Cooling Tower Best Practices: Improve Energy Efficiency

Michael Crocker, Director of Engineering at CQM Systems

By Michael Crocker, Director of Engineering at CQM Systems

Cooling towers provide cooling for air-conditioning, manufacturing processes, or power generation by utilizing the evaporation of water to transfer heat from the process or building to the atmosphere.  Therefore, cooling towers consume significant amounts of water and the cooling system is often a building’s largest energy consumer during the cooling season.   The following best practices and suggestions will help ensure that you are maximizing energy efficiency in your cooling system.

Overview
In order to understand how to optimize cooling tower efficiency, one must understand the basics of cooling tower operation.    The equation for the water balance on a cooling tower is below, as well as definitions for the basic terms utilized in cooling tower discussions:

Makeup = Evaporation + Blowdown + Drift

Makeup:In order to maintain the cooling system in proper working order, all water leaving the system must be replaced.This incoming water is referred to as makeup.Evaporation:Water that evaporates in the cooling tower and is rejected to atmosphere as vapor.Evaporation provides the necessary cooling for the system.Blowdown:When water evaporates from the system, the dissolved solids and minerals are left behind.Blowdown is water that is purposefully drained from the system to remove dissolved or suspended solids and prevent scale and corrosion problems in the cooling system.Drift:A very small amount of water will escape the cooling tower as mist or water droplets in the air, and is referred to as drift.Compared to evaporation or blowdown, water loss due to drift is insignificant, but can be a factor at the highest levels of water conservation efforts.

Cooling Tower Energy Efficiency
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.If a cooling tower is discharging blowdown from the cooling tower basin or cooling water supply pipe, a simple piping change to discharge blowdown water from the cooling water piping returning hot water to the tower can provide a 1-2% improvement in the energy efficiency of the HVAC cooling system. Implementation Cost:  Low   Difficulty:  Low·        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.Implementation Cost:  Low   Difficulty:  Low·        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.Implementation Cost:  Low   Difficulty:  Low

Summary
These simple tips and tactics to ensure your cooling tower is operating efficiently can help you significantly improve the energy efficiency of your cooling system.   A 1000-ton cooling system that achieves 5% efficiency improvements can save over 90,000 kW-hrs and almost $10,000 each year, and the corresponding sustainability impacts will make you feel good about being green while saving some green!

Michael Crocker is the Director of Engineering at CQM Systems. He has provided operations engineering and project management expertise for industry leaders such as BASF, Michelin, and CH2M Hill. He earned his B.S. in Chemical Engineering from Clemson University and M.B.A. from Portland State University.  Learn more energy saving tactics at www.cqmsystems.com.