From food processing to the electronic industry, colder process water equates higher productivity, better quality and lower costs. When using an open recirculating cooling system to generate cold water, fill media is regarded as the heart of the tower. Attention to its selection will pay significant dividends.
Proper fill selection and tower performance calculations take into consideration air properties, thermal requirements, the fill's thermal performance and various heat transfer theorems. Understanding the complexity of tower design is important; however, developing an understanding and appreciation for the key factors in fill selection is the first step.
A Look at Fill History
Water-cooling systems date back to pre-Roman and early Egyptian culture. Early water cooling consisted of river water channels directed through living areas to provide convective and evaporative cooling. These basic systems evolved into simple spray systems that provided colder water. The vestiges of these spray-cooling approaches can be seen in industrial operations that use spray pond cooling systems.
Cooling tower technology developed slowly and initially focused on various types of splash media that improved the cooling performance of a spray system. In the early 1950s, large improvements in tower performance were achieved when film fill designs that reduced pressure drop and increased water contact with air were introduced. The improvements in open recirculating cooling towers paved the way for reduced water pollution by providing an economical alternate to once-through cooling applications. The higher efficiency cooling tower provided water savings, produced colder water and furnished energy savings. Additionally, in cooling towers using splash fill, a change to film fill reduced capital expenditures and allowed a smaller tower footprint.
Counterflow and crossflow designs were used in early cooling tower development, but by the 1950s and 1960s, the crossflow tower became dominant. In a crossflow tower, the water flows vertically while the air passes horizontally and perpendicularly to the water flow. This tower design is well suited for splash fills because they can provide a large air-travel section. As film fills became more popular, the use of crossflow towers declined and counterflow towers became more common. The counterflow cooling tower flows the water counter to the airflow in a vertical path. Water flows down while the air passes up through the tower stack. Typically, counterflow towers provide a lower pump head, higher operating efficiency and lower maintenance.
Comparing Film Fills for Industrial Process Cooling Towers
In both splash and film fill designs, the objective is to increase air-to-water contact, driving up convection and evaporative cooling while reducing pressure drop in the system. Splash fill creates droplets that are in contact with the air while film fill creates a thin laminar flow of water in contact with the air. The success of film fill over splash fill is directly related to reducing pressure drop while increasing the water-to-air contact (figure 1).
Film fill technology has evolved from the use of wood panels, galvanized steel plates, brick and asbestos cement board to plastic film fills. The benefits of using plastic for film fill include:
Plastic can be shaped easily to provide the best thermal design.
Materials that are chemically stable in various water environments can be selected.
Most plastic can be made UV stable.
Plastic can be made in very thin sheets and assembled into strong packs.
The majority of film fills are made from polyvinylchloride (PVC) plastic. The recommended standard for fill material is Cooling Technology Institute (CTI) STD-136 (figure 2). It is important to specify material properties for the supply of raw material, and the manufacturing technique will affect the final sheet quality. The benefits of using PVC include durability, long service life, self-extinguishing characteristics, and the ability to create a uniform water film (wetting). Wetting is a condition where the surface allows water to form a film rather than droplets or beads. Fill material wettability varies as some plastics are more hydrophobic than others.
Film fill can be placed in three broad categories: cross-corrugated, vertical-offset and vertical. Each fill category will have fill designs with differing thermal performance. Cross-corrugated fills can be used in both counterflow and crossflow towers. They work by separating the water and air paths in opposing angles through the pack. Vertical-offset fills are used only in counterflow cooling towers. Water enters the pack from the top and flows vertically. Within the first 2 to 4" (51 to 102 mm), the water makes an angular transition to other vertical channels. These patterns of angular transition occur continuously through the vertical pack depth. Vertical flow fills also are used only in counterflow towers. They keep the water and air path oriented vertically, with no offsets, through the pack.
Each category offers variations in sheet shape and texture (figure 3). Design considerations include variations in microstructure (the small surface structure embossed on the sheet), corrugation angle, flow path and sheet spacing (flute height). Each design variation affects the fill's thermal performance. When equipped with a suitable microstructure, both cross-corrugated and vertical-offset fills are considered high performance fills as they provide thermal performance up to three times greater than older splash fill designs. High performance fills are used regularly in counterflow towers and typically are selected for industrial applications where good water quality makeup is used.
