Indirect cooling technologies
Cooling tower/coil systems
Another common IEC technology is the cooling tower/coil system combination. In this configuration, a cooling tower or other evaporative cooler is combined with a water-to-air heat exchanger coil. This system can have significant benefits because the cooling tower may be located remotely from the cooling coil itself. Cooling tower systems can be ideal for retrofit applications where space may be an issue. Another benefit to this type of IEC is that multiple air-handling units may be piped in various arrangements to a single cooling tower in one centralized location (see Figure 5).
In this method, a cooling tower is used to produce cooled water through an evaporative cooling process. The cooled water is then piped to an air-handling unit (AHU) cooling coil for cooling of the primary airstream. This configuration is a form of IEC by which the primary airstream does not come into direct contact with the water. Typically, this cooled water is used in a precooling coil application where additional downstream cooling methods are required for the design. The introduction of IEC for this type of system can increase capacity and reduce the electrical demand of a DX air conditioner or chiller.
The cooling tower can be selected in a wide variety of configurations to suit the design. Two basic types of evaporative cooling devices can be used: open cooling towers and closed-circuit cooling towers. The open cooling tower is a direct-contact device, which exposes water to the cooling atmosphere. The closed-circuit cooling tower contains an integral closed-circuit heat exchanger with two separate fluid circuits: one exposed to the atmosphere and a primary circuit used to transfer heat from the air-handling unit cooling coil.
Open cooling tower loops tend to collect airborne dirt and impurities that can clog cooling coils and are not recommended for indirect applications. If an open cooling tower system is used, it is recommended that the open and closed loops be separated by a plate and frame heat exchanger. Figure 5 shows a cooling tower installation.
Cooling tower/coil systems vary in size and complexity. One of the more attractive locations for IEC systems is in Las Vegas, mainly due to a considerably low design mean coincident wet-bulb temperature of 66 F. According to local measured bin weather data, the climate is well suited for IEC precool applications for more than 3,200 hours out of the year. In Las Vegas, it is common for casino properties to allow environmental tobacco smoke (ETS) on their gaming floors. The preferred HVAC industry design for spaces with ETS is to exhaust 100% and provide 100% OA as makeup. This consumes a large amount of energy in a climate with a design dry-bulb temperature of 108 F. An IEC cooling tower/coil system installed in a casino property with high OA quantities can offer significant energy savings, reducing plant tonnage and achieving shortened payback periods.
For example, a 50,000-sq-ft casino floor with ETS requires 200,000 cfm of 100% OA based on 4 cfm/sq ft. For a casino operating 24/7, this results in approximately 1,891,000 kWh of electricity to condition the 100% OA using water-cooled centrifugal chillers in a central plant. If a plate and frame heat exchanger is installed on the open-loop cooling tower condenser water system, and the primary side is piped to precool coils installed in each AHU, the energy savings can be evaluated.
The precool coil design reduces the entering 108 F dry-bulb air down to 80 F prior to the chilled water cooling coil, reducing the tonnage on the central plant chillers by approximately 460 tons. Throughout the year, the precool coil operates for approximately 3,200 hours where the ambient air temperature is above 75 to 79 F dry-bulb. The mean coincident wet-bulb temperature is as low as 52 F at 75 to 79 F dry-bulb ambient. For this example it can be demonstrated that the IEC cooling tower/coil system saves approximately 253,000 kWh per year in chiller operation. This equates to approximately $25,000 in annual electricity cost savings versus operating the chillers for the entire load and reduces the mechanical refrigeration load by an average of 130 tons. It should be noted that there is increased water consumption of approximately 780,000 gal per year, which reduces the annual operational cost savings by $1,600 based on the cost of potable water for that region. However, more properties are implementing water saving strategies such as rainwater harvesting and storage to offset cooling tower water consumption. There are several examples of similar IEC cooling tower/ coil systems currently in operation on the Las Vegas Strip.
Standards and guidelines
Several considerations, guidelines, and codes apply to evaporative cooling and associated systems. The most important are operational guidelines intended to minimize Legionella contamination of the makeup water system, such as ASHRAE Guideline 12-2000 – Minimizing the Risk of Legionellosis Associated with Building Water Systems.
