Basics of HVAC: Everything You Need to Know About Cooling Load, CFM, and Duct Sizing

1. Heat Load Calculation for Server Room

Given Data:

• Server Room Size: 9m x 3.6m
• Total Racks: 9
• Load per Rack: 6 kVA
• Total IT Load: 9×6=549 \times 6 = 549×6=54 kVA
• Efficiency Factor: 0.9 (typical for IT equipment)

Heat Load Formula:

Total Heat Load (BTU/hr) = kVA × 860 × Efficiency Factor

860 is the conversion factor from kVA to BTU/hr (1 kVA = 860 BTU/hr)

(1 Watt (KW) = 3.412 BTU/hr)

= 54×860×0.9 = 41,796 BTU/hrs

= 12,000/41,796 ​= 3.48 Tons

(1 Ton = 12,000 BTU/hr)

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2. Additional Cooling Factors to Consider

• People Load: Assumed negligible since server rooms are unmanned.
• Lighting Load: Minimal (usually LED). Consider 500W = 1,705 BTU/hr.
• Heat Loss from Walls & Ceiling: Typically 10-15% of IT load → Approx. 4,000 - 6,000 BTU/hr.
• Redundancy Factor: Add 15% extra cooling for efficiency and safety.

Total Cooling Load with Additional Factors:

Adjusted Load = 41,796+1,705+5,000 = 48,501 BTU/hr

Adjusted Load = 41,796+1,705+5,000=48,501 BTU/hr = 48,501/12,000 = 4.04 Tons – 6 TR

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3. FCU Sizing with 2 Working + 1 Standby

• Total Required Cooling: 6 TR Tons
• PMC Recommendation: Use 2 Working + 1 Standby Configuration.
• Each FCU Unit = 3 Tons

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4. CFM Calculation

Formula:

CFM = (BTU/hr)/ 1.08 × Temperature Difference​

(Assume ΔT = 20°F)

CFM = 48,501 / (1.08×20) = 2,245 CFM  

CFM per FCU (2 Units Working):

2,2452 = 1,122 CFM per FCU

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5. Duct Sizing Calculation

Formula:

A = CFM / Velocity (FPM)​

• Typical Air Velocity: 800 - 1200 FPM
• Assume Air Velocity = 1000 FPM

A = 2,245/1000 = 2.245 sq. ft

Using duct size conversion, an area of 2.245 sq. ft corresponds to:
✅ Duct Size: 20" x 16" (approx.)

Duct Size per FCU (1,122 CFM each):

A=1,122/1000 = 1.12 sq. ft

✅ Duct Size: 16" x 10" per FCU

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6. PMC Recommendation for Cooling Efficiency:

1. Use 2 FCUs (2 Tons each) in operation, with 1 standby to ensure continuous cooling.
2. Duct sizing of 20" x 16" for total airflow and 16" x 10" per FCU ensures proper airflow distribution.
3. Install precision cooling units (CRAC/CRAH) instead of standard FCUs for better humidity control.
4. Ensure hot aisle/cold aisle containment to improve cooling efficiency.
5. Use underfloor cooling or perforated tiles for optimal airflow distribution.
6. Install temperature & humidity sensors for real-time monitoring.
7. Consider backup cooling solutions (like a DX unit or emergency AC) for redundancy.

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7. Conclusion

For a server room (9m x 3.6m) with 9 racks (6kVA each), the PMC recommends:
✅ 2 x 2 Ton FCUs (2 Working + 1 Standby)
✅ Total Cooling Load = 4.04 Tons (including additional heat factors)
✅ Duct Size = 20" x 16" (Main) & 16" x 10" (Per FCU)
✅ Implement containment & precision cooling for efficiency

This ensures redundancy, optimized cooling, and energy efficiency for continuous operation.