This invention relates to a water treatment unit for the circuit water of open-circuit cooling systems, more particularly for cooling tower circuits, comprising a pipe with a filter unit connected to the circuit water stream of the cooling circuit for branching off and returning a part stream of the cooling water and a pipe for adding fresh water connected to the circuit water stream.
Water treatment units are known, for example from EP 0305897 (Henkel KGaA). The circuit water is passed through heat exchangers where it is heated and then pumped to the cooling tower. Here, the water is cooled by evaporation in an air stream and is re-used for cooling plants, reactors, etc. With open cooling systems such as these, the water lost through evaporation has to be replaced. In addition, additives for preventing corrosion of metal components and for preventing the formation of organic, inorganic or biological deposits have to be added to the cooling circuit water. It is known that the evaporated water can be replaced by untreated water, for example drinking water or ground water, or by pretreated water, for example decarbonized or deionized water. Through the addition of salt-containing water, i.e. water containing calcium or magnesium chloride, the salts are concentrated in the cooling system and generally lead to deposits of calcium and magnesium, particularly at places where the temperature is relatively high. In addition, relatively high chloride levels promote corrosion. It is known that part of the cooling water can be periodically xe2x80x9cflooded offxe2x80x9d to keep the salt concentration within tolerable limits.
In order to reduce the concentration of suspended solids, it is known that part of the cooling water, generally 5% of the circuit water, can be removed from the circuit as a part stream and passed through filters where the suspended solids are removed from the cooling circuit water. The filters are periodically cleaned by backwashing with the cooling circuit water, the backwash water being discharged as wastewater. The backwash water contains only about 1% of solids. As a result of the above-mentioned flooding off of the cooling water and the discharge of the backwash water as wastewater, water treatment chemicals are lost and have to be replaced. With relatively large cooling systems, this requires an elaborate measuring and control system as described, for example, in the above-cited EP 0305897 B1.
In addition, in view of legal requirements and the sensitive chemical environment in the cooling circuit, particularly so far as corrosion and deposits are concerned, the cooling circuit water has to be frequently monitored by chemical analysis. Known systems operated with low-salt or salt-free water are not free from wastewater either because the filters are cleaned by backwashing and the backwash water is discharged. As a result of this loss of filter backwash water, therefore, the cooling circuit water again has to be frequently analyzed and water treatment chemicals continuously added.
Accordingly, the problem addressed by the present invention was to provide a unit of the type mentioned at the beginning for the treatment of circuit water of open circuit cooling systems and a corresponding treatment process which would satisfy the following requirements. Suspended solids would have to be largely removed from the circuit water to prevent the deposits and reduce corrosion. This applies in particular to solids with an abrasive effect. The unit would have to lend itself to operation with low-salt and, more particularly, salt-free water so that few, if any, corrosion-promoting substances, for example chloride ions or sulfate ions, would be present in the cooling circuit water. Wastewater-free operation would have to be possible so that water treatment chemicals would not be discharged. On the one hand, this would reduce operating costs and, on the other hand, would avoid the introduction of chemicals into receiving waters. The quantity of water to be added would have to be minimal; in particular, only the water evaporated in the cooling tower would need to be replaced. This requirement would be fulfilled in particular by wastewater-free operation of the unit. Particularly stable operation of the cooling system would have to be possible in order significantly to reduce the expense of monitoring by chemical analysis in continuously and discontinuously operated systems. Finally, the need for forced circulation during stoppages as necessary in the prior art would have to be eliminated.