9.1 Low-Pressure System Fundamentals
Key Takeaways
- Low-pressure refrigerants boil above 50°F at atmospheric pressure (R-11: 74.9°F, R-123: 82.2°F)
- Low-pressure systems operate in vacuum — leaks pull air IN, not refrigerant out
- Centrifugal compressors use high-speed impellers and are used in large chillers (100-10,000+ tons)
- R-11 (CFC, phased out) and R-123 (HCFC, phasing out by 2030) are legacy low-pressure refrigerants
- R-1233zd and R-514A (HFOs) are next-generation low-pressure replacements
9.1 Low-Pressure System Fundamentals
Type III certification covers low-pressure appliances — primarily centrifugal chillers used in large commercial and institutional buildings. These systems operate under fundamentally different principles than high-pressure systems, and understanding their unique characteristics is essential.
What Are Low-Pressure Systems?
Low-pressure systems use refrigerants that boil above 50°F at atmospheric pressure. This means that under normal operating conditions, much of the system operates at below atmospheric pressure (in vacuum).
Key low-pressure refrigerants:
| Refrigerant | Type | Boiling Point | ASHRAE Class | ODP | GWP | Status |
|---|---|---|---|---|---|---|
| R-11 | CFC | 74.9°F (23.8°C) | B1 | 1.0 | 4,750 | Phased out (1996) |
| R-123 | HCFC | 82.2°F (27.9°C) | B1 | 0.02 | 77 | Being phased out (2030) |
| R-245fa | HFC | 59.5°F (15.3°C) | B1 | 0 | 1,030 | Subject to AIM Act |
| R-1233zd | HFO | 64.4°F (18.0°C) | A1 | 0 | 1 | Next-generation replacement |
| R-514A | HFO blend | 83.5°F (28.6°C) | B1 | 0 | 2 | Next-generation replacement |
Why "Low-Pressure"?
Because these refrigerants boil above room temperature (50°F+), the system must reduce pressure below atmospheric to make them boil at the temperatures needed for cooling:
- In the evaporator, the pressure is well below atmospheric (deep vacuum)
- Even the condenser may operate at or near atmospheric pressure
- This means any leak in the system allows air to enter rather than refrigerant to escape
- Air infiltration is the primary concern (rather than refrigerant leaking out)
Centrifugal Chiller Construction
Centrifugal chillers are large machines (typically 100-10,000+ tons of cooling) found in:
- Large office buildings
- Hospitals
- Universities
- Data centers
- Industrial facilities
- District cooling plants
Key components unique to centrifugal chillers:
| Component | Function |
|---|---|
| Centrifugal compressor | Uses a high-speed impeller to compress refrigerant vapor |
| Shell-and-tube evaporator | Large shell containing refrigerant, with water tubes running through |
| Shell-and-tube condenser | Similar construction; rejects heat to condenser water |
| Purge unit | Removes non-condensable gases (air) that leak into the system |
| Rupture disc | Safety device that bursts at 15 psig to prevent over-pressure |
| Motor/drive | Powers the impeller; may be open-drive or hermetic |
| Oil sump | Lubricates bearings; must be heated during shutdown to prevent refrigerant migration |
| Refrigerant charging valve | Allows liquid refrigerant to be added to the system |
Vacuum Operation
Since low-pressure systems operate below atmospheric pressure:
- Standard pressure gauges cannot accurately measure low-pressure operations — specialized vacuum gauges or compound gauges are needed
- Pressure is often measured in inches of mercury (in. Hg) vacuum or mm Hg absolute
- The evaporator typically operates at 6-10 in. Hg vacuum for typical cooling conditions
- Even the condenser may operate below atmospheric on cool days
For the Exam: Low-pressure refrigerants boil above 50°F at atmospheric pressure. The evaporator operates in vacuum, so leaks pull air IN rather than letting refrigerant OUT. Centrifugal compressors use an impeller. The purge unit removes air that enters through leaks.
What is the boiling point of R-11 at atmospheric pressure?
In a low-pressure chiller system, what happens when a leak occurs?
What type of compressor is used in centrifugal chillers?