Relationship Between Evaporation, Blowdown, and MakeupThe operation of cooling towers can be described by the relationship between evaporation, blow down, and makeup. Makeup water must equal blow down water plus water evaporation to maintain a constant operating water level in the system:EQUATION: M=B+EwhereM = makeup water, liters/sec (gpm)B = blow down, liters/sec (gpm) (all sources)E = evaporation, liters/sec (gpm)NOTE: Blow down (B) includes discharge to sewer, drift loss, and any leaks from the system

Cycles of Concentration (COC):One of the common terms used in Describing the water use efficiency of cooling tower water systems is COC. COC represents the relationship between the makeup water quantity and blow down quantity.COC is a measure of the total amount of minerals that is concentrated in the cooling tower water relative to the amount of minerals in the makeup water or to the volume of each type of water. The higher the COC, the greater the water use efficiency. Most cooling tower systems operate with a COC of 3 to 10, where 3 represents acceptable efficiency and 10 represents very good efficiency. It has been found that the range of 5 to 7 COC represents the most cost-effective situation.Calculating COC by Volume:If both makeup and blow down water volumes are known, COC by volume can be calculated. The term is defined as:EQUATION: C=MBwhereC = COC, no unitsM = makeup water, kg/hr (gpm)B = blowdown losses, kg/hr (gpm

Determining COC by Water Analyses:To determine COC, you mustknow the mineral content of both makeup and blowdown water. For example, you mustdetermine both the conductivity of the recirculating cooling tower water and theconductivity of the makeup water. (Note that the blowdown water will have the sameconductivity as the recirculating water.) Conductivity is commonly measured inmicromhos (mhos). You can also estimate COC by using other water quality parameters such as chlorides, silica, or sulfates. The relationship is represented by this

EQUATION: Bmhos BClC = ------ or ------ Mmhos MClwhereC= COC, no unitsBmhos = conductivity of blowdown (recirculating water), micromhos(mhos)Mmhos = conductivity of makeup water, mhosCl = chlorides in blowdown, ppmCl= chlorides in makeup water, ppm EXAMPLE 4-1:The measured conductivity of the blowdown (recirculating water) is 800micromhos and the makeup is 300 micromhos.The COC is:C = 800 300 = 2.67

Relationship Between Blowdown, Evaporation, and COCthe cooling water evaporation loss to calculate the blowdown rate that must be maintained to operate at a selected COC. The relationship between blowdown, evaporation, and COC is represented with this equation:

EQUATION: B = E (C - 1)

whereB = blowdown, liters per day or liters per second (gpd or gpm)E = evaporation, liters per day or liters per second (gpd or gpm)C = COC, no units

How to calculate cycles, blowdown, evaporation, makeupHere are the governing relationships for the makeup flow rate, the evaporation and windage losses, the draw-off rate, and the concentration cycles in an evaporative cooling tower system:

In the customary USA units:

M = Make-up water in gal/minC = Circulating water in gal/minD = Draw-off water in gal/minE = Evaporated water in gal/minW = Windage loss of water in gal/minX = Concentration in ppmw (of any completely soluble salts usually chlorides)XM = Concentration of chlorides in make-up water (M), in ppmwXC = Concentration of chlorides in circulating water (C), in ppmwCycles = Cycles of concentration = XC / XMppmw = parts per million by weight

A water balance around the entire system is:

M = E + D + W

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

M (XM) = D (XC) + W (XC) = XC (D + W)

and, therefore:

XC / XM = Cycles = M / (D + W) = M / (M E) = 1 + {E / (D + W)}

From a simplified heat balance around the cooling tower:

(E) = (C) (T) (cp) / HV

where:HV = latent heat of vaporization of water = ca. 1,000 Btu/poundT = water temperature difference from tower top to tower bottom, in Fcp = specific heat of water = 1 Btu/pound/F

Windage losses (W), in the absence of manufacturer's data, may be assumed to be:

W = 0.3 to 1.0 percent of C for a natural draft cooling towerW = 0.1 to 0.3 percent of C for an induced draft cooling towerW = about 0.01 percent of C if the cooling tower has windage drift eliminatorsConcentration cycles in petroleum refinery cooling towers usually range from 3 to 7.In some large power plants, the cooling tower concentration cycles may be much higher.

(Note: Draw-off and blowdown are synonymous. Windage and drift are also synonymous.)

In SI metric units:

M = Make-up water in m3/hrC = Circulating water in m3/hrD = Draw-off water in m3/hrE = Evaporated water in m3/hrW = Windage loss of water in m3/hrX = Concentration in ppmw (of any completely soluble salts usually chlorides)XM = Concentration of chlorides in make-up water (M), in ppmwXC = Concentration of chlorides in circulating water (C), in ppmwCycles = Cycles of concentration = XC / XMppmw = parts per million by weight

A water balance around the entire system is:

M = E + D + W

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

M (XM) = D (XC) + W (XC) = XC (D + W)

and, therefore:

XC / XM = Cycles = M / (D + W) = M / (M E) = 1 + {E / (D + W)}

From a simplified heat balance around the cooling tower:

(E) = (C) (T) (cp) / HV

where:HV = latent heat of vaporization of water = ca. 2260 kJ / kgT = water temperature difference from tower top to tower bottom, in Ccp = specific heat of water = 4.184 kJ / kg / C

Windage losses (W), in the absence of manufacturer's data, may be assumed to be:

W = 0.3 to 1.0 percent of C for a natural draft cooling towerW = 0.1 to 0.3 percent of C for an induced draft cooling towerW = about 0.01 percent or less of C if the cooling tower has windage drift eliminatorsConcentration cycles in petroleum refinery cooling towers usually range from 3 to 7.In some large power plants, the cooling tower concentration cycles may be much higher.

(Note: Draw-off and blowdown are synonymous. Windage and drift are also synonymous.)