The goal in fill design is to maximize thermal performance while minimizing pressure drop. Achieving this also ensures lower power consumption. In both cross-corrugated and vertical-offset fill designs, thermal improvement is achieved with a corresponding reduction in water film velocity through the pack. This reduction is due to the nonvertical water path and increased film area. Unfortunately, the lower water velocity also is a key factor in the increase of fouling potential. In lower velocity areas, small obstructions can create a deposit cell combining gelatinous biological activity with airborne material, scale, suspended solids from the water makeup, and, if present, process contamination. This deposit cell can grow rapidly into an overall fouling condition. It should be noted that light deposits on film fill do not have a significant affect on tower performance. However, more severe fouling inhibits uniform water film formation and obstructs airflow, thereby adversely affecting tower performance.
New vertical flow fills address fouling conditions and contribute to optimum tower performance. Many times, fouling conditions are due to process contamination - often seen in food processing plants and chemical plants - or where poor water makeup or airborne matter contaminates the circulating water.
New vertical flow fill designs offer the benefits of film fill's high performance with a reduced potential for fouling. These fills can accommodate poor water quality with characteristics as high as 500 ppm suspended solids and 25 ppm oil and grease. Typically, this is under conditions where good biological control is present. Biological matter is the glue that binds suspended solids to create fouling. An effective biological treatment program is always recommended for film fill and most splash fill applications. Also, using seawater in these vertical flow fills presents little problem.
Consistent thermal performance over the life of the tower is the greatest benefit of antifouling fills. Using higher performance fills may provide colder water in the short run, but in a fouling situation, your system can be starved for cold water over time.
Splash Fills for Industrial Cooling Towers
Capable of providing outstanding protection against fouling, splash fills are the oldest fill media used in cooling towers. The majority of splash fills are used in crossflow towers; however, splash fill packs designed for counterflow towers also are available. Thermal performance of splash fill varies significantly, and its thermal performance per unit depth is rated significantly below film fill. Splash fill's lower thermal performance requires a larger tower footprint and higher operating energy.
Most splash fills are installed on hangers in the tower. Made of stainless steel, PVC-coated steel wire or fiberglass hangers, hanging systems can be costly and time-consuming to install. In addition, they may impede maintenance activities because they restrict access inside the tower even when the splash fill is removed. Also, hanging systems are not as durable as bottom-supported systems. Bottom-supported systems use beams of wood, concrete, steel or fiberglass on appropriate centers under the fill. These systems have the ability to carry heavy loads.
Mixed Media Fills for Industrial Cooling Towers
Many tower owners have enjoyed the benefit of optimal thermal performance by combining fill designs. The most common combination is a 12" (305 mm) layer of cross-corrugated or vertical-offset fill above a vertical antifouling flow pack. Tower fouling rarely occurs in the top layer due to the washing effect of the water spray system. Using a mixed media fill pack - for example, combining a top layer of cross-corrugated or vertical-offset fill over vertical flow fill - will improve overall thermal performance without appreciably increasing the fouling potential. An additional benefit of using cross-corrugated fill on the top layer is improved water distribution.
It is less common to mix splash fills; however, process engineers and tower designers do mix splash and film fills for the following reasons:
To improve thermal performance. Higher thermal performance can be achieved using a top layer of film fill with splash fill in counterflow towers. Designs that mix bays of splash fill with bays of film fill in crossflow towers also are available and provide improved thermal performance.
In high temperature applications. In counterflow towers using PVC or high temperature PVC film fill, the recommended maximum operating water temperature is between 140 and 151°F (60 and 66°C). For higher water temperatures, using an alternate fill made of higher temperature plastic can increase operating temperature to approximately 180°F (82°C).
To expedite fill repair. Unexpected film fill damage can occur at the most inopportune time, and it seems to occur when the process needs require maximum cold water. This damage typically is confined to the top layers of fill. To expedite repair, maintenance personnel can use stacked horizontal grids or bottom-supported fill packs. Some fills are designed to nest for storage, which may allow owner-operators to easily store an emergency supply.
In conclusion, when engineering the heart of your cooling system, you should:
Select a high performance film fill when good quality water makeup systems are used with no process contamination and/or low airborne contamination.
Use vertical flow antifouling film fill for systems with poor water quality or other contamination.
Use splash fill when there is poor quality water makeup or high process contamination.
Use CTI material standards for PVC fill as a quality standard to help ensure a long fill life.
By considering the thermal performance of fill products and matching it to the application, end users can reduce capital and operating costs, optimize production and minimize maintenance costs.