What Is Free Cooling?
Free Cooling (also known as economizing) is an energy-efficient HVAC method that uses naturally cool outdoor air instead of mechanical refrigeration to provide cooling for a building or process. When ambient outdoor temperatures are lower than the air or water returning from the building, the system bypasses the energy-intensive mechanical cooling cycle and uses the free outdoor conditions to reject heat.
How Free Cooling Works
Free cooling leverages the temperature differential between the outdoors and the building's internal load. Trane systems typically utilize one of two primary methods:
1. Air-Side Free Cooling (Air-Side Economizer) This method is common in packaged rooftop units and air handling units.
The process:
- Sensors monitor outdoor air temperature and humidity. When conditions are favorable, typically cool and dry, the system opens its outdoor air dampers to bring in 100% fresh air.
- This cool air is filtered and distributed directly into the space, completely shutting off the compressors.
- If the outdoor air is cool but not cool enough to handle the full load, it can still be supplemented with conditioned air to reduce the mechanical cooling load. This is known as partial-free cooling.
2. Water-Side Free Cooling (Water-Side Economizer) This method leverages cold outdoor temperatures to cool the building's chilled water loop, reducing or eliminating the need to run the chiller’s compressors. Depending on the system design, this is typically accomplished in one of two ways:
- Cooling tower/heat exchanger method: Common in water-cooled systems, fluid from the chilled water loop is diverted through a heat exchanger, where it is cooled directly by the cold water coming from the cooling tower or fluid cooler.
- Integrated free cooling coils: Used in air-cooled systems, this is a key feature of Trane’s Air-Cooled Chillers with Integrated Free Cooling. When outdoor temps drop below the returning chilled water temp, a valve diverts the warm return water into built-in free cooling coils. Fans pull cool outdoor air across these coils to reject heat naturally.
The process:
- Regardless of the method used, the system monitors the outdoor conditions. If the outdoor air is cold enough, the water leaves the heat exchanger or free cooling coils fully chilled, and the compressors remain completely off. If the air is only moderately cool, the compressors run at a reduced capacity to "top off" the cooling.
3. Refrigerant-Side Free Cooling (Refrigerant Economizer) Often utilized on centrifugal chillers, this is another method of free cooling that relies on refrigerant migration rather than diverting the chilled water. When the condenser water from the cooling tower is colder than the chilled water, it naturally condenses the refrigerant. Gravity feeds the liquid refrigerant to the evaporator to cool the chilled water loop, providing cooling capacity without turning on the compressor.
Reasons to Consider Using Free Cooling in an HVAC System
Incorporating free cooling is one way to lower operational costs and improve system reliability.
- Significant energy savings: Because compressors are the largest energy consumers in an HVAC system, shutting them off or running them at part-load can significantly reduce annual cooling energy consumption in climates where the ambient air is often cooler than indoor temperatures.
- Extended equipment life: By offloading the work to the free cooling coils during cooler months, you reduce the run-hours and wear and tear on your compressors, extending the overall lifespan of your chiller or rooftop unit.
- Quick ROI in cooler climates: For buildings with year-round cooling loads, such as manufacturing facilities, hospitals, or data centers, free cooling can be utilized for a large portion of the year, offering a rapid return on investment.
- Sustainability & compliance: Free cooling lowers a building’s carbon footprint and helps facilities meet stringent energy codes like ASHRAE 90.1, which mandate economizers in many climate zones.
Free cooling can deliver substantial savings and performance benefits, but the right solution depends on your building's unique load profile, climate zone, and operational needs. Whether you're designing a new facility or retrofitting an existing system, our experts can help you determine if free cooling is the right fit and which technology will maximize your return on investment. Contact your local Trane representative today to discuss economizer options, integrated free cooling chillers, and custom solutions tailored to your facility.
