By Peter G. Demakos, P.E., Niagara Blower Co.
With the constant and ever-growing demand for more power in America, a power plant’s ability to be as efficient and dependable as possible will become paramount. Closed-loop, evaporative coolers can allow a facility to sell more power to the grid by means of lower horsepower requirements, sometimes by as much as 60 percent compared with other technologies. Using high quality components and rugged industrial construction allow a closed-loop, evaporative cooler to operate for many years with minimal maintenance.
The basic operating principle of a closed-loop, evaporative cooler is that heat is rejected by means of latent (evaporative) heat transfer (see Figure 1 schematic of a wet surface air cooler [WSAC]). The fluid/vapor to be cooled or condensed flows through tube bundles as part of a closed-loop system. A large quantity of water (generally 7 to 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 surfaces. 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 most 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:
- Maintains thermal performance
- Minimal (simple) maintenance
- Open-loop spray water never contaminates the process stream
- Poor quality water can be used as makeup source
- Higher cycles of concentration
- No exposure of process fluid to the environment.
The co-current flow of air and water allows for an unobstructed spray system fully accessible for observation and maintenance. 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 (no bare spots as seen with counter flow designs) to significantly reduce any fouling and freezing potential.
The closed-loop, evaporative cooling system is able to cool process fluids to within 5 F to 10 F of the wet bulb temperature. (Unsaturated airin other words air that has less than 100 percent relative humidityhas a wet bulb temperature lower than the dry bulb temperature.) For example, the closed-loop, evaporative cooler can provide process outlet temperatures as cool as 85 F even on a 110 F dry bulb day (75 F wet bulb).
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 includes two system loopsone open and one closedthat 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 10 F to 12 F of wet bulb. Since the heat transfer in the heat exchanger is sensible, a large quantity (flow rate) of water is required. Also, because this configuration utilizes 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 closed-loop, evaporative cooler 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 (8 F to 10 F) and requires less air (fan horsepower) to remove the total heat load.
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 ambient dry bulb, which is normally higher than the wet bulb. Additionally, due to the inefficiencies of sensible cooling, the realistic approach temperature to dry bulb is generally 20 F to 25 F.
The closed-loop, evaporative cooler 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), which is normally lower than the dry bulb. This is most significant in warmer climates. On a 100 F dry bulb, 75 F wet bulb day, the process outlet temperature of a dry cooler will be approximately 125 F, where the closed-loop, evaporative cooler can easily deliver 90 F.
Due to the efficient heat transfer and lower air flow requirements, the closed-loop, evaporative cooling system footprint is much smallertypically 25 percent of the plot area requiredand much less horsepower, typically 60 percent less than required for a dry cooler. Unlike a dry cooler that has a high potential for fouling and plugging in the closely spaced fins, the closed-loop, evaporative cooler 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. Within closed-loop, evaporative cooling systems, the re-circulating spray water is kept warm by direct contact with the heat source, acting as a buffer between the tubes and cold air.
The closed-loop, evaporative cooler is an efficient and effective heat rejection technology for several reasons. First, the closed-loop, evaporative cooling uses latent cooling, which is far more efficient than sensible cooling. This means a closed-loop, evaporative cooler can cool the same heat load with a smaller footprint than all-dry systems.
Second, because it has single source thermal responsibility, it provides the lowest process fluid outlet temperatures (as close as 5 F to 10 F to wet bulb).
Third, 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 when compared to a cooling tower.
Fourth, makeup water can come from almost any source (tower blowdown, reverse osmosis (R/O) and “de-min” blowdown, plant discharge, produced and so on).
Fifth, closed-loop, evaporative cooler systems require little maintenance. The spray nozzles are accessible for inspection and maintenance without shutting down the unit or removing any obstructions. And, because the cooler is closed-loop, maintenance and cleaning of heat exchangers is eliminated.
Tube Bundles: As previously discussed, the process fluid stays inside the closed-loop tube bundles. These bundles can be manufactured of almost any material based on the composition of the process stream (inside) and quality of spray water (outside). 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, 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 closed-loop, evaporative cooling system remains in service. Additionally, the tube bundles can be re-tubed using the existing headers. This bundle style offers the lowest process side pressure drop.
