Cooling Water Treatment Improves Heat Transfer in Industry

Improvement of Cooling Water Water is widely used in industry for heat transfer cooling purposes. (Heat of vaporization: 540 cal/gr) As water evaporates, it absorbs the necessary heat from itself and cools. Approximately, in the cooling tower, for every 10 ̊C temperature drop, 1% of the circulation flow rate evaporates. (For example, for a flow rate of 100 kg/h, 1 kg/h of water evaporates. 540 kcal/kg of evaporation energy is lost. The temperature of 100 kg of water drops by 5 ̊C.) The air entering the cooling tower is cooler than the inlet water. The exiting air is warmer than the incoming air and the outlet water is cooler than the inlet water. The cooling of the cooling tower is a function of the wet bulb temperature of the air. To achieve much lower temperatures, a refrigerant gas compressor is required. The compressed and heated refrigerant gas is cooled by the cooling tower water; when the cooled compressed gas is expanded, it cools to subzero temperatures.

Reasons for Improving Cooling Water

Increasing Heat Transfer and Energy Efficiency by Keeping Heat Transfer Surfaces Clean

Water contains dissolved chemicals. Particularly because the solubility of calcium and magnesium salts decreases with increasing temperature, these salts crystallize together with silicates through a complex mechanism, forming a layer on the surface that is difficult to scrape off. This layer is called scale or incrustation. Scale reduces heat transfer, causing energy loss. Heat transfer efficiency decreases. Cooling becomes insufficient. Insufficient cooling due to incomplete heat transfer can cause malfunction of systems requiring cooling or prevent them from operating or cause incomplete operation. In this case, production may stop or efficiency and quality may decrease. Costs increase. In a cooling system, the heat transfer surfaces in contact with water are the surfaces of the heat exchanger (condenser, evaporative condenser) where the refrigerant gas (freon or ammonia) heated after compression in the compressor is cooled by the cooling tower water. Scale formed on these surfaces prevents heat transfer. Cooling of the gas in the condenser slows down. The heat withdrawn from the condenser decreases. In this case, the compressor's work and pressure (head pressure) increase. An increase in compressor work means more electrical energy consumption and increased cooling costs. As a result of the compressor's peak pressure rising beyond limits, the compressor shuts down and cooling stops. Following thermodynamic calculations performed, it has been found that a scale layer 0.25 mm thick, the thickness of an eggshell, would cause 45% efficiency loss and excess electrical expenses.

Extending System Life and Reducing Maintenance Costs by Preventing Corrosion

Corrosion is the conversion of metal into a compound and its loss as a result of electrochemical reactions. Corrosion results in perforations. The cause of corrosion is dissolved oxygen, acidic water, microorganisms and the electrical potential difference created by deposits on surfaces. Metal oxides created by corrosion form sediment causing blockages.

Corrosion Coupon

For the simplest control of whether corrosion exists in systems, standard coupons made of the metals present in the system (steel, copper, etc.) are weighed and then attached to the water circulation line with appropriate and standard equipment. After a certain period, the coupons are removed and weighed again. Weight loss on the coupons quantitatively indicates the amount of corrosion. The details of the method are described in ASTM-D 2688.

Elimination of Disease-Causing Bacteria

Microorganisms and bacteria, especially water droplets dispersed from cooling towers, are transmitted to the atmosphere and to people through respiration, causing diseases. For example, Legionnaires' disease. To prevent algae, fungal and bacterial growth in the cooling tower, appropriate biocides should be fed into the system. The most dangerous among disease-causing organisms is:

Legionella

Resembles pneumonia. It begins 110 days after infection. Initial symptoms are diarrhea, weakness, headache, chills, dry cough. People with weakened immune systems are more susceptible to infection. The main route of transmission is inhalation of legionella in aerosol-form water droplets. Contaminated water particles from cooling towers, hot tubs and similar places transmit the disease. The conditions in the cooling tower are favorable for legionella multiplication. Water passing through the tower absorbs nutrient substances and organic dust from the air and promotes legionella growth. The tower also produces aerosols to be inhaled over a wide area around the tower. If Legionella CFU/litre <10 is 5 or higher, measures should be taken.

