1 Low-Pressure Appliances and Chiller Fundamentals
Learn how low-pressure chillers operate and how to recover refrigerant, evacuate equipment, and carry out service safely within applicable requirements.
Low-pressure operation
A has parts of its refrigerant circuit below atmospheric pressure during normal operation. Large centrifugal chillers are a common example, but their exact pressures vary with refrigerant and temperature; use the equipment’s pressure–temperature data rather than assuming one pressure applies to every chiller.
Because sections of the system operate under vacuum, air and other can enter through leaks. Their accumulation impairs heat transfer and can increase condenser pressure, reducing chiller capacity and efficiency.
Chiller cycle and components
A typical vapor-compression chiller transfers heat from building water to refrigerant, then rejects that heat to a condenser-water loop or another heat sink. The cycle proceeds through four main components:
Evaporator: Low-pressure refrigerant absorbs heat from return water and boils. The cooled water circulates to building coils or process equipment.
Compressor: In a centrifugal chiller, a motor-driven impeller raises the refrigerant vapor’s pressure and temperature.
Condenser: Hot refrigerant vapor releases heat to condenser water and condenses into liquid.
Expansion device: The liquid’s pressure is reduced before it returns to the evaporator, where it can absorb heat again.
Other common components include the motor and drive, lubrication system, controls and sensors, refrigerant charging and service connections, and pressure-relief devices. Many low-pressure centrifugal chillers also have a to separate and remove accumulated . Excessive purge activity can be a clue to air leakage or another system problem.
and
removes refrigerant from an appliance into suitable equipment and a cylinder. brings a system or component to a specified low-pressure endpoint; after repairs, technicians also evacuate systems as required to remove air and moisture before charging. Do not intentionally vent refrigerant during service. Use compatible, properly rated equipment and follow its instructions and the chiller manufacturer’s procedure.
For standard EPA Section 608 of a before opening it or disposing of it, the endpoint is absolute. This is an , not a vacuum reading of inches of mercury. Verify the applicable requirement and confirm that the endpoint has been reached before opening the appliance.
After reaching the required vacuum, wait a few minutes and watch for pressure rebound. A pressure rise can indicate refrigerant remaining in liquid or oil, so may need to continue.
Leaking appliances and exceptions
A leaking appliance may not reach the normal endpoint. Where possible, isolate leaking sections from non-leaking sections: evacuate the non-leaking sections to the specified level and take leaking sections to the lowest attainable level, not above .
A limited alternative applies to qualifying non-major repairs that are not followed by to the environment. Before opening a in this situation, bring it to using an allowable method. For most such systems, use controlled heat or the manufacturer’s built-in pressurization method, not a method that requires subsequent purging, such as nitrogen. Follow current EPA requirements for the specific situation.
and charging precautions
Liquid refrigerant can be removed first to speed , but vapor must also be recovered. Use the machine’s required controls and manufacturer-set limits.
During refrigerant , circulate water through the chiller or remove it as appropriate for the equipment and repair to prevent water-side freezing. When recharging, introduce vapor before liquid to reduce the risk of freezing water in the tubes. On centrifugal chillers, use the designated evaporator charging valve.
Oil, leak testing, and safety
Refrigerant can remain dissolved in compressor oil. Before removing oil, EPA requires reducing pressure to no greater than , or draining the oil into a receiver that is then brought to no greater than . EPA Type III test topics also identify heating oil to before removal as a technique to minimize refrigerant release. Follow the equipment procedure and use appropriate safeguards.
Low-pressure-system leak detection commonly prioritizes controlled hot water or a built-in heating or pressurization device. Nitrogen is another method where permitted, but never exceed the chiller manufacturer’s pressure limit. If using nitrogen, use a regulator and suitable pressure-relief protection; never use oxygen or compressed air.
Refrigerant can displace oxygen and cause cold injury. Use suitable personal protective equipment, ventilation, and site-required detection and emergency procedures. Do not energize equipment unless the manufacturer’s instructions permit it in the system’s current pressure condition.