Water efficiency guide for laboratories Part 1. Cooling towers and process equipment Editor’s note: This three-part article is one of a series of Best Practice Guides for laboratories, produced by Laboratories for the 21st Century (“Labs 21”), a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy. Geared toward architects, engineers, and facility managers, these guides provide information about technologies and practices to use in designing, constructing, and operating safe, sustainable high-performance laboratories. For more information about these free resources, see: www.Labs21century.gov/ toolkit/bp_guide.htm. The Labs21 website also provides full information about the agency’s upcoming annual conference, to be held in San Antonio Oct. 17-19. The first article, below, covers cooling towers and lab process equipment. The second part, which will appear in June, discusses lab-specific best practices and reviews alternative water sources. The final installment will be published in July, summarizing design considerations and providing a complete reference and resource list. Most laboratory buildings in our country use significantly more water per ft2 than standard commercial buildings do, primarily to meet their larger cooling and process loads. This greater need also provides laboratories with more opportunities to make cost-effective improvements in water efficiency, especially with respect to the amount of water they use in cooling towers and for special process equipment. A laboratory’s water efficiency can also be improved by making a few changes in other types of equipment, such as water treatment and sterilizing systems, as described in this guide. And alternative sources of water can often be effectively integrated into a laboratory’s operations. Laboratory cooling towers Cooling towers, which are part of many laboratory buildings, might represent the largest single opportunity for greater water efficiency. This is because laboratories usually have very large comfort-cooling and process loads. Laboratories often use 100% outside air for ventilation; this makes their comfort cooling loads higher than those of most office buildings. Additional cooling is often needed for special equipment such as lasers and electron microscopes (see the section on laboratory equipment). In fact, 30 to 60% of all the water used in multipurpose laboratories is for cooling. U.S. Department of Energy Energy Efficiency and Renewable Energy Federal Energy Management Program www.eere.energy.gov Cooling towers use water in three ways: evaporation, drift, and bleed-off. Fig. 1 (below) illustrates water use in a typical cooling tower. Evaporation (E) is fixed and controlled by thermodynamics; about 2.4 gal/min (gpm) of cooling water is used for every 100 tons of cooling. Bleed-off (B) contains the concentrated, dissolved solids and other material left behind from evaporation. Drift (D) losses are typically a function of tower design. Most of today’s tower designs reduce drift to about 0.05 to 0.2%. Since the amounts are small and they contain dissolved solids, they are usually included in bleed-off. Make-up (M) water replaces water lost because of E, B, or D. Cooling tower water management The primary methods for managing water use in cooling towers are operational ones. For example, cooling towers can be investigated to see if there should be an increase in the concentration ratio (CR) or cycles of concentration of water in the tower. The CR is an indication of how many times water circulates in the tower before it is bled off and discharged. Increasing the recycle rate of the tower reduces the consumption of make-up water and results in greater water efficiency (New Mexico Office of the State Engineer 1999).   Fig. 1. Water use in a typical cooling tower. Source: New Mexico Office of the State Engineer 1999; reprinted with permission. Click to enlarge. Fig. 2. Incremental water savings resulting from increasing the CR in cooling towers. Click to enlarge. Fig. 3. A hybrid cooling tower. Source: EPRI and CEC 2002. Click to enlarge. Fig. 4. Schematic diagram of counter-current rinsing process. Source: New Mexico Office of the State Engineer 1999; reprinted with permission. Click to enlarge. Fig.2 (left) shows the effect of the CR on make-up water use. Note that increasing the CR from two to five yields almost 85% of the savings that can be obtained by increasing the cycles from two to 10. Increasing the cycling above six does not significantly reduce make-up water use, but it does increase the likelihood that deposits will form and cause fouling of the system (Puckorius 2002). Any of several different parameters can be used to estimate the water savings for a specific tower, as shown in the sample calculation (below). In addition to savings on water and sewer costs, savings also result from having to purchase fewer chemicals to treat the water. As the volume of incoming fresh water is reduced, so is the amount of chemicals needed. Table 1 (below) shows approximate savings on chemical usage resulting from increasing the CR in a 10,000-gpm system. Perhaps the best way to increase the cycles of concentration is through better monitoring and management of the water chemistry. The first step is to understand the quality of the incoming water and what the controlling parameter should be, such as hardness, silica, or total dissolved solids. There will be a relationship between these parameters and conductivity, based on the water chemistry specific to a site. This relationship can help to establish a conductivity set point. The conductivity controller opens a blow-down valve as needed to maintain the control parameter within acceptable limits.     