What Is a Cooling Tower?

The cooling tower ^[1] (The cooling tower) is a device that uses water as a circulating coolant to absorb heat from a system and discharge it to the atmosphere to reduce the temperature of the water. Its cooling is the use of cold and heat exchange between water and air to generate steam The steam volatilization takes away the heat and reaches the principles of evaporation, convective and radiant heat transfer to remove the waste heat generated in industry or refrigeration and air conditioning to reduce the water temperature of the evaporative cooling device to ensure the normal operation of the system. The device is generally a barrel It is called a cooling tower.

The cooling tower is a comprehensive product integrating aerodynamics, thermodynamics, fluidics, chemistry, biochemistry, materials science, static and dynamic structural mechanics, and processing technology. Water quality is a multi-variable function, and cooling is a multi-factor, multi-variable and multi-effect synthesis process ^[2]

Divided by ventilation method: natural ventilation cooling tower; mechanical ventilation cooling tower; mixed ventilation cooling tower ^[3]

Some useful terms are commonly used in the cooling tower industry:

1. Analysis of fan energy-saving controller

The purpose of fan energy-saving control management is to realize closed-loop automatic control of fan operation. The water supply temperature is set in advance according to the needs of the production. The influence of the climatic and meteorological environment on the water temperature and the effect of changes in the system heat transfer conditions on the water temperature are reflected in time with the measured values of the temperature-sensing probe. Stable water supply temperature, realize self-control energy saving.

Generally speaking, the "frequency conversion speed regulation technology" is the ideal method to complete the above process. However, the application of variable frequency speed regulation technology in the control of circulating water cooling tower fans has the following limitations and defects:

"Variable frequency speed regulation technology" can achieve high temperature control accuracy, but this is not very important in the circulating cooling water system.

The energy loss of the inverter (average operating efficiency is less than 90%) affects the energy saving effect.

The variable-speed operation causes the angle of attack of the fan blade to change (windward angle), and the fan runs away from the operating point to reduce efficiency.

The low-speed operation of the motor out of the rated speed, and the non-linear relationship between speed, torque, and power consumption also greatly reduce the motor's operating efficiency.

The frequency conversion speed regulation system is relatively expensive (about 1,000 yuan per kilowatt), and new projects and old equipment transformation require large investment.

The design must also consider the problem of destructive resonance when the frequency converter is running at certain speeds, and the interference of other instruments caused by the strong electromagnetic pollution caused by the frequency converter.

2. Fan safety monitor analysis

The purpose of the fan safety monitoring and management is to automatically detect the changes in vibration, oil temperature, and oil level, and display and record them. At the same time, alarm and shutdown of the fans whose detection values exceed the limit, in order to achieve safe and stable operation The purpose is to reduce or even prevent the occurrence of fan damage. According to the actual situation of site management, the three parameters of "fan vibration", "oil oil temperature" and "decelerator oil level" were determined as the most important operating parameters to ensure the safety of the fan [3]. A total of 15 design parameters such as "measurement range", "measurement accuracy", and "inspection time" were determined for R & D and production. The system was first tested in a circulating water field in September 1993 and named "KR-939 Fan Safety Monitor".

The system uses multi-parameter combined probe technology, digital instruction coding technology and computer network management technology. The three-parameter combination probe is installed on the fixed base of the fan reduction box. The probe rod is directly inserted into the oil. The oil temperature, berth and equipment vibration value in the reduction box are directly converted into electrical signals and transmitted to the control room. Fan safety monitor. Each safety monitor can use a four-core cable to connect 8 combined probes to monitor the operating parameters of 8 fans in real time and complete digital display. Multi-phase functions such as over-limit alarm and over-limit shutdown. After many tests and modification design, it has been successfully applied to the equipment production site, and the parameters have reached the predetermined design requirements.

3. Achieve computer network control analysis

The two measurement and control systems introduced above can be connected to a management computer through a four-core communication cable (RS-422 standard serial interface). The computer can be a general-purpose PC or an industrial control computer. When equipped with the corresponding configuration monitoring management software (DCS-900 software), it can realize network control with multiple KR-933 and KR-939 monitors. The following functions have been added to the fan monitor after networking with the computer:

Simultaneously monitor the measurement parameters of all controllers in the network to achieve comprehensive management.

Modify the setting parameters of each controller in the network.

According to the change of operating parameters of each controller, the system is optimized and managed.

Record historical data and graphics to help analysis and query.

4. Effect analysis of fan management research

4. Ensure the safe operation of the fan

According to field experience, the characteristics of the oil temperature, oil level, and vibration curve of a fan in good condition are as follows:

Oil temperature curve: It gradually rises and falls from the moment of starting and stopping, and becomes an approximately straight and smooth curve in about 1h.

Berth curve: Regardless of whether the machine is turned on, it should be approximated by a horizontal straight line.

Vibration curve: In the state of starting, make an irregular curve with a narrow vertical oscillation around a virtual straight line.

5. Defect analysis

5.1 Large fans are not suitable for KR-933 energy-saving controllers

For the circulating water field of high-power and low-power unit fans, as each fan is turned on and off, it will have a great impact on the water temperature. Therefore, the application of KR-933 fan energy-saving controller cannot control the water temperature normally and stably. For example, there are three fans with a diameter of 8.53m and a power of 160kW in the sixth circulation water field. Assuming that a fan energy-saving controller is installed, the allowable difference in the set temperature rate is assumed. When the parameters such as temperature tolerance and execution period are inevitable, great contradictions will inevitably occur. It is difficult to select appropriate parameter values, and ultimately the purpose of energy saving and consumption reduction will not be achieved. In this case, the fan management is more suitable for the control management of the automatic frequency conversion speed regulation system. Preparations are also being made in this regard.

