Consequences of Low Flow in Cooling Towers

Cooling tower flow describes the minimum and maximum amount of water flow over/through its “fill”. The purpose of a fill is to spread the water out over a larger surface area for evaporation to easily take place. As the water droplets evaporate into the atmosphere, they carry heat away from the tower.

 

Low flow is the result of there not being enough water flowing over/through the cooling tower fill, specifically below the minimum amount. Manufacturers will specify the maximum and minimum.

When the amount of water passing through the tower fill is less than the designed minimum amount, it is considered “low flow”.

One of the main issues caused by low flow is solid debris buildup. Your makeup water’s hardness plays a factor in how much total dissolved solids, TDS, there is. During the normal operation of a cooling tower, a certain amount of dissolved solids is present in the water. This amount increases with normal evaporation, as the water is being evaporated, but the solids are still present in the water. As the hot water on top moves towards the bulk water in the sump of the tower, this will normalize. However, in a case of low flow, the amount of solids will be the same, but the amount of water will be significantly less. This will cause an increase in the concentration of dissolved problems. It can lead to scale and other mineral buildup. The amount of chemicals normally fed will not be enough to stop this, as the concentration of dissolved solids will be higher. While more chemicals could just be fed, it is better to fix the root issue of low flow, as it is causing inefficiency anyway, and there is no use in wasting money on extra chemicals.

 

Additionally, when the complete surface area of the fill is not covered due to low flow, this results in a loss of efficiency. If the flow is low enough, this will result in dry spots, which are areas of the fill surface not covered by water. These dry spots are not able to cool the tower, as there is no water on its surface to evaporate.

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.

As a best practice, your nominal flow rate should be near 3 gallons per minute for every ton of water. This describes the amount of water flowing through during normal operations.

E.g., A 100-ton tower should have a flow rate around 300GPM.

It is the cooling tower pump’s responsibility to deliver water at the nominal flow rate. First, ensure you are using the right kind of pump and that the advertised flow rate is correct. If this checks out, make sure that the pump’s filter is clean. From there, check the pump’s motor and make sure that it is still functioning properly.

 

Additionally, 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.

 

Lastly, your low flow issues may be caused by the valves installed on your tower. Try to further open your balance valves, and make sure to check that all of the valves that could limit the flow to the tower are fully open. Look for any areas of congestion where water abruptly stops flowing as quickly.

Chardon Labs is a full-service cooling tower water treatment company. No matter your cooling tower’s current circumstance, we can diagnose and treat the issue. Each situation is different and requires a tailored amount of chemical fed into it. We will test your water regularly to avoid common issues. For instance, we would be able to detect a high level of total dissolved solids and feed the right amount of chemical to fix the issue. Then, when the low flow rate issue is corrected, we would reduce the amount of chemical fed. Chardon Labs can evaluate your cooling tower, and recommend treatment options as a free site visit evaluation!