25 May 2005
(e.g., steam boiler, cooling tower, closed hot water system or chilled water system). This
process of external treatment is often referred to as "pre-treatment." In contrast, internal
treatment involves the treatment of water directly within the water system. Both external
and internal treatment methods may be used in a given system.
External Treatment. External treatment equipment processes and water
treatment chemicals reduce or remove impurities contained in the makeup water before
the impurities in the water stream enter the internal system. The most effective way to
protect the system, reduce boiler problems, and improve operating efficiency is to use a
process of removing impurities before they enter a system, particularly a steam boiler.
The required treatment methods and equipment are determined by the specific type and
amount of impurities that must be reduced or removed from the makeup water. Table 2-
1 lists the various external treatment methods that are available to remove the typical
impurities found in makeup water. Figure 2-1 illustrates the effects of these treatment
methods on raw water.
The treatment process may be applied to only a portion of the makeup water,
in which case the treated water is then blended with raw (untreated) water to achieve a
specific quality. Treatment involving this type of blending is known as "split-stream"
treatment. Split-stream treatment may also involve the blending of two different treated
waters to achieve a specific quality. Paragraphs 2-2.2 through 2-2.10 briefly describe
external treatment methods. External water treatment is required only for steam boilers
and high-temperature hot water systems, but its use may be justified for other industrial
water systems as well. Although several types of treatment may be available,
government installations most commonly use sodium zeolite softening (ion exchange)
for treating makeup water. Table 2-2 provides a guide for selecting external treatment
methods/equipment for steam boilers.