10.2 Purge Units and Air Management
Key Takeaways
A purge unit removes non-condensable gases (mostly air) that leak into the vacuum side of a low-pressure chiller
Non-condensables collect at the TOP of the condenser; the purge unit draws the mixture from there, recovers the refrigerant, and vents only the air
High purge rates (frequent or long purge cycles) indicate active air leaks that must be found and repaired
A high-efficiency purge unit returns nearly all refrigerant to the chiller; what it does release counts as an emission during normal operation (a permitted category), not a de minimis release
Rupture discs and condenser water-box leaks are common infiltration points; water-box leaks let cooling water into the refrigerant under vacuum
The Purge Unit: A Low-Pressure System's Air Pump
Because a low-pressure chiller lives in a vacuum, air leaks IN no matter how well it is sealed. The purge unit (purge recovery unit) is the device that continuously removes that infiltrated air — and any other non-condensable gas (NCG) — while saving the refrigerant. Understanding the purge unit is the heart of 9.2 because the exam treats it as the diagnostic "check-engine light" of the entire machine.
Why Non-Condensables Are a Problem
NCGs are gases that will not condense at the chiller's operating temperatures and pressures — chiefly the nitrogen, oxygen, and water vapor of infiltrated air. They cause real damage:
- Higher condenser pressure — NCGs occupy space and raise head pressure, so the compressor works harder and uses more energy.
- Lost capacity — a blanket of air on the condenser tubes insults heat transfer, cutting cooling output.
- Moisture damage — the water riding in with the air mixes with refrigerant and heat to form hydrochloric/hydrofluoric acid, copper plating, and sludge, which destroys bearings.
Air is lighter than refrigerant vapor, so NCGs rise and collect at the TOP of the condenser — which is exactly where the purge unit takes its suction.
How a Purge Unit Works
A purge unit does not simply vent the chiller to atmosphere. It separates refrigerant from air so the refrigerant goes back into the machine and only the air leaves:
- A mixture of refrigerant vapor and NCGs is drawn from the top of the condenser.
- The mixture is cooled in the purge unit's condensing coil, which liquefies the refrigerant (it condenses) but leaves the air as gas (it does not).
- The recovered liquid refrigerant is returned to the chiller.
- The remaining air/NCGs are vented to the atmosphere.
- The cycle repeats automatically whenever NCGs accumulate.
Reading the Purge Rate — The Key Diagnostic
| Purge behavior | What it means | Technician action |
|---|---|---|
| Rare / short purge cycles | Tight machine, minimal infiltration | Log it; routine monitoring |
| Rising purge frequency | Air is leaking in faster | Investigate — a leak is developing |
| Frequent / continuous purging | Significant active leak | Leak-test and repair before damage |
| Purge runs spike with no load change | New leak path opened | Find and seal immediately |
The single most tested idea here: high purge rates mean the system is leaking air. A modern purge unit logs its own runtime; reviewing that log at every service visit catches leaks early, before moisture wrecks the oil and bearings.
High-Efficiency Purge Units and Common Leak Paths
Older purge units vented some refrigerant along with the air. High-efficiency purge units condense out nearly all of it and are now standard on new low-pressure chillers.
| Feature | Older / low-efficiency purge | High-efficiency purge |
|---|---|---|
| Refrigerant lost with the vented air | Noticeable | Very little |
| Monitoring | Basic | Electronic logging |
| Releases | Larger | Minimal (emissions during normal operation) |
Refrigerant released by a purge unit during normal operation falls under EPA's separate permitted category for emissions during normal operation (like leaks) — it is not a "de minimis" release, a term reserved for small losses during good-faith recovery. Deliberately bypassing the purge to vent refrigerant is always illegal.
Where Air (and Water) Gets In
- Rupture-disc housing — a weeping or partly failed disc admits air on the vacuum side.
- Open-drive shaft seals — a classic infiltration point on older machines.
- Access plates, flanges, and gaskets on the shells.
- Water-box leaks — a cracked condenser/evaporator tube or failed water-box gasket lets cooling water get pulled INTO the refrigerant under vacuum, dumping both air and moisture into the charge at once.
Example: A 900-ton R-123 chiller historically purged twice a day. Over two weeks the log shows purge cycles climbing to every 30 minutes, yet building load is flat. The rising purge rate signals an active air leak. Soap-bubble and electronic testing trace it to a weeping rupture-disc housing. Sealing the housing returns purging to twice a day — and, just as important, stops moisture from continuing to enter and acidify the oil.
For the Exam: NCGs (air + moisture) collect at the TOP of the condenser. The purge unit draws that mixture, condenses and returns the refrigerant, and vents only the air. A HIGH purge rate = an air leak. Purge releases during normal operation are a permitted category of their own, not "de minimis" releases. Water-box / tube leaks pull cooling water into the refrigerant under vacuum.
What is the primary purpose of a purge unit on a low-pressure chiller?
To add refrigerant charge automatically when pressure drops
To remove non-condensable gases (air) that leak into the vacuum while recovering the refrigerant
To increase condenser pressure for better heat transfer
To filter oil and remove acid from the lubricant
A chiller's purge unit, which normally runs a few minutes a day, is now running almost continuously while the building load is unchanged. What does this most likely indicate?
The refrigerant charge is too high
An active air leak is allowing non-condensables into the system
The chilled-water temperature is set too low
The high-efficiency purge unit is malfunctioning and over-recovering
Where do non-condensable gases accumulate inside a low-pressure chiller, and from where does the purge unit take its suction?
At the bottom of the evaporator shell
In the oil sump
At the top of the condenser
In the compressor impeller housing
Put the steps of purge-unit operation in the correct order.
Arrange the items in the correct order
The recovered liquid refrigerant is returned to the chiller
Only the non-condensable air is vented to atmosphere
Refrigerant/air mixture is drawn from the top of the condenser
The mixture is cooled so the refrigerant condenses to liquid
Sections you finish are checked off in the contents.