Chillers: How Big Buildings Make Chilled Water for Cooling

When a building is too big for split ACs or VRF, its cooling comes from a chiller — a large refrigeration machine that doesn't cool air directly but makes chilled water, which is then piped to AHUs and fan-coil units all over the building to cool the air locally. The chiller is the heart of a central air-conditioning plant. This guide explains what a chiller is, how it works, air-cooled vs water-cooled, the compressor types, and how to read chiller efficiency when specifying one.

It builds on the HVAC Equipment Guide, part of the HVAC Knowledge Hub.

The safety line & scope. Chillers are major engineered plant — selection, sizing, installation, commissioning and servicing are qualified MEP/HVAC engineering work. This guide informs understanding and specification.

What a chiller is and how it works

A chiller runs the same refrigeration cycle as an AC, but instead of cooling air over its evaporator, it cools water:

• The chiller's evaporator cools water to ~6–7°C ("chilled water").
• Pumps circulate this chilled water to AHUs/FCUs around the building, whose coils cool the air.
• The warmed water returns to the chiller to be re-cooled — a closed loop.
• The heat the chiller absorbs is rejected outside — either straight to the air (air-cooled) or via a cooling tower (water-cooled).

So the chiller makes cold water; the AHUs make cold air from it. This "central plant + water distribution" is what makes cooling entire large buildings practical.

Air-cooled vs water-cooled

The biggest chiller decision:

Rule of thumb: water-cooled for large plants where efficiency dominates the life-cycle cost; air-cooled for smaller loads or where water is scarce or a cooling tower is impractical.

Compressor types

Chillers are classified by their compressor, which suits different sizes:

• Scroll — smaller chillers; multiple scrolls for staging.
• Screw — mid-range; robust, good part-load with variable-speed.
• Centrifugal — large chillers (hundreds to thousands of TR); very efficient at scale, especially with magnetic-bearing/oil-free and variable-speed drives.
• Absorption — driven by heat (waste heat, gas or solar) instead of much electricity; niche, used where cheap heat is available.

Reading chiller efficiency

How chillers are compared — worth understanding when specifying:

• kW/TR — kilowatts of electricity per ton of refrigeration of cooling. Lower is better (a good water-cooled centrifugal chiller can be well under 0.6 kW/TR; air-cooled is higher).
• COP / EER — coefficient of performance; higher is better.
• IPLV / NPLV — Integrated Part-Load Value — a weighted efficiency across typical part-load operation, which matters because chillers rarely run at full load. Often more meaningful than the full-load figure.
• ECBC (BEE) sets minimum chiller efficiency requirements for commercial buildings — see efficiency.

Because a chiller plant is a building's biggest energy user, small efficiency gains compound into large savings — which is why variable-speed drives and good controls matter so much.

The one-line answer

A chiller is a large refrigeration machine that makes chilled water rather than cooling air directly: it runs the refrigeration cycle to cool water to ~6–7°C, which pumps circulate to AHUs and fan-coil units all over a building to cool the air locally — the heart of a central plant. The heat it absorbs is rejected either straight to air (air-cooled — lower cost, no water, less efficient) or via a cooling tower (water-cooled — more efficient, uses water, best for large loads). Chillers are classified by compressor — scroll (small), screw (mid), centrifugal (large, most efficient) and absorption (heat-driven) — and compared by efficiency: kW/TR (lower is better), COP/EER (higher is better) and IPLV (part-load efficiency, often the most meaningful). Because it's a building's biggest energy user, chiller efficiency and controls dominate running cost — all specified, sized and commissioned by qualified MEP/HVAC engineers.

Where to go next

• The system it serves: Central Air Conditioning Guide.
• What it feeds: Air Handling Unit (AHU) Guide.
• The equipment overview: HVAC Equipment Guide.
• The efficiency angle: HVAC Energy Efficiency Guide.

References

• Bureau of Energy Efficiency (BEE) — Energy Conservation Building Code (ECBC), minimum chiller efficiency: https://beeindia.gov.in/
• ASHRAE Standard 90.1 (energy standard, chiller efficiency & IPLV); ISHRAE — chiller plant design guidance.
• National Building Code of India, SP 7 (Part 8 — Building Services; verify current edition), Bureau of Indian Standards.

Chiller selection, sizing, installation, commissioning and servicing are qualified MEP/HVAC engineering work. Verify any standard's current status via the BIS catalogue before relying on it.