Tube sheet thicknesses are designed to meet TEMA/ASME standards. Tube material, diameter, wall thickness, length, number of tubes deep and wide and so on 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, sea cure, titanium or copper alloys.
Basin: Many factors need to be considered when selecting construction materials for the basin. Metal basins are generally most economical for smaller units. They are usually constructed of carbon steel, hot-dipped galvanized after fabrication. Many other material selections including stainless are available. Concrete basins are widely used for larger units and can be extended vertically to support tube bundles and fan plenums.
Depending on cost, fiberglass reinforced plastic (FRP) can be used. This selection can be used for the fan plenum (center section) or the entire structure. A three-foot-high (approximate) “swimming pool” must be poured to enclose the basin water level. FRP is corrosion resistant, fire-rated and in some instances can be less expensive than concrete.
Final determination of basin and structural materials is made based on shipping constraints, installation location, cost of materials, labor and spray water quality.
Direct-drive, mill-chemical, severe-duty or totally enclosed air over (TEAO) self-lubricating fan motors operate directly in the air stream for designs requiring 5-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 totally enclosed fan-cooled (TEFC) motors located outside the air stream are used in conjunction with fiberglass- reinforced epoxy fan blades.
The spray water distribution system for all closed-loop, evaporative cooling systems uses a low-pressure and high-flow design (generally 7 to 10 gpm/ft2 of coil surface area). 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 (3/4”) non-clogging nozzles are used.
While the closed-loop, evaporative cooler is an effective heat transfer device, there are several application limitations. When cooling liquids, the inlet temperature must be lower than 180 F to prevent spray water flashing off the exterior tube surfaces. For hotter inlet temperatures, a dry cooler is used to lower closed-loop, evaporative cooling system inlets to an acceptable temperature. As with cooling towers, siting and proximity to other plant components both need to be considered. And, for direct steam condensing, the closed-loop, evaporative cooler needs to be located as close to the steam turbine outlet as possible. This will minimize the length of steam duct that needs to be installed.
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 closed-loop, evaporative cooling system can handle: simple cycle aux loop cooling, combined cycle steam condensing and refrigerant condensing for inlet air chilling systems.
The closed-loop, evaporative cooler 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 (see Figure 2).
Regardless of fuel energy source, steam condensing efficiency is important for steam turbine performance and ultimately power output. In a closed-loop, evaporative cooling system, the steam flows from the steam duct and is condensed directly inside the tube bundles. This is an alternative to a cooling tower, pumps and piping and steam surface condenser.
For systems used to cool the inlet air to the gas turbine, the closed-loop, evaporative cooler is used as a condenser on the refrigeration system.
There are many plants that have seen their thermal performance degrade over time. This is usually due to fouling in air-cooled condensers and coolers. The closed-loop, evaporative cooling system can be used as a supplemental steam condenser (see Figure 3). During periods when the vacuum exceeds optimal design, some of the steam can be condensed in a closed-loop, evaporative cooler, resulting in a lower backpressure on the turbine.
If the aux loop coolant temperature gets too high, problems with the turbine and other mechanical components can result. When the loop temperature exceeds safe limits, the closed-loop, evaporative cooling system can be used as a “trim cooler” to lower the temperature (see Figure 4).
In both cases, blowdown from other sources or poor quality water can be used for spray makeup.
A closed-loop, evaporative cooler has multiple approaches to power savings. From smaller fan motors, lower capacity pumps and less overall maintenance a closed-loop, evaporative cooling system can have a dynamic influence on running costs and therefore can increase plant efficiency.
The effectiveness of direct steam condensing and/or fluid cooling reduces the parasitic (fan and pump) energy by as much as 50 percent versus competing technologies. This unused power can then be sent into the grid for sale.
A recently completed condenser (163,000 lb/hr) comparison between a closed-loop, evaporative cooler and a conventional cooling tower/heat exchanger for a power project in the northwestern United States shows the closed-loop, evaporative cooler saving the facility approximately $42,000 annually just in horsepower saving over the competition (see Figure 5).
