Industrial heat exchangers are responsible for many cooling process fluids operating in the world’s harshest environments. A failure to reject heat due to poor thermal performance or construction can result in unplanned downtime and lost profit. It also can negatively affect other important plant processes.
Material and component selection varies from application to application, depending on the technical and environmental requirements of each customer. Industrial process-cooling solutions must meet these demanding requirements.
A custom-engineered, closed-loop, evaporative cooling system can be a cost-effective technology to achieve the coldest process outlet temperature in the harshest conditions. Wet-surface air coolers and condensers are used in many heavy industrial applications — fluid cooling, vapor condensing and refrigerant desuperheating, condensing and subcooling — to improve plant capacity.
Operating Principles for Wet-Surface Air Coolers
The basic operating principle of a closed-loop, evaporative cooler — also known as a wet-surface air cooler (WSAC) — is that heat is rejected by latent (evaporative) heat transfer (figure 1). During operation, the fluid or vapor to be cooled or condensed flows through tube bundles as part of a closed-loop system. A large quantity of water from the unit basin is sprayed over the tube surface. Simultaneously, fans induce airflow over the bundles in a cocurrent direction. Evaporative cooling takes place at the exterior of the tube surfaces.
The saturated airstream leaving the tube bundle makes two 90° turns into the unit’s fan plenum at a lower velocity. There, most of the large water droplets fall into the basin, and the air is discharged through the fan stacks.
Keeping the process stream inside the tubes is important for many reasons. This approach:
- Maintains thermal performance.
- Requires minimal (simple) maintenance.
- Separates the process fluid from open-loop spray water, which means the process stream is never contaminated.
- Makes it possible to use poor quality water as a makeup source.
- Allows higher cycles of concentration to be achieved.
- Prevents the process fluid from being exposed to the environment during cooling.
The cocurrent flow of air and water allows for an unobstructed spray system that is fully accessible for observation and maintenance. Additionally, in cold environments, because the air passes over the spray-water system before and during contact with the tube bundle, the mixed water temperature remains above freezing. This protects the tubes from freezing — even in northern Canada. Cocurrent flow also ensures complete coverage of the tube surfaces — bare spots are not seen as with counterflow designs — to significantly reduce fouling and freezing potential. An advantage of using closed-loop technology is that — unlike open cooling towers where clogging can occur — spray water does not enter the heat exchangers.
A closed-loop evaporative cooler is able to cool process fluids to within 5 to 10°F (2.7 to 5.6°C) of the wet-bulb temperature. (This temperature will always be lower than the ambient dry-bulb temperature.) For example, a wet-surface air cooler system can provide process outlet temperatures as cool as 80°F (27°C), even on a 110°F (43°C) dry-bulb (75°F [24°C] wet-bulb) day.
General Components and Material Selection for WSACs
Tube Bundles. Materials of construction for tube bundles are specified based on the composition of the process stream being cooled (inside) and quality of the spray water (outside). Bundles can be designed for high pressure use per ASME and TEMA codes.
Two basic tube bundle types are used: serpentine and straight-through (cleanable). Serpentine bundles, which are fabricated with a continuous tube circuit, are less expensive and can be designed to accommodate pressures up to 2,500 psi. Straight-though or cleanable bundles have removable headers that allow complete internal access for inspection and cleaning in place while the balance of the wet-surface air cooler system remains in service. Additionally, straight-though tube bundles can be retubed using the existing headers. This bundle style offers the lowest process-side pressure drop.
Tubesheet thicknesses are designed to meet TEMA/ASME standards. Tube material, diameter, wall thickness, length, tube bundle depth and width, etc., can be optimized to provide the most cost-effective thermal performance. Typical material choices include carbon steel, stainless steel, admiralty brass, titanium or copper alloys (figure 2).
Basin. Many factors need to be considered when selecting materials of construction for the basin. For smaller units, metal basins generally are most economical. Typically, they are constructed of carbon steel and hot-dip galvanized after fabrication. Some manufacturers offer a polyurethane coating for specific applications or stainless steel construction.
For larger units, concrete basins are most common, and the concrete can be extended vertically to support the tube bundles and fan plenums. Depending on cost, fiberglass-reinforced plastic (FRP) can be used for the fan plenum (center section) or the entire structure.
Mechanical Components. Direct-drive, mill-chemical, severe-duty or totally enclosed fan-cooled (TEFC), self-lubricating fan motors operate directly in the airstream for designs requiring 5’ 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 airstream are used in conjunction with fiberglass-reinforced epoxy fan blades.
The spray-water distribution systems for all wet-surface air coolers and condensers use a low pressure, high flow design. The entire spray system is constructed of galvanized carbon steel for factory-assembled units and PVC material for field-erected systems. To ensure reliable and complete coverage, large-orifice, non-clogging nozzles are used.
Materials to be Cooled, Waters to Perform Cooling
Because industrial applications vary greatly, custom-engineered solutions often are used to meet specific design conditions. Thermal sizing is specific to the process stream properties and flow rate, and mechanical components are chosen based on customer requirements or environmental considerations.
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 wet-surface air cooler can handle. They include refrigerant desuperheating, condensing and subcooling; process water cooling; and vapor condensing.
Closed-loop evaporative coolers are used in all major, heavy industries, including power, oil and gas, refinery, metal and petrochemical. A partial list of fluids cooled includes:
- Water and wastewater.
- Sour water.
- Quench water.
- Lean amine.
- Machine oils.
- Carbon dioxide (CO2).
- Natural gas.
The most important feature of a custom-engineered, closed-loop evaporative cooling system is the ability to use poor-quality water as makeup and operate at higher cycles of concentration. Water sources can include:
- Blowdown from other cooling towers.
- Reverse osmosis demineralization blowdown.
- Plant discharge.
- Produced water from drilling and mining operations.
- Brackish water and seawater.
- Flue-gas discharge (FGD) wastewater.
- Sewage plant effluent.
As discussed, the spray water drenches the tube bundles that then carry the rejected heat away from the tube surface. By contrast, one alternative evaporative cooling technology — a cooling tower — must maximize water/air contact to cool the water, which then is used to cool the process in a heat exchanger.
With any evaporative cooling technology, 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 where not enough water is available to use evaporative cooling for the entire load, a hybrid unit incorporating both dry (finned) and wet (tube) sections can be used. Hybrid wet-surface air coolers can be designed to operate either wet or dry, further reducing the need for makeup water.
Industrial applications are uniquely demanding and will continue to evolve as processes are conceived and continually improved. Heat transfer solutions should be custom-engineered specifically to each application, using materials appropriate for the environment. This helps ensure they provide the best thermal performance under harsh conditions.
Innovative heat transfer systems such as closed-loop, evaporative coolers deliver the required temperatures and improve plant performance while utilizing water streams currently considered unusable with conventional towers and heat exchangers. Also, as a result of lower process outlet temperatures, a wet-surface air cooler system can improve plant capacity. PC