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ADM-APN014-EN (01/05): Selecting Cooling Towers

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ADM-APN014-EN (01/05): Selecting Cooling Towers engineers newsletter volume 34–1 ● providing insights for today’s hvac system designer selecting cooling towers for efficiency: Tower selection 101 Range or approach? The thermodynamic realm of cooling towers can be defined by just three temperatures … from the editor … When was the last time you revised It’s tempting to rely on ARI standard your specifications or selection rating conditions for flow rates and parameters for cooling towers? Or do temperature differences when you specify unique parameters for designing chilled water systems. cooling tower selection on every job? But as valuable as these benchmarks Some HVAC designers specify 3 gpm are for verifying performance, they are of cooling water per ton of chiller unlikely to reflect optimal conditions for capacity; others specify less. Still … the “hot” water entering the cooling the entire system … especially as others base their selections on tower, the “cold” water leaving the mechanical efficiencies improve and something other than flow, such as a tower, and the design ambient wet customer requirements change. condenser ∆T of 85/95 in humid bulb of the geographic region where climates or 80/90 in less sultry locales. The same caveat applies to current the tower will be used. rules of thumb, such as a 10°F ∆T 99/85/78 95/85/78 Approach is the temperature across the cooling tower or a 90/80/71 102/83/78 difference between what is being condenser water flow rate of 3 gpm/ produced and the “power source” that ton. Basing the design of the Are your tower selection guidelines creates the product. In the case of a condenser water loop on either of listed above? Do you know what each cooling tower, the “product” is cold these parameters may short-change number represents and why those water leaving the tower and ambient the performance potential of the particular values are significant? wet bulb is the driving force that system and overlook opportunities to creates the cold water. If a cooling reduce costs. tower produces 85°F cold water when In this issue, veteran applications the ambient wet bulb is 78°F, then the engineer, Don Eppelheimer, explores cooling tower approach is 7°F. the chiller–tower relationship by The effectiveness of a heat exchange demonstrating that a wider cooling process can be gauged by examining tower range not only delivers cost the approach temperature. For savings but may also improve example, a cooling coil that can the efficiency of the entire chilled produce 48°F leaving air with 42°F water system. entering water (an approach of 6°F) is more effective than a cooling coil that only can produce 50°F leaving air with the same 42°F entering water (an CTI STD-201. Just as the Air- approach of 8°F). The same will hold Conditioning and Refrigeration Institute (ARI) develops performance standards to true for cooling towers. For a given certify chillers, the Cooling Technology type of cooling tower, a closer (smaller) Institute (CTI) provides a certification program to validate the performance of cooling towers. Unlike chillers, however, there is no standard set of selection conditions for cooling towers. Towers that receive certification under CTI STD-201 will provide predictable performance within the operating limits illustrated above. For more information, visit the CTI web site at www.cti.org. © 2005 American Standard Inc. All rights reserved ●1 approach temperature indicates a more that increasing cooling tower range effective tower.1 Selecting a cooling Precepts of tower sizing. Four from 9.4°F to 14°F or more will reduce tower with a close approach will supply fundamental factors affect tower capital cost AND annual energy cost.2 size: heat load, range, approach, and the chiller condenser with cooler water ambient wet-bulb temperature. If three … but the capital cost and energy of these factors remain constant, then consumption of the tower will be changing the fourth factor will affect higher, too. tower size in this way: Opportunity to engineer • Tower size varies directly and linearly Still, the cooling tower isn’t the with the heat rejection load. Now, we come to the fun part of the most grievous energy consumer in a • Tower size varies inversely with design process … the opportunity to chilled water system. Different tower range. exercise a bit of engineering judgment. selections can afford opportunities • Tower size varies inversely with There is a thermodynamic price to pay to increase the overall efficiency of approach. when the cooling tower range is the system. • Tower size varies inversely with wet- increased. That penalty occurs at the bulb temperature. Mechanical efficiency refers to the chiller. We can pay that price now by fan power that’s required to circulate [From Cooling Tower Performance: Basic specifying a more