Effect of Changes in Water Composition
Cooling water operation changes water composition radically causing the circulating water composition really affecting the system’s operation and life
Factors affecting physical and chemical properties of water
- Temperature change
- Evaporation
- Air contact
- Product contamination
Cooling Water Treatment
Why treatment is required?
- Scaling and fouling control
- Corrosion control
- Microbial growth control
- pH control
Scaling and Fouling of Cooling Tower
Main causes:
- Scaling – Due to salts that deposit on high temp surface due to retrograde solubility
- General fouling – Due to insoluble suspended particles forming deposits on the surface
- Biological fouling -This results from abundant growth of Algae, Fungi or Bacteria on the surface
Scale Control in Cooling Tower
- Blowdown control
- Increased Blow-down limits cycles of concentration.
- Treatment is required to keep the dissolved solids in the non-scale forming state.
- Scale inhibitors do this permitting operation at supersaturated conditions
- Acid Dosing- Sulfuric Acid
- Relatively inexpensive method
- The acid treatment removes the bicarbonate acidity by converting HCO3- to SO4 and CO2
- CO2 is released to atmosphere and Sulfate remains as by product
- Caution – Overdosing causes corrosion
Fouling Control in Cooling Towers
- Fouling occurs when insoluble suspended particles like iron, mud, silt, and other debris deposit on the surfaces.
- Removal of suspended matter from make-up water, re-circulating water, basin by use of Side-stream filters (filtering 1-5% of the total circulation)
- High Water Velocities- A design stage measure for deposit control. This is always not possible ( Shell side)
- Dispersants-Keep the particulates in suspended form
- Surfactants-Keep the Hydrocarbons in emulsified form
Controlling Corrosion in Cooling Tower
- Achieved by dosing a Corrosion inhibitor. This primarily works by forming a protective film on the surface.
- Different types of treatments:
- Chromate based program
- Phosphate/ Phosphonate Programs
- Di-anodic
- Alkaline Zinc
- Alkaline Phosphate Program
- All Organic
- Molybdate based
Micro-Biological Control in Cooling Tower
- Microorganisms enter through wind, dust and make-up water
- Results in Slime and algae growth
- Important to minimize slime and algae growth as they reduce
- heat transfer
- decrease cooling water flow
- localize corrosion
- serve as a mortar for the rapid buildup of deposits
- Control is done by dosing chemicals
- Biostatic agents: Chemicals that inhibit the growth of microbes
- Biocides: Chemicals that kill the microorganisms
- Advisable to use biocides in the cooling water treatment – two classes :
- Oxidizing Biocides: Chlorine, bromine, Hypochlorite, Chlorine dioxide, Ozone
- Non-Oxidizing Biocides: Methylene-bis-thiocyanate (MBT), Organotin compounds, Aldehydes, Chlorophenols, Sulphones and Thiones, Carbamates, Isothiazoline
- Biofouling – accumulation of deposits of microorganisms forming films
- Biodispersants used for dislodging of biofilms and prevention of their formation
- Followed by the addition of biocide which kills the released bacteria
pH Control in Cooling Tower
- Important in minimizing the rate of corrosion in a heat transfer/cooling system
- To avoid corrosion problems, controlling the pH in an alkaline range of between 8 and 9 is important
- Alkaline pH has a greater tendency towards scaling from dissolved solids
- Conductivity measurement measures the level of TDS in water.
