The function of an important part of the water used in industry is cooling. Cold water is spring and takes the calories to be extracted from a hot spring. Calorie change or heat transfer usually occurs above the metallic surface without cold and hot weld contact. for this process cooling tower conditioning is necessary.
There are 3 types of cooling circuits.
- open circuits
- half open
- closed circuits
Open Cooling Tower Circuits Conditioning
They form “One Pass” water circuits. Water is thrown back after it passes directly through various cooling devices. A large amount of water is lost in this type of circuits. These circuits are encountered in old systems with a lot of water and low cost.
What are the main problems: Corrosion
Scale (Stone binding) Sludge (deposit) Biological formations. Semi-open cooling is the same problem that will be studied in refrigeration circuits. But here the problems are milder. Since the water is single-pass, there is no significant change in its chemical and physical properties. The water is not concentrated. The temperature difference of ΔT is less significant.
If the water used in the open circuit is natural water with low salinity, the problem to be solved depends on the analysis of the chalcocarbonic balance. In this case, a corrosive water (CO2RYZNAR stability index is used to determine whether it is in balance or scale form.
Control of corrosion;
- Use of a chelator in accordance with the RZYNAR index to regulate pH.
- Prevention of corrosion by filmogen effect
Difficulties in the process arise from the amount of water processed. Because the use of excess water leads to the consumption of large amounts of chemicals. Continuity of conditioning is necessary to maintain film quality.
In open circuit corrosive conditions, it is economically preferable to choose a corrosion resistant material.
Control of scale formation;
With the addition of acid to the system (usually sulfuric acid is used), the pH balance of the water is regulated and the scale form is removed, but at the same time, excess acid that will cause corrosion can be added. The difficult part of this process is to set the acid dosage correctly and to ensure that it remains stable. Materials that stabilize water with a low concentration and prevent calcium carbonate deposits should be used with chemical dosing.
Control of deposits;
Deposit problems, which are usually caused by suspended substances in water, are not so significant. Intermittent treatments with dispersant complexes can be done when agglomerates are so large that they form muddy deposits.
If the water is well water containing iron, it is oxidized to give trivalent iron and iron hydroxide and rust deposits are formed. In this case, some phosphate complexes or phosphonate complexes that retain the iron as iron are used.
Periodic chlorination is applied to prevent biological developments.
Semi-Open Cooling Tower Circuits Conditioning
A semi-open cooling circuit is a recirculating water circuit on the air cooler. The air cooler provides the evaporation of some of the water by the water-air contact and removing the incoming calories through the heat exchangers. With each pass, its temperature rises at the level of the exchangers and cools at the level of the water tower. The efficiency of the tower depends on the technology in perfecting the water-air contact.
- Normal draft tower (Hyperbolic shape)
- Forced draft tower – with ventilating fans
- Equipment with extractor fans:
- Type wood, plastic, fiber cement, steel, galvanized
- System for adjusting droplets
- Air routing or division system
- droplets separator
In a semi-open cooling system, there is always a make-up water to compensate for the water losses caused by evaporation and deconcentration blowdown.
Deconcentration bluffing relies on the preferential removal of water to prevent increased salinity leading to sediments and agglomerations. Evaporation consists of pure water supplemented with a certain salinity, except for the real loss of scattering (wind). As a result, a salty concentration event occurs, which is restricted by the deconcentration bluff.
Properties of semi-open circuit
Each circuit has a certain number of parameters. It is necessary to know these before starting the conservation processes. These parameters are:
- m3as volume(V)
It is the total amount of water in the circuit. (Hot water containers, cold water containers, exchangers, machinery, piping etc.)
- m3Circulation flow in /h(Q)
- Tower inlet and outlet temperature difference (ΔT) in ᵒC
- Existing metal types in the system
- m3Evaporation rate in /h(E)
It is the flow rate of pure water evaporated in the tower to cool the water in the circuit. It is calculated by the empirical formula below.
E = Q /560 x ΔT Q =m3/h
ΔT = ᵒC
E = m3/h
- m3drift (wind) loss flow rate in /h(ES)
It is the flow rate of the water that enters the cooling tower and mixes with the atmosphere in droplets. The chemical composition of this water is the same as that of the water in the circuit and should be taken into account when calculating the blowdown. (TOS) varies according to the tower type.
- % 0.2 – 0.3 for normal draft towers(Q)
- % for forced draft towers 0.3 – 0.5(Q)
In systems it can be wind loss preventer that limits this loss to % 0.1 – 0.15.
m3blowdown flow in /h (B)
In order to avoid excessive concentration of mixed salts leading to significant accumulations
flow rate of water discharged from the circuit.
m3total blowdown flow in /h (BT)
From wind loss (ES), blowdown (B) and various water losses (leakage losses B) which must be negligible.K, traction losses BCetc.) the sum of the flow rates
BT= ES+ B + BK+ BC+ ……..
