Cooling Tower Best Practices: Chiller Energy Efficiency (Pt. 2)
Maximizing the energy efficiency of a cooling system requires evaluation of the entire cooling system (cooling tower, chiller, heat exchangers, etc.) and can often be significantly improved by maximizing the cleanliness of the heat transfer surfaces in the tower and cooling system heat exchangers.
- Locate Blowdown on “Hot” Water Side: Cooling water returning to the cooling tower from the building or process is typically about 10 degrees Fahrenheit warmer than the water supplied from the cooling tower basin. The purpose of the cooling tower is to transfer heat from a building or process to atmosphere, so it makes sense to ensure that the blowdown water is from the high temperature source. For example, a 500-ton cooling tower is designed to remove heat at a rate of 6 million BTU/hr. If the tower operates at 3 cycles of concentration, the blowdown flow rate will be about 7 gpm. Discharging 7 gpm of 80-degree water from the tower basin removes heat at a rate of 168,000 BTU/hr, while discharging 7 gpm of 90-degree water returning to the tower removes heat at a rate of 203,000 BTU/hr, reducing the cooling load on the tower itself. If a cooling tower is discharging blowdown from the cooling tower basin or cooling water supply, a simple piping change to discharge blowdown from the cooling water piping returning to the tower can provide a 1-2% improvement in the energy efficiency of the cooling system.
- Install a Sun Shade: Many cooling towers are plagued by algae growth on the top of the tower, which inhibits proper water distribution and flow over the cooling media. This, in turn, reduces the efficiency of the tower and the overall cooling system. This problem can often be alleviated by installing a sun shade or covering over the tower decks—preventing sunlight from reaching the cooling tower decks and inhibiting or preventing algae growth.
- Utilize Side Stream Filtration: Cooling towers pull air through the falling water flow, inducing evaporation and providing the cooling for the building or process, but this also means that cooling towers often scrub dust, pollen, or other particles from the air. These particles increase the suspended solids in the cooling water, and can cause deposits on the cooling tower fill media or heat exchanger tubes and decrease the cooling system’s energy efficiency. Installation of a side-stream filtration system will reduce the suspended solids in the system, resulting in cleaner heat transfer surfaces and improved chiller energy efficiency.
- Evaluate Your Water Treatment Program: The cooling tower should be periodically inspected to ensure the tower fill media and heat transfer surfaces are free from scale, biological growth, corrosion, and particulate deposits. Accumulation of these foulants on the tower will inhibit the cooling efficiency of the tower, and can reduce energy efficiency of the overall cooling system by 5% or more. Include a visual inspection of the tower on your maintenance log, and if your water treatment vendor is unable to effectively control these issues, consider alternative treatment options.