How to Properly Size Industrial AC Cooling Capacity

Accurate sizing of air conditioning units is critical in industrial settings. Improper sizing can lead to significant issues in both cooling effectiveness and operational efficiency. There are two primary concerns when selecting an air conditioning unit:

1. Undersized units fail to achieve adequate cooling in high-temperature conditions.
2. Oversized units can lead to frequent cycling, inadequate dehumidification, non-uniform cooling, and excessive energy consumption.

This guide outlines a systematic approach to determining the appropriate air conditioning capacity for industrial spaces, with the goal of maintaining a consistent temperature of 24°C and relative humidity between 30 and 50 percent. Please note that this is a rule-of-thumb method, to quickly estimate required cooling capacity. Be sure to contact an expert for more specific situations.

Understanding Cooling Capacity

Cooling capacity is quantified in British Thermal Units per hour (BTU/h), representing the amount of heat an air conditioning unit can remove from a space in one hour. Higher BTU/h ratings indicate greater cooling power.

Understanding BTU and Common Conversions

BTU, or British Thermal Unit, is a traditional unit of heat. One BTU is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at sea level. In the context of air conditioning, BTU/h (BTU per hour) is commonly used to measure cooling capacity. For reference, 1 BTU/h is equivalent to approximately 0.293 watts (W). To convert BTU/h to kilowatts (kW), divide the BTU/h value by 3,412. In refrigeration, another common unit is the ton of refrigeration (RT), where 1 RT equals 12,000 BTU/h or about 3.52 kW. These conversions are crucial when comparing specifications of cooling equipment from different manufacturers or regions that may use varying units of measurement.

Step-by-Step Sizing Process

Step 1: Calculate Floor Area

Begin by accurately measuring the floor area of the space requiring cooling:

1. Measure length and width in meters (or feet).
2. Calculate area by multiplying length by width.
3. For multiple connected spaces, sum the individual areas.

Example calculation: Length: 6.1 m (20 ft) Width: 3.7 m (12 ft) Floor Area: 6.1 x 3.7 = 22.6 m² (240 sq ft)

Step 2: Determine Basic Cooling Capacity

Utilize the following table to determine the basic cooling capacity based on total floor area:

Total Floor Area (m²)	Total Floor Area (sq ft)	Basic Cooling Capacity (BTU/h)
9–14	100–150	5,000
14–23	150–250	6,000
23–28	250–300	6,500
28–33	300–350	7,250
33–38	350–400	8,000
38–41	400–450	8,750
41–46	450–500	9,650
46–51	500–550	10,500
51–65	550–700	12,500
65–93	700–1000	15,000
93–111	1000–1200	17,700
111–149	1200–1600	19,000–24,000
149–167	1600–1800	24,000–27,000
167–260	1800–2800	27,000–33,000

Note: For cooling capacities exceeding 12,000 BTU/h, consider multiple smaller units instead of a single large unit. This approach may mitigate the need for higher amperage circuits or dedicated 240-volt circuits.

Step 3: Adjust for Environmental and Operational Factors

Refine the basic cooling capacity estimate by accounting for specific environmental and operational factors:

1. Space Orientation:
   • North/northeast facing or heavily shaded spaces: Reduce basic cooling capacity by 10%.
   • High solar gain spaces (e.g., west/southwest facing): Increase basic cooling capacity by 10%.
2. Insulation Quality:
   • Poorly insulated spaces: Increase basic cooling capacity by 15%.
3. Heat-Generating Equipment:
   • Spaces with significant heat-generating equipment: Add 4,000 BTU/h to the basic cooling capacity.
4. Occupancy:
   • For each person beyond two regularly occupying the space, add 600 BTU/h.
5. Operational Hours:
   • For nighttime-only operation, reduce basic cooling capacity by 30%.

Final Calculation

To determine the final cooling capacity requirement:

1. Start with the basic cooling capacity from Step 2.
2. Apply adjustments based on factors identified in Step 3.

Example calculation:

• Basic cooling capacity for 22.6 m² (240 sq ft): 6,000 BTU/h
• High solar gain space: +600 BTU/h (10% of 6,000)
• Significant heat-generating equipment: +4,000 BTU/h
• Total: 6,000 + 600 + 4,000 = 10,600 BTU/h

In this scenario, an air conditioning unit with a cooling capacity of approximately 10,600 BTU/h would be appropriate.

