Overview
Chillers produce chilled water (typically 6-7°C supply) for air conditioning, process cooling, and industrial refrigeration. They are the largest single energy consumer in most facilities — often 30-50% of total electrical load. Selecting the right type, size, and efficiency level has enormous energy and cost consequences over the 20-30 year life. Major types: centrifugal, screw, scroll, reciprocating, and absorption.
Chiller Types and Applications
Centrifugal Chillers (Dynamic Compression)
- Uses centrifugal compressor with one or more impellers (one-stage, two-stage, or magnetic-bearing)
- Capacity range: 150 to 5,000+ tons (500 kW to 18 MW)
- Efficiency: 0.50-0.65 kW/ton (COP 5.4-7.0) at full load; excellent at large sizes
- Best for: large commercial buildings (offices, hospitals, malls), campus cooling, large process loads
- Capacity control: inlet guide vanes + variable speed drive (VSD); turndown to 10-20% load
- Magnetic-bearing models: oil-free, very low maintenance, high part-load efficiency; premium cost
- Drives: direct-drive (high speed) or geared (lower speed)
- Refrigerants: R-134a, R-1234ze, R-514A (low-GWP replacements)
Screw Chillers (Positive Displacement — Helical Rotary)
- Twin helical screws compress refrigerant between meshing rotors
- Capacity range: 50 to 1,000 tons (175 kW to 3.5 MW)
- Efficiency: 0.60-0.80 kW/ton (COP 4.4-5.9)
- Best for: medium to large process loads, industrial cooling, district cooling, buildings with variable loads
- Fewer moving parts than reciprocating; reliable; smooth operation
- Capacity control: slide valve, variable speed, or both; turndown to ~10%
- Refrigerants: R-134a, R-1234ze, R-513A; ammonia for industrial
Scroll Chillers
- Spiral scroll compressor (one fixed, one orbiting) — positive displacement
- Capacity range: 10 to 200 tons (35 kW to 700 kW); often modular (multi-scroll)
- Efficiency: 0.75-1.0 kW/ton (COP 3.5-4.7)
- Best for: small to medium commercial, light industrial, process cooling, modular plants
- Very reliable (few moving parts); low vibration; quiet
- Capacity control: multiple fixed stages (e.g., 4 compressors = 4 steps) or variable-speed scroll
- Refrigerants: R-410A, R-32, R-454B (low-GWP trend)
Reciprocating Chillers (Piston Compressor)
- Piston-cylinder compression — old but robust
- Capacity range: 5 to 200 tons (17-700 kW)
- Efficiency: 0.90-1.2 kW/ton (COP 2.9-3.9) — lower than modern types
- Best for: small industrial, high-temperature process applications, low-temperature brine
- Tolerant of high pressure ratios and varied refrigerants
- More maintenance (valves, rings); higher noise/vibration
- Declining market share in favor of scroll/screw
Absorption Chillers (Heat-Driven)
- Uses heat (steam, hot water, natural gas, waste heat) instead of mechanical compression — lithium bromide/water cycle (water = refrigerant, LiBr = absorbent)
- Capacity range: 10 to 2,000+ tons
- Electric input minimal (pumps only): COP ≈ 0.7-1.2 single-effect; 1.2-1.4 double-effect
- Best for: facilities with waste steam (cogeneration, power plants, industrial), low-cost gas, or where electricity is unavailable/expensive
- No compressor, no refrigerants (water is refrigerant); very low electrical load
- Higher capital cost; lower electrical efficiency but fuel/heat-driven
- Types: single-effect (low-pressure steam/hot water); double-effect (higher pressure steam or direct-fired gas)
Efficiency Metrics
Full Load: COP and kW/Ton
- COP (Coefficient of Performance) = kW_cooling / kW_electricity input. Higher is better.
- kW/ton = kW electricity per ton of cooling. Lower is better (1.0 kW/ton = COP 3.5; 0.5 kW/ton = COP 7.0).
Part Load: IPLV / NPLV
Chillers operate at part load 90%+ of the time. IPLV (Integrated Part Load Value) weights efficiency at 25/50/75/100% load:
IPLV = 0.01 × A + 0.42 × B + 0.45 × C + 0.12 × D
(A = 100%, B = 75%, C = 50%, D = 25% efficiency in kW/ton or COP)
IPLV reflects typical building operation better than full-load COP. For variable-load operation, select for lowest IPLV/NPLV rather than lowest full-load kW/ton.
