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In coreless induction melting systems, water is vital to the success of a complete operating system. It needs the high quality water to maximize system reliability and component longevity for the cooling of power supplies and furnaces. In a coreless induction furnace much of the heat loss by the metal passes through the furnace lining. Heat is also generated in the power coil or induction coil itself by the passage of current. To prevent damage and overheat to the coil it must be water cooled. A sample of the cooling water passing through inside the thick-walled copper tubing is shown in Figure 2.8.

Figure 2.8. A Sample Induction Coil with Cooling Water

Figure 2.9. Sample of the Damaging Induction Coil

Flow velocity and monitoring of all water circuit should be considered for the cooling of induction coil. Bailey [10] recommended that all cooling-passages should be designed so that the flow velocity is not less than 1 meter per second, to prevent any suspended solids settling-out in the system. All complete water circuits should be designed so that the flow can be monitored, either by open-ended pipes or by instrument indication. Monitoring with instrument indication may be expensive, but accuracy is good and reliable for the whole system. Temperature should also be monitored at each outlet. Flow switches should be provided at each outlet to ‘trip out’ the furnace power supply in the event of a failure. The over flow-bucket types are preferred in an open system.

If the cooling water cannot be sufficiently provided to the induction coil and the necessary components in some installations such as the frequency-conversion equipment, the power cables, the control panel and the capacitors, the coil may be damaged and exploded to the surrounding where the employees will be working

inside the foundry shop. Simultaneously, it will affect the productivity, the mental and physical power of workers and all works of industry. A sample of the damage of induction coil is shown in Figure 2.9.

Sometimes, it may also be necessary in some installations to cool the water in the frequency-conversion equipment, the capacitors and power cables. In channel furnaces the coil and the inductor casing are usually water-cooled. The cooling water supply temperature should not be below 25ºC, to prevent condensation on the cooled components. The upper limit of water temperature leaving the coil should be no more than 70ºC, and that from the capacitors and frequency-conversion equipment should not exceed the value specified by the manufacturers. If too cold water is allowed to return to the system (cold temperature is defined as water temperature lower than the ambient air temperature), condensation will then form on the electrical parts and the coil. The life expectancy of these components is related to their operating temperature and maintenance.

There are various types of cooling system to support the induction coil, frequency- conversion equipment, the capacitors and the control panel. They are installed and constructed in many foundry shops according to the requirements of installation space, the annual operating costs, the furnace sizes and capacities, and the environmental conditions, and the area of the industry. The types of water cooling system used in most of the application for coreless induction melting systems will be described in section 2.7.

2.6.1. Water Requirements

The quality and quantity of water required to cool a coreless induction melting system should be specified in the equipment manufacturers’ literature or quotation. If a new coreless induction melting system is proposed to be installed in an existing facility with established plumbing in place, several design factors relating to water flow and pressure must be considered. Additional water supply must exist within the plant. Then, there is adequate flow and pressure to satisfy the equipment manufacturers’ specifications. The present water quality characteristics do meet the specifications of the induction furnace manufacturer. The addition of the new system will affect flow and pressure to the existing and new system may be required. If a new line is required, it should be designed to eliminate friction losses along with assuring that there is an adequate supply of emergency water.

2.6.2. Effects of Water Quality

There are three detrimental effects of poor water quality in melting equipment cooling paths are:

(a) The reduction in the ability to transfer heat that leads to subsequent damage to the components from overheating.

(b) Electrochemical corrosion of tubing.

(c) Degradation of the electrical performance of the melting equipment due to the water having too high an electrical conductivity.

All of these effects are directly related to impurities in the water scale formation, fouling due to products of corrosion or fouling due to biological growth. When this fouling does occur, in order to maintain the same heat transfer, the temperature difference between the water and the component will increase. As the fouling continues to build up, the temperature increases and the components fails. This process is further aggravated by the reduction of water flow caused by the reduction in the cross sectional area of the path.

Electrochemical corrosion is the deterioration of solids by liquid electrolytes. In this case, the electrolyte is the contaminated cooling water, which attacks metal components in the system. Under severe corrosion conditions the components can corrode or rust in less than a year time. High electrical conductivity is directly related to the amount of dissolved solids in the water. The resulting problems are the distortion of the electrical control signals to solid-state devices and the desensitizing of the ground detector circuits.

