Corrosion Control in Close Loop System

Corrosion control in close loop system

Overview

Corrosion damage in closed heating and cooling systems is a common problem. As an integral part of heating, ventilation, and air conditioning systems, they circulate hot water in the winter and cold water in the summer.

The quality of the water used to fill the closed system has an affect on the performance of the corrosion control program. Typically, raw water as received from a municipality or private well is used to fill a closed system. However, this may not be the best alternative. Another option is to use softened or demineralized water. Hardness poses a scaling problem in hot water systems. It is best to soften the make-up water if it exceeds 10 ppm total hardness.

Overall, the water used in the closed system should be of the best quality available. As a general rule, demineralized water is better that softened water which is better than raw water. Some engineers resist the use of demineralized water because of the common belief that it is more aggressive or corrosive than raw or softened water. Although it is true that untreated, oxygenated demineralized water is very corrosive, particularly at hot water temperatures, corrosion inhibitors such as sodium nitrite passivate metal surfaces resulting in a final system water that is noncorrosive.

Chemical Treatment Options

Several chemical treatment methods have been developed for closed loop systems. The selection of one method over another is determined by the water quality, the type of antifreeze used, the system metallurgy, and any environmental or safety issues that must be considered.

If the closed loop system is new, it should be chemically cleaned prior to the start of the treatment program. Chemical cleaning removes oil, mill scale, dirt, welding fluxes, and other contaminants that can interfere with the