On a low-pressure centrifugal chiller, the purge unit is mounted directly on or near the condenser section of the machine. It connects to the condenser’s outlet header, where non-condensable gases like air and moisture naturally collect. The unit is typically a compact, self-contained assembly that sits on top of or beside the chiller’s condenser shell, with short piping runs linking it to the points where trapped air accumulates.
Why the Condenser Is the Collection Point
Low-pressure refrigerants, including R-123 and the newer R-1233zd, operate below atmospheric pressure on the evaporator side. That means any small leak in a gasket, valve stem, or pipe fitting doesn’t push refrigerant out. Instead, it pulls outside air in. Once air enters the refrigeration circuit, it travels with the refrigerant vapor through the compressor and into the condenser.
Inside the condenser, refrigerant vapor cools and turns back into liquid. Air and other non-condensable gases can’t condense, so they get swept along with the flow and pile up at the condenser coil’s outlet header. This is the lowest-pressure zone in the condenser circuit and the natural dead end where air settles. The purge unit taps into this exact spot because that’s where the highest concentration of trapped air exists.
How the Purge Unit Connects to the Chiller
The purge unit’s suction line connects to the condenser outlet, sometimes through a vertical pipe called an “air leg.” This air leg acts as a collection column: air rises and accumulates in it, then feeds into a line (called the foul gas line) that carries the air-refrigerant mixture into the purge unit. The air leg’s diameter typically matches the condenser outlet pipe size up to 4 inches. For larger outlets, the air leg is usually half the outlet diameter, but never smaller than 4 inches. A larger leg prevents liquid refrigerant from being siphoned into the foul gas line.
A return line runs from the purge unit back to the evaporator, sending recovered liquid refrigerant back into the chiller circuit. So in simple terms, the purge unit sits between the condenser (where it pulls in contaminated gas) and the evaporator (where it returns clean refrigerant).
What the Purge Unit Actually Does
The purge unit separates air from refrigerant so the air can be vented while keeping refrigerant loss to a minimum. It does this by chilling the gas mixture it draws in from the condenser. As the mixture cools, the refrigerant vapor condenses back into liquid and drips down into a collection area. Air can’t condense at these temperatures, so it stays in gas form and becomes increasingly concentrated as more refrigerant drops out.
A high-efficiency purge unit, like the YORK TurboGuard system, uses a refrigerant-cooled condenser tank filled and drained with chilled oil from the compressor’s lubrication system. A small gear pump boosts the oil pressure to drive this cycle. During the drain phase, pressure inside the purge tank drops below condenser pressure, which pulls fresh foul gas in through a check valve. The concentrated air is then vented to the atmosphere in small, controlled bursts, while the recovered refrigerant liquid returns to the chiller.
One important point: the purge unit doesn’t “find” air inside the system. It only processes whatever the foul gas line delivers from the condenser outlet. If air is trapped elsewhere in the circuit, it has to migrate to the condenser before the purge unit can remove it.
Inside a Typical Purge Unit
Despite handling an important job, purge units are relatively simple assemblies. A standard high-efficiency unit contains a refrigerant-cooled condenser tank, a magnetic drive oil pump with suction and discharge valves, interconnecting tubing, strainers, driers, and a self-contained control system. The unit starts and stops either manually or automatically based on the concentration of non-condensables detected in the condenser.
Most units also include an exhaust indicator that tracks either total exhaust time or the number of discharge cycles. This data serves double duty: it confirms the purge is working, and it flags potential problems. Frequent purge cycling usually means the chiller has a significant air leak that needs to be found and repaired.
Why Low-Pressure Systems Need Purging
High-pressure refrigerants operate above atmospheric pressure throughout the entire circuit, so leaks push refrigerant out rather than pulling air in. Low-pressure refrigerants are the opposite. The evaporator side runs in a vacuum during normal operation, making air infiltration inevitable over time, even in a well-maintained system.
Trapped air raises condenser pressure, which forces the compressor to work harder and increases energy consumption. Moisture that enters with the air can form acids that corrode internal components. Without a functioning purge unit, efficiency drops steadily and the risk of expensive damage climbs.
To reduce air infiltration when the chiller is idle, many systems use a prevention-of-vacuum system. This applies heat to the evaporator to maintain enough internal pressure that outside air can’t seep in through potential leak paths. It also eliminates the need to pressurize the system with nitrogen or dry air for leak testing, which could introduce additional non-condensables.
Maintenance That Keeps the Purge Running
Purge units need regular attention to stay effective. The pressure relief set point should be checked and adjusted at least monthly. Once a year, the unit should be opened for a full inspection. Technicians look for oil fouling and corrosion inside the tank, and mechanical parts like the purge float valve get inspected and replaced if worn.
Oil and water levels inside the purge unit should be checked on units equipped with level indicators. An unusually high water level is a red flag. It can mean air is leaking into the system at a high rate, or it can point to a tube leak in the condenser or evaporator that’s allowing water from the chilled or condenser water circuits to enter the refrigerant side.
Automatic monitoring of the purge unit’s run time and discharge frequency is standard practice on modern chillers. A sudden increase in purge activity that can’t be explained by seasonal changes or recent service work warrants a thorough leak check of the entire low-pressure circuit.