Blowdown Control to Eliminate the Disadvantages Listed Above

For scale and corrosion control, the critical factor is maintaining the chemical saturation level in water below saturation. For this purpose, a portion of saturated water is discharged from the cooling system. (blowdown) The amount of water discharged is determined by the number of cycles. Increasing the number of cycles reduces blowdown. Decreasing the number of cycles increases blowdown. It is clear that low blowdown means the cooling tower operates at lower cost in terms of water conditioning of the system. At this point, we are concerned with four quantities: 1) Make-up water (M), 2) Evaporation (E), 3) Blowdown (B), 4) Cycles of concentration (C). Rarely are flow measurements given as quantities. However, we can analyze make-up water and circulating water. We can measure the temperature of hot and cold water and the wet bulb temperature of incoming air. If we know two of these four quantities, we can calculate the other two using the following relationships. M= E+B

Blowdown

%B=100 xB(kg)/ M(kg) %B= 100xchloride content in make-up water/chloride content in blowdown Cycles of concentration (C) is the ratio of certain components in circulating water to the same components in blowdown water. C=circulating water in ppm (gr/ton) (chloride or TDS) /make-up water in ppm (gr/ton) (chloride or TDS) C=conductivity of blowdown water / conductivity of make-up water Here, if hardness value is used, it should not be forgotten that if scale formation occurs, the hardness in circulating water may be lower than in make-up water. Actually it is a good test for scale formation.

Evaporation

Evaporation (E)= Blowdown(B)(C-1) If make-up water cannot be measured, evaporation can be calculated from the cooling range (R=difference between water inlet and outlet temperatures) and the circulation rate (L). E=0.00075 R L For every 10 F, 0.75% of the circulating water evaporates. A much more precise formula includes the wet bulb temperature of inlet air (Twb). E= L R (0.0000036 Twb +0.0000014R +0.000416).

Indices

There are indices that show the formation of scale and corrosion based on the properties of the water used. LSI Langelier's Index LSI=pH-pH (saturation) Positive values of the LSI index indicate the likelihood of scale formation. RSI Ryznar's Index RSI=2pH (saturation)-pH RSI index values greater than 6 indicate no likelihood of scale formation; values less than 6 indicate scale may form. PSI Practical Scale Index PSI=2pH(saturation)-pH(equilibrium) PSI index values greater than 6 indicate no likelihood of scale formation; values less than 6 indicate scale may form. pH(saturation)=9.30 +A+BCD A is a function of TDS, B of temperature, C of calcium hardness (as ppm CaCO3) D of alkalinity (as ppm CaCO3). pH (equilibrium) 1.465 log (total alkalinity)+4.54 These indices give correct results when no chemicals are added to the water. Chemicals modify the index indicators to reduce the likelihood of scale. Although blowdown is necessary, specially selected chemicals must be dosed into the water. Phosphate, phosphonate compounds, inorganic acids and various polymers can be used together or separately. Phosphate compounds increase blowdown and pollute the environment. The economically and effectively active group is polymers.

Use of Polymers:

Polymers are effective in preventing scale formation through three mechanisms: A) They disrupt the crystal structure of scale. (crystal modification) As seen in the attached image, CaCO3 crystals are regular cubic in shape. Polymers enter between the cubes and prevent crystallization. [caption id="" align="alignnone"] Improvement of Cooling Water[/caption] B) Threshold effect. By using amounts below the stoichiometric ratio, they convert salts into soluble form; for example, acrylic acid polymers with a molecular weight between 2,000 and 3,000, when dosed at 210 ppm, prevent over 90% of scale formation. C) Calcium sequestration. Polymers bind calcium and other metals in complex form at stoichiometric ratios, making them soluble. Among many and different types of polymers, those most suitable to the structure of the cooling system and its working conditions should be selected. Our products carry the WTC-C code. The following number varies according to system characteristics. WTC-C products are developed utilizing polymer technology. They prevent scale formation without chemical reaction, do not form deposits, minimize blowdown requirements, and reduce water consumption and energy losses. Effective dispersant. Disperses calcium sulfate, phosphate, carbonate and iron oxide. Prevents crystallization. WTC-CD, while the system is operating, slowly removes deposits and scale and discharges them from the system with blowdown. Prevents corrosion. Prevents scale formation without using acid. Acts as a dispersant. WTC-E10; in silver mines, prevention of pipe blockage from milk of lime fed to raise pH. In coal-fired thermal power plants, by feeding milk of lime to exhaust gas, sulfur and sulfur gases are captured. Prevents calcification of calcium sulfate in the system. Prevents problems that CaSO4 2H2O gypsum stone would create in geothermal water. WTC-DS series is developed for scale removal. Does not cause corrosion on stainless steel and copper equipment. Hilmi Bodur Chemical Engineer Partner / Prokim Kimyasal Ürünler Pazarlama Ltd. Şti.