Many Canadian buildings need cooling in winter. Interior office zones, data rooms, laboratories, hospitals and process loads produce heat year round, and the chillers that serve them often keep running while snow falls outside. A waterside economizer takes advantage of cold outdoor conditions to produce chilled water directly, using cooling towers or dry coolers and a heat exchanger instead of compressors.
Canada's climate makes waterside free cooling especially attractive. Many locations have long periods with wet-bulb temperatures well below what is needed to produce chilled water. This guide explains how waterside economizers work, how to arrange them and the design issues specific to cold climates.
How a waterside economizer works
In cold weather, a cooling tower can produce condenser water colder than the chilled water return temperature. A plate-and-frame heat exchanger transfers heat from the chilled water loop to that cold condenser water. When conditions allow, the heat exchanger carries the full load and the chillers shut off. In partial economizer mode, it pre-cools return water before it reaches the chillers.
Alternative arrangements include:
- Closed-circuit fluid coolers or dry coolers with glycol, connected through a heat exchanger or directly to a glycol chilled water loop.
- Air-cooled chillers with integral free cooling coils, which pass returning glycol through an outdoor coil ahead of the chiller evaporator.
- Strainer cycle or direct tower water use, where tower water passes directly into the chilled water system. This is generally discouraged because it contaminates the closed loop.
When free cooling is available
Free cooling capacity depends on the chilled water temperature the building needs and the approach temperatures of the tower and heat exchanger.
Roughly, the tower must produce water colder than the required chilled water supply temperature by the heat exchanger approach, often about 1 to 2 K (2 to 4 F). The tower in turn produces water warmer than the outdoor wet-bulb by its approach, which widens at low wet-bulb and part load.
| Chilled water supply needed | Approximate wet-bulb needed for full free cooling |
|---|---|
| 7 C (45 F) | Around or below freezing |
| 10 C (50 F) | A few degrees above freezing |
| 13 C (55 F) | Cool spring and fall weather |
| 18 C (64 F) or higher (some data centres) | Mild weather; many more hours per year |
These are indicative only; actual performance depends on equipment selection, load and controls. The key lesson is that warmer chilled water dramatically increases free cooling hours. Designing air handlers, fan coils or chilled beams to work with higher chilled water temperatures in winter, and resetting supply temperature upward when loads allow, is often the biggest lever. Data centres using higher chilled water temperatures, as permitted by the ASHRAE thermal guidelines, can achieve free cooling for a large part of the year in most of Canada. See free cooling and economizers for data centers in Canada.
Integrated vs non-integrated arrangements
Integrated (series) economizer
The heat exchanger is piped in series with the chillers on the chilled water return, upstream of the chillers. Return water passes through the heat exchanger first, then the chillers trim the remaining load. This allows partial free cooling across a wide range of conditions and captures many more hours of savings.
Non-integrated (parallel) economizer
The heat exchanger is piped in parallel with the chillers. The plant operates either in economizer mode or chiller mode, not both. This is simpler, but loses partial economizer hours, and transitions between modes can be difficult because chillers may struggle to start with very cold condenser water.
Energy codes generally favour integrated economizers. NECB includes economizer requirements for cooling systems, with water economizers recognized as one compliance approach under defined conditions. Consult the applicable code edition and provincial amendments.
Design considerations for Canadian climates
Freeze protection
In free cooling mode, condenser water is intentionally cooled to near freezing in sub-zero weather. Strategies include:
- Controlling tower leaving temperature to a minimum, typically a few degrees above freezing.
- Using indoor remote sumps and heated basins.
- Maintaining full flow through operating tower cells to prevent icing.
- Using a closed-circuit fluid cooler or dry cooler with glycol, isolated by a heat exchanger.
See cooling tower selection and winter operation and glycol systems.
Heat exchanger selection
Plate-and-frame heat exchangers can achieve close approaches economically. Size for the required winter load at the design free cooling condition, and allow for fouling on the condenser water side. Strainers or side-stream filtration on the condenser side protect the narrow plate channels.
Chiller start-up and transitions
When the economizer can no longer carry the load, chillers must start with cold condenser water. Most chillers need a minimum entering condenser water temperature or a minimum pressure differential at start-up. Condenser water bypass valves or head pressure control help. In integrated arrangements this is handled more smoothly because the heat exchanger shares the load during transition.
Pumping
The heat exchanger adds pressure drop to both loops. Pumps must be selected for economizer mode as well as chiller mode, and bypass valves around the heat exchanger reduce pump energy in summer.
Controls
A typical integrated economizer sequence:
- Enable economizer mode when outdoor wet-bulb (or tower supply temperature) is low enough to provide useful pre-cooling, based on a calculated approach.
- Start condenser pumps and towers to produce cold water, with tower leaving temperature limited to prevent icing.
- Open the heat exchanger valves and route chilled water return through it.
- Stage chillers down as the economizer carries more load.
- When the economizer alone holds chilled water supply setpoint, shut chillers off.
- Revert to chiller operation when the economizer can no longer contribute, with smooth transition logic.
Water use
Free cooling with open towers still evaporates water, although at a reduced rate in cold weather. Dry coolers use no water but require larger surfaces and glycol. For data centres tracking water usage effectiveness, this trade-off can matter.
Frequently asked questions
Is an airside economizer better than a waterside economizer?
Airside economizers avoid chillers and towers entirely but bring large volumes of cold, dry outdoor air into the building, which can create humidity and freeze issues. Waterside economizers suit buildings with central chilled water and limited outdoor air paths. See economizers: types, controls and common failures.
Can an existing chiller plant add a waterside economizer?
Often yes, if space is available for a heat exchanger and towers can be operated in winter. The analysis should include winter cooling loads and achievable chilled water temperatures.
Key takeaways
- Waterside economizers produce chilled water from cold condenser water through a heat exchanger.
- Canada's long cold seasons provide many free cooling hours, especially with warmer chilled water.
- Integrated (series) arrangements capture partial free cooling and ease chiller transitions.
- Freeze protection, heat exchanger fouling and chiller start-up need careful design.
- Free cooling potential is best quantified with energy modeling using local weather data.
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