by Michael Arny, PE, LEED AP, and Mary Reames, LEED AP O+M
Leonardo Academy, Madison, WI — This is the second part of our series of articles on how implementing LEED® for Existing Buildings: Operation and Maintenance reduces building operating costs. Improved cooling tower operation is one of these opportunities. The first article was on insurance discounts, The “Hidden” Benefits of LEED-EB Certification: Insurance Discounts
A Quick Overview of Cooling Tower Operation
Cooling towers are designed to remove heat from a system, such as an HVAC system, by cycling hot water through a tank of cooler water. The heat from the cycled water is transferred to the cooling tower water, reducing the temperature of the water being cycled through the tower. The cooled water returns to the HVAC system, and the heat that has been transferred to the cooling tower water is dispersed to the atmosphere by evaporation of the cooling tower water.
Cooling Tower Water Loss
Cooling towers often use potable water as makeup water. Cooling towers lose water, in a variety of ways and this results in both environmental and economic costs.
Cooling towers lose water is in four ways:
- Evaporation: This is the goal, as cooling occurs by evaporating some of the cooling tower water. Thus, evaporation is part of proper cooling tower function.
- Escape from the system as a mist, also called “drift”
- Overflow
- Blow-Down / Bleed-Off: As the water evaporates, the minerals in the water become more concentrated. When the concentration of minerals in the water gets too high, scale (mineral deposit) forms on the cooling tower. To avoid excessive scale, the mineral concentration in the cooling tower water is reduced by dumping some of the cooling tower water (called blow-down or bleed-off water) and replacing with fresh water (makeup water). This process keeps the amount of dissolved minerals and other impurities in the cooling tower water at an acceptable level.
Measuring the Mineral Concentration in the Cooling Tower Water
Mineral concentration in cooling tower water is measured using a conductivity meter. The measurement of mineral content is then used to determine the concentration ratio, which is the ratio of the level of dissolved minerals in the cooling tower water to the level found in the makeup water entering the cooling tower. This ratio is also called the cycles of concentration; the number of cycles is the number of times dissolved minerals in the water are concentrated compared with makeup water.
Actions to Increase Water Efficiency
There are several ways to operate cooling towers more efficiently, thus reducing the amount of potable makeup water that is needed.
- Meter water use: Both makeup water and blow-down water can be metered. By monitoring what you are putting and taking out of the cooling tower, you can identify and address excess water use.
- Control drift: Drift can be reduced by using appropriate shielding of the cooling tower to prevent water from being blown out of the cooling tower.
- Monitor mineral concentration in cooling tower water: Often, blow-down or bleed-off is set to occur on a regular schedule, whether it is needed or not. Sometimes blow-down or bleed-off are set to occur at a lower than needed cycles of concentration. One way to reduce water use and costs is to reduce the volume of blow-down, by setting controls so this happens only when needed. With the help of a conductivity meter to measure the concentration of minerals in the cooling tower water, the system can be set to replace water only when mineral concentrations get too high; this will keep mineral concentration in the desired range but will avoid unnecessary water changes. The conductivity meter should be set to maintain the highest concentration of minerals in the cooling tower water that is acceptable for cooling tower operation.
- Additional measures such as chemical treatment and filtration can be implemented to allow the cooling tower to be operated with increased cycles of concentration.
- Use non-potable makeup water: Potential sources of non-potable make up water include condensate from air conditioning systems, rain water, and fire system testing water.
Preventing Biological Fouling of Cooling Tower Water
Warmer water makes a good breeding ground for bacteria such as legionella, the bacteria that causes Legionnaire’s disease. Chemicals are added to the cooling tower water to prevent both biological fouling of the cooling tower water and the growth of biological pathogens.
How Efficiency of Cooling Tower Operation is Addressed in LEED-EB O+M
LEED for Existing Buildings awards points for efficient management of cooling towers. Under LEED-EB v2009 (available for use until October 31, 2016), one point may be earned by developing and implementing a water management plan for the cooling tower that addresses chemical treatment, bleed-off, biological control, and staff training as it relates to cooling tower maintenance. A point can also be earned if the cooling tower uses at least 50% non-potable water for makeup water, as demonstrated by metering both the potable and non-potable makeup water.
LEED-EB v4 (released in 2013 and mandatory after October 31, 2016) provides points based on the number of cooling tower cycles achieved without exceeding a set of predetermined filtration levels. Two points are earned for achieving up to 10 cycles before adding makeup water; three points are earned for achieving 10 or more cycles, or for achieving fewer cycles but using makeup water that contains at least 20% non-potable water.
Under both versions of LEED-EB, separately metering cooling tower makeup water can also contribute toward earning points.
By earning the points associated with cooling tower management and metering, building owners will be operating their cooling towers at peak efficiency. Monitoring the mineral levels in the water and only bleeding off when necessary will reduce water use while avoiding excessive buildup of scale within the tower. Additionally, regular metering of makeup water use will ensure that leaks or other problems will be discovered in a timely fashion and will also document makeup water reductions. Both of these management techniques can provide environmental and economic savings.
Recommended Approach
If you are ready to take action on reducing cooling tower water use for your building, start by installing and maintaining meters for makeup water and tracking the meter readings. Then, implement the cooling tower improvements you select, such as chemical controls or conductivity meters, and use the makeup water meter readings to document the water use reduction and calculate the resulting savings.