A chilled‐water system is a common HVAC (heating, ventilation, and air‑conditioning) arrangement in medium‑to‑large buildings (offices, hospitals, campuses) where cooling is provided by circulating cold water through coils or heat exchangers. Rather than each fan coil or air‑handling unit having its own refrigerant circuit, a central chiller plant produces chilled water which is then piped to distributed terminals.

1. Major Components

• Chiller(s): Produce chilled water (typically 4–7 °C supply) by removing heat from the water via a vapor‑compression or absorption cycle.

• Pumps: • Primary pump(s): circulate water through chillers;
  • Secondary (and tertiary) pump(s): distribute chilled water to building loads and return it to the chiller.

• Piping network: Supply and return mains connect the chiller to terminal units; often arranged in looped (ring) or riser configurations to balance flow and provide redundancy.

• Terminal units: Fan coil units, air‑handling units (AHUs), or induction units—each contains a chilled‑water coil through which air is blown to be cooled/dehumidified.

• Cooling tower (if water‑cooled chiller): Rejects heat extracted by the chiller’s condenser to the atmosphere; includes tower, condenser‑water pumps, basin, and drift eliminators.

• Controls & instrumentation: Temperature sensors, flow switches, variable‑frequency drives (VFDs) on pumps, building‑automation system (BAS) logic for sequencing, fault detection, and energy optimization.

2. Types of Plant Arrangements

• Single‐Pumping (Direct): One pump loop serves both chiller and distribution; simpler but less flexible under partial load.

• Primary‐Secondary: Separate “primary” loop through chillers and “secondary” loop to the building, hydraulically decoupled—allows variable flow on the secondary side without affecting chiller flow stability.

• Variable‐Primary‐Flow: Eliminates the secondary loop; uses VFD‑driven pumps to adjust flow directly through chillers based on load—more complex controls but higher efficiency at light loads.

3. System Operation

• Chilled‑Water Generation: Chiller absorbs heat from the process water and rejects it at the condenser.

• Distribution: Primary pumps push chilled water into the supply header. Secondary pumps draw from that header and send it out to coils.

• Air‑side Heat Exchange: Building air passes over chilled‑water coils; heat is transferred into the water, which warms slightly (e.g. returns at 12 °C).

• Return Loop: Warmed return water flows back to the chiller’s evaporator inlet to be re‑cooled.

Temperature and flow resets, based on outdoor temperature or space‑temperature resets, can optimize energy use: raising supply temperature when load is light reduces chiller lift and pump power.

4. Design Considerations

Cooling Load:

The cooling load refers to the total amount of heat energy (usually in BTUs or watts) that needs to be removed from a space or process to achieve the desired cooling effect. This could be based on factors like:

Internal heat gains (from equipment, lighting, people, etc.)

External heat gains (solar heat, ambient temperature)

The desired temperature differential (how much cooler the system needs to be than the surroundings)

ΔT (Delta‑T): Typical design ΔT (supply‑to‑return) is 5–7 K; affects flow rate and pipe sizing.

 

The flow rate (m˙) required for a given cooling load can be calculated using the formula:

m˙ = Cooling Load (W) / Specific Heat Capacity of Fluid (J/kg\K) ×Temperature Difference (K)​

Cooling Load is the heat to be removed (in watts).

Specific Heat Capacity of Fluid is a property of the cooling fluid (for water, it's about 4.18 J/g·K).

Temperature Difference is the difference between the temperature of the incoming and outgoing fluid.

Flow rates: Determined by cooling load:

m˙=Q / cp ΔT ​

where Q = load (W), cp​ ≈ 4.18 kJ/kg·K, ΔT = temperature difference.

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Example: Let’s say you have a cooling load of 10,000 W (which is the amount of heat that needs to be removed), and you are using water as the coolant. The temperature differential (ΔT) across the system is 5°C (or 5 K). The flow rate required would be:

m˙=10,000 / 4.18 × 5 = 478.4 kg/hr

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• Piping: Insulated to prevent heat gain and condensation; designed for pressure drop and pump horsepower.

• Controls strategy: Staging of multiple chillers, pump sequencing, supply‑temperature reset, chilled‑water differential‑pressure control.

• Redundancy & safety: Emergency isolation valves, bypass lines to maintain minimum flow through chillers, low‑pressure cut­outs.

5. Advantages & Limitations

AdvantagesLimitations

Centralized, easier maintenance & controlHigher first cost (chiller plant, pumps, piping)

Potential for high efficiency (especially with variable flow)Distributed coils need careful balancing

Scalable: add more chillers/pumps as load growsRequires skilled operators & good controls

Flexibility: supports many different terminal types and zoning patternsLarge equipment footprint and noise considerations

6. Typical Applications

• Large commercial towers or campus installations

• District cooling systems (one plant serving multiple buildings)

• Data centers (supporting precision‑air cooling units)

• Hospitals and labs (centralized chilled‑water for process loads and comfort)

7. Energy‑Saving Strategies

• Free cooling / economizer: Use cooling tower water or outside air to cool the loop when outdoor conditions permit.

• Thermal storage: Ice or chilled‑water storage tanks shift cooling load to off‑peak hours.

• Heat recovery chillers: Capture waste heat from chiller condenser for space‑heating or DHW.

• Variable‑speed drives: On pumps and condenser fans to modulate based on real‑time demand.

By understanding these elements—generation, distribution, terminal exchange, controls and design—you can size, specify, operate, and optimize a chilled‑water system that best fits your building’s cooling requirements.