Calculating AHU Tonnage for Commercial Fit outs -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

## Jun 20, 2024

Accurately calculating the Air Handling Unit (AHU) tonnage is essential for maintaining a comfortable indoor environment in commercial spaces. This guide will walk you through the detailed process of calculating AHU tonnage, including important considerations and a helpful thumb rule table for quick reference.

Key Factors in AHU Tonnage Calculation

1. Space Area (Square Feet)
• The total area of the space that needs to be conditioned.
• Occupancy Load: Heat generated by occupants.
• Equipment Load: Heat generated by office equipment and machinery.
• Lighting Load: Heat generated by lighting fixtures.
• Solar Heat Gain: Heat gained through windows and walls exposed to sunlight.
• Ventilation Load: Heat gain or loss due to fresh air ventilation requirements.
• The total cooling load in BTU/hr is the sum of all heat load factors.
4. Conversion to Tonnage
• 1 ton of cooling is equivalent to 12,000 BTU/hr.

Detailed Steps for AHU Tonnage Calculation

1. Calculate Space Area
• Determine the total square footage (sq ft) of the area to be conditioned.
• Occupancy Load: Generally, each person adds approximately 400 BTU/hr. Occupancy Load = Number of Occupants×400 BTU/hr
• Lighting Load: Sum the wattage of all lighting fixtures and convert to BTU/hr. Lighting Load=∑Lighting Power (W)×3.412 BTU/hr
• Solar Heat Gain: Estimate based on window area and orientation. Use appropriate multipliers for direct sunlight exposure. Solar Heat Gain=Window Area (sq ft)×Solar Factor (BTU/hr/sq ft)
• Ventilation Load: Account for the heat load due to fresh air intake. This includes sensible and latent heat components.
4. Convert Total Cooling Load to Tonnage
• Divide the total cooling load in BTU/hr by 12,000 to convert to tons. Required Tonnage=Total Cooling Load (BTU/hr)/12,000

Practical Example

Let’s calculate the AHU tonnage for a 10,000 sq ft office space with the following details:

• Occupants: 50 people
• Equipment Load: 10 kW (10,000 W)
• Lighting Load: 5 kW (5,000 W)
• Window Area: 500 sq ft with a solar factor of 50 BTU/hr/sq ft

1. Solar Heat Gain

Solar Heat Gain=500×50=25,000 BTU/hr

1. Required Tonnage

Required Tonnage=116,18012,000≈9.68 tons

Thumb Rule Table for Quick Reference

For quick estimations, the following thumb rule table can be used. Note that these are general guidelines and actual requirements may vary based on specific conditions.

 Type of Space Area per Ton (sq ft/ton) Office Space 200 - 300 Retail Space 250 - 350 Server Rooms 100 - 150 Restaurants 150 - 250 Conference Rooms 100 - 200 Industrial Spaces 300 - 400

Conclusion

Accurate AHU tonnage calculation ensures efficient and effective HVAC system performance in commercial fitouts. By considering all heat load factors and using detailed calculations, you can design a system that meets the cooling demands of your space. For quick estimates, refer to the thumb rule table, but always verify with detailed calculations for precise requirements.