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Type III certification covers centrifugal chillers that operate at sub-atmospheric (vacuum) conditions. These are large-building cooling systems -- hospitals, universities, office towers, airports. A single chiller may cool 500-5,000 tons. Refrigerant charges can be thousands of pounds. Understanding why these systems are fundamentally different from high-pressure equipment is the key to passing the Type III exam.
| Refrigerant | Class | ODP | GWP | Evap Pressure (40 deg F) | Status |
|---|---|---|---|---|---|
| R-11 | CFC | 1.0 | 4,750 | ~3-4 psia (deep vacuum) | Banned 1996; legacy systems only |
| R-113 | CFC | 0.9 | 6,130 | ~1-2 psia (extreme vacuum) | Banned 1996; legacy systems only |
| R-123 | HCFC | 0.012 | 77 | ~3-5 psia | Active; dominant in modern centrifugal chillers |
| R-1233zd(E) | HFO | 0 | 1 | ~5-7 psia | Emerging replacement for R-123 |
The shift from R-11 to R-123 reduced ODP by over 98% (from 1.0 to 0.012). The next transition to R-1233zd(E) will reduce GWP from 77 to essentially zero.
R-123 at 40 degrees F evaporator temperature has a saturation pressure of approximately 3-5 psia -- roughly 10 to 12 psia below atmospheric pressure. In gauge terms this reads as -10 to -12 psig, or 24-28 inches Hg vacuum.
The critical consequence: any breach allows atmospheric air (and moisture) to enter the system, rather than refrigerant to escape. Air infiltration is the central enemy of low-pressure chiller operation. The system must prevent air from getting in, not prevent refrigerant from getting out.
When leak-testing a low-pressure system, use dry nitrogen only. Introducing compressed air creates a potentially combustible mixture of refrigerant and oxygen. Oxygen alone creates an explosive hazard. Dry nitrogen is inert and safe for all refrigerant system applications.
Because low-pressure chillers inevitably experience some air infiltration through shaft seals, valve stems, and other ingress points, virtually all centrifugal chillers have a purge unit.
How it works: The purge unit draws a mixture of refrigerant vapor and non-condensable gases from the top of the condenser (where non-condensables accumulate). It compresses and cools this mixture to condense the refrigerant while keeping the non-condensables in vapor form. The condensed refrigerant returns to the chiller; the non-condensables (air) are vented or captured. Modern "zero-emission" purge units recapture the refrigerant vapor that would otherwise be lost.
25 mm Hg ABSOLUTE PRESSURE -- ALL LOW-PRESSURE EQUIPMENT
| High-Pressure Recovery | Low-Pressure Recovery | |
|---|---|---|
| System pressure | Above atmospheric -- refrigerant can be pushed out | Below atmospheric -- must apply heat to create vapor pressure |
| Recovery equipment | Standard self-contained recovery machine | Dedicated low-pressure recovery equipment |
| Recovery cylinder | Standard R-22 or R-410A specific cylinders | R-123-specific recovery cylinders ONLY |
| Speed | 1-2 hours typical | Several hours for large charge |
R-123 reacts with moisture to form hydrochloric acid (HCl) and hydrofluoric acid (HF) -- highly corrosive to metals, copper, and elastomers. The massive refrigerant charge means even small moisture levels produce significant acid quantities. Annual refrigerant analysis (moisture, acid, non-condensables) is standard practice for large chillers.
Low-pressure chillers use centrifugal compressors with high-speed rotating impellers. Surge occurs when the pressure lift exceeds the compressor's capability at a given flow rate -- causing violent pressure fluctuations, noise, and potential compressor damage.
Common causes of surge: Low load with high condenser pressure, non-condensables elevating head pressure, insufficient refrigerant charge, or rapid load changes. Modern chillers have antisurge controls to prevent damage.
Operates in VACUUM -- below atmospheric, opposite of high-pressure systems
Air ENTERS if there is a leak -- opposite of high-pressure (where refrigerant escapes)
Purge unit removes air -- high purge rate = air infiltration = locate the leak
25 mm Hg absolute -- evacuation level, all low-pressure equipment, no exceptions
Dry nitrogen only -- for leak testing; never air or oxygen
R-123 replaces R-11 -- 99% lower ODP; same low-pressure operating principle