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There are three sources of water available.
1. Sea Water.
2. Surface water.
3. Under ground water.

INTRODUCTION

  • Every industry need water for various purposes like process, boilers, for cooling purposes etc.,
  • Water is sufficiently available, economical and has more specific heat than any liquid.
  • Thus water is used for cooling purpose.
  • But water carries lot of impurities which pose serious threat if it is used without proper treatment.

INTRODUCTION

  • In any cooling process water takes heat from process and cools the system.
  • By the way it gets heated up and in turn it need to be cooled to reuse the same.
  • It gives up the heat through a process called evaporation.

TYPE OF COOLING SYSTEMS

  • There are three types of cooling systems which are in use in industry:
    Once through system
  • Closed recirculating system
  • Open recirculating system

ONCE THROUGH SYSTEM

  • Once through systems normally present nearby a sea or river.
  • In this system water is drawn from the source and pass it through the system once and discharge it back into the source.

CLOSED RECIRCULATING SYSTEM

  • In closed recirculating system water is recirculated continuously.
  • The heat taken from the process is cooled in an another heat exchanger which in turn cooled by cooling tower water.
  • Thus in other words it is called as a PRIMERY CIRCUIT.

OPEN RECIRCULATING SYSTEM

  • In this system water is recirculated continuously through the process where it is required to take heat.
  • The heated water gets cooled in a cooling tower through evaporation method.
  • The loss of water through evaporation is compensated with fresh makeup water.
  • The open recirculating systems are prone to many external contaminations which calls for proper treatment control.

COOLING WATER TREATMENT

  • The following problems are encountered in cooling water:
  • SCALING
  • CORROSIONFOULING
  • MICRO BIOLOGICAL FOULING

SCANLIN

  • In a cooling system water is constantly recirculated to remove heat from heat exchangers. In this process water is continuously evaporated to give up heat, while the minerals it contained are left back in the remaining water. This leads to the increase in mineral concentration in the cooling water. These minerals start precipitating after attaining saturation and form crystalline like substance in heat exchanger system. This crystalline growth is called Scale.

CAUSES OF SCALING

  • pH: pH has got a significant effect on scaling. Solubility of salts of calcium & magnesium reduces as pH increases. Hence they tend to precipitate at higher pH and form scale.
  • Hardness is one of the main elements that induce scale formation.
    Higher alkalinity leads to the precipitation of salts of calcium & magnesium and also leads to the formation of silicate scales.
  • Temperature: Salts of calcium & magnesium have inverse solubility i.e., as the temperature increases their solubility decreases. Thus they precipitates and form scales.
  • Silica precipitates as Calcium silicate or magnesium silicate at higher pH. This forms hard scales.

REMENDIAL MEASURES TO CONTROL SCALING

  • Scaling is control with addition of scale inhibitors which form water soluble complexes with calcium & magnesium ions and prevent them from precipitating.
  • While adding above chemicals the following parameters to be monitored:
  • Maintain pH at less than 8.5.
  • Maintain regular blow down as recommended to control build up of mineral concentration.
  • Identify low velocity areas in cooling system and correct the same.

CAUSES OF CORROSION

  • Corrosion is nothing but physical loss of metal when the
    metal turns to it’s oxide form. It occurs due to the following:
  • Low pH
  • Presence of oxygen. Oxygen combines with electrons and water at cathode. Continuous contact of cathode with oxygen keeps corrosion process going on by way of inducing electron flow from anode and this results in loss of metal ai anodic sites.
  • Presence of Corrosive anions like Chlorides, sulfates, etc.,
    Presence of Bacteria viz., Sulfate reducing bacteria, Nitrifying bacteria, iron reducing bacteria.
  • Dissimilar metals in the system cause galvanic corrosion.


PROBLEMS DUE TO CORRESION

  • Reduces heat transfer due to build up of corrosion products in the heat exchanger.
  • Corrosion increases maintenance costs as heat exchangers frequently fouled by corrosion products.
  • Corrosion increases repairs and replacement costs and in turn it increases down time losses.
  • Corrosion develops leaks in heat exchanger system due to which cooling water gets mixed up with process fluids and contaminate the process.

REMEDIAL MEASURES TO CONTROL CORROSION

  • Passivate anodic sites to prevent electron transfer with the help of anodic inhibitors. Similarly passivate cathodic sites with the help of cathodic inhibitors also.
  • Use of filming inhibitors suitable to the metallurgy of the system to control corrosion.
  • Use of suitable biocides to control corrosion causing bacteria.


