Open recirculated systems

Open recirculating systems These are more amenable to inhibition since it is possible to maintain a closer control on water composition. Corrosion inhibition in these systems is closely allied to a number of other problems that have to be considered in the application of water treatment. Most of these arise from the use of cooling towers, ponds, etc. in which the water is subject to constant evaporation and contamination leading to accumulation of dirt, insoluble matter, aggressive ions and bacterial growths, and to variations in pH. A successful water treatment must therefore take all these factors into account and inhibition will often be accompanied by scale prevention and bactericidal treatments. 

Figure 8-3 Three basic types of cooling water systems. Top the once-through system where the cooling water is used once and then discharged. Middle the open recirculation system where the water is cooled and recycled through a system in which it comes in direct contact with air. Bottom a closed recirculation system where the water is cooled and recycled without coming in direct contact with the atmosphere.

Widespread practice has always existed, unfortunately, by owners and operators of cooling systems, or their general contractors and consultants, to specify chemical treatment operating standards in isolation, with little or no actual contact with prospective vendors of water treatment service companies before the issuance of such standards. There is now a growing tendency for particularly low corrosion rates to be specified as a part measure of the quality of water treatment and technical services to be provided in water management contracts (say below 1 mpy for steel in closed-loop systems and below 2 mpy for steel in open-recirculating systems). These low rates are often unattainable in practice. 

Precommission Cleaning and Passivation of Open Recirculating Systems... 

Fig.4.3-7. An open recirculation system with divided flow...

The following scheme is an open recirculating system with a recycle of magnitude Qzi. 

In open recirculating systems evaporation is the major factor in heat disposal. In these evaporative systems circulating water is continuously scrubbed with air, therefore saturated with dissolved oxygen. In contrast, water in closed recirculating cooling systems usually will contain minimum dissolved oxygen, even though the systems may include vented expansion tanks. 

Open recirculating systems with cooling towers. 241... 

It is advisable that counts should not exceed lO total germs per milliliter in an open recirculating system and 5 10 in make up water. 

Table 4 shows the levels found in the open recirculating system of a thermal power plant fed by highly infected river water (before carbonate removal) undergoing effective antialgae chlorination treatment. 

Fig. 73 Open recirculating system with cooling tower...

An understanding of the environment of an open recirculating system is very important, as the cooling tower functions as an "air scrubber". Pollutants from the air can be ... 

Comparison of properties of the four main types of corrosion inhibitors for open recirculating systems with cooling towers... 

Given the short residence time, very good protection can be achieved with smaller amounts of inhibitor than in an open recirculating system. Inhibitors can be cut down to values of around 5 to 10 mg l of P2O5. 

Conditioning similar to what is proposed for open recirculating systems if residence time is under one week. The quality (scale-forming or corrosive) of the available water must be taken into account. 

Many studies were performed with the use of plate count techniques. Experience demonstrates that the concentration of microorganisms in the cooling water depends on many parameters and is site-specific. However, the normal functioning of cooling water system is reported when bacterial counts are below 10 colony-forming units (cfu) per milliliter of cooling water in open recirculating systems, 10" cfu/ml in closed systems or 400 cfu/ml in a makeup supply (Drew, 1994). 

There are certain fundamental considerations which should be understood in relation to open recirculated systems. First is the concept of cycles of concentration. If three cups of boiling water in a tea kettle were allowed to boil away to one cup, the residual cup would contain a threefold concentration of soluble water salts, assuming that only pure steam was driven off. The water would be said to be at three cycles of concentration. 

Scale control in open recirculated systems is affected primarily by limiting the concentration of the scale-forming species and related parameters, especially by controlled acid additions to bring the water to a suitable pH range. Many of the commercial inhibitor formulations include scale-controlling additives (e.g., polyphosphates and chelating agents). 

Chlorination to residual chlorine content of a few tenths of a ppm is the usual treatment for control of biological growths in open recirculated systems. In order to discourage the development of chlorine-resistant strains, it is advisable to surprise the microbes at irregular intervals with other biocides (e.g., quaternary amines). Other oxidizing biocides (e.g., chlorine dioxide and ozone) used in municipal waters are normally too expensive for cooling water systems. 

Excessive overfeed of some types of quats may contribute to foaming problems, especially in open recirculating systems with organic contaminations... 

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