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Performance is an important part of the water used in the cooling industry. Cold water is a spring and takes calories from a hot water source. Calorie change or heat transfer usually occurs above the metal surface without contact with hot and cold welding. This process requires cooling of the cooling tower.
There are 3 types of cooling tower circuits.
Open circuit
Semi-open
Closed circuits
Open circuit cooling tower ventilation
Water is thrown back after passing directly through various cooling devices. In this type of circuit, a lot of water is lost. These circuits in old systems are faced with high water and low cost.
What is the main problem: corrosion
Scale (binding stone) sludge (sediment) of biological formations. Semi-open cooling is the same problem that will be investigated in refrigeration circuits. But here the problems are milder. Since water is monolayer, there is no significant change in its chemical and physical properties and water is not concentrated. The temperature difference 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 calcicarbonate equilibrium. In this case, the RYZNAR stability index is used to determine whether corrosive water (CO2 toner product performance) is present at equilibrium or scale.
Ventilation solutions
Corrosion control
Use scaling agent according to RZYNAR index to adjust pH.
Prevention of corrosion by the effect of phylogeny
Problems 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. Continuous ventilation is essential to maintain film quality.
In open circuit corrosion conditions, it is economically preferable to choose a corrosion-resistant material.
Control the formation of scale
By adding acid to the system (usually sulfuric acid is used) the pH balance of the water is adjusted and the sediment is removed, but at the same time excess acid can be added which causes corrosion. The difficult part of this process is adjusting the acid dose correctly and making sure it stays stable. Substances that stabilize water at low concentrations and prevent calcium carbonate deposition should be used in chemical doses.
Accumulation control
Sediment problems that are usually caused by suspended solids are not very significant. Intermittent treatments with scattered complexes can be performed when the agglomerates are large enough to form mud deposits.
If the well water contains iron, it oxidizes and trivalent iron is obtained, and iron hydroxide and rust deposits are formed. In this case, some phosphate complexes or phosphonate complexes are used that retain iron as iron.
Biological control
Periodic chlorination is applied to prevent biological changes.
Ventilation of semi-open circuit cooling towers
A semi-open cooling circuit is a circulating water circuit on an air conditioner. The air conditioner provides evaporation of some water by contacting the weather and removing calories received through heat exchangers. With each pass, its temperature rises on the surface of the converters and cools on the surface of the water tower. The efficiency of the tower depends on the technology in completing the weather call.
Tower type:
Normal traction tower (hyperbolic shape)
Forced traction tower – with ventilation fans
Equipment with extraction fans
Type of wood, plastic, cement fiber, steel, galvanized
Drop adjustment system
Routing or air sharing system
Dropper separator
In a semi-open cooling system, there is always a cosmetic water to compensate for water losses due to evaporation and decentralized blowing.
Reduction of concentration relies on preferential removal of water to prevent salinity increase leading to sediments and accumulations. Evaporation involves pure water with a certain salinity, except for the actual loss of dispersion (wind). As a result, a salt concentration event occurs that is constrained by a decentralized bluff.
Properties of semi-open circuit
Each circuit has a certain number of parameters. It is important to know this before starting protection processes. These parameters are:
Volume in m 3 (V)
This is the total amount of water in the circuit. (Hot water containers, cold water containers, converters, machines, plumbing, etc.)
Circulating flow in m 3 / h (Q)
The difference between the inlet and outlet temperature of the tower (ΔT) in degrees Celsius
Types of metals in the system
Evaporation rate in m 3 / h
The rate of pure water flow that evaporates in the tower to cool water in the circuit. It is calculated by the following experimental formula.
E = Q / 560 x ΔT Q = m 3 / h
ΔT = ᵒC
E = m 3 / h
Fluctuation (wind) loss flow rate m 3 / h (E S)
It is the speed of water flow that enters the cooling tower and mixes with the atmosphere in drops. The chemical composition of this water is similar to the water in the circuit and must be taken into account when calculating its tail. (ES) varies according to the type of tower.
For conventional tensile towers, 0.2 – 0.3% (Q)
0.3 – 0.5% (Q) for forced traction towers
In systems, it can be a wind loss prevention that limits these losses to 0.1-0.15%.
Explosion current in m 3 / h (B)
In order to prevent excessive concentration of mixed salts that lead to significant accumulation
Speed of water discharged from the circuit
Total blowing current (BT) in m 3 / h (B T)
The sum of flow rates due to wind losses (E S), wind losses (B) and various water losses (leakage losses B K, traction losses B Ç, etc.) should be negligible.
B T = E S + B + B K + B Ç + …… ..
B T is the sum of decentralization bluffs (preferred or non-preferred) in the circuit.