Cooling tower concentration ratios and savings To calculate the concentration ratio (CR) and associated water savings: Since the CR represents the relationship between the concentration of dissolved solids in bleed-off (CB) to the concentration in make-up water (CM), it can be expressed as CR = CB/CM If a cooling tower is metered for bleed-off and make-up water, CR can be calculated as follows where M is the volume of make-up water and B is the volume of bleed (in gal) CR = M/B The amount of water saved by increasing the CR can be calculated as Vsaved = M1 * (CR2 - CR1)/CR1(CR2 - 1). Here, V is the total volume saved, M1 is the initial make-up water volume, CR1 is the initial concentration ratio, and CR2 is the desired or final concentration ratio. Conductivity and flow meters should be installed on make-up and bleed-off lines. Meters that display total water use and current flow rate provide useful information about the status of the tower and cooling system, so they should be checked regularly to quickly identify problems. For example, the conductivity of make-up water and bleed-off can be compared with the ratio of bleed-off flow to make-up flow. If both ratios are not about the same, the tower should be checked for leaks or other unwanted draw-offs. It is important to select a chemical treatment vendor carefully—one who understands that water efficiency is a high priority. Vendors should provide estimates of the quantities and costs of treatment chemicals, bleed-off water volumes, and expected CR. Criteria for selecting a vendor should include the estimated cost of treating 1,000 gal of make-up water and the highest recommended cycle of concentration for the water system.   Cooling tower chemical use and concentrations Cycles Makeup (gpm) Change(gpm) Chemical needed at 100 ppm (lb) Change (lb) 1.5 - 300 240 - 2 200 100 120 120 4 133 67 40 80 5 125 8 30 10 10 111 14 13.3 16.7 Source: GC3 Specialty Chemicals 2000. Service Document; www.gc3.com/srvccntr/cycles.htm. Table 1. Chemical savings resulting from increasing the concentration ratio of a cooling tower in a 10,000-gpm system. New construction and renovation projects are excellent opportunities to design for greater water efficiency. A plume abatement or hybrid tower is one design that can have an impact on water use, even if the primary reason for it is to reduce the visible plume emanating from large industrial towers. (A plume is the visible column of saturated air exiting a conventional cooling tower.) A smaller plume is desirable in many residential areas and in areas where visibility is important, such as near airport runways. Hybrid towers have both a wet and a dry cooling section (Fig. 3, above). The tower can be run in wet mode in the summer, when the plume is less problematic, at the highest efficiency. In winter, the tower can be run in either dry or wet/dry mode. When operating in this mode, the dry section warms the exit air stream to raise the temperature above the dew point of the surrounding air, reducing humidity and thus the size of the plume. Hybrid cooling tower performance depends on the location and environmental characteristics of the site. Energy and water costs also play a crucial role in the decision to use hybrid cooling towers, because making some of these towers more water-efficient could have a negative impact on energy efficiency. Laboratories for the 21st Century U.S. Environmental Protection Agency Office of Administration and Resources Management www.labs21century.gov Another option for new and retrofitted cooling tower designs is to pipe blow-down water to a storage tank. This water can then be reused for nonpotable needs, such as bathroom commodes or fire suppression systems. Facilities should exercise caution when using blow-down water, however, as it can be extremely high in dissolved solids as well as chemical by-products from the water treatment process. The quality of blow-down water should be checked to make sure that it will not clog, foul, or otherwise damage other systems. Special water-efficient features Special features of towers and water systems that promote water efficiency include side-stream filtration, sunlight covers, alternative water treatment systems, and automated chemical feed systems. Side-stream filtration systems cleanse the water with a rapid sand filter or high-efficiency cartridge filter. These systems increase water efficiency and use fewer chemicals because they draw water from the sump, filter out sediment, and return filtered water to the tower. Side-stream filtration is particularly helpful for systems that are subject to dusty atmospheric conditions. Sunlight covers can reduce the amount of sunlight (and thus heat) on a tower’s surface. They can also significantly reduce biological growth, such as algae. Alternative water treatment options, such as ozonation or ionization, can reduce water and chemical usage. Such systems can have an impact on energy costs, however, so managers should carefully consider their life-cycle cost. Automated chemical feeds should be installed on cooling tower systems larger than 100 tons. An automated feed system controls bleed-off by conductivity and adds chemicals according to the make-up water flow. Such systems minimize water and chemical use while optimizing the control of scale, corrosion, and