Maintaining Coreless Induction Furance Water Systems.

Quality water, regular treatment and proper filtration are critical measures to ensure efficient coreless induction furnace water systems and melt campaigns.

In today's coreless induction melting systems, water is critical to the cooling of power supplies and furnaces. Operating at increased voltages with high energy levels, melt systems require water-cooling to ensure that the furnace components remain at their optimum operating temperature. In addition, furnace manufacturers demand high quality water in their cooling systems to maximize reliability and component longevity.

But as the reliance on water systems has increased, so has the responsibility for regular water system maintenance. In today's foundries, more than 50% of all furnace downtime can be attributed to water-related problems. Efficient melt campaigns are dependent upon each furnace component operating at its peak efficiency.

This article examines the various components of a coreless induction furnace water system and provides maintenance tips for foundries to follow to ensure quality water and an efficient water system.

Water Systems

Today's induction furnaces generally utilize two types of water-cooling systems--evaporative closed-loop water systems and dry air closed-loop water systems.

Evaporative closed-Loop System--This system consists of two separate water-cooling stations as shown in Fig. 1. Each of these systems is a closed cooling water system. The main water system is used to cool the furnace coil and its related equipment (furnace-shorted turn, coil-shorted turn, water-cooled shunts, and bus and power cables from the bus to the furnace coil). This main system also supplies cooling water to a plate-and-frame type water-to-water heat exchanger on the exchanger's primary (raw cooling water) side. The secondary (process water) side of the plate-and-frame heat exchanger has its own pumping system, which cools the capacitors, diodes, reactors, chokes and transformers in the power supply. This type of dual-loop system is preferred as the total volume of water with the strictest water quality specifications (the power supply-cooling loop) is reduced. Water pumps are used to circulate the water through the equipment in both the main cooling loop and the power supply loop.

Most primary water systems include a closed loop evaporative water tower as the means for removing heat from the water. The primary heated water passes through the tubes of the water tower and the water within is cooled by an open water spray loop. This spray system water is evaporated into the atmosphere and the lost water is transferred to a sump with fresh and/or well water.

Since the spray water system is open to the atmosphere and the water is evaporated in the cooling process, it is necessary to continually add water. When the added water has a high content of dissolved minerals, it also may be necessary to treat it. Water quality is diverse throughout the U.S. and varies dramatically from one region to another. Water treatment must be designed to address the water problems for the particular region in which the melting system is located.

Dry Air Closed-Loop System--In cooler climates, a dry type cooling tower (Fig. 1) can be used in place of an evaporative cooler. In this system, the primary water flows through the tube bundles inside the cooler. Fans blow ambient air past the tube bundles and heat is transferred from the tubes to the air. The advantage of this type of cooler is that there is no spray loop and the required maintenance of the water on the spray loop is eliminated. The drawback of this system is that the water can never be cooled to temperatures lower than the ambient air temperature. On hot days, this may require that the cooling system be supplied with a trim cooler to provide enough additional cooling for operation.

If necessary, the trim cooler could be supplied as either an evaporative cooling tower or it could be a water-to-water heat exchanger based upon the specific installation requirements to remove the additional heat load lost due to the dry air cooler's reduced heat rejection capacity at elevated ambient temperatures. In some instances, a dry cooler can be used without a water-to-water heat exchanger and a separate power supply pump.

Water Quality

Although the water system's main function is furnace cooling, if it isn't operating at the height of efficiency due to poor water quality, several areas of the furnace can be affected. Poor water quality leads to:

* the reduction of the water system's ability to transfer heat and keep the furnace components from over heating. A reduction in the heat transfer in the water-cooling passages may be caused by scale formation or fouling due to products of corrosion or biological growth. When this fouling does occur, the temperature difference between the water and the component will increase to maintain the same heat transfer rate. As the fouling continues to build up, the temperature increases and the component fails. This process is further aggravated by the reduction of water flow caused by the reduction in the cross sectional area of the path (Fig. 2);

* electrochemical corrosion of tubing. 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 corrosive conditions, the components can corrode or rust in less than 1 yr;

* degradation of electrical performance of the melting equipment due to the water having too high an electrical conductivity. If the water has excessive amounts of dissolved solids in it, the result will be distorted electrical control signals to the furnace's solid state devices. In addition, the ground circuit detector circuits will be desensitized.