5.2 The oil level measurement technology of KR-939 safety control system needs to be improved

KR-939 safety monitor still has shortcomings. Its main problem is oil level monitoring. Due to the influence of harsh conditions, hot wire scaling and oil breakage due to water content are more likely to occur. If the probe is not overhauled in time, you need to carry out a manual inspection on the tower.

To strengthen the scientific modernization management of wind turbines, continuous improvement should be made on the existing basis.

1. The circulating water volume is gradually lost due to the following factors during the operation of the cooling tower:

A During the heat exchange process between hot water and cold air in the tower, part of the water volume will become gas and evaporate;

B Because the cold air is pumped by mechanical power (motor and windmill), under high wind speed conditions, some water will be pumped out;

C Due to the repeated circulation of cooling water, the solid concentration in the water is gradually increasing, affecting the water quality and prone to algae, so it must be partially discharged and supplemented with fresh water.

2. Calculation of supplementary water:

A Water loss due to evaporation (E)

E = Q / 600 = (T1-T2) * L / 600

E represents the amount of evaporated water (kg / h); Q represents the heat load (Kcal / h);

600 represents the latent heat of evaporation (Kcal / h); T1 represents the temperature of entering the water (° C);

T2 stands for effluent temperature (); L stands for circulating water (kg / h)

B splash water loss (C)

The splash loss of the cooling tower is determined by the cooling tower design type and wind speed. Under normal circumstances, its value is about 0.1 ~ 0.2% of the circulating water volume. C. Periodic water loss (D)

Periodic water loss depends on factors such as water quality or solid concentration in the water. Generally, it is about 0.3% of the circulating water volume.

D make-up water (M)

The total replenishment of circulating water in the water tower is equal to M = E + C + D

When the cooling tower is used for air conditioning, the temperature difference is designed at 5 ° C. At this time, the amount of supplemental water required by the cooling tower is about 2% of the circulating water amount.

Because most cooling water contains calcium, magnesium ions and acid

Painting primer:
After the surface treatment of the cooling tower is qualified, the prepared epoxy primer is applied within 6 hours. The primer plays a supporting role and has excellent adhesion to the substrate. Before the first primer is applied, cotton yarn is applied Dip gasoline to scrub the surface of the cooling tower first to remove debris and floating dust on the surface of the cooling tower. The primer must be uniform, no flowing, no leak coating, no pinholes, and no air bubbles. . The film thickness is 25 35um. The amount of primer is 80-90 g / m2.
Brush the corners and yin and yang corners again. When painting, the facade should be painted up and down, left and right, difficult first, and easy, crisscross. After the first primer is cured naturally, debris and floating dust are removed from the cooling tower surface, and the second epoxy coal flake primer can be painted in the same way. The thickness of the second primer is 35-40um, and the dosage is about 100g / m2.
The pits, cracks and leaks on the surface of the concrete cooling tower should be filled with epoxy putty, and then the second coat of epoxy coal glass flake primer is applied.
Painting intermediate paint:
After the epoxy primer has been cured for 24 hours and basically dried, the experience is acceptable. That is, the ring-coal glass flake intermediate paint is painted by the same painting method. The amount of each coating is 110-125 g / m2, and the thickness of the paint film is 45um. And to ensure that there are no pinholes, no flow, no leakage, the surface of the cooling tower should be flat, uniform, full, and the gloss is consistent.
After the intermediate paint is completely dry and qualified, remove the dust on the surface of the cooling tower, then you can apply the first coat of epoxy coal glass flake topcoat, the dosage is 100-125 g / m2, and the film thickness is 40-50um; After the topcoat is dried, apply a second topcoat in order, with a thickness of 80 to 100um. The last topcoat is a key process. When painting, make sure that the paint liquid is full, uniform in color, smooth and flat, and has a certain gloss. The appearance is beautiful and consistent.

The materials of cooling towers are mainly carbon steel, stainless steel and copper. Among them, when carbon steel tube sheets are used as cooling towers, the welds between the tube sheet and the tube are often corroded and leaked, and the leakage will enter the cooling water system. Causes pollution to the environment and waste of materials.

In the manufacture of cooling towers, the manual welding of tubesheets and tubes is usually performed by manual arc welding. There are different degrees of defects in the shape of the weld, such as depressions, pores, slag inclusion, etc., and the stress distribution of the weld is uneven. When in use, the tube sheet part is in contact with industrial cooling water, and impurities, salts, gases, and microorganisms in the industrial cooling water will cause corrosion to the tube sheet and the weld. Studies have shown that whether industrial water is freshwater or seawater, there will be various ions and dissolved oxygen. The changes in the concentration of chloride ions and oxygen play an important role in the shape of metal corrosion. In addition, the complexity of the metal structure also affects the corrosion morphology.

For the anticorrosion of cooling towers, the traditional method is mainly repair welding, but repair welding can easily cause internal stress inside the tube sheet, which is difficult to eliminate, which may cause the weld of the cooling tube tube sheet to leak again. At present, most western countries use the method of polymer composites for protection ^[2] .