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TREATMENT OF HOT WATER SYSTEM

In a typical circulating hot water system, control of corrosion is the primary water treatment concern. While waterside deposits can develop, the source can generally be traced to improper control of corrosion. In engineering terms, the "closed system" is one that does not employ evaporation and has a water loss of less than 5% of the circulating rate. This definition describes a typical circulating hot water system. When operated within this definition, controlling corrosion is easily attainable and inexpensive. Uncontrolled, corrosion can result in deposits, substantial energy waste, costly repairs and in extreme instances, replacement that can run into thousands of dollars

In order to fully appreciate the ramifications of corrosion, the mechanisms involved and the cause and effect cycle which it produces must be understood. Corrosion is an electrochemical process by which metal undergoes destruction through reaction with its environment. In this process oxygen is one of the primary factors involved. By diffusion, oxygen is liberated at the metal surface and reacts to accelerate the corrosion process. In general, as the temperature of water rises, the amount of oxygen released from the water increases. For example, a rise in temperature from 70⁰F to 170^oF may increase the rate of corrosion by 400%. Since circulating hot water systems function in the higher temperature ranges, controlling the amount of oxygen available is critical to minimizing the rate of corrosion.

The amount of oxygen available will depend to a very large degree on the condition of the system. Oxygen enters the system as a part of the makeup required to replace any losses. A system with leaks requires excessive amounts of makeup and therefore will continually add excessive amounts of oxygen. In addition, oxygen can seep into the system water at vents, pipe joints, valves or pump packing. A well maintained system, particularly one that keeps makeup below 5% of the circulating rate, makes a major contribution to minimizing corrosion.

Once underway, corrosion removes metal and initially, these solids are held in solution by the system water. However, the amount of solids that can be held in solution is limited. When that point is exceeded, the metal starts to precipitate (fall out of solution) to form waterside deposits. Since deposits form first on heat transfers surfaces, efficiency is affected resulting in energy waste. Once deposits form, controlling the rate of corrosion becomes more difficult. Inhibitors added to the system water aid in holding down the rate. The type corrosion inhibitors most commonly used function by forming a film on the metal surfaces. This film results in limiting the ability of oxygen to participate in the corrosion process. Deposition of any type, reduces the ability of the inhibitor to get to the metal surface to provide this protective barrier. As a result corrosion, underneath the deposit, can go on unabated.

As solids are added to the system water, another factor in the corrosion mechanism is brought into play. The electrical charge involved in the process, can more easily flow through the water and participate in the corrosion process. Once underway, the process of corrosion then sets up a cause and effect, self destructive cycle.

Oxygen is the primary driving force that sets the corrosive mechanism in motion. If the amount available for diffusion to the metal surface can be controlled, fuel to feed the process is reduced and corrosive minimized. As pointed out earlier, oxygen enters with makeup water. Limiting the losses from the system therefore reduces the amount of oxygen available. However, some oxygen will enter the system and however small that amount may be, it must be controlled within the system to minimize its effect in the corrosion process. Inhibitors added to the system water minimize the ability of any oxygen present to participate in the mechanism. Limiting the amount of oxygen entering the system and maintaining inhibitor in the system water constitutes the thrust of corrosion control in a circulating hot water system.