- ▸Two graphite parts in a centrifugal chiller: impeller labyrinth strips (control internal leakage) and shaft-end carbon rings (isolate bearing chamber from refrigerant chamber) — different jobs, different failure modes
- ▸Every 0.1 mm of labyrinth clearance growth costs roughly 1–2 points of machine efficiency (varies with impeller diameter and pressure ratio) — the basis for wear-based replacement
- ▸Carbon rings work as controlled-leakage seals: 0.08–0.15 mm clearance with a seal-gas barrier; contaminated seal gas poisons the refrigerant circuit
- ▸Overhaul triggers: COP/IPLV down more than 5%, seal-gas flow creeping up, abnormal bearing temperature — any one warrants teardown
- ▸Replacement usually rides on bearing overhauls (20,000–40,000 h); carbon stacks are replaced as sets, labyrinth strips individually by measurement
1. The Two Graphite Parts
A centrifugal chiller compresses refrigerant with a high-speed impeller (typically 8,000–15,000 rpm, geared or direct-drive). Refrigerant side and oil side are two separate systems, and graphite parts appear at both interfaces.
1.1 Impeller Labyrinth Strips
Between the impeller rim and the casing sits an annular gap; high-pressure gas leaks back through it (internal leakage), directly costing compression efficiency. The control is a labyrinth strip — segmented carbon graphite bars in the casing (or on the impeller) holding a small clearance over the impeller: wear-resistant and non-galling to the impeller.
Typical material: resin- or metal-impregnated carbon graphite; some OEMs use polyarylate or aluminum strips. Graphite's advantages are self-lubrication, non-galling behavior and low thermal expansion.
1.2 Shaft-End Carbon Rings
The impeller shaft passes through the bearing chamber (oil-lubricated) with the refrigerant chamber (oil-free, possibly sub-atmospheric) on the other side. At startup suction, oil is drawn toward the refrigerant side, fouling heat exchangers and degrading heat transfer. A shaft-end carbon ring (the TP-series three-segment rings on this site are such components, rated ≤ 10 MPa, ≤ 15 m/s) isolates the two sides, with seal gas forming the pressure barrier.
2. Clearance Design
2.1 Labyrinth Clearance
The strip-to-impeller radial clearance is one of the machine's core efficiency parameters:
| Machine class | Typical clearance | Notes |
|---|---|---|
| Small semi-hermetic (impeller on motor shaft) | 0.25–0.40 mm | Lower speed, smaller diameter |
| Medium open-drive (geared) | 0.35–0.55 mm | Mainstream commercial machines |
| Large two-stage | 0.50–0.80 mm | Thermal growth and assembly tolerance both need room |
Design tensions:
- Thermal growth: casing and impeller heat differently (the impeller absorbs compression work); carbon CTE (2–5×10⁻⁶/°C) is far below steel and impeller alloys, so cold clearance must be computed for hot condition
- Assembly and wear: too small and bearing wear, shaft vibration or balance degradation let the impeller touch the strips; too large costs efficiency
- Start/stop rub: at startup the oil film is not yet established and the shaft sits differently from steady state — clearance must cover this transition
2.2 Carbon Ring Clearance
Shaft-end carbon ring diametral clearance is typically 0.08–0.15 mm (50–100 mm shaft machines), consistent with this site's article on oil-free screw compressor carbon rings. Seal gas is mandatory:
- Source: clean dry air or nitrogen, dew point ≤ -20°C, oil ≤ 0.01 mg/m³
- Pressure: 0.01–0.05 MPa above refrigerant side so flow points at the refrigerant side, keeping oil mist in the bearing chamber
- Flow: minimum effective value per the leakage curve
The most dangerous mistake is ordinary instrument air as seal gas: oily air injects oil straight into the refrigerant circuit; capacity drops and oil-separator load rises, yet the field finds no cause — because leak detection watches refrigerant, not oil.
3. Wear and Criteria
3.1 Labyrinth Strips
| Failure | Symptom | Criterion |
|---|---|---|
| Uniform wear | Clearance grows, COP slowly falls | Measured clearance at overhaul above 2× new value, or unacceptable efficiency loss |
| Localized wear (rub) | One side of strips polished, nicked | Evidence of shaft vibration or balance degradation — fix the root cause before replacing |
| Fracture / detachment | Fragments in the gas path | Stop immediately; inspect impeller for damage |
Wear rate correlates strongly with shaft vibration, start/stop count and liquid slugging. Machines with frequent cycling (commercial buildings with swinging load) wear faster than continuous-duty machines.
3.2 Carbon Rings
| Stage | Symptom |
|---|---|
| Stable | Flat seal-gas flow, normal oil return |
| Degradation | Flow creeping up (clearance opened by bore grooving) |
| Failure | Flow jump (ring fracture/spring relaxation); oil traces in refrigerant side |
Criteria: seal-gas flow up 50% or more over initial value, or abnormal bearing temperature rise — schedule replacement.
4. Overhaul Inspection Order
At overhaul (usually synced with bearing replacement), inspect graphite parts in this order:
1.Record before teardown: seal-gas flow, bearing temperature, current, refrigerant in/out temperatures — without pre-teardown data there is no trend
2.Shaft-end carbon rings: measure actual bore-to-shaft clearance; inspect bore grooving and butt-joint wear; measure spring free length (replace beyond 5% deviation)
3.Labyrinth strips: measure clearance to impeller at four equally spaced positions minimum; record wear pattern
4.Shaft and bearings: radial runout, vibration, bearing clearance — these are root causes of abnormal strip wear; fix before resealing
5.Seal-gas system: source quality (dew point, oil content), regulator stability, line filter ΔP
6.Reassembly: select or trim strips to measured clearance; replace carbon stacks as sets; establish seal gas before starting
5. Site Management
- Efficiency trend is the best health check: weekly COP or kW-per-ton logging catches problems earlier than any point measurement
- Liquid slugging kills strips: expansion-valve faults and sudden load drops cause it; schedule inspection after any slugging event
- Quarterly seal-gas quality test: dew point, oil content, particles — archive the data
- Spares strategy: carbon stacks stocked as sets (springs and retainers included); strips stocked 1–2 sets per machine count — uneven wear is normal
- Retrofit evaluation: replacing legacy aluminum strips with metal-impregnated carbon graphite at overhaul protects the impeller on rub and is more economical long-term
Conclusion
Neither graphite part in a centrifugal chiller is complicated — a few strips, a few ring segments — but both sit at the hinge of efficiency and oil management. Design the clearances correctly (hot-condition labyrinth, controlled-leakage carbon rings), treat COP trend and seal-gas flow as health indicators, and at overhaul follow "fix the shaft line first, then install the seals." Machine efficiency will then track design values over the long term.