biological growth (Vickers 2002). Laboratory process equipment: Cooling Three broad areas in which the water efficiency of a wide range of laboratory process equipment can be improved are cooling of equipment, rinsing, and flow control. These areas can be addressed individually or together to increase the water efficiency of most laboratories. Single-pass cooling typically consumes more water than any other cooling method in laboratories. In single-pass or once-through cooling systems, water is circulated once through a piece of equipment and then discharged to a sewer. Single-pass systems use approximately 403 more water than a cooling tower operating at five cycles of concentration to remove the same heat load. The equipment typically associated with single-pass cooling includes CAT scanners, degreasers, hydraulic equipment, condensers, air compressors, welding machines, vacuum pumps, ice machines, X-ray equipment, air conditioners, process chillers, electron microscopes, gas chromatographs, and mass spectrometers. Sometimes, research staff members order and install these and other types of equipment that require cooling without consulting facility management. The equipment is usually connected directly to a public water supply, and it drains to a sewer. The best way to combat the water waste associated with single-pass cooling is to use a process or cooling loop. This loop provides water at a preset temperature to cool researchers’ equipment. A small packaged chiller or central plant towers can reject the heat from these systems. Other efficient options include reusing single-pass discharge water for irrigation or initial rinses, or for recovering the heat from one process for use in another. Often, the equipment in this category is used only intermittently. So it can be quite difficult to determine how much of a laboratory’s total water use goes to process equipment. A water meter on the process loop can provide this kind of information. By separating laboratory water from domestic, irrigation, or other cooling water, facility managers can better monitor water quality and usage across the whole facility. The more complicated equipment used in today’s laboratories often requires tighter or more stable temperature control (or both) than a centralized system can provide. Small packaged chillers allow this control and reduce water usage. Such chiller systems consist of a compressor, condenser, evaporator, pump, and temperature controller in one small package. The packaged unit recirculates temperature-controlled fluid to a laboratory application to remove heat and maintain a constant temperature. The recirculating fluid picks up heat from the application and returns to the chiller to be cooled to a specified set point before circulating back to the application. Packaged chillers work in somewhat the same way that large comfort-load chillers do. Laboratory managers may want to compare the amount of energy used by different packaged chillers at both part and full loads, and select the most efficient one that meets their needs. Removing the chiller’s heat can be done by rejecting the heat to either air or water. If an air-cooled condenser is used, it is better to use a design that rejects heat to the outside air rather than to conditioned laboratory space. The second option would increase inside temperatures and the amount of energy needed for space conditioning. An alternative is to reject the heat to water (Krupnick 2000). In this case, the cooling water should be recirculated chilled water, or recirculated through a cooling tower. Using once-through cooling water for this purpose is not recommended. Rinsing equipment and flow control Rinsing equipment can often be made more efficient. A counter-current rinsing operation is typically the most efficient method (Fig. 4, above). In counter-current rinsing, the flow of rinse water is opposite to that of the workflow. The basic premise is to use the cleanest water only for the final or last stages of a rinse operation; water for early rinsing tasks, when the quality of the rinse water is not as important, is obtained later in the process. Other efficient rinsing options include batch processing, in which several pieces are cleaned at the same time, and using rinses from one process in another one. Flow control is another key efficiency strategy. Many pieces of lab equipment are “on” continuously, even when the process runs only a few hours per day or a few days per year. Often, the water flow to some of this equipment is only a few gpm. However, a continuous 1.5-gpm trickle flow through a small cooling unit adds up to 788,400 gal per year. Using a control or solenoid valve in these applications allows water to flow only when the unit is being used. Another option is to use shut-off valves or timers to turn equipment off after normal working hours and when a process is shut down for maintenance or other reasons. Stephanie Tanner at the National Renewable Energy Laboratory, Golden, Colo., was the principal author of this document. The author thanks Bill Hoffman, City of Austin water department, for information on rainwater harvesting and A/C condensate recovery, and James Kohl, URS Corp., for initial research. Roy Sieber of Eastern Research Group and Otto Van Geet, PE, Nancy Carslile, AIA, and Sheila Hayter, PE, all of NREL, provided helpful comments and peer reviews. Paula Pitchford and Susan Szepanski of NREL provided editing and the graphic design of the original Best Practices Guide.