Cooling Tower Fill Deposits

Overview

Cooling towers serve an important role in many industries and processes. Regardless of the variations, they all exist to eliminate heat through evaporation. Over the years, manufacturers have worked to optimize evaporation rates, and many systems now use film fill. This fill consists of tightly packed plastic sheets. With this type of fill, water flows down in thin sheets, optimizing the air-water interface to enhance evaporation.

As a result, similar cooling capacities can be achieved in smaller footprints, minimizing capital costs. However, the benefits of film fill tower packing come with a few challenges that need to be considered.

The Challenge

Depending on several variables, film fill tower packing can experience Evaporative Tower Deposits (ETD). ETD is not a type of scale, but an accumulation of hard water deposits.

This is similar to the build-up of minerals around a sink faucet, where repeated wetting and evaporation cause deposits to form over time.

While in most cases ETD’s downside is simply aesthetics, there are instances where the accumulation becomes so excessive that it leads to airflow restriction and reduces the efficiency of the cooling tower.

The Cause

With cooling tower fans on, air is drawn in and pulls the water film closer to the center of the tower. This movement may be less than 1 inch but can be more. When the fans stop or slow down, the effect is reduced, and the leading edge of the water film moves back toward the outside of the fill. Alternating between fans running at high speed and turning off or running at low speed causes this area of the fill to cycle between wet and dry. The frequency of these fan operation changes influences the number of wetting and evaporation cycles.

While most systems have constant condenser water recirculation flow, some systems can have variable flow rates. In these types of systems, the flow rate across the cooling tower can vary, thereby causing the film of water flowing across the tower fill to push outward closer to the edges. Then when the recirculation rate is reduced the leading edge of the film of water migrates closer to the center of the tower. This also causes repetition of wet-dry areas in the film fill.

Prevention

ETD can be challenging to prevent in systems with high dissolved solids concentration and frequent changes in mechanical operation. The following are potential actions that can help reduce the accumulation of ETD:

1.    Reduce cooling tower cycles of concentration to lower the dissolved solids content in the cooling tower.

2.    Pretreat the cooling tower make-up using water softeners. This will not reduce the occurrence of ETD, but it will make the deposits more easily removable.

3.    Make sure the flow is properly balanced across multiple tower cells to ensure the flow across the fill is distributed evenly.

4. In systems with lead/lag, staggered fans, or variable fan speeds, allow one tower cell to run for a longer duration, such as weekly, instead of cycling between cells more frequently, such as hourly or daily. This reduces the number of wet-dry cycles and balances mechanical run-time.

5.    Increase or supplement cooling tower dispersants to try and reduce the tenacity of the ETD accumulations.

Remedy

A common misperception is that the chemical treatment program is failing, but no additives can prevent ETD because water completely evaporates and leaves all mineral content behind on the fill. As ETD accumulates and ages, it becomes more adherent and difficult to remove.

Regular washdowns of the tower fill can help make removal more manageable. Pressure washing is often the best method, but care is needed to avoid damaging the thin and brittle film fill.