteria: V_z = (Q_zc / Q_zc,tot) × V_sys,adj
• With CFM/sqft criteria: V_z = V_sys,adj (A_z / A_sys)

Space Airflow Equation:
V_s = (Q_csc / Q_zc) × V_z

Best Used For: Building zones with predictable usage patterns where spaces typically peak together.

Method 2: Peak Zone Load with Individual Peak Space Loads

This method calculates zone airflow based on maximum zone sensible load, but sizes space airflows according to each space’s individual peak load, regardless of when it occurs.

Key Principle: Space airflows reflect individual maximum loads, potentially exceeding the zone total when peaks occur at different times.

Practical Application:
For a zone with 1,000 CFM capacity at August 1600, containing:

• Space 1: Peak load at August 1300 requiring 550 CFM
• Space 2: Peak load at August 1700 requiring 600 CFM
• Result: Sum of space airflows (1,150 CFM) exceeds zone airflow (1,000 CFM)

Zone Airflow Calculation: Same as Method 1

Space Airflow Equation:
V_s = Q_msc / [ρ_aC_paK (T_zc-T_sc)]

Best Used For: Zones with diverse space types that experience peak loads at different times, requiring flexibility in space-level distribution.

Method 3: Sum of Space Airflow Rates

This approach sizes each space based on its individual peak load, then sets zone airflow as the sum of all space airflows.

Key Principle: Zone capacity equals the sum of independent space requirements, ensuring adequate capacity for all spaces simultaneously.

Practical Application:
For a zone with two spaces:

• Space 1: Peak load requiring 550 CFM at August 1300
• Space 2: Peak load requiring 600 CFM at August 1700
• Zone airflow = 550 + 600 = 1,150 CFM

Zone Airflow Equation:
V_z = Sum of V_s for all spaces in zone

Space Airflow Equations:

• With supply temperature criteria: V_s = Q_msc / [ρ_aC_paK (T_zc-T_sc)]
• With supply CFM criteria: V_s = (Q_msc / Q_msc,tot) × V_sys,adj
• With CFM/sqft criteria: V_s = (Q_msc / Q_msc,tot) × V_sys,adj

Best Used For: Critical facilities requiring conservative sizing or buildings with highly variable space loads.

Strategic Selection of Calculation Methods

The choice between these methods significantly impacts system performance and capital costs:

Method	Advantages	Considerations	Typical Applications
1: Peak Zone/Coincident Space	Most economical; prevents oversizing	May undersize spaces with non-coincident peaks	Office buildings, educational facilities
2: Peak Zone/Individual Space	Balanced approach; ensures space comfort	May require terminal unit throttling	Mixed-use spaces, retail
3: Sum of Space Airflows	Maximum comfort security; handles diverse loads	Most conservative; higher equipment costs	Healthcare, laboratories, data centers

Engineering Implications

Selecting the appropriate airflow calculation method requires balancing several factors:

1. Load Diversity: Buildings with high temporal diversity in space loads benefit from Method 3, while Method 1 is suitable for spaces with synchronized peak loads.
2. System Efficiency: Method 1 typically produces the most energy-efficient design by avoiding system oversizing.
3. Comfort Reliability: Method 3 provides the highest certainty for maintaining comfort conditions but at increased equipment cost.
4. Future Flexibility: Method 2 or 3 accommodates future space repurposing more effectively than Method 1.

Understanding these calculation methodologies enables HVAC engineers to make informed decisions that balance performance, efficiency, and cost-effectiveness in cooling-only system designs. The variable definitions provided in our technical documentation offer further precision for applying these calculations to specific projects.

Next Steps in Your Design Process

After calculating airflows using these methods, proceed to the subsequent steps in our 8-step design methodology to ensure comprehensive system design:

• Step 4: Calculate required zone equipment sizes
• Step 5: Calculate system airflow rates
• Step 6: Simulate air system operation
• Step 7: Identify peak coil loads
• Step 8: Report results

For personalized assistance with your specific cooling system design challenges, contact our engineering team or explore our additional technical resources.