In cooling towers, water is used in three different ways. First and foremost, it is evaporated. The evaporation of water is how cooling towers remove heat, and the amount of water evaporated is a measure of the system load. The evaporated water contains no minerals.
The second method of water use is loss in the form of drift and splash-out. Drift is water that is carried out of the tower by the force of the air moving through the cooling tower. Drift is recirculated water that contains all of the minerals, chemicals and bacteria that are in the tower. Basically, drift is an uncontrolled loss usually listed as a percent of tower recirculating flow. The drift percent varies among tower designs, but 0.008 percent is a reasonable estimate for a typical new tower in good mechanical condition.
The third method of water use is blowdown (also referred to as bleed or discharge), which is recirculating water containing all of the minerals, chemicals and bacteria that are intentionally being removed from the tower. Blowdown is a controlled water-removal process to prevent the minerals in the recirculating water from becoming too concentrated. Blowdown is preferentially accomplished with a conductivity controller operating a motorized ball valve, although other methods sometimes are used.
Water Use by EvaporationThe following is an example of water use by evaporation and energy balance on a cooling tower operating with a mechanical chiller.
A ton of cooling equals the removal of 12,000 BTU/hr from water. A mechanical chiller adds approximately 3,000 BTU/hr of parasitic heat load. Therefore, a ton of cooling in a cooling tower is the removal of 15,000 BTU/hr of heat. (Please note: An absorption chiller has a much greater parasitic heat load. For each ton of chilled water produced, an absorption chiller generates 18,000 BTU of heat. Thus, a tower for an absorp-tion chiller must remove 30,000 BTU/ton.)
Rule of Thumb 1.Assume there are 3 gal/min of recirculating flow per ton of cooling with a 10°F temperature differential (ΔT). This will result in 15,000 BTU of heat being removed from the system in 1 hour, as shown in the equation that follows.
Note that the rule of thumb with an absorption heater is 4 gal/min per ton with a 15°F ΔT.
Rule of Thumb 2.There are 1.8 gal/hr of evaporation per ton of cooling. Evaporation of 1 lb of water takes about 1,000 BTU of heat. Evaporating 1.8 gal of water requires 15,000 BTU of heat, as shown in the equation that follows.
Note that for an absorption chiller, because twice the amount of BTU must be removed, twice that amount of water must be evaporated. Thus, for an absorption chiller, the rule of thumb is 3.6 gal/hr/ton.
Water Use by DriftDrift is the uncontrolled blowing of water droplets from the tower. A new tower in good condition would lose about 0.008 percent of the recirculating water via drift. The water lost per ton of cooling in one hour is:
This is only a small quantity of the water used in the tower. However, a chemically treated 500-ton tower operating around the clock will result in quite a large uncontrolled release of chemically contaminated water.
A rule of thumb for drift is that it will amount to about 10 gal/month/ton.
Spills and leaks also can be large contributors to potential contamination. The spill conditions are affected by strong winds and operating with less than full fan power.
Water Use by BlowdownAs noted before, blowdown is the intentional, controlled removal of mineral-laden water. It is quantified by the term “cycles of concentration,” or cycles. Cycles are equal to the ratio of makeup water volume to blowdown water volume, assuming that drift and spills are negligible. Cycles can be calculated by the ratio of the concentration of a soluble mineral (chloride is often a good choice) in the recirculating water to the concentration of that mineral in the incoming water. So, if the mineral were twice as concentrated in the recirculating water as in the makeup water, the tower would be running at two cycles. If the mineral were four times as concentrated, the tower would be running at four cycles.
The equations that relate these parameters are as follows:
The amount of water evaporated by the cooling tower is a function of the load on the system. The amount of water lost through drift and spillage is a function of the tower design and its mechanical maintenance. It is the amount of blowdown that can be affected by water treatment.