BTis the sum of the deconcentration bluffs (preferential or non-preferential) in the circuit.
Support water in m3/h (make-up) (A)
(V) is the flow rate to be added to the circuit to maintain its volume.
A = E + BT
A = E + ES+ B + BK+ BC+ . . . . .
Concentration increase coefficient ( c )
It is the ratio between the concentration of salts mixed with the water in the circuit and the concentration of the support water. Practically, this concentration ratio is determined by the amount of chloride. Because the amount and stability of this toner in the circuit can be adjusted easily.
Chloride amount in the circuit / Chloride amount in the support water
(A) and (B)T) Since they are constant, they reach an equilibrium state in a circuit with a certain concentration ratio ( C ) and are equal to the amount of mixed salts entering the support water.
S x A = CS x BT As a result C = A /BT
The concentration ratio is equal to the flow of the support water divided by the flow of the blowdown water.
Working process ( T )
Let's assume a semi-open circuit andx0) at concentration ( t0) instantly inject an item ( x ) . If there is no support water, over time due to blowdown ( Cx0It is clear that ) will decrease. The working process ( T ) is the same as when the concentration of ( x ) is halved. It is found as follows;
T = 0.7 X (V/BT)
V = m3
BT= m3/ h
T = h (hours)
The importance in the selection of the concentration ratio depends on the blowdown flow concentration ratio.
C = A/BT
A = E + BT
C = (E + BT ) / IT
Here is the following important result.
BT = E / (C – 1 )
If the concentration ratio and the blowdown are calculated for a particular circuit, the gains of the cooling circuits will also be seen. Because these calculations will greatly reduce water losses.
Example: Q = 1000 m3/ h , E = 20 m3/ h , BT = 20 / (C – 1) ΔT = 11.2ᵒC
Thus, the following table can be made.
Two important results are obtained from this table.
C = 1.05 ( P = 400 m3/h ) of
C = 2.0 ( P = 20m3/h) to a significant water savings ( 380 m3/h ) allows to do.
Transition from C = 3 to C = 5 only 5 m3/h gains.
Choosing the concentration ratio is an important event, as corrosion and scale-deposit problems increase with the concentration ratio.
Concentration rate ;
— It must be high enough for the system to operate in economic conditions.
— But it must not exceed a certain limit.
Problems with semi-open cooling circuits
The operating conditions of a semi-open circuit are determined by the various parameters of the circuit and especially by the choice of the concentration ratio.
— They must preserve the heat exchange, the most important task of the circuit is cooling.
They should keep the installation intact for as long as possible without aging.
— Operating costs should be reduced as much as possible. (Water, chemical consumption etc.)
When it starts to work in the circuit, the operator encounters the 4 most common classical problems.
- Stone binding (scale)
- puddle ( sludge)
Corrosion presents issues with circuit uptime and depreciation. Scale and sludge reduce heat transfer efficiency and cause local corrosion. Microorganisms are responsible for corrosion and organic sludge deposits.
Corrosion problems in semi-open cooling circuits are quite complex. Because the reasons are innumerable. Physical, chemical and biological factors may interfere. With all these factors, the corrosion rate increases in semi-open circuits.
The water of semi-open circuits is oxygenated to a certain extent. In other words, air-water contact will oxygenate the water in each passing through the cooling tower.
Concentration of mixed salts
The concentration of water in semi-open circuits increases the salinity of the originally available support water.
conductivity greatly increases and also increases the corrosion rate. In addition, the stimulating effects of chlorides and sulfates on corrosion are also affected. High toners can be achieved in semi-open circuits with the same concentration ratio.
In semi-open circuits, there is a large temperature difference between the coldest and warmest places. High temperatures increase the corrosion rate in hot spots .
— Presence of various metals
The most important is the presence of copper or copper compounds. Although the corrosion of copper is weak, Cu+2gives rise to ions. These accumulate on mild steel and then cause significant localized corrosion by micropiles (galvanic corrosion).
— Deposit corrosion with differential aeration
Residual corrosion is due to the equal concentration of the mixed oxygen. (EVANS effect) The metallic part with the residue where the oxygen concentration is weak becomes anodic and causes the tube to perforate. This event is important in terms of collecting the suspension materials contained in the support water or contaminating the system with the air-water contact in the cooling tower.
— Bacterial activation
Some types of bacteria (such as reducing sulfates or types that lose iron in reactions) cause corrosion.