Legionella, the bacteria responsible for Legionnaire’s disease, markedly thrives in aquatic environments at temperatures of 77 to 108 F and at conditions containing scale and sediment, biofilms, and stagnation, all or some of which can be present in evaporative cooling systems. ASHRAE 12-2000 includes separate recommendations for direct and indirect evaporative coolers. Indirect evaporative cooling with fluid coolers and cooling towers typically operate at water temperatures ranging of 85 to 95 F, within the range of accelerated Legionellosis growth, and consequently are very likely to contain detectable levels of the bacteria. It is recommended that a water treatment program be provided for this equipment that minimizes the conditions listed above.
An effective water treatment program might include scaling and corrosion inhibitors as well as oxidizing or nonoxidizing biocides approved by the local authority’s environmental regulatory agency (the U.S. Environmental Protection Agency lists these for each state). The selection of these different agents requires knowledge of water chemistry, water microbiology, and characteristics specific to the cooling application; therefore, it is recommended that a qualified water treatment specialist be involved and his or her services used to oversee the treatment. Additionally, quarterly equipment cleaning to reduce the buildup of sediment and debris is recommended to control microbial growth, as well as system draining during shutdown periods to prevent stagnant water conditions.
DEC units using wetted evaporative media operate at water temperatures close to ambient wet-bulb temperature (typically well below 77 F), presenting less risk for accelerated growth of bacteria. It is recommended that the recirculation water be continually bled off or periodically purged at a rate dependent on water quality and airborne contaminant levels to minimize scale and nutrient buildup. Figure 6 is an example of scale buildup on evaporative cooling media that reinforces the importance of water treatment and scale inhibitors. Daily drying of the evaporative media should be applied with dry fan operation (without the recirculating water pump) when possible.
Chemical-free scale inhibitors (electromagnetic) and biocide (electromagnetic as well as ozone-generation) water treatment solutions exist; however, they do not eliminate the need for periodic blow-down nor are they specifically required per the guidelines. Counterintuitively, very pure water from reverse osmosis, deionization processing, or otherwise is not recommended for use with evaporative coolers because of its corrosive nature and decreased ability to wet most types of evaporative media. Given that liquid-to-air heat exchanger water is typically chemically treated or at risk of bacterial contamination, the supply of makeup water (typically potable water) to the IEC unit must be installed in accordance with the International Plumbing Code (IPC) to prevent backflow contamination. Additionally, drain lines and overflow lines required for IEC unit blow-down and excess water must be indirectly connected to an approved disposal location, such as the sanitary sewer system, and not discharged to grade or to a storm sewer.
Related to code restrictions, a potential benefit of IEC units is that they are not directly affected by the various regulations regarding OA intake proximity to building exhaust discharge. As a DEC cooler generally operates by cooling and introducing 100% OA to the building, its intake opening location must be located at least 10 ft horizontally from hazardous or noxious contaminant sources such as vents, streets, alleys, parking lots, and loading docks per the International Mechanical Code (IMC). The effect of contaminated secondary air contact with the IEC heat exchanger should still be considered. Cooling tower discharge itself is considered a contaminated exhaust source and cannot be located proximate to OA intakes.
Is this the right option?
Indirect evaporative cooling is a modern space conditioning approach using a simple form of cooling. It reduces or eliminates the inherent drawbacks of direct evaporative cooling without significantly reducing the well-known energy savings compared to conventional refrigerant-based cooling systems. Where standards, local restrictions, and applicable guidelines do not preclude its use, the potential to supplement or replace refrigerant-based cooling systems in a growing number of applications can translate to considerable cost savings and should be evaluated when designing HVAC cooling systems.
While not specifically discussed in this article, passive forms of indirect evaporative cooling, such as chilled beams, can benefit projects based on similar principles and are worth further discussion and study.
Stephen A. Haines is a senior project engineer at jba consulting engineers. He has been designing indirect evaporative cooling systems in Nevada, Arizona, and California for more than 5 years on various types of gaming, hospitality, health care, and office projects.
James A. Freeman is a senior mechanical designer at jba consulting engineers. He has provided consulting services on large mixed-use hospitality and data center projects in the Southwestern United States and Asia that have incorporated indirect evaporative cooling systems.