salt content in fresh water drift (below 2,000 ppm) compared to the salt content of sea water drift (60,000 ppm). Use as a flue-gas stack At some modern power stations equipped with flue gas purification, such as the Großkrotzenburg Power Station and the Rostock Power Station, the cooling tower is also used as a flue-gas stack (industrial chimney), thus saving the cost of a separate chimney structure. At plants without flue gas purification, problems with corrosion may occur, due to reactions of raw flue gas with water to form acids. Sometimes, natural draft cooling towers are constructed with structural steel in place of concrete (RCC) when the construction time of natural draft cooling tower is exceeding the construction time of the rest of the plant or the local soil is of poor strength to bear the heavy weight of RCC cooling towers or cement prices are higher at a site to opt for cheaper natural draft cooling towers made of structural steel. Operation in freezing weather Some cooling towers (such as smaller building air conditioning systems) are shut down seasonally, drained, and winterized to prevent freeze damage. During the winter, other sites continuously operate cooling towers with water leaving the tower. Basin heaters, tower draindown, and other freeze protection methods are often employed in cold climates. Operational cooling towers with malfunctions can freeze during very cold weather. Typically, freezing starts at the corners of a cooling tower with a reduced or absent heat load. Severe freezing conditions can create growing volumes of ice, resulting in increased structural loads which can cause structural damage or collapse. To prevent freezing, the following procedures are used: The use of water modulating by-pass systems is not recommended during freezing weather. In such situations, the control flexibility of variable speed motors, two-speed motors, and/or two-speed motors multi-cell towers should be considered a requirement. Do not operate the tower unattended. Remote sensors and alarms may be installed to monitor tower conditions. Do not operate the tower without a heat load. Basin heaters may be used to keep the water in the tower pan at an above-freezing temperature. Heat trace ("heating tape") is a resistive heating element that is installed along water pipes to prevent freezing in cold climates. Maintain design water flow rate over the tower fill. Manipulate or reduce airflow to maintain water temperature above freezing point. Fire hazard Cooling towers constructed in whole or