Water support in m3 / h (arrangement) (A)
(V) is the flow rate that must be added to the circuit to maintain its volume.
A = E + B T
A = E + E S + B + B K + B Ç +. . . . .
Concentration increase coefficient (c)
The ratio is the concentration of salts mixed with the water in the circuit and the concentration of the holding water. In practice, this concentration ratio is determined by the amount of chloride. Because the amount and stability of this toner in the circuit is easily adjustable.
The amount of chloride in the circuit / the amount of chloride in the holding water
Since (A) and (B T) are constant, they reach equilibrium in a circuit with a certain concentration ratio (C) and is equal to the amount of mixed salts introduced into the holding water is written as follows.
S x A = CS x B T As a result C = A / B T
The concentration ratio is equal to the holding water flow divided by the blower water flow.
Work Process (T)
Let’s assume a semi-open circuit and inject a substance (x) at time (t 0) at a concentration (C x0). If there is no backup water, it is clear that (C x0) decreases over time due to bluffing. The process (T) is the same as when the concentration (x) is halved. Can be found as follows;
T = 0.7 X (V / B T)
V = m 3
B T = m 3 / h
T = h (hours)
The importance in choosing the concentration ratio depends on the ratio of the blower current concentration.
C = A / B T
A = E + B T
C = (E + B T) / BT
Here is the important result below.
B T = E / (C – 1)
If the concentration-blow ratio is calculated for a particular circuit, the gain of the cooling circuits is also seen. Because these calculations will greatly reduce water losses.
Example: Q = 1000 m 3 / h, E = 20 m 3 / h, B T = 20 / (C – 1) ΔT = 11.2ᵒC
Therefore, the following table can be prepared.
C 1.05 1.1 1.2 1.3 1.5 1.8 2.0 2.5 3 4 5
BT (m3 / h) 400 200 100 66.6 40 25 20 13.3 10 6.6 5
Two important results are obtained from this table.
C = 1.05 (P = 400 m 3 / h)
Switching to C = 2.0 (P = 20m 3 / h) saves significant water (380 m 3 / h).
Switching from C = 3 to C = 5 saves only 5 m 3 / h.
Choosing the concentration ratio of an event is important because the problems of corrosion and sediment deposition increase with the concentration ratio.
Note:
It must be high enough for the system to work in economic conditions.
But it should not exceed a certain limit.
Problems with semi-open circuit cooling towers
The operating conditions of a semi-open circuit are determined by various circuit parameters, especially by selecting the concentration ratio.
– They must maintain heat exchange, the most important task of the cooling circuit.
They should keep the installation intact for as long as possible without aging.
– Operating costs should be reduced as much as possible. (Water consumption, chemicals and…)
When starting a circuit, the operator encounters 4 common classic problems.
Corrosion
Stone Binding (Scale)
Pit (sludge)
microorganisms
Corrosion causes problems with circuit operation time and wear. Sediment and sludge reduce heat transfer efficiency and cause local corrosion. Microorganisms are responsible for the corrosion and deposition of organic sludge.
Corrosion
Corrosion problems in semi-open cooling circuits are very complex. Because the reasons are countless. Physical, chemical and biological factors may interfere. With all these factors, the amount of corrosion in semi-open circuits increases.
Oxygen concentration
Semi-open water circuits become somewhat oxygenated, in other words, the contact of air and water causes oxygen to be supplied to the water passing through the cooling tower.
Concentration of mixed salts
Concentration of water in semi-open circuits increases the salinity of the primary support water.
Conductivity increases sharply and also increases the corrosion rate. In addition, the irritating effects of chlorides and sulfates on corrosion are also affected. High toners can be obtained in semi-open circuits with the same concentration ratio.
– Heat
In semi-open circuits, there is a large temperature difference between the coldest and warmest places. High temperatures increase the rate of corrosion in hot spots.
– Existence of different metals
The most important of these is the presence of copper or copper compounds. Although copper corrosion is weak, it produces Cu + 2 ions. They accumulate on mild steel and then cause significant local corrosion by microcandles (galvanic corrosion).
– Sedimentary corrosion with differential aeration
The residual corrosion is due to the equal concentration of oxygen in the mixture. (EVANS effect) The metal part with the residue in which the oxygen concentration is weak, anodes and causes the pipe to puncture. This event is important in terms of collecting the suspension in the support water or contaminating the system with air-to-water contact in the cooling tower.
Bacterial activation
Some types of bacteria (such as reducing sulfates or those that lose iron in reactions) cause corrosion.
– Environmental effects
The interaction of a semi-open circuit with the environment is important. We know that the cooling tower also plays the role of an air washer. If this air is polluted, the same elements will appear in the surrounding water in the water circuit. Good ;
– Corrosive substances (H 2 S, SO 2, NH 3….)
