Cooling tower operational variables

Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up flow rate, evaporation and drift losses, blowdown rate, and the concentration cycles:

Referring to the schematic diagram in Fig., water pumped from the basin at the bottom of the cooling tower is the cooling water routed through the process stream cooling and condensing heat exchangers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C).

The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts the fan-induced upward flow of ambient air. That contact causes a portion of the water (E) to evaporate into water vapor that exits the tower as part of the water saturated air. A small amount of the water also exits with the air as entrained droplets of liquid water called drift losses (D). The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate.

The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the circulating water from becoming too high, a portion of the water, referred to as blowdown (B, is drawn off for disposal. Fresh water make-up (M) is supplied to the tower basin to compensate for the loss of evaporated water, the drift loss water and the blowdown water.

Defining the various terms:

• M = Make-up water in m3/hr
• C = Circulating water in m3/hr
• B = Blow-down water in m3/hr (also called draw-off)
• E = Evaporated water in m3/hr
• D = Drift loss of water in m3/hr (also called windage)
• X = Concentration in ppmw (of any completely soluble salts … usually chlorides)
• XM = Concentration of chlorides in make-up water (M), in ppmw
• XC = Concentration of chlorides in circulating water (C), in ppmw
• Cycles = Cycles of concentration, XC ÷ XM (dimensionless)
• ppmw = parts per million by weight

A water balance around the entire system is:

• M = E + D + B

Since the evaporated water (E) has no salts, a chloride balance around the system is:

• M (XM) = D (XC) + B (XC) = XC (D + B)

and, therefore:

• XC ÷ XM = Cycles of concentration = M ÷ (D + B) = M ÷ (M – E) = 1 + [E ÷ (D + B)]

From a simplified heat balance around the cooling tower:

• E = C · ΔT · cp ÷ HV

where:

• HV = latent heat of vaporization of water, 2,260 kJ / kg
• ΔT = water temperature difference from tower top to tower bottom, in °C
• cp = specific heat of water, 4.184 kJ / (kg ⋅ °C)

Modern cooling towers have demisters known as drift eliminators to reduce the amount of drift losses (D) from large-scale industrial cooling towers. However, some older cooling towers have no drift eliminators. In the absence of manufacturer's data, drift losses may be assumed to be:

• D = 0.3 to 1.0 percent of C for a natural draft cooling tower without drift eliminators
• D = 0.1 to 0.3 percent of C for an induced draft cooling tower without drift eliminators
• D = about 0.005 percent of C (or less) if the cooling tower has drift eliminators

Cycles of concentration

The cycles of concentration represent the accumulation of dissolved minerals in the recirculating cooling water. Blowdown of a portion of the circulating water (from the tower basin) is the principal means of controlling the buildup of these minerals.

The chemistry of the makeup water including the amount of dissolved minerals can vary widely. Makeup waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Makeup waters from ground water supplies (wells) are usually higher in minerals and tend to be scaling (deposit minerals).

As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the heat exchangers and/or in the cooling tower itself. . The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. Often a professional water treatment consultant will evaluate the makeup water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening, pH adjustment, and other techniques can affect the acceptable range of cycles of concentration.

Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States the majority of water supplies are well waters and have significant levels of dissolved solids. On the other hand, one of the largest water supplies in the United States (located in the city of New York) has water that is quite low in minerals and cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.

Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered and also be dosed with biocides and algacides to prevent growths that could interfere with the continuous flow of the water. Corrosion inhibitors may also be used, but caution should be taken to meet local environmental regulations as some inhibitors use chromates which are toxic.