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 recovers nearly all refrigerant (often <0.1 lb lost per lb of air removed); its small releases are de minimis
- 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 — often losing less than 0.1 lb of refrigerant per pound of air removed — and are now standard on new low-pressure chillers.
| Feature | Older / low-efficiency purge | High-efficiency purge |
|---|---|---|
| Refrigerant lost per lb air vented | Noticeable | < 0.1 lb |
| Recovery efficiency | ~85–90% | 99%+ |
| Monitoring | Basic | Electronic logging |
| Releases | Larger | De minimis (minimal) |
Releases from a properly operating high-efficiency purge unit are considered de minimis and are not a Section 608 violation — provided the unit is maintained per manufacturer specs. 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. High-efficiency purge releases are de minimis. 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?
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?
Where do non-condensable gases accumulate inside a low-pressure chiller, and from where does the purge unit take its suction?
Put the steps of purge-unit operation in the correct order.
Arrange the items in the correct order