The Key Challenges of Cooling Tower Deposits and Film Fill Systems

Oct 7, 2024 | Case Studies

Overview

Cooling towers serve an important role in a wide variety of industries and a wide variety of processes. Regardless of the variations, the one common denominator is they exist to eliminate heat via the process of evaporation. Over the years, cooling tower manufacturers have worked to optimize the evaporation rates and many systems now employ film fill. This type of fill is essentially sheets of plastic that are tightly packed. The evaporation rates are optimized with this type of fill because the water flows down the fill in very thin sheets, and it optimizes that air-water interface to enhance the evaporation rates of the system. The result is being able to get similar cooling capacities in smaller footprints, thereby minimizing the capital costs. However, the benefits realized with film fill tower packing is offset with a few challenges that need to be considered.

Clean Film Fill Tower Packing
Clean Film Fill Tower Packing
Evaporative Tower Deposits
Evaporative Tower Deposits

The Challenge

Depending on several different variables, film fill tower packing can experience Evaporative Tower Deposits (ETD). ETD is not necessarily a type of scale, rather an accumulation of hard water deposits that exist. This phenomenon is like hard water deposits around a sink faucet where overtime there gets to be a build-up of minerals due to the repetitive wetting and evaporation to dryness.

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

The Cause

Under normal operation with cooling tower fans on, the air is drawn in from the outside and essentially pulls the film of water closer to the center of the cooling tower. This induced movement of the tower film may be less than 1” but it can be more. Then, when the tower fans either stop or slow down, the effect on the film of water is reduced and the leading edge of the water film moves back towards the outside of the fill. The alternation in fans running (or running at high speed) and fans turning off (or running at low speed) causes this area of the tower fill to go from wet-dry-wet-dry. Depending on how often the change in fan operation (either on/off or high/low) occurs will influence the number of wetting and evaporation cycles encountered.

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 flow is properly balanced across multiple tower cells to ensure the flow across the fill is distributed evenly.
  4. In systems with lead/lag, stagged fans, variable fan speeds, etc. allow one tower cell to run for a longer duration (i.e. weekly) rather than more frequent cycling between tower cells(i.e. hourly or daily). This reduces the number of wet-dry cycles encountered while also balancing out the mechanical run-time.
  5. Increase or supplement cooling tower dispersants to try and reduce the tenacity of the ETD accumulations.

Remedy

While a common misperception is that the chemical treatment program is failing, there are no additives that can prevent ETD accumulations because the fact that the water completely evaporates and leaves all mineral content behind on the fill. As ETD accumulations build up and age, they can become more adherent and difficult to remove. For this reason, regular washdowns of the tower fill can help make their removal more manageable. While pressure washing is often the best method for these washdowns, care needs to be taken to not damage the often thing and brittle film fill.

In more advanced cases of ETD accumulations, chemical cleaning may be required. In these situations, an Apex Water + Process representative can determine the correct chemistry to apply as well as the process and safety considerations.

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