Closed-loop, evaporative cooling systems can save water

By Peter G. Demakos, Niagara Blower Co.

Power generation accounts for a significant amount of water use in the United States. With limitations on water availability and the importance of water quality, closed-loop, evaporative cooling systems can be a cost-effective technology for both heat transfer and water conservation. Closed-loop evaporative coolers (wet surface air coolers) are used in a variety of industries including simple and combined cycle power plants. Applications include auxiliary fluid loop cooling, direct steam condensing and gas turbine inlet air chilling.

Rejected Heat
The basic operating principle of a wet surface air cooler (WSAC) is that heat is rejected by means of latent (evaporative) heat transfer. The fluid/vapor that needs to be cooled or condensed flows through tube bundles as part of a closed-loop system. A large quantity of water (generally 7-10 gpm/ft2 per coil face area) from the unit basin is sprayed downward over the tube surface. Simultaneously, fans induce air over the bundles in a co-current direction. Evaporative cooling takes place at the exterior tube surface. The saturated air stream leaving the tube bundle then makes two 90 degree turns into the unit's fan plenum at a lower velocity, dropping almost all of the large water droplets back into the basin. The air is then discharged out of the unit through the fan stacks.

Keeping the process stream inside the tubes is important for many reasons:

  • It maintains thermal performance
  • It enables minimal (simple) maintenance
  • It ensures that open-loop spray water never contaminates the process stream, which:
    • Allows poor quality water to be used as makeup source.
    • Allows higher cycles of concentration.
    • Prevents process fluid exposure to the environment.

    The co-current flow of air and water allows for an unobstructed spray system that is accessible for observation and maintenance. Additionally, in cold environments, because the air passes over the spray system water before and during contact with the tube bundle, the mixed water temperature remains above freezing. This protects the tubes from freezing even when the ambient air temperature is below freezing. Co-current flow also ensures complete coverage of tube surfaces, which reduces any fouling and freezing potential.

    The WSAC is able to cool process fluids to within 5-10 degrees Fahrenheit of the wet bulb temperature (which will always be lower than the ambient dry bulb temperature.) For example, the WSAC can provide process outlet temperatures as cool as 90 F even on a 110 F (ambient) day.

    Alternative Technologies
    Two other system designs can accommodate heat transfer applications: an open tower with a heat exchanger and a dry, air-cooled system.

    An open tower configuration has two system loops – one open and one closed –. These require two heat-transfer devices to complete the duty. Open-loop water flows through a heat exchanger where heat is transferred from the process fluid through sensible heat transfer and then is pumped to the cooling tower where it is cooled via evaporation. This means there are two approach temperatures, a (sensible) approach in the heat exchanger and a second (latent) approach to the wet bulb in the cooling tower. This limits the system's practical ability to cool the process fluid to within 15-20 F of wet bulb.

    Also, because this configuration uses an open tower and a heat exchanger, the cycles of concentration in the cooling tower are limited based on makeup water quality and fouling potential in the heat exchanger(s). The WSAC directly cools the process fluid via the more effective latent heat transfer and does not require an additional heat transfer device to complete the heat removal service. It has only one approach to the wet bulb temperature (5-10 F) and requires less air (fan horsepower) to remove the total heat load. Typically, spray-pumping horsepower also is less, which means lower operating costs.

    In a dry (fin-fan) system, the closed-loop fluid is cooled directly via sensible heat transfer. Due to this form of heat transfer, a dry cooler's approach temperature is based on the dry bulb (ambient). Also, due to the inefficiencies of sensible cooling, the realistic approach temperature is between 20-25 F. The WSAC also directly cools the process fluid using a much more efficient form of (latent) heat transfer. This means that its approach temperature is based on the wet bulb (which takes into account the moisture in the air) that is always lower than the dry bulb. This is most significant in warmer climates.