When compared to a dry cooler of similar capacity, a closed-loop, evaporative cooling system can begin to approach horsepower savings of up to 60 percent.
Efficiency is important to the overall net output of the plant. The closed-loop, evaporative cooler in the majority of cases will have a lower kw/ton (higher efficiency) than other systems (see Figure 6).
During hot weather periods, turbine (and plant output) is limited by high steam condensing temperature/pressure. Many power plants have found success by installing a closed-loop, evaporative cooler to handle the aux load; taking the aux load off the cooling tower will improve tower capacity, provide additional capacity for steam condensing and increase power output (see Figure 7).
Another feature of the closed-loop, evaporative cooling system is the ability to use poor quality water as makeup and operate at higher cycles of concentration. These sources can include:
- Blowdown from other cooling towers
- R/O; demin blowdown
- Plant discharge
- Produced water from drilling and mining operations
- Brackish and seawater
- FGD wastewater
- Sewage plant effluent.
As discussed, 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. There are tradeoffs between tube material, water quality and water 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 closed-loop, evaporative cooler can be designed to operate either wet or dry, further reducing the need for makeup water (see Figure 8).
Drift is 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 closed-loop, evaporative cooling systems; drift is a larger issue with open cooling towers because of counter-current flow of air and water, and the high flow rate of water. Due to the closed-loop, evaporative cooler 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 based on manufacturer claims.
The drift droplets discharged out of the fan stack contain a variety of materials. When the water droplets evaporate, the fine 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) in accordance with PM10 (particles less than 10 micrometers in size). According to the EPA, the standard for PM10 is 50 µg/m3 (annual mean) and 150 µg/m3 (daily concentration). With co-current flow design and through the use of drift eliminators, closed-loop, evaporative coolers’ discharge is significantly less than these standards.
Plume is the visible “cloud” that is created above a cooling tower or closed-loop, evaporative cooling system, a result of vapor condensing in cold weather. In today’s society, the aesthetics of a facility are becoming increasingly important. As a result, certain areas have plume restrictions. There are several methods of plume abatement including cold air introduction, reheat coils and partial wet/dry operation, all of which are available on the closed-loop, evaporative cooler.
A 240,000 lb/hr steam condenser in Toronto, Canada, currently operates a plume abatement system using steam reheat coils located in the fan plenum of the cooler. When in operation, the system effectively reduces the visible plume to zero.
With more stringent limitations on water use and increasing costs for water purchase and discharge, many are under the impression that a dry cooler is the only system option. However, this may not be the case. The closed-loop, evaporative cooler technology can actually reduce the amount of water required by making use of water previously discharged.
Since the closed-loop, evaporative cooler spray only has to pass over widely-spaced tubes, poor quality water can be used. In contrast, cooling tower water must flow through strainers, heat exchangers and other components that can become plugged or fouled. The constant flushing of the tubes allows for the ability to use poor quality water without outside fouling.
Therefore, blowdown from plant devices can be used for closed-loop, evaporative cooler makeup, reducing the amount of fresh water required, and also the amount of water discharged from the plant.
The closed-loop, evaporative cooler 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 there is not enough water 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 closed-loop, evaporative cooling system handles the remainder. Use of this type of “hybrid” system can save as much as 50 percent of annual water use requirements (see Figure 8).
Water (resource) issues will continue to have an increased impact on plant design and operation. Closed-loop, evaporative coolers can help deliver required cooling water temperatures and maintain plant performance while utilizing water streams currently considered to be unusable with conventional towers and heat exchangers. These systems are versatile and can provide solutions to water use, water quality and outlet temperature. Also, as a result of better efficiency, the closed-loop, evaporative cooler can contribute to a reduction in plant emissions.
Author: Peter G. Demakos, P.E., is president of Niagara Blower. This article is based on a paper given at the Electric Power Research Institute Cooling Tower Technology Seminar and Conference held in Cincinnati, Aug. 18-20, 2009.