General Assist Technical Training
VR-II Refrigerant Principles and Heat Recovery Cycle
Three-pipe heat recovery, RB unit operation and high-reliability oil control
AIRSTAGE VR-II training overview for technicians, designers and commissioning personnel
The AIRSTAGE VR-II is a three-pipe heat recovery VRF system. It can provide simultaneous cooling and heating by transferring heat between indoor zones and using the outdoor heat exchanger only for the remaining difference between the cooling and heating loads.
Training note: This article explains operating principles. Always refer to the applicable installation, design and service manuals for model-specific piping, charge, addressing and commissioning requirements.
1. System overview
The VR-II series builds on Fujitsu General's established VRF architecture and adds heat recovery operation through three refrigerant pipes:
| Pipe | Typical function |
|---|---|
| High-pressure gas | Supplies heating indoor units. |
| Low-pressure gas | Returns refrigerant from cooling indoor units. |
| Liquid | Distributes liquid refrigerant to the required circuits. |
2. Why Fujitsu uses a three-pipe refrigerant cycle
The training material identifies the three-pipe arrangement as the most common heat recovery format in the market. Its key advantages are familiar system design, flexible piping and reduced design or installation risk.
Flexible system design
Single and multi RB arrangements can be selected to suit the layout, making later indoor-unit or floor-plan changes easier.
Single and multi RB arrangements can be selected to suit the layout, making later indoor-unit or floor-plan changes easier.
Improved RB installation
Smaller, slimmer RB units, fewer brazing locations and flexible control-box positioning reduce field work.
Smaller, slimmer RB units, fewer brazing locations and flexible control-box positioning reduce field work.
Industry familiarity
Consultants and installers can apply a familiar heat recovery concept without a specialised two-pipe piping layout.
Consultants and installers can apply a familiar heat recovery concept without a specialised two-pipe piping layout.
Three-pipe and two-pipe heat recovery comparison
| Aspect | Three-pipe heat recovery | Two-pipe heat recovery |
|---|---|---|
| Simultaneous cooling and heating | Supported | Supported on systems designed for heat recovery |
| Branch arrangement | Single or multi RB installation can be selected. | Typically relies on centralised multi-port branch controllers. |
| Layout changes | Generally easier to adapt. | May require significant redesign around the branch controller. |
| Cooling-only branch connection | Permitted within the applicable design rules. | May be restricted depending on system architecture. |
3. Heat recovery operating principle
During simultaneous operation, heating indoor units condense refrigerant and release heat, while cooling indoor units evaporate refrigerant and absorb heat. The system transfers this energy between zones before using the outdoor heat exchanger to make up the remaining load difference.
Core principle: Refrigerant condensed by heating indoor units is reused by cooling indoor units for evaporation. The system controls refrigerant flow so both heating and cooling indoor units receive the required capacity.
Mainly cooling operation
- The compressor speed is adjusted to maintain the required low pressure and cooling capacity.
- Where indoor heating demand does not provide enough condensing capacity, the outdoor heat exchanger operates as a condenser.
- The outdoor expansion valve regulates liquid pressure and refrigerant flow.
Mainly heating operation
- The compressor capacity is adjusted to maintain the required high pressure and heating capacity.
- Where indoor cooling demand does not provide enough evaporation capacity, the outdoor heat exchanger operates as an evaporator.
- The outdoor expansion valve regulates liquid pressure and balances flow.
4. RB unit cooling and heating switching
The Refrigerant Branch (RB) unit routes refrigerant to each connected indoor unit according to the requested operating mode.
| Indoor-unit demand | RB unit action |
|---|---|
| Heating | Routes high-pressure gas to the indoor heat exchanger and returns condensed liquid. |
| Cooling | Routes liquid refrigerant to the indoor unit and returns low-pressure gas. |
| Mode change | Internal valves balance pressure and change the refrigerant path while limiting noise and chattering. |
Important: The RB unit is a refrigerant routing device. Compressor capacity and outdoor heat-exchanger operation are still controlled by the outdoor unit according to total system demand.
5. High-reliability control methods
Stable oil return and prevention of liquid refrigerant backflow are critical to compressor reliability. VR-II uses several coordinated controls.
| Control feature | Purpose |
|---|---|
| Oil separation and return circuit | Separates compressor oil and returns it to the lubrication circuit. |
| Automatic oil recovery | Runs at approximately three-hour intervals during normal operation. |
| Enlarged accumulator | Helps prevent oil accumulation and reduces liquid refrigerant reaching the compressor. |
| Receiver tank | Manages excess refrigerant volume under changing operating conditions. |
| Superheat and liquid-temperature control | Uses thermistor feedback to maintain stable refrigerant condition and prevent constant liquid return. |
| Compressor operating pattern | Balances compressor operation and controls refrigerant velocity for oil return. |
Oil recovery by indoor-unit state
| Unit state | Oil recovery behaviour |
|---|---|
| Cooling indoor unit | Cooling operation and the indoor fan continue while oil is recovered. |
| Heating indoor unit | The fan is stopped temporarily to prevent cool air discharge during recovery. |
| Stopped indoor unit | Refrigerant is circulated through the inactive circuit to draw back collected oil. |
| Outdoor unit | Compressor speed is adjusted to maintain the refrigerant velocity needed for oil return. Completion is judged using compressor suction temperature. |
6. Key takeaways
- VR-II uses a three-pipe heat recovery cycle to provide simultaneous cooling and heating.
- Heat rejected by heating zones can be reused by cooling zones, reducing the outdoor-unit load.
- The outdoor unit operates as either a condenser or evaporator to balance any difference between heating and cooling demand.
- RB units direct high-pressure gas, low-pressure gas and liquid refrigerant to each indoor unit.
- Automatic oil recovery, accumulator and receiver controls protect compressors and maintain long-term reliability.
Training conclusion: The three-pipe architecture provides a familiar, flexible and serviceable heat recovery solution, while VR-II reliability controls recover oil in the shortest practical time according to actual system conditions.
Source: Fujitsu General Limited engineering training material, “Fundamental of Heat Recovery”, 2012. Product specifications and control details may vary by model and market.