Chilled water system design fundamentals

A chilled water system produces cold water at a central plant and distributes it to coils in air handlers, fan coils, chilled beams or process equipment. Chilled water is the dominant cooling approach in large commercial, institutional and data centre buildings because water carries heat efficiently over long distances and allows plant equipment to be centralized, maintained and optimized.

Good chilled water design comes down to a handful of decisions: supply temperature, temperature difference, pumping arrangement, piping layout, coil selection and controls. This guide explains each and how they interact.

Main components

• Chillers: produce chilled water using a refrigeration cycle. Types include centrifugal, screw, scroll and absorption, either water-cooled or air-cooled.
• Heat rejection: cooling towers, fluid coolers or air-cooled condensers.
• Pumps: circulate chilled water and condenser water.
• Distribution piping: supply and return mains, risers and branches.
• Terminal units: coils in air handlers, fan coils, chilled beams, CRAHs and process heat exchangers.
• Controls: valves, sensors, VFDs and the plant control system.
• Accessories: expansion tank, air separator, dirt separator, chemical treatment and makeup.

Supply temperature and delta T

Supply temperature

Conventional comfort systems often use a chilled water supply temperature around 6 to 7 C (42 to 45 F), cold enough for coils to dehumidify air to typical indoor humidity levels. Systems serving chilled beams, radiant panels or data centre cooling can use much warmer water, 12 to 18 C (54 to 64 F), because they handle sensible load only.

Warmer supply water improves chiller efficiency, since the compressor lift is lower, and extends free cooling hours. Colder water reduces flow and pipe size, and can support low-temperature air distribution. The choice is a trade-off that depends on dehumidification needs.

Delta T

The temperature difference between supply and return water (delta T) determines flow. For a given load, flow is inversely proportional to delta T:

Flow (L/s) = Load (kW) / (4.19 x delta T in K) for water. In imperial units, gpm = Btu/h / (500 x delta T in F).

Historical designs used a delta T of about 5.5 K (10 F). Many modern designs use 7 to 9 K (12 to 16 F) or more. A higher delta T means:

• Less flow, smaller pipes and pumps, lower pumping energy.
• Larger, deeper coils to achieve the higher water temperature rise.
• Potentially better chiller efficiency at higher return temperature, depending on the chiller.

Achieving the design delta T in operation matters as much as the design value. Many systems suffer from low delta T syndrome, where return water comes back too cold, flows rise and chiller capacity is wasted.

Pumping arrangements

Constant primary flow

Older systems often pumped constant flow through chillers and used three-way valves at coils. This wastes pump energy and is rarely used in new designs.

Primary-secondary

Constant flow primary pumps circulate water through the chillers, and variable flow secondary pumps distribute it to the building. A decoupler pipe hydraulically separates the loops.

Variable primary flow

Variable speed pumps circulate water directly through chillers and the distribution system, with a minimum flow bypass to protect chillers. This reduces pump count and energy but needs more careful controls.

See primary-secondary vs variable primary flow for a detailed comparison.

Piping design

1. Size for velocity and friction: keep velocities and friction rates within accepted limits to control noise, erosion and pump energy.
2. Use direct return for most variable flow systems: with two-way valves and pressure-independent valves, reverse return is usually unnecessary.
3. Provide isolation and balancing: isolation valves at equipment and branches, and balancing provisions sized for the design flow.
4. Insulate with a vapour barrier: chilled water piping and fittings must be fully insulated and sealed to prevent condensation.
5. Plan for air and dirt removal: air separators and dirt separators at the plant, and air vents at high points.
6. Allow for expansion and pressurization: expansion tanks sized for the system volume and temperature range.

Coil selection

Coils should be selected to match the system delta T. Key points:

• Select coils for the design entering and leaving water temperatures, not just the capacity.
• More rows and closer fin spacing increase water temperature rise but also air pressure drop.
• Check coil performance at part load; coils that are oversized or poorly selected lead to low delta T.
• Keep coil face velocities within limits to avoid moisture carryover, commonly about 2.5 m/s (500 fpm) or less for cooling coils.

Control valves

Two-way modulating valves are standard in variable flow systems. Valve sizing and authority affect whether coils achieve design delta T. Pressure-independent control valves maintain flow at each coil regardless of system pressure changes, improving control and reducing overflow.

Pump control

Variable speed distribution pumps are typically controlled to maintain a differential pressure setpoint at a remote point in the system. Resetting this setpoint based on valve positions reduces pump energy further.

Free cooling in Canadian climates

Canadian climates offer long periods when outdoor conditions can produce chilled water without running chillers. Options include:

• Waterside economizer: a plate heat exchanger between condenser water and chilled water.
• Dry coolers with glycol, common where towers are not practical in winter.
• Chillers with integral free cooling coils, common on air-cooled units.

Warmer chilled water supply temperatures extend free cooling hours considerably.

Plant controls and sequencing

Plant controls decide how many chillers, pumps and towers run, and at what setpoints. Good sequencing considers chiller efficiency at part load, condenser water temperature reset, chilled water temperature reset and the cost of pump and fan energy. See chiller plant sequencing and optimization.

Frequently asked questions

Is a higher delta T always better?

Generally, a higher design delta T reduces pumping and pipe costs, but it requires coils and controls that can actually achieve it. An unrealistic design delta T can lead to poor dehumidification or chillers that cannot be loaded fully.

Should chilled water systems use glycol?

Only where freezing is a risk, such as coils exposed to outdoor air, outdoor piping or dry coolers. Glycol reduces heat transfer and raises pumping energy, so many systems use a glycol loop only where needed, separated by a heat exchanger.

Key takeaways

• Supply temperature and delta T drive pipe sizes, pump energy and chiller efficiency.
• Modern designs favour higher delta T and variable flow with two-way valves.
• Coil selection and control valves determine whether design delta T is achieved in practice.
• Warmer chilled water extends free cooling in Canadian climates.
• Good controls and sequencing are essential to realize design efficiency. Proper water balancing verifies flows at commissioning.