    For example, on a 100 F (38 C) dry bulb, 75 F (24 C) wet bulb day, the process outlet temperature of a dry cooler will be approximately 125 F (52 C), where the WSAC can easily deliver 90 F (32 C). Due to the efficient heat transfer and lower air flow requirements, the WSAC footprint is much smaller – typically 25 percent of the plot area required – and much less horsepower – typically 60 percent less than required for a dry cooler. Unlike a dry cooler, which has a high potential for fouling and plugging in the closely-spaced fins, the WSAC uses all prime surface coils (no fins). Also, in fin-fan units, cold air passes directly over the tubes, which can lead to freezing inside the tubes during low ambient operation. With the WSAC, the re-circulating spray water is kept warm by the direct contact with the heat source that acts as a buffer between the tubes and cold air.

    WSAC Technology Summary
    The WSAC is an efficient and effective heat rejection technology for several reasons:

    • It utilizes latent cooling, which is far more efficient than sensible cooling. This means that a WSAC can cool the same heat load with a smaller footprint than all-dry systems.
    • Because it has single-source thermal responsibility, it provides the lowest process fluid outlet temperatures (as close as 5 F to wet bulb).
    • Because it is a closed-loop with no plastic "fill" or heat exchanger to become clogged or scaled, it can have an extremely low blowdown rate compared to that of a cooling tower.
    • Makeup water can come from almost any source (tower blowdown, R/O and demin blowdown, plant discharge, produced and so on).
    • It is subject to lower parasitic energy use (approximately 60 percent less than a dry cooler).
    • The WSAC systems require little maintenance. The spray system is accessible for inspection and maintenance without shutting down the unit or removing any obstructions such as tower fill. Because the cooler is closed-loop, maintenance and cleaning of heat exchangers is eliminated.

        General Specifications
        As previously discussed, the process fluid stays inside the closed-loop tube bundles. These tube bundles can be manufactured from almost any material based on the composition of the process stream (inside) and quality of spray water (outside). All bundles can be designed for high-pressure use per ASME and TEMA codes.

        There are two basic tube bundle types: serpentine and straight-through, cleanable. Serpentine bundles are less expensive, are fabricated with a continuous tube circuit and can be designed to accommodate pressures up to 2,500 psi. Cleanable bundles offer removable headers for complete internal access for inspection and cleaning in place while the balance of the WSAC remains in service. Additionally, the tube bundles can be re-tubed using the existing headers. Tube sheet thicknesses are designed to meet TEMA/ASME standards. This bundle style offers the lowest process side pressure drop.

        Tube material, diameter, wall thickness, length, depth and width can be optimized to provide the most cost-effective thermal performance for any application. Typical material choices include black or galvanized carbon steel, stainless steel, admiralty brass, duplex stainless steel alloy, titanium or other copper alloys.

        Many factors need to be considered when selecting material of construction for the basin. Metal basins are generally most economical for smaller units. They are generally constructed of prime carbon steel, hot dipped galvanized after fabrication. Many other material selections including stainless are also available.

        In many parts of the world, it is less expensive to construct the basin on-site out of concrete; this is especially advantageous with larger unit sizes.

        A third alternative in material choice is fiberglass reinforced plastic (FRP). This selection can be used for the fan plenum (center section) or the entire structure. A threeñfoot-high (approximate) "swimming pool" must be poured to enclose the basin water level. FRP is corrosion resistant and in some instances, can be less expensive than concrete.

        Direct-drive, mill-chemical, severe duty, totally enclosed air over (TEAO), self lubricating fan motors operate directly in the air stream for designs requiring five-foot-diameter or smaller fans. The fan blades for these smaller units are heavy duty, epoxy coated plastic with adjustable pitch. For units requiring larger fans, right angle gear drive designs with TEFC motors located outside the air stream are used in conjunction with fiberglass reinforced epoxy fan blades.

        The spray water distribution system for all WSAC uses a low-pressure and high-flow design (generally 7-10 gpm/ft2 of coil surface). The entire spray system is constructed of galvanized carbon steel for factory-assembled units and PVC material for field erected systems. To ensure reliable complete coverage, large orifice non-clogging nozzles are used.

