Choosing a Water Screen for Your Cooling Tower System
Water screens made from a variety of metals and alloys play a crucial role in preventing trash and debris from damaging pumps and other sensitive components used in industrial cooling tower systems. Many types of industrial process plants rely on cooling towers to remove waste heat, including oil and gas refineries; chemical, fertilizer and plastics manufacturers; power plants and even paper mills.
Their cooling system pumps draw cool water at a consistent temperature and flow from a nearby waterway or retention pond and direct it over a heat sink in the tower. As the water runs over the heat sink, the waste heat is siphoned off into the water, which evaporates into steam. Large fans at the top of the cooling tower pull the steam out into the air so that the cooling process can continue. Any water that is not evaporated falls to the bottom of the cooling tower; then, it is returned to its source.
If system pumps become obstructed with debris and water flow is impeded, cooling tower temperatures rise and performance suffers. The system must consume even more energy to achieve proper cooling levels, and premature equipment failure can result. Even worse, debris could damage a pump or plug a heat exchanger, leading to a costly shutdown while equipment is serviced.
Filters typically used in cooling tower systems include static water screens, trash racks and traveling water screens. Three factors help determine filter choice:
- The anticipated amount of debris.
- The anticipated size of debris material.
- Availability of manpower for maintenance and cleaning.
This article will take a closer look at each type.
Static Water Screens
The most commonly used filters in cooling towers are static water screens. They are the best choice when debris accrues infrequently.
Ranging in size from two-by-three feet to 10-by-15 feet and even larger, static screens are offered by some manufacturers. The screen specifications and metal mesh openings are determined by how much water is needed to flow through to the cooling tower.
Using two screens per water-flow opening with the screens set about three feet apart is recommended. Each screen has a debris catch-basket at its base. The use of two screens is necessary because when one screen is pulled for maintenance, some trash will fall off its face and be reintroduced to the water system. The second screen then catches that debris. Having screens installed in a series also allows plant personnel to clean the screens without shutting down any equipment.
Although the frequency varies depending upon location and season, for best maintenance, static screens should be lifted from the water monthly. A crane raises the screens, and a hose or rake is used to scrub the metal surface.
Static screens typically are made from T304 stainless steel. For more corrosive environments, T316SS often is used. Stainless screens are durable and will last 20 to 40 years if they are mostly catching leaves and litter from a retention pond. When water is pumped from waterways where larger debris is a factor, the screens are subject to more extreme wear and may need to be refurbished every four to five years.
Other metal options include epoxy-coated or galvanized mild steel, which can be less expensive, but they can rust and deteriorate more quickly. It also is possible to passivate T316SS screens in a citric acid bath. This process uses the citric acid to dissolve the free iron on the surface of the stainless steel, leaving a surface layer rich in chromium, nickel and molybdenum. Passivation helps T316SS screens further resist corrosion.
Many screens are made for replacement in existing applications. In producing static screens for both new and existing customers, fabrication engineers first look at cooling system drawings and make hydrodynamic calculations to determine water flow. Typically, they then design for “full dam,” the highest water pressure the system could possibly encounter. This situation represents the forces a screen would see if it were to become fully plugged with debris so that no water is allowed through.
This worst-case scenario tells engineers how big the static screen structural members need to be and how far apart they should be spaced. To account for water pressure, the members are placed closer together at the bottom and then moved farther apart as you advance up the screen. Variable spacing allows the screen manufacturer to use the minimal amount of material necessary, which lowers costs and improves water flow. Reducing material in the water stream lessens the drag force from the screens.
The engineer then produces a rough design that gauges the amount of material needed and estimates a project price for the customer. Once an order is received, engineers create a model using CAD design software. The drawing is sent to the customer for final approval.
At some fabricators, production on the structural members begins by taking a flat sheet of steel, cutting it on a laser and then bending it on a press break. The members then are welded to form the frame. Custom-built members are recommended over frames made with standard structural angles, eye beams and channels by some manufacturers. Also, custom-built members may provide savings that can be passed on to the customer.
Normally, customers will specify the opening size that is desired in the mesh. Experienced screen design engineers also can look at pump manuals to determine the biggest-sized particle the sump can take in without getting damaged. Once the opening size has been selected, the mesh is sheared to size and welded to the face of the frame. Some manufacturers have the ability to weave metal mesh in house. The opening sizes available are infinitely customizable, and the best choice for the customer can be selected.
Screens are transported by flatbeds and installed in existing systems. For new systems, anchor guides may need to be mounted. If the guides have rusted out or been damaged at a facility that is replacing existing screens, new hardware may be required.
Trash racks are a variation of static screens and are designed for heavy-duty use. They are constructed with coarser filtration that is intended to catch larger debris such as logs and masses of aquatic grass often found floating in active waterways.
Trash racks normally are constructed from welded 0.25” thick flat bars rather than woven wire mesh. This durable construction allows them to withstand the impact of a log coming down a river at high speed during flood conditions or other high stress situations.
Traveling Water Screens
For plants looking to reduce manpower associated with regular maintenance and cleaning, traveling water screens provide a low maintenance option. The rotating screens operate by motors with timing mechanisms for periodic cleaning.
Because they are automated and self-cleaning, far less attention from plant personnel is required. Access to power and water lines, however, is needed for their operation. While they are more efficient, the upfront material and manufacturing costs for traveling screens are greater.