BAT approach methodology presented in this document to arrive at a a definition of BAT for industrial cooling systems and to identify techniques within this definition
Bioconcentrationfactor the capacity of a substance to bioaccumulate defined as the ration between the concentration of a substance in an organism and its concentration in the water (in a state of equilibrium). The bioconcentration is always determined by experiment.
Blow down (BD, kg/s) the intentional draining of a cooling system to balance the increasing concentration of solids in the cooling system and in practice it is the water that has to be withdrawn from an evaporative cooling system in order to control the cycle of concentration. It is calculated as BD = E*1/(x-1), where E is the evaporation loss and x is the concentration factor. Calculating the blowdown usually includes the non-evaporative losses such as windage, drift and leakage.
Biocide chemical that kills or slows down the growth of undesirable
organisms. In cooling water systems a biocide kills or slows down the growth of macro- and micro-fouling organisms, thereby minimising organic pollution in the cooling system. The most important biocides are: chlorine, sodium hypochlorite, ozone, quaternary ammonium and organic bromide.
Biocide demand the quantity of biocide that is reduced or converted to inert or less active forms of the biocide by substances in the water or the quantity of biocide that give complete reaction with all biocide-reactable materials.
Biochemical oxygen demand (BOD) a measure of the oxygen required to break down
(also named Biological oxygen demand) organic materials in water. Higher organic loads require larger amounts of oxygen and may reduce the amount of oxygen available for fish and aquatic life below acceptable levels. It can be assessed using a standard 5-day (BOD5) or 7-day (BOD7) test.
Bio-slime or bio-slime is defined as the bacterial film that develops on any substrate immersed in water. It consists of algae and sessile microbial population, comprising slime producing bacteria and anaerobic sulphate reducing bacteria. Microfouling promotes the deposition of macrofouling.
Breakpoint the demand of oxidyzing biocide by water impurities which must be overcome before a viable biocide-concentration in the cooling water is available.
Chemical oxygen demand (COD) a measure of the oxygen-consuming capacity of inorganic and organic matter present in water or waste (discharged cooling) water;
the amount of oxygen consumed from a chemical oxidant in a specific test. (normally referring to analysis with dichromate oxidation)
Coatings materials applied to surfaces to form either a low friction surface to reduce pump losses, or a protective layer to reduce erosion, corrosion and fouling.
Concentration factor (CR) concentration factor or cycles of concentration is the ratio of concentration of any particular solute in the recirculating cooling water to that in the makeup water. Calculated as CR = MU/BD, where MU is the make up water and BD the blow down.
Condenser cooler used for condensation of a gas flow (or steam). Condensation places extra demands on the heat exchanger: there must be space for the vapour volume. Condensers of power stations are therefore extremely large and specifically designed.
Coolant synonym for cooling medium. In many cases the coolant is water or air, but can also be water mixed with an antifreeze substance or a medium such as oil or a gas.
Corrosion can be defined as the destruction of a metal by (electro-)
chemical reaction with its environment.
Corrosion inhibitors are chemical substances that are able to slow down the corrosion process in water. They are de-aeration substances, passivating inhibitors (eg chromate, nitrite, molybdate, and orthophosphate), precipitating inhibitors (zinc phosphate, calcium carbonate and calcium orthophoshate), and adsorption inhibitors (glycine derivates, aliphatic sulfonates and sodium silicate).
Counter flow is the principle where the air flows in the opposite direction within the heat exchanger. In counter flow towers air moves upward opposed to the downward flow of the cooling water. This design provides good heat exchange because the coolest air contacts the coolest water. Headers and spray nozzles are used to distribute the water.
Cross flow is the principle where air flows perpendicular to the process fluid within the heat exchanger. In cross flow towers air flows horizontally across the downward stream of the cooling water.
Cycles of concentration (or “cycles”) are a comparison of the dissolved solids level of the blowdown with that of the makeup water. Thus, it is defined as quotient of salt concentration in the blow-down and the salt concentration in the make-up.
Dispersion substances or dispersants are chemicals that prevent the growth and deposit of particles present in water by increasing the electric charge resulting from absorption. As a result the particles repel each other and remain suspended.
Drift eliminators devices that change the direction of airflow, imparting centrifugal force to separate water droplets from the air.
Drift loss the loss of water due to small droplets, which are emitted into the draught air, leaving the top of a cooling tower.