efficient chiller, or ambient air over the cooling tower fill. Theory and Practice, a June 1986 paper we can pay it later by allowing the published by Marley Cooling Technologies and increased cooling tower range to Different types of cooling towers differ available online at http://www.marleyct.com/ in their mechanical efficiencies. pdf_forms/CTII-1.pdf] diminish chiller COP. The following example illustrates this concept. Experience leads us to the best thermal efficiency for cooling towers Alternative 1: Base design. A middle used in a particular market or the efficiency of the chilled water school in Tennessee requires a chilled geographic location. It’s quite likely that system is the cooling tower range. water system with 800 tons of cooling the same cold water temperature has capacity (Alternative 1 schematic). To been used to select cooling towers in Range is the temperature difference meet the specification, the engineer your area for years. However, approach between the hot and cold water at the has proposed an 800-ton centrifugal temperature only represents the tower. Increasing the range will reduce efficiency of the cooling tower’s the capital cost and energy cost of the evaporation process. It not only says tower; it also will reduce the capital little about the efficiency of the chilled cost and energy consumption of the 2 Tumin Chan echoes this sentiment in his water system, but the effect of tower condenser water system. However, Engineered Systems article, “A Chiller approach on chilled water system increasing the cooling tower range is Challenge.” You can find it at <http:// www.esmagazine.com/CDA/ArticleInformation/ efficiency also is limited. What drives only possible if the chiller is capable of features/BNP__Features__ Item/ producing warmer leaving condenser 0,2503,76249,00.html>. water. Selecting chillers for warmer leaving condenser water will increase 1 Note that effectiveness refers to the thermal chiller energy consumption and may efficiency of the cooling tower fill and the also increase the dollar-per-ton cost of Alternative 1: Base design evaporative process; do not confuse it with the the chiller. mechanical efficiency of the cooling tower fan. This begs the question: What cooling tower range results in the lowest capital cost for the chilled water system? Further, what cooling tower range results in the lowest annual energy cost for the chilled water system? This author firmly believes 2 ●Trane Engineers Newsletter volume 34–1 providing insights for today’s HVAC system designer chiller; the unit under consideration cooling tower and condenser water was selected at ARI conditions and is “Having your cake and eating it, pump exceeded the chiller’s additional the lowest cost centrifugal machine too.” In most cases larger ∆Ts and the power consumption. Ultimately, the associated lower flow rates will not only that complies with ASHRAE Standard save installation cost, but will usually projected energy consumption for the 90.1’s minimum efficiencies. Pressure save energy over the course of the year. entire chilled water system is 8 percent drops through the evaporator and This is especially true if a portion of the less than the base design: condenser do not exceed 25 ft. first cost savings is reinvested in more efficient chillers. With the same cost ANNUAL ENERGY USE cooling tower range 9.4°F 14°F chillers, at worst, the annual operating The engineer also proposed a two-cell, cost with the lower flows will be about centrifugal chiller 259,776 278,389 kWh cooling tower with two 20-hp, variable- equal to “standard” flows but still at a cooling tower 66,911 64,878 speed fan motors. The tower’s cross- lower first cost. condenser water pump 85,769 33,936 flow design was selected for its TOTAL CONSUMPTION 412,456 377,203 kWh [From CoolingTools Chilled Water Plant Design reliability, ease of maintenance, and Guide, Pacific Gas and Electric (PG&E), <http:// low height. The tower selection is www.hvacexchange.com/cooltools/>] Alternative 3: Wider range, based on a range of 9.4°F and a flow optimized system. The school-district rate of 2400 gpm, which is provided by administration in our example was a 40-hp condenser water pump. With concerned about the capital costs of the help of energy modeling software, the condenser water piping wasn’t their buildings and equipment, but the engineer estimates annual energy resized: even more attentive to energy/ consumption as follows: PRESSURE DROPS condenser water flow 2400 gpm 1600 gpm operation and maintenance costs—the ANNUAL ENERGY USE condenser 26.41 ft 12.34 ft total cost of ownership. cooling tower range 9.4°F cooling tower 12.23 ft 12.16 ft centrifugal chiller 259,776 kWh condenser piping 11.56 ft 5.32 ft Since available space for the cooling cooling tower 66,911 condenser water pump 85,769 tower wasn’t a selection issue, the Reselecting the centrifugal chiller design team adopted a different tack.
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