Cooling towers reject heat from water-cooled chiller plants, process loads and heat pump loops by evaporating a small portion of the circulating water. Selecting the right tower means balancing thermal performance, energy, water use, noise, footprint and maintenance. In Canada there is an additional challenge: many towers have to keep running when outdoor temperatures are well below freezing.
This guide covers tower types, the main selection criteria, siting, and the freeze protection and control strategies that make winter operation reliable.
Tower types
| Type | Description | Typical strengths | Typical limitations |
|---|---|---|---|
| Induced draft crossflow | Fan on top pulls air horizontally through fill as water falls by gravity | Easy access to distribution basins, low pump head | Larger footprint; more prone to icing on air inlets |
| Induced draft counterflow | Fan on top pulls air upward against falling water | Compact, efficient heat transfer | Higher pump head; spray distribution less accessible |
| Forced draft (centrifugal or axial) | Fan at the base pushes air through | Can be ducted, suits indoor installation, lower sound with centrifugal fans | Higher fan energy; risk of recirculation |
| Closed-circuit fluid cooler | Process fluid runs inside a coil; spray water and air cool the coil | Keeps loop fluid clean and closed; can run dry in winter | Higher cost and weight; larger for same duty |
| Hybrid or adiabatic cooler | Dry coil with water spray or pre-cooling pads on hot days | Very low water use | Larger, and less effective at peak than evaporative towers |
Open towers are usually paired with a chiller condenser. Closed-circuit fluid coolers are common for water loop heat pump systems and for winter free cooling, because the loop fluid can contain glycol and is not exposed to air.
Selection criteria
Thermal performance
A tower is selected for a flow rate, an entering and leaving water temperature (the range) and a design wet-bulb. The difference between leaving water and wet-bulb is the approach. A closer approach requires a larger tower. Typical comfort cooling designs use approaches of about 3 to 4 K (5 to 7 F). Design wet-bulb data for Canadian locations is available in the NBC climatic data and ASHRAE handbooks.
Specify towers with thermal performance certified by the Cooling Technology Institute (CTI) under its certification standard, so capacity claims are independently verified.
Energy
Larger towers with more fill and lower fan power can produce the same water temperature with less fan energy. Variable speed fan drives are standard practice and are required by NECB for many applications. Oversizing tower capacity somewhat often pays back through lower fan energy and colder condenser water, which improves chiller efficiency.
Materials
- Galvanized steel: lowest cost, but vulnerable to white rust and aggressive water.
- Stainless steel: for basins and wetted parts in harsher conditions or longer life.
- Fibreglass-reinforced plastic (FRP) and engineered plastics: corrosion resistant, common in field-erected and some factory towers.
Match materials to water quality, the treatment program and expected life.
Sound
Tower fans and falling water generate noise. Low-sound fans, larger slower fans, attenuators and careful siting help where municipal noise bylaws or neighbours are a concern.
Maintenance access
Specify access doors, platforms, ladders with safety cages or fall protection, and easy access to fill, eliminators, nozzles and basin. Hard-to-service towers are poorly maintained towers.
Siting
- Avoid recirculation. Hot, humid discharge air must not be drawn back into the tower's inlet. Keep clear of walls, enclosures and other towers, following manufacturer clearance guidance.
- Keep away from air intakes. Drift can carry legionella. ASHRAE Standard 62.1 sets minimum separation distances between cooling towers and outdoor air intakes, and some jurisdictions require more. See outdoor air intake placement and design.
- Plan for plume. In cold weather, towers produce visible vapour plumes and can ice nearby surfaces, roofs and walkways.
- Check structure for operating weight with full basins.
- Consider pump suction. Towers must be high enough above condenser pumps to provide adequate net positive suction head.
Winter operation
Many Canadian plants need condenser water in winter for process loads, data rooms, interior zones or heat recovery. Others shut towers down and switch to a waterside economizer or dry cooler. Either way, freezing must be managed.
Freeze risks
- Water in the basin freezing when the tower is idle.
- Ice forming on air inlet louvres and fill when warm water meets very cold air at low heat load.
- Frozen makeup lines, drains, overflow lines and exposed piping.
- Ice damage to fans and fill from ice falling or building up.
Protection strategies
| Strategy | How it works |
|---|---|
| Indoor remote sump | Tower drains by gravity to a tank inside the heated building; tower basin stays dry when idle |
| Basin heaters | Electric or steam heaters keep basin water above freezing when idle |
| Heat tracing and insulation | Protects makeup, drain and exposed condenser piping |
| Bypass to basin | Diverts warm water directly into the basin at low load to keep water temperature up; should discharge into the basin, not bypass straight to pump suction |
| Fan speed control and cycling | Reduces airflow at low load to keep water warm |
| Fan reversal | Briefly reverses fans to melt ice on inlets, where the manufacturer allows |
| Cell isolation | Run fewer cells at full flow to keep good distribution and warmer water, draining idle cells |
| Minimum leaving water setpoint | Keeps leaving temperature above a limit that avoids icing and protects chillers |
Maintaining full design flow through operating cells is important in winter: low flow leaves dry areas in the fill where ice forms. Closed-circuit fluid coolers can run with spray water off and the coil filled with glycol, avoiding wet operation entirely in the coldest weather.
Free cooling transition
When the plant switches to waterside economizer mode, condenser water is cooled to temperatures near freezing to cool the chilled water loop through a plate heat exchanger. This requires careful control of tower leaving temperature to avoid ice. See waterside economizers and free cooling in Canada.
Frequently asked questions
Should I drain my cooling tower for winter?
If the tower is not needed in winter, draining the basin and exposed piping, isolating makeup and following the manufacturer's lay-up procedure is the simplest protection. Water treatment and legionella plans should address start-up after lay-up.
Do towers lose capacity in winter?
Capacity rises as wet-bulb falls, so towers usually have excess capacity in winter. The challenge is preventing overcooling and ice, not shortage of capacity.
Key takeaways
- Choose tower type based on duty, space, sound, water quality and winter operation needs.
- Specify CTI-certified performance and variable speed fans; modest oversizing often saves energy.
- Site towers to avoid recirculation and to keep drift away from air intakes.
- Winter protection relies on indoor sumps, basin heaters, heat tracing and controls that maintain flow and leaving temperature.
- Verify winter sequences during building commissioning, ideally with seasonal testing.
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