SPECIAL SECTION | Valves & Actuators
Achieving the Full Benefits of Oil-Free HVAC Systems
How oil-free check valves and oil-free ball valves can help boost efficiency and reliability.
Paul Nuno | Danfoss
| IMAGE 1: Oil-free compressor with OFC and OFB installed (Images courtesy of Danfoss)
| IMAGE 2: OFC component breakdown
The push for better performance and higher efficiency in the global market has led to a demand for new and innovative technologies. In the heating, ventilation and air conditioning (HVAC) world, one example of this can be found in oil-free compressor technologies. Oil-free compressors operate without any oil circulating throughout the refrigerant circuit, and as a result, evaporators and condensers remain free from oil contamination, helping to maintain heat transfer performance, system efficiency and cooling capacity throughout the life of the chiller.
Instead of gradually losing efficiency due to oil fouling, oil-free systems can continue to operate much closer to their original design performance for many years. The simplified system design eliminates common refrigeration components such as sump heaters, oil separators and oil‑level sensors. For units that can struggle with oil return, installation and maintenance demand is also reduced, increasing the reliability of the overall system.
Within these oil-free designs are two components that are critical to improving performance within a system. These are the oil-free check valve (OFC) and the oil-free ball valve (OFB).
Oil-Free Check Valves
The OFC is an assembly that integrates a damped check valve, an isolation valve, a diffuser elbow, a pressure tap and a sight glass into a single compact module at the compressor discharge. Its main purpose is to help reduce reverse flow into the compressor, whether that be during system surging or refrigerant migration during the off cycle.
It accomplishes this using a direct axial cone design. This implementation strategy offers a few key benefits. When a compressor is operating at part load conditions, refrigerant velocities will naturally tend to be lower. At these slower flow rates, swing valves will be partially open and may even begin to flutter. This can cause eddy currents in the refrigerant flow and a restriction, leading to higher pressure drops. It can also give off a chattering noise and cause the flow to oscillate as the swing door closes and then opens as pressure builds behind it.
The parabolic design of the OFC creates a pressure differential around the cone of the valve, which forces it to open all the way, even at low velocities. Because the path of the refrigerant over this aerodynamic surface is relatively unhindered, the refrigerant is easily able to transition downstream of the valve, thereby also reducing the overall pressure drop across the component.
Materials for the OFC keep metal-on-metal contact wear to a minimum and prevent seizing. The OFC is not merely a check valve; it comes with a ball valve, which is located at the inlet before the staging port within the elbow envelope. This provides local isolation of the compressor discharge for service or staged operation without requiring additional external isolation hardware. It also includes a pressure tap at the inlet before the ball valve to provide a convenient measuring point for discharge pressure during commissioning and diagnostics. An integrated sight glass in the elbow gives visual confirmation of flow condition/phase in the discharge line, aiding troubleshooting and maintenance.
Oil-Free Ball Valves
The OFB is a tight shutoff suction valve engineered to protect oil-free systems by preventing refrigerant migration during extended standby, thereby reducing the risk of liquid slug upon compressor restart. It has a full-bore, smooth-cone design, which encourages low turbulence and therefore lower pressure drops. This also keeps intake losses low and minimizes turbulence at the compressor eye, supporting chiller efficiency and stable compressor operation across the operating map. The valve requires a low breakaway torque, allowing the use of smaller, more efficient actuators. This enables the unit controller to operate the ball valve as needed.
A reinforced stem sealing system with guide bearings and top-mounted support improves alignment and reduces wear, thus maintaining tightness over time. Traditional valves rely primarily on mechanical compression to maintain sealing. Over time, this can contribute to wear, relaxation and increasing operating torque. The OFB also includes a spring-loaded seat. At low pressures, the spring ensures the seat remains in contact with the ball. As pressure increases, refrigerant pressure acts behind the seat and automatically increases sealing force. The seat’s material is generally low friction and resists creep, maintaining stable operating characteristics and consistent sealing performance.
Government regulations have placed fugitive refrigerant emissions as a top concern. These emissions are unintended releases of refrigerant from valves, compressors, flanges and other pressurized equipment. Industry studies identify valves and connectors as one of the largest contributors to refrigerant leakage from mechanical systems.
For oil-free systems, refrigerant containment is particularly important, because maintaining the performance of the compressor and refrigeration circuit is essential to preserving the efficiency advantage of the entire installation. The OFB adheres to International Organization for Standardization (ISO) 15848-1 fugitive emission requirements, ensuring that refrigerant containment performance can be maintained throughout the equipment life cycle.
OFC and OFB valves are engineered complements to oil-free compressor platforms. The OFB prevents liquid ingress during a system restart and the OFC prevents backflow on discharge during transitions. Together, they manage both sides of the compressor to reduce stress events and improve restart quality.
| IMAGE 3: OFB with actuator assembly
Low suction loss (OFB) plus conditioned discharge (OFC) supports the compressor’s aerodynamic performance and part load efficiency, which are critical benefits of oil-free compressor-based systems. The OFC consolidates multiple components (check valve, isolation valve, diffuser, sight glass, pressure tap) into one factory-engineered assembly. The OFB’s flange system and actuator readiness simplify suction-line design, which reduces installation time, potential leak paths and mismatch risks.
For OEMs and end users, the combination of these valves preserves the core advantages of oil-free chillers: high efficiency at full and part load, reliable operation and lower maintenance while streamlining commissioning and service. In modern air and water-cooled chiller systems, especially those embracing low global warming potential (GWP) refrigerants and oil-free architectures, OFC and OFB provide tangible value in performance, protection and total cost of ownership. They will help ensure that the lifetime efficiency advantages of oil-free chiller systems remain efficient and reliable for years to come.
Paul Nuno is an applications engineer supporting Danfoss Climate Solutions. He has over eight years of HVAC experience, including designing and testing residential and light commercial heat pumps. For more information, visit danfoss.us.
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