— Impact of the environment
The interaction of a semi-open circuit with the environment is important. We know that the cooling tower also plays the role of air washer. If this air becomes polluted , the same elements appear in the surrounding water in the circuit water . Well ;
–corrosive substances ( H2S , SO2, NH3…. )
-suspended substances and sludges (sand, mineral and organic dusts)
Corrosion problem in semi-open cooling circuits should be considered as an important problem. To determine whether there is significant corrosion in a semi-open circuit;
- Calculation of corrosion with the RYZNAR index
Parameters depending on the working subjects of the circuit
— Analysis of support water
— Analysis of circuit water
— Concentration increase coefficient of the circuit
All this will serve to achieve the pH of theoretical saturation. pHsIt is given by the chart and the index is obtained according to the working pH of the circuit. The RZYNAR index is the most realistic for semi-open cooling circuits.
- After RZYNARIndexPhSadding the effect of other corrosion factors that are not in the calculation
– Metallics — Air pollution
– Circulation rate — Bacteria activities
- The amount of iron in the circuit water
It is an important factor in terms of corrosion. Because iron is formed by putting the steel walls of the circuit directly into the solution. The greater the amount of iron, the more important it is in corrosion. (The iron present in the support water should be taken into account.) If the iron present in a circuit shows significant corrosion, it is impossible to determine the amount of iron with corrosion rate, concentration amount and pH. Because the iron is dissolved or sedimented.
If pH < 7.5, the iron is in solution and if there is 1 ppm iron in the circuit, corrosion is negligible.
If pH > 8.0, iron in solution precipitates and the amount is meaningless.
- Internal examination of the circuit
In order to see the importance of sedimentation, it is necessary to take advantage of disassembly, cleaning and repair processes and to carefully examine the condition of metallic surfaces, especially heat exchangers. Even if it is known that there is corrosion, it is necessary to examine the inner surfaces of the circuit in order to determine its importance. The type of corrosion should be determined by analyzing whether the existing corrosion is uniform or in local and deep holes.
- analysis of residues
With the iron oxide percentage in the residues, it can be determined very well whether there is corrosion in the circuit or not. This analysis is especially used in circuits with high pH. Because for them, the amount of dissolved iron is not important as the iron precipitates.
- corrosion test coupons
It is a method of calculating corrosion. Pre-prepared and weighed witness corrosion test coupons are placed at selected locations of the circuit. These coupons are removed at intermittent times and re-measured and weighed. Thus, the rate of corrosion in a circuit can be determined by the measured weight loss.
The units used are:
Micron / year = 10-6mm / year , Milipus / year ( MPY ) = 25 Micron / year
The corrosion rate values obtained with the test coupons belong to a uniform corrosion, the weight loss is distributed over the entire metallic surface. For the determination of the corrosion type, the surface condition of the coupons should be examined.
There are instruments available to measure both the rate of snapshot corrosion and the tendency to pits. These instruments measure the corrosion rate with the electric current passing through the electrodes that are activated.
- Stone binding ( scale)
Scale is the formation of sticky and hard deposits on the walls of a heat exchanger or plumbing pipes resulting from the crystallization of some mineral salts in solution in water.
After these salts exceed the solubility limit, they precipitate due to the properties of the semi-open circuit.
Semi-open water circuits are very suitable for the formation of scales. There are two main reasons for this.
— Concentration increase event caused by evaporation
A water that does not have stone binding feature in open circuit may form scale in semi-open circuit where it will be concentrated many times.
If the wall temperature is too high in semi-open circuits, this phenomenon causes precipitation on the heat transfer surfaces of salts whose solubility decreases with temperature.
- puddle ( sludge)
Depending on biological developments, amorphous objects that are formed by the collection of solid particles such as organic or mineral sand, clay, dust, organic residues are called sludge.
The significance of the event depends on the amount of suspended solids present in the water in the circuit. But this event is not only dependent on the quality of the support water. Because no matter how clean and clear the support water is, suspended solids constantly rise in the cooling towers in semi-open circuits. It is well known that a cooling tower acts as a true air scrubber, 500 or 1000 volumes of air pass through one volume of water.
It should be known that the atmosphere is more or less dusty and also contains many pollutants compared to the surrounding industrial environment. For these reasons, site selection is very important when a semi-open circuit is to be made.
The circulation rate of water is of great importance in circuits containing suspended solids. Because these substances accumulate especially in places where the transition velocity is weak. ( < 0.5 m/s ) In semi-open circuits, especially biological deposits and incorrect conditioning programs cause deposits to form. Accumulation problems are the cause of many drawbacks.
— Thermal exchange losses
— Pressure loss increases that decrease the circulation flow
— Danger of clogged pump holes
— Deposit corrosion
— Bacterial corrosion
Algae are inferior cryptogams that do not contain chlorophyll. They occur mainly in cooling towers. Because all the necessary factors for them are available here.
Fungi are inferior cryptogams containing chlorophyll. This determines their difference with algae. Fungi destroy wood.
These fungi form on the wet parts of the frame and fixtures. They degrade the cellulose into a black and gelatinous mass of 2-3 mm thick.