– Suspended materials and sludge (sand, mineral and organic dust)
Result ;
The problem of corrosion in semi-open cooling circuits should be considered as an important problem. To determine if there is significant corrosion in a semi-open circuit.
Corrosion calculation with RYZNAR index
The parameters depend on the working topics of the circuit
– Supported water analysis
– Water circuit analysis
– Circuit concentration increase coefficient
– Heat
All this is useful for achieving theoretical saturation pH. PH s is given by Saab 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.
Add the effect of other corrosion factors that are not present in the PH s account after the RZYNAR index
– Metals – Air pollution
– Circulation rate – Bacterial activity
– Residues – Heat
The amount of iron in the water circuit
It is an important factor in terms of corrosion. Because iron is formed by placing the steel walls of the circuit directly in solution. The higher the amount of iron, the more important it is in corrosion. (The iron in the retaining water must be considered.) If the iron in the circuit is significantly corroded, it is impossible to determine the amount of iron at the corrosion rate, concentration, and pH. Because iron dissolves or precipitates.
If the pH is less than 7.5, iron is in solution, and if 1 ppm of iron is in the circuit, corrosion is negligible.
If the pH is> 8.0, iron precipitates in solution and its value is meaningless.
Internal circuit inspection
To understand the importance of sedimentation, one must use the processes of separation, cleaning and repair, and carefully examine the condition of metal surfaces, especially heat exchangers. Even if corrosion is found, the internal surfaces of the circuit should be inspected for significance. The type of corrosion should be determined by analyzing whether the existing corrosion is uniform or in localized and deep holes.
Residue analysis
With the percentage of iron oxide in the residues, it is possible to determine whether there is corrosion in the circuit or not. This analysis is especially used in high pH circuits. Because they do not care about the amount of dissolved iron because iron precipitates.
Corrosion test coupons
This is a way to calculate corrosion. Pre-prepared and weighted control corrosion test coupons are placed at selected locations in the circuit. These coupons are removed at regular intervals and re-measured and weighed. Therefore, the corrosion rate in the circuit can be determined by the weight loss measured.
The units used are:
Microns / year = 10 -6 mm / year, millipus / year (MPY) = 25 microns / year
Corrosion rate values obtained with test coupons belong to a uniform corrosion, weight loss is distributed over the entire metal surface. To determine the type of corrosion, the surface condition of the coupons must be checked.
کوریمتر
There are tools for measuring the degree of corrosion of the snapshot and the tendency of the hole. These instruments measure the amount of corrosion by passing an electric current through the activated electrodes.
Binding stone (scales)
Sediment is the formation of viscous and hard sediments on the walls of heat exchangers or piping pipes, which is the result of 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.
reasons
Semi-open water circuits are very suitable for forming scales. There are two main reasons for this.
Concentration event due to evaporation
Water that does not have the property of connecting a rock in an open circuit may cause sediment in a semi-open circuit where it is concentrated many times.
– Mood
If the wall temperature in semi-open circuits is too high, this phenomenon will cause salts to settle on the heat transfer surfaces whose solubility decreases with temperature.
Pit (sludge)
Depending on biological evolution, amorphous bodies that form from the collection of solid particles such as organic or mineral sand, clay, dust, and organic debris are called sludge.
The significance of this event depends on the amount of suspended solids in the water in the circuit. But this event does not depend only on the quality of the support water. Because no matter how clean and clear the water retainer is, suspended solids are constantly rising in cooling towers in semi-open circuits. It is well known that a cooling tower acts as a real air scrubber, 500 or 1000 volumes of air passing through a volume of water.
It should be noted that the atmosphere is more or less dusty and also contains more pollutants than the surrounding industrial environment. For these reasons, the choice of location is crucial when building a semi-open circuit.
The speed of water circulation in circuits containing suspended solids is of great importance. Because these materials accumulate especially in places where the transfer rate is low. (<0.5 m / s) In semi-open circuits, especially biological sediments and improper ventilation programs cause sedimentation. Accumulation problems are the cause of many problems.
– Heat exchange losses
– Pressure drop increases which reduces blood flow
– Risk of clogging of the pump hole
– Sedimentary corrosion
Bacterial corrosion
microorganisms
Algae
Algae are post-cryptograms that do not contain chlorophyll. They occur mainly in cooling towers. Because all the necessary factors for them are available here.
Mushrooms
Fungi are lower cryptogams that contain chlorophyll. This distinguishes them from algae. Fungi destroy wood.
– Ascomists
These fungi form on the wet parts of the frame and equipment. They break down cellulose into a black and gelatinous mass 2-3 mm thick.