FOULING

  • Foulants are particles of oil, dust, silt, dead organisms, scale & corrosion products and other debris in suspended state in cooling water. They enter the system through internal and external sources.
  • These foulants stick together and gradually settle on metal surfaces of the cooling lines and create deposites.

PROBLEMS OF FOULING

  • Obstructs water flow.
  • Reduces heat transfer efficiency.
  • Reduces the performance of scale and corrosion inhibitors.
  • Induces corrosion which leads to increased maintenance and replacement costs.

Remedial measures to control fouling

  • Addition of dispersants help control fouling.
  • Low velocity areas are to be identified and corrective action is to be taken.
  • Remove foulants regularly with the help of side stream filteration help control fouling.

MISCROBIOLOGICAL CONTAMINATION

  • Water is an ideal place for micro organisms to live and breed.
  • Bacteria, algae and fungi are the most common microbes that are encountered in cooling water systems.
  • These microbes cause serious damage to the cooling water system.

TYPES OF BACTERIA

  • Iron reducing bacteria
    These bacteria survives by converting iron to iron oxides.
  • Sulfate reducing bacteria
  • These bacteria survives by converting sulfates in cooling water to sulfites there by reduces cooling water pH.
  • Nitrifying bacteria
  • These bacteria survives by converting ammonia to nitrites and there by reduces cooling water pH.
  • Slime forming bacteria
  • These bacteria secrets slime in cooling water system.
  • Apart from the above cooling water is conducive for the growth of Algae & Fungi.


PROBLEMS ENCOUNTERED WITH MISCROBES

  • Iron reducers cause iron oxide deposites in heat exchanger system. This reduces heat transfer efficiency.
  • Sulfate reducers cause reduction in cooling water pH thereby cause severe corrosion in cooling system.
  • Nitrifying bacteria causes reduction in cooling water pH and induce severe corrosion.
  • Slime forming bacteria secret slime in cooling system which is sticky. This slime entraps suspended particles and fouls the system. Also it reduces heat transfer efficiency of heat exchangers. It reduces water flow rate in the cooling system.
  • Algae grows with the help of sun light.
  • Fungi is produced by microbes that thrives on cellulose. These destruct cooling tower wooden structures.


MEASURES TO CONTROL MICROBIAL PROBLEMS

  • Control entrainment of process contamination like phosphates, ammonia etc.,.
  • While designing phosphate corrosion inhibitor program care should be taken to see that phosphate reserves are not exceeding 10ppm.
  • Recommend non – oxidising biocides compatible to the dispersants.
  • pH is one of the important factors which enhances the performance of biocides.
  • Select effective combination of Oxidizing and non – oxidizing program to suit the system’s operating condition and other scale & corrosion programs.
  • Supplement the biocide program with
    bio - dispersants to improve the efficacy of biocides.


COOLING WATER CALCULATIONS EVAPORATION

          0.1% evaporation takes place for every
          10 F rise in temperature.
          Evaporation rate (M3/Day) = 0.001x Rec. Rate x ∆T 0F x 1.8 x 24 Hrs
          Blow Down (M3/Day)= evaporation/C – 1
          C = Cycle of concentrations.

LANGLIER INDEX

  • This helps us to find out tendency of cooling water, i.e., whether the water is on scaling side or corrosive side.
  • With the help of the table we first evaluate saturation pH. Saturation pH is the pH at which caco3 becomes saturated in the cooling water.
  • L.I = pH – pHs pH = analytical pH.
  • pHs = saturation pH
  • If LI is on +ve side indicates Scaling. If LI is on - ve side indicates corrosion. If LI is 0.0 indicates chemically balanced and prone to pitting.
  • The optimum LI range is between + 0.5 & + 1.0.
    Evaluation of Corrosion Rate

          Corrosion Rate = W1 – W2 x534
                                    ---------------- MPY
                                       D x A x T
          MPY = mills per year.

          W1 = Initial Weight of Coupon in mg.
          W2 = Final weight of Coupon in mg.
          D = Density of Coupon
          A = Surface area of Coupon in in2.
          T = Time of exposure in Hours.
          The minimum time of exposure should be 30 days.

          Standard corrosion rate allowed is 3 to 5 mpy.