        Power Plant Applications
        Because the process stream flows within a closed-loop, there is virtually no limit to what type of process stream (liquid, vapor, or gas) the WSAC can handle:

        • Simple cycle auxiliary loop cooling: The WSAC is used to cool the turbine oil loops, fireye and other auxiliary equipment. Advantages of separating this cooling loop are that it can be run independently and also use the various plant blowdown sources as spray water makeup.
        • Combined cycle steam condensing: Whether the plant is fossil fuel or solar, steam condensing efficiency is important for steam turbine performance. In a WSAC, the steam is condensed directly from the steam duct into the tube bundles. This is an alternative to a cooling tower and steam surface condenser. The effectiveness of direct steam condensing reduces the parasitic (fan and pump) energy by as much as 50 percent. This is power that can be put into the grid for sale.
        • Refrigerant condensing for inlet air chilling systems: For systems used to cool the inlet air to the gas turbine, the WSAC is used as a condenser on the refrigeration system. As with steam condensing, efficiency is important to the overall net output of the plant. The WSAC in the majority of cases will have a lower kw/ton (higher efficiency) than other systems.

        "Thermally Challenged" Plants
        Many plants have seen their thermal performance degrade over time. This is usually due to fouling in air-cooled condensers and coolers. The WSAC can be used as a supplemental steam condenser (Figure 2). During periods when the vacuum exceeds optimal design, some of the steam can be condensed in a WSAC, resulting in a lower backpressure on the turbine. If the auxilliary loop coolant temperature gets too high, problems with the turbine and other mechanical components can result. When the loop temperature exceeds safe limits, the WSAC can be used as a "trim cooler" to lower the temperature (Figure 3). In both cases, blowdown from other sources or poor quality water can be used for spray makeup.

        The most important feature of the WSAC is the ability to use poor-quality water as makeup and operate at higher cycles of concentration. As discussed above, the spray water is deluged over the tube bundles that then carry the rejected heat away from the tube surface. In contrast, a cooling tower must maximize water/air contact to cool the water that is then used to cool the process in a heat exchanger. Tradeoffs exist between tube material and water quality and treatment. Consequently, a water treatment professional should be part of the design process.

        For water-limited applications, when not enough water is available to use evaporative cooling for the entire load, a hybrid unit incorporating a dry (finned section) and a wet section can be used. The WSAC can be designed to operate either wet or dry, further reducing the need for makeup water. This can include:

        • Blowdown from other cooling towers
        • R/O; demineralization blowdown
        • Plant discharge
        • Produced water from drilling and mining operations
        • Brackish and seawater.

            Drift consists of the water droplets created by sprays that are discharged into the environment. Drift droplets have the same particulates and chemicals that are in the spray water system.

            Drift must be controlled for both cooling towers and WSACs. It is a major issue with open cooling towers because of counter-current flow of air and water. Due to the WSAC's co-current design, the drift is approximately 0.02 percent of the recirculating spray rate. With high efficiency drift eliminators installed, drift can be reduced to 0.0005 percent.

            The drift droplets discharged out of the fan stack contain a variety of materials. When the water droplet evaporates, the particulate materials end up in the air and are either carried away by wind or dropped to the ground. Many plants are required to calculate the total amount of material discharged (lbs/yr). With co-current flow design and the use of drift eliminators, the discharge from WSACis below imposed standards.

            Plume is the visible "cloud" created above a cooling tower or WSAC, the result of vapor condensing in cold weather. Today, a facilityís aesthetics are increasingly important. As a result, certain areas have plume restrictions. Several methods of plume abatement are available on the WSAC, including cold air introduction, reheat coils and partial wet/dry operation.

            Water Conservation
            With more stringent regulations on water use and increasing costs for water purchase and discharge, many are under the impression that a dry cooler is their only system option. However, this may not be the case. WSAC technology can actually reduce the amount of water used in a plant. These systems can also make use of water previously discharged.

            The WSAC is commonly used for adding capacity in "thermally challenged" plants. This allows for additional direct cooling without having to add more tower capacity or purchase additional makeup water.

            When not enough water is available to handle the entire heat load, a dry/wet system can be used. The dry cooler takes care of the first portion of heat transfer and the WSAC handles the remainder. Use of this type of "hybrid" system can save as much as 50 percent of annual water use requirements.

            Peter G. Demakos, P.E., is president of Niagara Blower Co., headquartered in Buffalo, NY.

Sponsored by FLSmidth

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