2.6.3. Water Purification/Maintenance

The highly de-ionized water has very corrosive properties and it can cause damage to the induction coils. Corrosion of iron in the piping can add enough iron in suspension to affect conductivity. Therefore, the newer water systems will usually include a de-ionizer to main the conductivity of the water at acceptable levels. The de- ionizers are used to maintain a water conducting level of 50 micromhos/cm or lower. It is generally accepted that an operational water conductivity range of 100 to 300 micromhos/cm is adequate for operation for the water system.

In a closed water system if the water is not changed periodically a microscopic organism will develop. This organism will attack the copper surfaces of the water system and if not addressed will eventually lead to water leaks throughout the system.

By removing a hose on the furnace coil and inspecting the inside diameter of the copper tubing it can be determined if there are microorganisms present. The inside of the copper tubing will show a shiny black surface and will be very slipping. Treatment for microscopic organisms can be done by draining the system of all water, then acid wash the entire system with water. Then refill the system, making sure to remove all of the entrapped air.

2.6.4. Filtration

Many filtration units have been used with high maintenance requirements. The centrifugal separator, one of the filtration units, is used in water systems to remove solids from liquids. Many advantages of using these devices are as follows:

1. No moving parts to wear out

2. No screens, cartridges, cones or filter elements to replace 3. No backwashing

4. No routine maintenance or downtime requirements 5. No standby requirement needs

6. Low and steady pressure loss 7. Easily automated

By removing the solids from the water, the life of the pumps can be extended, fouling of cooling towers and heat exchangers can be virtually eliminated and allow for optimum efficiencies.

2.6.5. Effects of Impurities

It is important that there are the effects of impurities in circulating water system. Typical water impurities affect water quality. High water conductivity can result in distortion of control signals and it can lead to corrosion of pipe nipples. If the water is over saturated with calcium bicarbonate, calcium carbonate will form on the piping interior. This deposited scale will restrict water flow and decrease heat transfer. The suspended solids can also accumulate in equipment, particularly at low points, causing clogging and reducing heat transfer. Suspended solids in makeup and circulating water can be removed by either filtration or centrifugal separation.

Water that contains a high amount of free mineral acid is required. Acidity is evidenced by effervescence when in contact with carbonate. This makes the water very corrosive. The measure of pH of a solution is a measure of acidity of the solution.

Acid solutions have a pH of less than 7. Other effects of impurities are alkalinity, slime and algae biological fouling, and dissolving oxygen and corrosion. If the alkalinity is determined to be in excess, treatment of water with acid may be necessary. Slime and algae biological fouling can offer and occur in once through and open circulating systems. It is formed by the excessive growth or accumulation of lower forms of plant life. Chemical treatment, usually chlorine, may be used for control of these growths to avoid loss in heat transfer and to minimize biological fouling on metal surfaces.

Dissolving oxygen and corrosion is accelerated by dissolved gases such as oxygen, ammonia, carbon dioxide or sulfur dioxide, dissolved solids and high temperature. The gases mentioned cannot be removed by mechanical means because they tend to ionize in the water. The life of electrical conducting components in induction systems relies heavily on the quality of the water supplied by the water system. Nevertheless, the selection of a high quality cooling system for coreless induction melting systems is of prime importance.

2.6.6. Emergency Water Supply and Cooling System

In all coreless induction furnace systems, a source for emergency water must be used to supply cooling water to the furnace during times when the water system loses power or has a pump failure. Many water systems are provided with a standby pump in case of primary pump failure; but in a case where there is a power outage and the recirculating pumps cannot be run, an emergency water system is the only alternate source for cooling water. This is due to the fact that both the molten metal in the furnace and the refractory system have significant amount of stored energy that must be removed through the recirculating water at all times. Energy transfer to unrecirculated water in the coil will cause the temperature of the water contained within it to rise. The temperature will continue to elevate until the water turns to steam where it will expand in volume.

Since the water is closed, the pressure in the coil will increase until hoses blow off of the coil and all of the water contained within will be expelled. At this point there is nothing to remove the stored energy in the furnace and it will transfer to the coil and raise its temperature to that exceeding the ratings of materials in contact with it. This will result in a significant expense to the foundry as regards to equipment damage as well as loss of production due to loss of service of the equipment. In this

situation, if possible, there should be a procedure to empty the furnace immediately of molten metal, thereby eliminating the largest amount of the stored energy that needs to be removed.

The emergency cooling system should be provided to cool the furnace coil in the event of power failure. The emergency water should be gravity-fed from a high- level storage tank, supplied from the mains, and connected directly to the furnace coil via a check valve that should be opened automatically when the pressure in the normal, pumped supply falls. The emergency water will flow through the coil to the buffer tank, and then to the drain through an overflow pipe.