Typical Efficiency by Type (AHRI conditions)
| Chiller Type | Full Load kW/ton | IPLV kW/ton | Full Load COP |
|---|---|---|---|
| Magnetic-bearing centrifugal | 0.48-0.55 | 0.30-0.40 | 6.4-7.3 |
| Standard centrifugal | 0.55-0.65 | 0.35-0.50 | 5.4-6.4 |
| High-efficiency screw | 0.60-0.70 | 0.40-0.55 | 5.0-5.9 |
| Standard screw | 0.70-0.85 | 0.50-0.70 | 4.1-5.0 |
| Scroll (modulating) | 0.70-0.85 | 0.55-0.75 | 4.1-5.0 |
| Scroll (staged) | 0.80-1.0 | 0.65-0.90 | 3.5-4.4 |
| Reciprocating | 0.90-1.2 | 0.75-1.0 | 2.9-3.9 |
| Absorption (double-effect steam) | 0.05 (pump only) | 0.05 | COP 1.2 (thermal) |
Condenser Type
Water-Cooled
- Cooling tower rejects condenser heat
- Lower condensing temperature → higher efficiency (chiller runs at lower head pressure)
- Requires cooling tower + condenser water pumps + piping
- Most efficient option overall for large systems
- Standard for centrifugal, screw, absorption chillers >100 tons
Air-Cooled
- Ambient air blown over condenser coils
- No cooling tower, no water consumption, simpler installation
- Higher condensing temperature (higher than wet bulb, at dry bulb) → higher kW/ton (typically 1.1-1.4 kW/ton)
- Best for small systems, water-scarce areas, locations where towers are impractical
- Common for scroll and screw chillers to ~200 tons
Evaporative-Condenser
- Hybrid: condenser coil in a tower-like unit, water sprayed on coil + air drawn through
- Between water-cooled and air-cooled in efficiency
- Used in some industrial and refrigeration applications
Refrigerant Landscape (2026+)
Refrigerants are rapidly transitioning to low-GWP (global warming potential) due to regulatory phase-downs (Kigali Amendment, US AIM Act, EU F-Gas):
| Refrigerant | GWP | Status |
|---|---|---|
| R-134a | 1430 | Phase-down; still common in existing centrifugal |
| R-410A | 2088 | Phase-down; used in scroll/screw |
| R-1234ze | 1 | Low-GWP replacement for R-134a (centrifugal); mildly flammable (A2L) |
| R-1234yf | 1 | Low-GWP for automotive/small systems |
| R-514A | 2 | Low-GWP for centrifugal |
| R-454B | 466 | Low-GWP R-410A replacement (scroll/screw) |
| R-32 | 675 | Lower GWP alternative; A2L flammable |
| Ammonia (R-717) | 0 | Industrial refrigeration; toxic, efficient |
| Propane (R-290) | 3 | Very efficient, hydrocarbon; flammable, charge limits |
| Water (R-718) | 0 | Absorption systems |
New installations should specify low-GWP refrigerants. Check local codes for A2L (mildly flammable) refrigerant handling requirements.
Selection Workflow
- Determine cooling load (peak tons, minimum load, diversity)
- Select chiller type by capacity (see table above)
- Choose condenser type (water-cooled for >100 tons unless water-limited)
- Define chilled water temperatures:
- Standard: 44°F supply / 56°F return (6.7°C / 13.3°C) = 12°F ΔT
- Low-temperature: 20-40°F (brine/chilled water for process)
- Higher chilled water temperature = higher chiller efficiency (each 1°F higher supply improves efficiency by ~1.5-2%)
- Specify efficiency: compare full-load kW/ton AND IPLV; premium efficiency often pays back in 2-3 years
- Redundancy: N+1 (one standby for facilities requiring continuous cooling); N-1 (one failure tolerated)
- Controls: BMS interface; VFD for capacity modulation; remote monitoring
- Acoustics: specify sound requirements (urban sites may require low-noise variants)
- Vibration: vibration isolation pads, flexible connections
Plant Configuration
Single Chiller
Simplest; appropriate for small loads; zero redundancy.
Multiple Chillers (Series or Parallel)
- Parallel (most common): chillers share load; each can run independently; provides redundancy; staging matches part load efficiency
- Series counterflow: for very high ΔT (15-20°F) or low temps; higher efficiency at part load; more complex
- Use unequal sizes where load profile is skewed (e.g., 200 ton + 400 ton = better staging than two 300 ton)
Variable Primary Flow (VPF)
Chilled water pumps vary flow through chiller evaporator with VFD — saves pumping energy (most modern plants). Requires careful chiller minimum-flow protection.
Free Cooling
In cold weather (low wet bulb), plate heat exchanger bypasses chiller to cool water directly via cooling tower (water-side economizer). Saves 100% of chiller energy in winter — major savings in cold climates.
Chilled Water System Design
- Chilled water ΔT: 5-8°C (10-15°F) standard; higher ΔT reduces pumping power
- Chilled water flow: typically 2.4 gpm/ton at 10°F ΔT (or 2.0 gpm/ton at 12°F ΔT)
- Variable primary flow with VFD pumps for energy efficiency
- Buffer tank for small systems (prevents short cycling)
- Expansion tank, air separator, makeup water
Maintenance Considerations
- Annual efficiency loss without maintenance: 5-15%
- Tube cleaning: fouled condenser tubes increase kW/ton by 10-30%
- Refrigerant charge: low charge reduces capacity and efficiency
- Oil analysis (for non-magnetic-bearing units): detects compressor wear
- Water treatment: fouled tubes are #1 energy waster
- 20+ year life with proper maintenance
Summary
Chillers are the largest energy consumer in most facilities. Select by capacity: scroll for <50 tons, screw for 50-800 tons, centrifugal (especially magnetic-bearing) for 800+ tons, and absorption when waste heat is available. Efficiency matters most over the 20-year life — prioritize IPLV (part-load) over full-load COP as chillers run at part load most of the time. Water-cooled chillers are most efficient; air-cooled simpler for small systems. Specify low-GWP refrigerants (R-1234ze, R-454B, R-513A) for new installations. Premium efficiency models pay back in 2-3 years; fouled tubes can negate that premium — maintain the chiller and cooling tower.