Scaling is one of the most common types of foulants to occur on heat exchanger surfaces. When we refer to scale, we most often mean mineral scale that develops most commonly as calcium or hardness based salts such as calcium carbonate. These salts are unique in that they exhibit reverse solubility e.g. they become less soluble in water as the temperature rises. While the relationship between temperature and the level of salts in the water will determine if scale will form on the heat exchanger, there are relatively easy ways to control this type of scale in most situations.
Cooling tower scaling is typically caused by three main factors. Let's take a look at each and some potential ways to monitor for the impact of scaling.
1. Inadequate Chemical Scale Inhibitors
Chemical scale inhibitors can be used to control scaling tendencies in cooling tower water systems. While there are many different types of scale inhibitors, they all perform different functions in controlling different types of scales in a cooling tower system. As a result, if a calcium carbonate inhibitor is used in a system with high silica limits, it is a sure bet that silica problems will develop over time.
2. Degradation of the Chemical Dispersant/Scale Inhibitor
The importance of choosing the proper scale inhibitor, understanding how that product gets along with the other components of the chemical program can impact the long term success as well. For instance, some scale inhibitors are degraded by the use, or overuse, of oxidizing biocides. If the scale inhibitor is degraded, the obvious impact will be seen by the formation of scale and loss of heat exchange performance.
3. Poor Makeup Quality or Inadequate Water Chemistry Control
At the outset, a thorough review of the water chemistry is required to design the water treatment program and also the parameters needed to control the water conditions in the cooling system to prevent scaling. If the makeup quality is very poor, pretreatment may be required such as softening to remove hardness salts. Cycles can also be adjusted as needed to control the scaling species in the cooling tower. To control cycles and chemical feed and control, a robust equipment system must be in place. Without proper control, problems will develop resulting in poor heat exchange performance. Routine review of water chemistry and operating parameters of the cooling system is paramount to the success of the water treatment program.
How Can I Monitor and Prevent Scaling?
The tools used to monitor performance can be from the very simple to the sophisticated. Data tracking of chemical residuals, heat exchanger approach temperature monitoring, deposit coupons, back pressure monitoring, calculating U-coefficients are all various methods to monitor heat exchanger performance and can be indicators of a developing scale problem.
Cooling tower scaling will greatly reduce the efficiency of your cooling tower. To effectively prevent scaling, speak to a QualiChem Water Treatment Expert.
Stay tuned for the next post in this series where we will address microbiological fouling.