Spray-type cooling towers effectiveness is reliant on how well the fill material exposes the water to the mass air flow. The spray nozzles are designed to widely distribute the water onto the fill material, and the fill material needs to be able to expose the maximum amount of surface area of the water. The more water exposed to the mass air flow through the cooling tower, the greater the efficiency.
For specific designed systems the cooling tower needs to be designed so that the period of time that the water is in contact with the air is sufficient to achieve the desired, or necessary cooling effect. The greater the demand from the industry the greater duration of intimate contact will be necessary in the fill to achieve proper enthalpy. Enthalpy is a measure of the system’s ability to cope with the overall demand.
One way to increase the contact between water and air in a spray-type cooling tower is to make the tower higher. The greater height allows for increased fall time for the water. The longer the water is falling through the air the cooler it becomes. There are limits to this effect dictated by psychrometric limits. This means that given a tower of infinite height the cooling towers ability to cool water would be limited to the air coming into the tower. The air exiting the tower would also be limited to the hot water entering the cooling tower.
Clearly there are constraints on how tall towers could or should be constructed. Not to mention what the monolith free fall spray type cooling towers would cost. There would also be simple structural concerns, where wind would also come into play and introduce horizontal wind shear factors. This is where fill material comes in and shines. Fill simply slows the water down in its journey through the cooling tower. This increases the amount of time the water is in contact with the air without having to have monolithic cooling towers.
Many times fill is referred to as a heat transfer surface, this is not actually true in the strictest sense. The heat transfer is achieved through the water surface contacting the mass air flow. The fill simply helps increase the duration, and therefore the overall effect of the heat transfer.
Types Of Cooling Tower Fill
There are two basic types of fill used today. There are film type and splash type fill options. Either type of fill can be used in cross or counter flow cooling tower designs. Both types of fill have their pros and cons which is why there are still two options, and that isn’t likely to change. There are operational advantages which can offset the costs and make one better for one situation than another.
Splash Fill System
The cooling towers which use splash fill have a series of splash bars which are positioned over each other. This causes the water to successively cascade from one layer to another. Not only is the amount of time the water spends falling from one layer to another a function of the efficacy but the repeated pauses that the water takes while flowing across the splash bars.
Because the water’s movement through a splash fill system is mostly vertical the bars must be positioned with the wide surface of the bars orientated horizontally. This ensures that the water is efficiently distributed and broken up for maximum water to air surface contact. Splash fill delivers the least about of opposition to airflow along the horizontal plane. For this reason splash fill is more popular in crossflow cooling towers than it is in counterflow towers.
Splash fill systems to enjoy a resilience to imperfect water distribution and impure water. But they do require some maintenance. Without being maintained and cleaned the water will move to the lower end of the fill quickly. Without the water spreading evenly there is loss contact between the air and water and the system’s efficiency is reduced. A significant number of the splash fill systems are found in crossflow cooling towers. This is because the cooling tower has a full height air inlet, and the splash fill bars are easy to visually inspect. This makes for diagnosis of problems much quicker when you can clearly see into the unit before shutting it down and getting inside of it.
Film Fill Systems
Fill film is gaining a market share because of its compact performance. Film fill’s intricate inner workings provide the maximum water to air contact in that given space. It’s very widely used; in fact about half of all crossflow and practically all counterflower systems utilze film fill.
Capitalizing on the hydrostatic property of water, film fill material channels water along its corrugated surface. Unlike splash fill bars where water is only making contact with the air on one side of it the film is getting contact on both sides of the inner walls and a channel of air is passing over it. The protrusions or valleys and hills created by the corrugated surface cause intentional turbulence within the fill to maximize the air to water contact and promote the highest possible cooling effect.
From the perspective of most manufacturers film fill is the best option. This partly because film fill allows the manufacturer to build smaller cooling towers. Splash fill systems simply take up more room. In fact some manufacturers have simply stopped offering a line of splash fill cooling tower. There are a few negative aspects of film fill which we will examine.
During the vertical progress of the water through the fill material splash fill allows for water redistribution, film does not. This means for film fill the initial distribution of water at the time of installation is critical. Water must be evenly distributed down each of the channels so air will flow evenly through the fill. If this is not done correctly or build-up occurs in the fill the air will naturally take the course of least resistance and follow the path without water. When the air isn’t being forced to flow by and through the water filled areas the cooling affect is diminished.
When using film fill water quality becomes a point of concern. Because of the narrow spacing between the internal edges of the film fill it can become clogged easily. Things like leaves, high turbidity, debris, or the presence of slime, algae, or fatty acids can reduce the passage for the air to travel, a bit like clogged arteries. Any time there is a diverted air flow or restricted path for the air the overall cooling effect of the tower is reduced. Unlike splash fill systems you can’t just take a look inside to examine the condition of film fill. In fact if owners or technicians can already see clogs in the fill the level of clogging is likely to be extreme. Newer materials have helped improve the pass through of debris and build up which still delivering maximum surface area for water to air contact. Internal texturing helps increase the amount of time the water stays in the fill which improves the air to water contact time and increases the cooling towers effect.
Attention must be used during the cooling tower design phase and any upgrades to the tower as structural blockages to the input air flow to film fill will cause measurable disruption to the air distribution. If this is left unchecked it can cause spotty air distribution which leads to diminished thermal performance. Ultimately to maintain thermal performance both types of fill need to be maintained in all cooling towers.
Cooling Tower Fill Conclusions
Which type of cooling tower fill is best for a specific application really depends on the purity of the water used in the cooling tower. Either system can be effective if proper maintenance procedures are followed. However in particularly harsh environments splash fill can simplify maintenance and be more resilient to excessive dirt and debris. This is especially true for industries where the water can be contaminated. Such as dairy, food processing, steel production, hydrocarbon processing, foundries, paper processing, soaps/cosmetics, and ammonia stripping. Apart from these industries splash fill should be highly considered where there is going to be an un-purified water source or the area is particularly forested or dusty.
Phoenix Cooling Tower Maintenance
Clearly with either type of cooling tower fill regular maintenance is the key to optimal longevity and thermal performance. The professionals at All Kote Inc. have the knowledge and tools to keep your cooling tower running at peak performance, which ultimately translates into energy savings.