How does the refrigeration circuit in a recirculating chiller work?
A recirculating chiller removes heat from a process fluid and transfers it away from the process using a refrigeration system.
Within a typical ATC recirculating chiller, there are two important circuits:
- The refrigeration circuit, which provides the cooling
- The coolant circuit, which circulates process fluid between the chiller and the customer’s equipment
These circuits are separate but interact through a heat exchanger. Heat is transferred from the process fluid into the refrigeration circuit before ultimately being rejected from the chiller.
What is the refrigeration circuit?
The refrigeration circuit uses a vapour-compression refrigeration cycle.
Refrigerant circulates continuously through the system, changing pressure, temperature, and physical state as it passes through four main stages:
- Compression
- Condensation
- Expansion
- Evaporation
Together, these stages allow heat to be absorbed from the process fluid and rejected elsewhere.
1. Compression
Refrigerant enters the compressor as a low-pressure vapour after absorbing heat in the evaporator.
The compressor raises the pressure of the refrigerant, which also increases its temperature. The resulting high-pressure, high-temperature refrigerant then travels to the condenser.
The compressor therefore provides the driving force that keeps the refrigerant moving through the refrigerant cycle.
2. Condensation
The hot, high-pressure refrigerant passes through the condenser, where heat is rejected from the refrigeration circuit.
As heat is removed, the refrigerant condenses from a vapour into a high-pressure liquid.
How this heat is rejected depends on the chiller configuration.
In a standard air-cooled chiller, heat is transferred to the surrounding air. This is why adequate ventilation and unrestricted airflow around the chiller are important.
Where a Water-Cooled configuration is available, condenser heat is instead transferred to a suitably facility cooling-water system.
3. Expansion
The high-pressure liquid refrigerant then passes through an expansion device.
The expansion device creates a pressure drop between the high-pressure and low-pressure sides of the refrigerant circuit. As the refrigerant pressure falls, its temperature also falls, preparing it to absorb heat in the evaporator.
The low-pressure refrigerant then moves into the evaporator.
4. Evaporation
The evaporator is where heat is transferred from the process coolant into the refrigeration circuit.
The colder refrigerant absorbs heat from the process fluid as it passes through the evaporator. As it absorbs this heat, the refrigerant evaporates and returns to a low-pressure vapour.
The refrigerant then returns to the compressor and the refrigeration cycle begins again.
How does the coolant circuit work with the refrigeration circuit?
The refrigeration circuit does not normally circulate through the customer’s process.
Instead, a separate coolant circuit carries the process fluid between the chiller and the equipment being cooled.
The process absorbs heat and the warmed fluid returns to the chiller. Inside the chiller, heat is transferred from this fluid to the refrigerant through the evaporator.
The cooled process fluid is then circulated back to the application, creating a continuous cooling loop.
Maintaining an appropriate and continuous coolant flow path is important for correct chiller operation.
Where does the heat ultimately go?
A chiller does not make heat disappear. It moves heat from one location to another.
In an air-cooled system, the heat removed from the process is ultimately rejected into the surrounding environment through the condenser.
This means that the installation area needs sufficient ventilation to manage the heat produced during operation. Hot discharge air should not be obstructed or allowed to recirculate back into the chiller.
With a Water-Cooled configuration, this heat is instead transferred into the facility chilled water circuit.
Which refrigerants do ATC chillers use?
The refrigerant depends on the chiller model.
ATC’s current product strategy focuses on lower-GWP refrigerants, including R290 and R454C, as the refrigeration industry transitions away from older higher-GWP refrigerants.
R290 is ATC’s preferred long-term refrigerant where its use is technically and safely appropriate, while R454C provides a lower-GWP solution for applications and product platforms where R290 is not currently practical.
The refrigerant specified for a particular chiller should not be changed by the customer. Refrigerant selection forms part of the design, safety, performance, and regulatory compliance of the complete refrigeration system.
Do I need to maintain the refrigeration circuit?
The refrigeration circuit should not normally require routine intervention by the equipment operator, but the chiller as a whole should be correctly maintained.
Preventative maintenance can help to identify developing issues, maintain reliable operation, and support the service life of the equipment.
Work on the refrigeration circuit should only be carried out by an appropriately qualified personnel and in accordance with the requirements applicable to the equipment and refrigerant.
If you suspect a refrigeration fault, contact ATC rather than attempting to open or modify the refrigeration circuit.
