Editor's note: This is a follow-up to the May 2014 article titled "Understanding LEED v4 Certification." The article shows a table of water-efficiency credits and LEED v4's requirements to meet and obtain these credits. Credit 4 applies to cooling tower makeup water, and we want to expound on this topic for those who are unfamiliar with cooling tower operation and where the water is used. Water use is becoming a hot topic in many areas because of scarcity and drought conditions, and rising costs of water in the future will contribute to the need to conserve more water.
The HVAC cooling system has cooling towers that are usually well hidden and overlooked as a source of water use reduction. Before we discuss how water use can be reduced in a water cooling tower, let's look closer at how one operates.
How it works
A cooling tower reduces water temperature by evaporative cooling. The water is pumped to the top of the tower and distributed to a wet deck, or in some cases a distribution manifold with spray nozzles like those of a sprinkler. In both cases the water is distributed over fill (vertical or horizontal surfaces of tubes) and then evaporates as it drops by gravity to the basin below. The evaporation is promoted by fans that pump air up through the dropping water and fill to the atmosphere. As water evaporates, the remaining water cools. The amount of cooling depends on the geometric design of the tower, with taller towers resulting in more cooling than shorter ones.
Conventional technology uses water conductivity to control the time the water resides in the cooling tower. When the conductivity reaches a set limit, usually three times the conductivity of the incoming makeup water, a valve opens to blow down water (called "blowdown") until the lower fresh makeup water dilutes the conductivity to below the set limit by some allowable tolerance. This is done to prevent scaling of the surfaces inside the tower, heat exchangers and piping, which degrade the capacity of the cooling system.
A thousand-ton cooling tower will evaporate 26,000 gallons of water per day and blow down 13,300 gallons a day in a conventional operation. The blowdown, which costs $5 per thousand gallons, is wasted to the sewer and could cost just as much for disposal.
Hardness and controls matter
New technologies have been introduced to reduce the blowdown volume, since nothing can be done about the evaporation that is needed to achieve cooling. Since the conductivity limit is set due to scaling, a method of reducing or eliminating scaling is employed. New chemistries called HighCycle have been developed to increase the reuse of the water retained in the circulating system. HighCyle works on medium- to low-hardness makeup water.
High-hardness makeup water above 10 grains can be handled by softening the water and then using a corrosion inhibitor proven capable of controlling corrosion. ProChemTech International has a patent on this method, which also uses a multimedia filter to take out solids and eliminate blowdown completely. The only wasted water is from backwashing the filter and softener. That backwash volume is one-tenth (or less) of the blowdown volume.
Newer controls have been developed that use closed-loop feedback controls, where a tracer element introduced with the corrosion inhibitor can be detected and used to determine when chemical is needed. These controls can maintain corrosion-control chemistry very accurately and also furnish alarms to the building management and water management personnel to ensure that the system is functioning properly.
There are systems in the market that can take the data from the controls and send an email to the designated parties when an alarm condition occurs. These systems also can be used to adjust the control remotely and provide real-time status of the system parameters.
LEED and the savings
LEED v4 is giving credits for sustainable wastewater management of two credits, cooling tower makeup water of two credits and landscape water use reduction of one credit.
Higher cycles of water use in a cooling tower will reduce makeup water on a thousand-ton system by 13,000 gallons a day. Office personnel and employees in general generate 35 gallons per day of water use.
The cooling tower water use method that eliminates blowdown saves the equivalent of the amount of water used daily by 370 people. The cost savings at the above-estimated utility costs for water and sewage saves $3,900 in a typical month at full load. If your cooling tower is in an industrial application running 24/7 at the full load, then that is a water savings of $47,000 a year. The water savings is above 4,000,000 gallons a year in these conditions. On an HVAC system, the resulting savings is estimated at 1,700,000 gallons in the northeastern U.S., where winter and cooler temperatures prevail for half the year and nights are not full load.
There are economic and environmental gains to be achieved by addressing the water use in your cooling towers. In addition to the water savings, a good chemical inhibitor can keep your tower clean.
In addition, the use of liquid bromine as a biocide to control bio growth can improve the efficiency of the chilled water system by reducing pumping and electrical loads on your chiller. An additive for the chilled water and cooling tower circulating water has recently been developed that lowers the surface tension of the water and has shown to further improve heat transfer in these systems, from five to 15 percent.