Choosing an air-cooled chiller involves more than just matching a model to a specific tonnage. The ideal unit must meet the building or process cooling load under design conditions while operating efficiently during the many hours when demand is below peak.
Here the experts at Hahn-Mason Air Systems, Inc. explain what to consider when choosing an air-cooled chiller.
How Is Air-Cooled Chiller Capacity Measured?
Chiller capacity is usually expressed in refrigeration tons or British thermal units per hour (BTU/hr). One refrigeration ton equals 12,000 BTU/hr of heat removal.
For an existing chilled-water system, a basic water-side capacity estimate can be calculated with the following formula:
Cooling capacity (BTU/hr) = GPM × 500 × temperature difference
GPM is the water flow rate, and the temperature difference is the change between the entering and leaving water temperatures. Divide the result by 12,000 to convert it to tons.
For example, a system circulating 240 gallons per minute with a 10°F temperature difference has an estimated capacity of 100 tons:
240 × 500 × 10 ÷ 12,000 = 100 tons
This formula is useful for an initial estimate, but it is not a substitute for a complete load calculation. The factor also changes when the system uses glycol or another fluid because its density and heat-carrying properties differ from those of water.
What Should You Compare Besides Chiller Tonnage?
Tonnage is only one factor in selecting your air-cooled chiller. That’s because two models with the same nominal capacity might perform differently once efficiency, operating range, installation requirements, and site conditions are considered.
Important specifications to consider include:
- Full-load and part-load efficiency
- Integrated Part Load Value (IPLV) or Non-Standard Part Load Value (NPLV)
- Minimum operating capacity and turndown
- Sound levels
- Unit dimensions, weight, and required clearances
- Electrical requirements
- Controls and building automation system compatibility
- Refrigerant type and applicable code requirements
- Maintenance access and manufacturer support
If you have any questions or need help selecting the right air-cooled chiller, you can contact the team at Hahn-Mason Air Systems, Inc. for help. Our dedicated team serves residential, commercial, and industrial markets in Charlotte, NC, and surrounding areas.
How Do You Determine the Required Cooling Load?
The required cooling load is the amount of heat the chiller must remove under peak operating conditions. For comfort cooling, the calculation should account for factors such as:
- Building size and insulation
- Outdoor design temperature
- Sun exposure
- Number of occupants
- Lighting and equipment
- Ventilation and outdoor air
- Operating schedule
Process cooling applications may also require calculations based on equipment heat output, fluid flow rate, target fluid temperature, and production demands. Because cooling requirements can change throughout the day or year, a qualified HVAC professional should calculate both the peak load and expected part-load conditions before selecting an air-cooled chiller.
What Conditions Can Change a Chiller’s Available Capacity?
An air-cooled chiller’s rated tonnage is typically based on specific operating conditions. Its actual capacity may change depending on:
- Outdoor air temperature
- Required leaving-water temperature
- Chilled-water flow rate
- The amount of glycol in the system
- Installation altitude
- Condenser airflow and coil cleanliness
For example, an air-cooled chiller generally has to work harder as outdoor temperatures rise. Requiring colder supply water can also reduce its available capacity and efficiency. Glycol protects the system from freezing, but at higher concentrations, it transfers heat less effectively than water alone.
Because published capacity ratings may not reflect the conditions at your facility, compare the calculated cooling load with the manufacturer’s performance data at the expected design temperature, fluid mixture, and leaving-water temperature.
Why Should You Avoid Oversizing an Air-Cooled Chiller?
Choosing an air-cooled chiller that’s larger than the calculated load might seem like a safe way to prepare for peak demand, but an oversized unit might operate at low loads or cycle on and off more frequently than intended. This can reduce efficiency, create unnecessary equipment wear, and make it harder to maintain consistent leaving-fluid temperatures.
Oversizing can also increase the initial equipment cost, electrical requirements, and space needed for installation. A properly sized chiller should meet the facility’s peak cooling load while also operating efficiently under the part-load conditions it will most often experience. If future expansion is planned, additional demand should be evaluated carefully rather than covered by an arbitrary safety factor.
What Information Is Needed Before Selecting a Chiller?
Before requesting equipment selections, gather the peak and part-load profiles, fluid temperatures and flow rates, fluid type, outdoor design temperature, elevation, electrical requirements, sound limits, space constraints, and redundancy goals. Complete project information can help our team help you find the right air-cooled chiller for your unique needs.
Hahn-Mason Air Systems supports commercial and industrial clients throughout North and South Carolina with products and guidance for engineered HVAC systems. Review our HVAC product portfolio or request information about the needs and constraints of your next project.
You can also call (704) 523-5000 to speak with the Hahn-Mason team directly.