This type of fungus thrives in moist environments that are not wet. They penetrate deep into the wood, destroying the cellulose and reducing the mechanical resistance.
Many kinds of bacteria live in semi-open circuits and it is very difficult to prevent their presence. Support water and air are very rich in bacteria. The main point is to prevent these bacteria from forming disturbing layers. As with algae, there are many factors (air, light, heat, CO) that cause their development.2etc.) are available in cooling systems.
Some bacterial families participate in the classical electrochemical corrosion mechanism that occurs between the anode and the cathode, which is well known for its corrosive effects.
— Aerobic bacteria
They live in an oxygen environment. Aerobic iron bacteria accelerate electrochemical corrosion by anodic polarization. They enable the bivalent iron to become trivalent by means of oxygen, and at the end of this reaction, Fe (OH) is formed.3occurs.
— Anaerobic bacteria
It is found in oxygen-poor areas. Reducing sulfate bacteria are the most well-known because of the damage they cause. ( Desulfobrio desulfurican ) It reduces the sulfate ion by using the molecular hydrogen produced by the cathodes. ( Cathodic depolarization ) This event accelerates corrosion. In practice, bubbles appear on the iron, the bottom of which is hollow like a crater, and it is seen that the iron has dissolved.
Conditioning of semi-open refrigeration circuits
Conditioning a water means radically changing its condition by adding very small amounts of chemicals without changing the composition of that water. These chemicals are called inhibitors.
We have briefly examined the problems caused by water in cooling water systems above. To date, many chemical conditioning programs have been developed to prevent these problems. In general, these conditioning programs take two different approaches.
- Barren It is formed by the crystallization of salts when the dissolved solid concentration exceeds the solubility limit. In order to prevent this crystallization, the RYZNAR stability index of the water is set in the range of 7.0 – 7.5 (corrosive prone) and the pH value in the range of 6.0 – 7.0. Thus, salts that can form scales are kept in a soluble form and their precipitation is prevented. In this case, the character of the water is corrosive and the metal surfaces are clean. A corrosion inhibitor should also be used to protect these surfaces from corrosion. In this type of conditioning program, the corrosion inhibitor must be very effective. This can only be achieved with chromates. However, the use of chromate has been abandoned in today's technology and due to the interaction of the systems with the environment. Because chromate compounds are extremely toxic and difficult to treat. In addition, due to the various properties of chromate compounds, max. For protection, very good control and monitoring of the system is required.
- In this second conditioning program, where organic inhibitors are generally used, the structure of the water is the opposite of the above program. By keeping the RYZNAR stability index of the water at 5.5 – 6.5 and the pH lower limit at 7.1 (no upper limit is the advantage of application.) the water becomes less aggressive and corrosive, its corrosive property min. is downloaded. Corrosion is completely stopped with the corrosion inhibitors used. Heat transfer surfaces are kept clean by adding some chemicals that act as crystal growth modifiers, dispersants and separators to the water. Acceptable corrosion rate max. It is 2.0 MPY. This conditioning program is also called high pH programs, where the water seeks its own pH. The addition of acid in the system is minimized, there is no problem with the salinity index, and besides the prevention of scale, the accumulation of scale in the system can be dispersed with this system and with the right dispersants. It has no harmful effects on the environment and human health. They are biodegradable (biodegradable) products. Due to these superiorities and being economical, they have become the preferred program in today's technology.
As a result, it is necessary to keep the RYZNAR stability index and the water character within the desired range, regardless of what kind of conditioning is done in all systems. However, from the chemical additive to be used after that, max. efficiency can be achieved. If it is ; As can be seen from the table,
1- Given max. The blowdown to be made in the system according to the coefficient increase can be achieved by keeping the system pH value within the desired range.
Conditioning Closed Cooling Tower Circuits
A closed circuit is a recurrent circuit, and the water in it only serves for calorie transfer. In the closed circuit, the water is not exposed to any evaporation or any concentration change. Therefore, bluffing is not done to adjust the concentration. Theoretically negligible amount of support water is needed. Water additions are usually caused by leakage problems, uncontrolled water intakes, repairs etc. In a closed circuit, more quality and conditioned water is required. No corrosion and stone formation is required in this type of business.
Since the water of a closed circuit is not constantly saturated with oxygen, the corrosion problem can be considered minor. In the application, since the oxygen is frequently and lightly, corrosion occurs in the form of deep cavities at the hottest points. ( Pitting ) In addition, galvanic corrosion caused by different metals is also encountered in closed circuits.
Theoretically, if the water of the circuit is not concentrated by evaporation and the amount of support water is small, scale formation is relatively unimportant. In closed circuits, if the amount of support water is important, the scalding phenomenon cannot be neglected. In order to prevent the formation of calcium and magnesium-based stones, the supporting water must be purified and chemical conditioning must be done.