In recent years, induction heat treatment technology has been rapidly developed and applied in the heat treatment industry due to its advantages of energy saving, high quality, high efficiency, low cost, easy mechanization, and automation, etc. Heat treatment technicians have done a lot of research on the degree of automation of the induction heat treatment process and equipment and have made a lot of achievements. However, there are few reports on the fouling problem of cooling water of induction heat treatment equipment, and insufficient attention is paid to the scaling problem of cooling water, resulting in unstable equipment and high failure rate in the daily management of induction heat treatment, resulting in unstable product heat treatment quality. According to our use of induction heat treatment equipment for many years, the problem of cooling water scaling of induction heat treatment power supply is an important factor affecting the quality stability of induction heat treatment products, which should be paid enough attention to.
1. Influence of scale on induction heat treatment equipment
Once the scale is formed in the circulating cooling water system, it will bring the following harms to the system: it will affect the heat transfer of the heat exchanger, increase the flow resistance in the system, intensify the corrosion process of the system equipment, and increase The Times of equipment shutdown and cleaning, etc.
Scale generation is mainly in the cooling inner wall of inductive power supply electrical components, transformer cooling copper tube, inductor copper tube, as shown in Figure 1, Figure 2, figure 3.Scale can directly reduce the cooling effect of power supply, reduce the life of electrical components, and reduce the effective output power of power supply.
FIG. 1 Pipeline scaling
Figure 2 Sensor scaling
FIG. 3 Scale of SCR cooling water jacket
2. Characteristics and causes of scale
Scale, also known as hard scale or inorganic scale, is the scale formed when insoluble or slightly soluble salt in cooling water deteriorates in operating conditions of circulating water.The common scale includes calcium carbonate, calcium phosphate, calcium sulfate, magnesium hydroxide, calcium silicate, iron oxide, etc. Among them, a carbonate scale with calcium carbonate as the main component is relatively common.
At present, it is believed that the scale formation process has the following six aspects:
(1) Crystallization
The formation of scale is mainly caused by the crystallization of micro-soluble salts, as shown in the following reaction:
Ca（HCO3）2→CaCO3+CO2+H2O
Mg（HCO3）2→MgCO3+CO2+H2O
MgCO3+H2O→Mg（OH）2+CO2
3Ca2++2PO43-→Ca3（PO4）2
The crystalline core may be a rough place in the pipe or a suspended substance in the water.
(2) Sedimentation
The existing suspended particles and formed crystals in the water rely on gravity to settle on the surface of the metal equipment, and this settlement produces dirt that is much more serious in the stagnant area than in the flowing area. This is also why we often find in the production site cooling water flow slows down, the main reason for the acceleration of cooling water blocked waterways, but also in the design of the power cooling water system, the pump head to choose a larger, faster flow rate reason.
(3) Combination and polymerization
When circulating water contains oil and hydrocarbon organics, they will increase the cohesion and bonding force of sludge, thus accelerating the growth of sludge and adhesion on the metal surface. Reasons why a closed cooling system is superior to the open cooling system in power supply cooling water.
(4) Microbial growth
In addition to producing microbial slime, microorganisms themselves are also the crystallization core of suspended substances, and their reproduction will increase the deposition of dirt.
(5) Corrosion
Corrosion products produced by corrosion are part of the sludge, and corrosion will make the surface of the metal rough, which is beneficial for scaling and settling.
(6) Baking effect
When dirt is baked at high temperatures, it becomes hard and difficult to remove.
The six processes described above are independent and interact with each other. In these processes, crystallization and sedimentation are the main processes of fouling deposition.
3. Anti-scale of cooling water system
In industrial production, there are currently two types of carbonate scale prevention methods:
(1) External treatment, that is, before the supplementary water enters the cooling water system, the scaling substances are removed or reduced, such as lime precipitation method and sodium cation exchange method. The sodium ions on the sodium-type cation exchange resin are ion-exchanged with the calcium ions in the supplementary water. The result is that the calcium ions in the water bind to the resin, while the sodium ions on the resin enter the water. The equation is as follows:
R (SO3Na) 2+Ca (HCO3) 2 — R (SO3) 2Ca+2NaHCO3 (softening reaction)
The softened water contains no or only a small amount of calcium ions. After the softened water enters the cooling water circulation system, the calcium hardness of the water is still very low, so it is not easy to scale the whole cooling water system. That is, the externally treated cooling water is added to the power supply cooling system for recycling, and the water quality is regularly tested. This method has been widely used in the induction heat treatment cooling water system.
(2) Internal treatment, that is, some drugs are added to the circulating cooling water to transform the scaling substances in the water into non-scaling substances, or to make the scaling substances in the water deform, disperse and stabilize them in the water, such as adding acid method and water quality stabilizer method. Scale inhibitor method: In the circulating cooling water, adding a small number of chemical agents can increase the hardness of its limit carbonate and prevent scale formation. This agent is called a scale inhibitor. Due to the need to add some drugs, internal treatment may bring water pollution, in the heat treatment induction heat treatment system is still less used.
4. Conclusion
For the induction heat treatment cooling system, in order to ensure that the cooling system does not scale or less scale, the following points should be paid attention to in the design and daily management of the induction heat treatment water cooling system:
(1) The use of softened water in power supply cooling water is the premise to ensure no scale formation or less scale formation of water. Meanwhile, the hardness of softened water is regularly tested.
(2) Closed circulation system is better than an open cooling system for power supply cooling water.A closed cooling system can effectively reduce the entry of foreign microorganisms, dust, and other pollutants. Reduce the combination and polymerization of water scale.
(3) In the cooling system, pumps with a high head should be selected to increase the flow rate of water in the pipeline and reduce the settlement effect of scale.
(4) In the powerful cooling system, the pipe shall be made of stainless steel pipe, copper pipe, or reinforced plastic pipe, avoiding the use of galvanized steel pipe.
(5) In the cooling system, the control of water temperature is the key point, so it is necessary to monitor the water temperature in the cooling system and give an alarm when the temperature is overheated. Generally, the water temperature should not be higher than 35℃.
(6) Cleaning and testing the powerful cooling system regularly every year is also a necessary condition to ensure the stability of the cooling system.Such as the regular cleaning plate, plastic hose aging replacement, water temperature, pressure monitoring instrument detection.