Evaporative loss (E, kg/s) the mass of cooling water, which is evaporated per time unit during the operation of an evaporative cooling-system.
Free oxidant (FO)/ applied measure of free oxidants in the discharge of
Total residual oxidants (TRO) cooling water systems. Also referred to as TRO or total chlorine (TC) or free chlorine (FC).
Free available chlorine (FAC) free chlorine represents an equilibrium mixture of
or free residual chlorine hypochlorous acid and hypochlorite ion OCl−in the cooling water system.Both are oxidants, but OCl− is far less effective than HOCl.
Hardness stabilisers are chemical substances, which, added to water, are able to prevent the deposit of hardness salts by hindering the crystallisation process through absorption of the nucleation nuclei of the crystals. In this way the growth of amorphous crystals, which are relatively easy to keep in suspension and give less cause to deposits, is encouraged.
Hazardous substances substances or groups of substances, that have one or several dangerous properties, such as toxic, persistent, bioaccumulative, or are classified as dangerous for the human or environment according to the Directive 67/548 (Dangerous substances directive).
Macrofouling undesirable organisms in cooling water circuits, which are visible with the naked eye. Macrofoulingis represented mainly by mussels, barnacles, and serpulid polychaetes that encrust the walls of the circuits with their calcareous walls, filamentous organisms such as hydroids, and other organisms such as sponges, bryozoans, and tunucates.
Make up (M, kg/s) is defined as all water mass per time unit, which is added to the system to compensate the loss of water due to evaporation and blow down.
Maximum allowable risk-level the concentration of a substance in surface water where 95% of the species are protected. The toxicity and degradability are important aspects.
Mechanical draught tower cooling towers equipped with fans to push cooling air through the tower (forced draught) or to pull cooling air through the tower (induced draught).
Micro-fouling or bio-slime is defined as the bacterial film that develops on any substrate immersed in water. It consists of algae and sessile microbial population, comprising slime producing bacteria and anaerobic sulphate reducing bacteria. Microfouling promotes the deposition of macrofouling.
Natural draught tower large size cooling towers without fans, but designed to take advantage of the density difference between air entering the tower and the warmer air inside the tower to create a flow of cooling air.
Non-oxidising biocides mostly organic substances used for cooling water treatment particularly in recirculating cooling systems. Their working is more specific than that of oxidising biocides oxidising some species more effectively then others. They exert their effects on micro-organisms by reaction with specific cell components or reaction pathways in the cell.
Oxidising biocides mostly inorganic substances particularly applied in open once-through systems against fouling. They attack organisms via a non-specific mechanism. The biocide oxidises the cell wall or enters the cell and oxidises the cell components. These biocides are fast working and because of their non-specificity have a broader spectrum than the non-oxidising biocides.
Plume is the visible re-condensation of evaporated water in the discharged air of a cooling tower.
Precipitation softening this process is used to reduce water-hardness, alkalinity, silica, and other constituents. The water is treated with lime or a combination of lime and soda ash (carbonate ion). Water with moderate to high hardness (150-500 ppm as CaCO3) is often treated in this fashion.
Process medium the process medium will always refer to the medium to be cooled.
Scaling process of precipitation in CWS that occurs when the concentration of salts in the water film near the heat exchanger exceeds the solubility.
Sound Pressure Level (Lp) the measure for the immission of sound – the amount of sound at a defined direction and distance from the sound source. It is measured in dB per frequency band or A weighted as dB(A). The measure is logarithmic, this means that doubling the sound pressure level is equal to an increase 6 dB(A).
Sound Power Level (Lw) the measure for the amount of sound-energy, which is radiated (emitted) from a sound source. It measured in dB per frequency band or A weighted as dB(A). The measure is logarithmic, which means that doubling the sound power level is equal to an increase of 3 dB(A).
Hydraulic half time is defined as the time needed to reduce the initial concentration of a non-degradable compound to 50% of its initial concentration.
Total available chlorine (TAC)/ the sum of free chlorine and combined chlorine,
total residual chlorine (TRC) with combined chlorine as the available chlorine in chloramines or other compounds having a N-C link, in cooling water system.
Total residual oxidants (TRO) oxidant capacity measured in cooling water system via stoichiometric method (iodide.iodine). TRO is numerical and operational equivalent to TRC and TAC.
Variable speed drive a way of controlling the speed of a motor, usually electronically using an inverter. The speed can be varied manually, but is more often controlled via a signal from the process, e.g. pressure, flow, level, etc.