– Basidiomycetes
This type of fungus grows in humid environments that are not wet. They penetrate deep into the wood, destroying cellulose and reducing mechanical strength.
bacterias
Many types of bacteria live in semi-open circuits and are very difficult to prevent. Backup climates are very rich in bacteria. The main thing is to prevent the formation of annoying layers of these bacteria. Like algae, there are many factors (air, light, heat, CO 2) that cause them to grow in cooling systems.
Some bacterial families participate in the classical electrochemical corrosion mechanism that occurs between the anode and cathode, which is known for its corrosive effects.
– Aerobic bacteria
They live in an oxygen environment. Aerobic iron bacteria accelerate electrochemical corrosion by anodic polarization. They make divalent iron by trivalent oxygen, and at the end of this reaction Fe (OH) 3 is formed.
Anaerobic bacteria
Oxygen is found in poor areas. Sulfate-reducing bacteria are best known for the damage they cause. (Desulfobrio desulfurican) Reduces sulfate ions by using molecular hydrogen produced by cathodes. (Cathodic depolarization) This event accelerates corrosion. In practice, bubbles appear on the iron, the bottom of which is like a hollow opening, and it is observed that the iron has dissolved.
Air conditioning of semi-open refrigeration circuits
Water conditioning means a fundamental change in its state by adding very small amounts of chemicals without changing the composition of that water. These chemicals are called inhibitors.
Above we have briefly examined the problems caused by water in cooling water systems. To date, many chemical ventilation programs have been developed to prevent these problems. In general, these conditioning programs represent two different approaches.
If the concentration of soluble solids exceeds the solubility limit, it is formed by the crystallization of salts. In order to prevent this crystallization, the RYZNAR water stability index is set in the range of 7.0 – 7.5 (corrosion prone) and the pH value is in the range of 6.0 – 7.0. Therefore, the salts that can form scales are kept in solution and their deposition is prevented. In this case, it is water corrosive and the metal surfaces are clean. A corrosion inhibitor should also be used to protect these surfaces from corrosion. In this type of ventilation program, the corrosion inhibitor must be very effective. This can only be achieved with chromates. But the use of chromate in today’s technology has been abandoned due to the interaction of systems with the environment. Because chromate compounds are highly toxic and difficult to treat. In addition, due to the different properties of chromate compounds, the maximum. Required for excellent protection, control and monitoring of the system.
In this second ventilation program, which usually uses organic inhibitors, the water structure is the opposite of the above program. By keeping the RYZNAR water stability index at 5.5-6.5 and the low pH at 7.1 (no upper limit is an advantage), water becomes less aggressive and corrosive, its corrosiveness is minimal. Will be downloaded. With corrosion inhibitors, corrosion is completely stopped. Heat transfer surfaces are kept clean by adding some chemicals that act as crystal growth modifiers, diffusers and separators to the water. The maximum acceptable corrosion rate is 2.0 MPY. This ventilation program is also called high pH program, where water follows its own pH. The addition of acid in the system is minimized, there is no problem in terms of salinity index, and in addition to preventing sediment, the accumulation of sediment in the system can be dispersed with this system and with suitable dispersants. It has no harmful effects on the environment and human health. They are biodegradable (biodegradable) products. Due to these advantages and cost-effectiveness, they have become the preferred program in today’s technology.
As a result, regardless of what type of ventilation is provided in all systems, the water specificity must be kept within the desired range with the RYZNAR stability index. However, from the chemical additive used thereafter, the maximum. Performance is achievable. If it’s; As shown in the table,
1- According to the maximum. By maintaining the pH of the system in the desired range, blowing in the system can be achieved due to the increase in coefficient.
Ventilation of CCTV cooling towers
A closed circuit is a return circuit and the water in it only works to transfer calories. In a closed circuit, water is not subject to evaporation or concentration changes. Therefore bluffing is not done to adjust the concentration. Theoretically, a small amount of support water is needed. Water additions are usually caused by leakage problems, uncontrolled dewatering, repairs, etc. In a closed circuit, more quality water is needed. In this type of job, there is no need for corrosion and stone formation.
Corrosion
Since the water in a closed circuit is not constantly saturated with oxygen, the corrosion problem can be considered minor. In application, since oxygen is frequent and low, corrosion occurs in the form of deep cavities in the hottest spots. (Hole) In addition, galvanic corrosion caused by various metals is also observed in closed circuits.
Formation of chips
Theoretically, sediment formation is relatively insignificant if the water circuit is not concentrated by evaporation and the amount of water retaining is low. In closed circuits, the burn phenomenon can not be ignored if the amount of water retention is important. To prevent the formation of calcium and magnesium stones, the retaining water must be treated and chemically ventilated.
Resources:
Wikipedia
Solechem website
Awt.org Association
Simon Fraser University
