A cooling tower is designed to remove heat by evaporation of water. The tower is also designed to have a minimum and maximum amount of water flow over (or through) the tower’s “fill”. The fill in the tower is designed to increase the surface area of the water that can be exposed to air. The larger the surface area of the water passing through the fill, the more efficient and faster the heat transfer to the atmosphere.
When the amount of water passing through the tower fill is less than the designed minimum amount, it is considered “low flow”.
Chardon understands that during normal operation of a cooling tower, the required evaporation will increase the concentration of the dissolved solids. Therefore, the hot water at the top of the tower will increase in dissolved solids during the evaporation process until it hits the bulk water in the sump of the tower. When the normal amount of water is passing through the fill, the resulting concentration of dissolved solids in the sump is usually negligible during a single pass.
However, when less than the minimum amount of water is passing through the fill, the evaporation process will still remove the same amount of water from that low flow. The concentration of the dissolved solids in the low flow water will increase much faster. It may increase so rapidly that the current cooling tower treatment chemicals in the water may not be able to keep the dissolved solids dissolved any longer and some precipitation may occur!
By inspection of the tower, normally the “hot decks” (or distribution pans) are supposed to be filled to about one inch from the top of the pan. If you observe that the water level in the hot deck is much less than one inch from the top of the pan, then you may have the problems mentioned above. Typical towers usually have about 4 to 6 inches of water depth in the hot deck. If the fan on the tower is running and you have low flow, then the problem of over-concentration of dissolved solids is going to be more severe.
You should verify from the tower manufacturer the amount of designed water flow for that individual tower. The tower manufacturer may need to know the orifice number to determine the normal water depth in the hot deck. If the water in the hot deck does not approach the estimate of the tower manufacturer, then notify the person responsible for the facility of your findings.
Some possible solutions are: open the balancing valves further, clean pump strainers, check valves that limit flow to the tower, clean rust chips from the balancing valves, etc.