Our hazardous area air conditioners are available in sizes up to 14 kW!

Advanced Considerations for Industrial Cooling

Heat-Generating Equipment

The presence of heat-generating equipment significantly impacts cooling requirements. The 4,000 BTU/h addition mentioned earlier is a general guideline, but in industrial settings, this can vary greatly depending on the specific equipment.

To more accurately account for heat-generating equipment:

1. Identify all major heat sources (machinery, computers, lighting, etc.).
2. Determine the heat output of each source in watts or BTU/h. This information is often available in equipment specifications.
3. Sum the total heat output from all sources.
4. Add this total to your cooling capacity calculation.

For example, if you have:

• 2 industrial machines generating 5,000 BTU/h each
• 10 computers generating 500 BTU/h each
• Lighting generating 2,000 BTU/h

Total additional heat load: (2 × 5,000) + (10 × 500) + 2,000 = 17,000 BTU/h

This more precise calculation would replace the general 4,000 BTU/h addition in our earlier example.

Adjusting for Different Target Temperatures

The calculations provided earlier assume a target temperature of 24°C (75°F). If a different target temperature is required, you’ll need to adjust the cooling capacity. As a general rule:

• For every degree Celsius below 24°C, increase the cooling capacity by about 3-5%.
• For every degree Celsius above 24°C, decrease the cooling capacity by about 3-5%.

For example, if targeting 20°C instead of 24°C:

• Difference: 4°C
• Increase: 4 × 4% (average) = 16%
• If your calculated cooling capacity was 10,000 BTU/h for 24°C, for 20°C it would be: 10,000 + (10,000 × 16%) = 11,600 BTU/h

Heat Flow Rate Calculations

For more precise HVAC sizing, especially in complex industrial environments, it’s crucial to consider heat flow rates. This involves calculating:

1. Space Heat Gain: The rate at which heat enters or is generated within the space.
2. Space Cooling Load: The amount of heat that needs to be removed to maintain the desired conditions.

The ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Transfer Function Method (TFM) provides a standardized approach to these calculations. This method accounts for various factors including:

• Solar radiation through windows
• Heat conduction through roofs and walls
• Heat generated by occupants, equipment, lighting, and appliances
• Thermal mass effects

The TFM involves complex calculations that typically require specialized software. It uses conduction transfer functions for walls, roofs, and glazing, and room transfer functions for internal heat sources.

The general form of the TFM equation is:

Q = Σ(A × U × ΔT) + Σ(Internal Heat Gains)

Where:

• Q is the total heat gain/loss
• A is the area of each surface
• U is the heat transfer coefficient of each surface
• ΔT is the temperature difference between inside and outside
• Internal Heat Gains include equipment, lighting, occupants, etc.

This method provides a more accurate estimation of cooling needs by accounting for the dynamic nature of heat transfer in buildings. It considers not just the instantaneous heat gain, but also the thermal storage effects of the building structure.

For industrial applications with critical temperature control requirements or complex heat-generating processes, employing the TFM or similar advanced calculation methods is highly recommended. These methods can be particularly valuable when dealing with:

• Large variations in heat load throughout the day
• Spaces with significant thermal mass
• Areas with complex solar exposure patterns
• Processes with precise temperature and humidity requirements

While these advanced calculations often require specialized software and expertise, they can lead to more efficient and effective cooling solutions, potentially resulting in significant energy and cost savings over time.

Conclusion

Proper sizing of industrial air conditioning units is crucial for maintaining optimal environmental conditions, ensuring equipment longevity, and maximizing energy efficiency. While this guide provides a solid foundation for estimating cooling requirements, complex industrial environments may benefit from consultation with HVAC professionals who can account for additional factors such as equipment heat loads, process requirements, and specific climate conditions.

Accurate sizing not only ensures consistent temperature and humidity control but also contributes to reduced energy consumption, lower operational costs, and improved overall system performance.

Efficiency Consideration: After determining the appropriate cooling capacity, prioritize units with high Coefficient of Performance (COP) or Energy Efficiency Ratio (EER) ratings to optimize energy utilization.

Further Information: For more comprehensive guidance on industrial cooling solutions, refer to ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) standards and guidelines.