When it comes to HVAC equipment, when it’s working properly, you won’t notice it. While this is great when everything is running smoothly, it means equipment can be easily overlooked or forgotten. It takes something dramatic like full system shutdown or a surprise high energy bill to realize something isn’t right.
Cooling towers are no different. They can sit overlooked on top of the roof or behind the building, and while still technically doing their job, your energy bills may rise, water usage can increase, and you may struggle to fight peak summer heat. Understanding how cooling towers lose efficiency and what to look out for is key to preventing unnecessary operating costs before they catch you unawares.
How Cooling Towers Affect Energy Use
Cooling towers are heat-rejection systems – they remove heat from your building that is built up by equipment use and occupants. Hot water from HVAC chillers is sprayed over internal fill media (material that creates a large surface area to maximize heat transfer through evaporation to cool water) while air moves through the tower. As a portion of the water evaporates, heat is carried away and cool water is returned back to the chiller as the cycle continues.
As water is evaporating in the process, you may wonder if that leads to potential problems. But this is a normal and necessary part for the process to work. As a rule of thumb, about 1% of the circulating water flow evaporates for every 10°F of cooling achieved. The issue that cooling towers run into is everything that interferes with the heat transfer, not evaporation. When a cooling tower becomes less effective, the chiller must work harder to reject heat, which then correlates directly to increased electric bills.
Common Cooling Tower Energy Drains
1. Fouled or Scaled Fill Media
With a large surface area, the cooling tower fill media will collect minerals, dirt, and biological growth that coats or clogs the material over time (see #3 for additional information). Even mild scaling (less than 0.5 mm thick) can reduce cooling efficiency by 5–10%, and moderate fouling can reduce efficiency by 10–25%. Reduced efficiency forces chillers to operate at higher condenser temperatures, increasing energy use across the system. This is a problem that can easily be overlooked because the cooling tower will still be operating, and appear fine, yet won’t be cooling as well as it should.
2. Drift Loss and Aging Drift Eliminators
Drift occurs when small droplets of water unintentionally escape with the exhaust air stream. This poses problems as the droplets often carry the same chemicals, salts, and bacteria as the circulating cooling water. They can cause equipment corrosion, mineral deposits, and even release unhealthy particulates and bacteria as they evaporate. Drift eliminators are used to prevent these droplets from leaving the tower, reducing water loss, but older or damaged drift eliminators allow more water to leave the tower – up to .05% in older designs versus .002% in modern ones. Drift is not just a water issue; it can change water chemistry through increased makeup water, accelerate scaling, and reduce thermal efficiency.
3. Poor Blowdown Control
We’ve already mentioned that scale buildup occurs over time. Blowdown control is the calculated removal of a portion of circulating water to purge this scale. It replaces water with fresh makeup water and prevents scale buildup and corrosion. This is crucial to the efficiency and longevity of cooling towers. Poorly managed blowdown leads to higher dissolved solids, more scaling, and increased fan and chiller energy use.
4. Leaks and Overflow You Don’t See
Like other hidden issues, cooling towers may still appear to operate normally but could be experiencing leaking basins, stuck float valves, and cracked distribution piping. To fill in for the lost water, makeup water will be used, but this leads to additional problems. Continuous makeup water means extra pump runtime, altered water chemistry, and unnecessary energy consumption.
How to Prevent Energy Loss in Cooling Towers
Most energy losses begin in the spring, not the summer. In addition, winter shutdowns can leave behind scale, biological growth, damaged belts, worn bearings, or cracked spray nozzles. Once the summer heat arrives, there’s little room to recover lost efficiency without emergency repairs. Spring is the critical window to perform full inspections and maintenance tasks on cooling towers when corrections are cheaper and less disruptive.
Spring-time Inspection Checklist
- Inspect fill for scale, slime, or airflow blockage.
- Verify drift eliminator condition and alignment.
- Check spray nozzles for clogging or uneven distribution.
- Inspect basin for leaks, corrosion, or overflow at float valve.
- Measure cycles of concentration and verify proper blowdown rate.
- Inspect fan belts, bearings, and motor amperage for inefficiencies.
It’s not complete shutdowns on cooling towers you need to worry about; it’s the hidden issues that lead to slow efficiency loss over time, increasing energy, water, and maintenance costs. Understanding these losses and addressing issues early is the best way to protect your equipment and budget.