Warm water is recirculated continuously from a heat source, such as an air conditioning system or process equipment, to the cooling tower.  In most cooling tower systems, warm water (or water to be cooled) is pumped to the top of the tower where it is sprayed or dripped through internal fill. The fill creates a large surface area for a uniform thin film of water to be established throughout the tower. Fans pull or push air through the tower in a counterflow, crossflow or parallel flow to the falling water in the tower. Water is evaporated carrying away the heat. For most efficient cooling, the air and water must mix as completely as possible. Cooling is reduced when dew points are high.
 

 Cooling Tower System Schematic

Evaporation

Cooling occurs in a tower by the mechanisms of evaporative cooling and the exchange of sensible heat. The loss of heat by evaporation (approximately 1,000 British thermal units per pound of water) lowers the remaining water temperature. The smaller amount of cooling also occurs when the remaining water transfers heat (sensible heat) to the air. The rate of evaporation is about one percent of the rate of flow of the recirculating water passing through the tower for every 10° F decrease in water temperature achieved by the tower. The decrease in water temperature will vary with the ambient dew point temperature. The lower the dew point, the greater the temperature difference between water flowing in and out of the tower. Another rule of thumb for estimating the rate of evaporation from a cooling tower is as follows: evaporation equals three gallons per minute per 100 “tons” of cooling load placed in the tower. The term “ton,” when used to describe cooling tower capacity, is equal to 12,000 Btu per hour of heat removed by the tower. When the dew point temperature is low, the tower air induction fans can be slowed by using a motor speed control or merely cycled on and off, saving both energy and water evaporation losses.

Blowdown

Blowdown is a term for water that is removed from the recirculating cooling water to reducecontaminant buildup in the tower water. As evaporation occurs, water contaminants, such as dissolved solids, build up in the water. By removing blowdown and adding fresh makeup water, the dissolved solids level in the water can be maintained to reduce mineral scale build-up and other contaminants in the tower, cooling condensers and process heat exchangers. Thermal efficiency, proper operation and life of the cooling tower are directly related to the quality of the recirculating water in the tower. Water quality in the tower is dependent on make-up water quality, water treatment and blowdown rate. Optimization of blowdown, in conjunction with proper water treatment, represents the greatest opportunity for water efficiency improvement. Blowdown can be controlled manually or automatically by valves actuated by timers or conductivity meters. 

Drift Losses

Drift is a loss of water from the cooling tower in the form of mist carried out of the tower by an air draft. A typical rate of drift is 0.05 to 0.2 percent of the total circulation rate. Reduction in drift through baffles or drift eliminators will conserve water, retain water treatment chemicals in the system, reduce "spotting" around the tower area and improve operating efficiency. 

Make-up Water

Make-up water is water added to the cooling towers to replace evaporation, blowdown and drift losses. The amount of make-up water added directly affects the quality of water in the systems. The relationship between blowdown water quality and make-up water quality can be expressed as a “concentration ratio” or a “cycle of concentration.” The most efficient use occurs when the concentration ratio increases and blowdown decreases. 

Water Balance

A simple water balance on a cooling tower system can be determined if three of the four following parameters are known: make-up, evaporation, drift and blowdown.

Blowdown Optimization

Water consumption of cooling towers can be reduced significantly by minimizing blowdown in coordination with an integrated operationand maintenance program. Blowdown is minimized when the concentration ratio increases. Historical concentration ratios are 2-to-3, and generally can be increased up to six or more with generic treatment options. Automation and 24-7 online monitoring can often allow cycles to be pushed to ten. Some states have passed laws governing the quality level in a cooling tower as an attempt to promote efficient cooling tower water use. The volume of water saved by increasing the cycles of concentration can be determined by this equation:

     V = Mi  x  CRi - CRf/(CRi)(CRf - I)
    

      V=volume of water conserved

     Mi=initial make-up water volume(before modification)

     CRi= concentration ratio before increasing cycle

     CRf=concentration ratio after increasing cycles

For example, increasing concentration ratio from two to six will save 40 percent of the initial make-up water volume.  The maximum concentration ratio at which a cooling tower can still properly operate will depend on the make-up water quality, such as pH, TDS, alkalinity, conductivity, hardness and microorganism levels. The use and sensitivity of a cooling system will also control how much blowdown can be reduced. Scale, corrosion, fouling and microbial growth are four critical parameters that must be controlled in cooling towers. Minimum blowdown rates must be determined in tandem with the optimum water treatment program for the cooling tower.  

Controlling Blowdown

To better control the blowdown and concentration ratio, facilities can install submeters on the make-up water feed line and the blowdown line. Submetering allows operators to carefully control water use. In some areas, evaporative water loss, as determined by submetering and water balances, can be subtracted from local sewer charges. Submeters can be installed on most cooling towers for less than $1,000.

Recirculating water systems are blown down when the conductivity of the water reaches a preset level. Blowdown can be done manually or automatically. Automation generally allows for higher cycles. Typically, towers are blown down in a “batch” process, releasing water to lower the tower volume until the make-up turns on and begins to reduce the concentration in the tower. Once tower levels are replenished the cycle repeats. This produces a saw tooth pattern. If the mechanicals and tower load allow it to be done, proportional or continuous make-up and blowdown systems can reduce the saw tooth and increase overall cycles. 

Recovering Sewer Charges

Because all cooling towers lose significant quantities of water through evaporation, some wastewater utilities allow these evaporative losses to be subtracted from utility bills. The utilities that allow this billing adjustment typically will require that a submeter be installed on the make-up water line to the cooling tower(s). Some system of reading this submeter monthly and requesting a reimbursement can be established where allowed. Submeters can be installed on most cooling towers for less than $1,000.