To combat these water quality issues, foundries must perform regular maintenance on the water itself as well as the melt equipment's water system.

Water Purification/Maintenance

The quality of the water used in a water cooling system begins with its purity. For the primary portion of the water cooling system, water with a conductivity range of 100-300 micromhos/ cm is acceptable. In this part of the system, deionizers often are used to maintain the water conductivity. If the foundry is experiencing a changing ground resistance with their furnace, it is often the product of a faulty delonizer instead of the usual association with water leaks, faulty capacitors or furnace lining problems. If the water used in the system is highly ionized, a foundry must be careful because it will exhibit corrosive properties. Corrosion of iron pipes can add enough iron in suspension to affect conductivity.

In the power supply (secondary) water system, deionizers are used to keep the conductivity below 50 micromhos/cm. Due to this low conductivity requirement, the water system must be carefully monitored to ensure that the corrosive effects of relatively pure water do not create undue corrosion of the power supply components.

Water temperature also plays an important part in water conductivity. In some monitored systems, hot water will show an increase in conductivity. Trapped air bubbles in the system also will create the same effect. It is difficult to distinguish the difference between a true furnace ground and the effect of conductivity. This problem can be solved by the installation of a water conductivity meter in the system.

When conductivity has been increased due to excessive water treatment additives, the water must be diluted or the entire system must be flushed with fresh water. If a synthetic coolant is used, pure ethylene glycol must be added to the water. Commercial automotive antifreeze contains corrosion inhibitors, making the solution highly conductive and not suitable for use. Adding too much glycol to the water decreases its capacity to remove heat and not adding enough may allow the water to freeze in cold weather. When choosing the percentage of ethylene glycol to be used in the water system, the furnace manufacturer must be consulted.

In a closed water system, the water must be changed periodically to avoid the development of microscopic organisms. These organisms will attack the copper surfaces of the water system and lead to water leaks throughout it. To determine if organisms are present in a system, remove a hose on the furnace coil and inspect the inside diameter of the copper tubing. If organisms are present, the inside of the copper tubing will show a shiny black surface and feel very slippery. To eliminate the organisms, the system must be drained of all water and then acid-washed. Follow the acid wash with a basic solution and then rinse the entire system with water. While refilling the system, be sure to remove all entrapped air.

Filtration is one form of continual maintenance used to combat organisms in the water. In recent years, melt equipment has begun to incorporate the use of centrifugal separators (Fig. 3) instead of traditional filters. The advantage of this device is:

* no moving parts to wear out;

* no screens, cartridges, cones or filter elements to rep]ace;

* no backwashing;

* no routine maintenance or downtime requirements;

* no standby equipment needs;

* low and steady pressure loss;

* ease of automation.

The separators use centrifugal action to remove solids from liquids. By removing the solids from the water, the life of the pumps can be extended and fouling of the plate-and frame heat exchangers and the small tubing in the power supply components can be eliminated.

Water Impurities

The types of water impurities that can affect the induction furnace cooling system vary depending on the water used. Table 1 is a list of common impurities and the water property they affect. Following is a description of each of those water properties and their effect on the furnace water system:

* Conductivity--High conductivity is caused by high total dissolved solids. If DC potentials are present in induction melt equipment, high conductivity water will lead to corrosion of the pipe nipples. High water conductivity can result in ground detector tripping and distortion of control signals. In induction melt systems, water is intended to be a coolant, not a current path.

* Hardness--The solubility of calcium bicarbonate in water depends on pH, alkalinity and temperature. 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. This is particularly detrimental because the scale will tend to be deposited on those surfaces, which are already the hottest.

* Suspended Solids--These are impurities that are insoluble but remain in suspension due to turbulence of the water. These solids can accumulate in equipment, particularly at low points, causing clogging and reduced heat transfer. Suspended solids in makeup and circulating water can be removed by either filtration or centrifugal separation.

* Acidity--Water can be encountered that contains a high amount of free mineral acid. Acidity is evidenced by effervescence when in contact with carbonates, which makes the water corrosive. Whenever a standard determination of pH indicates an acidic condition, the watershould be neutralized with an alkaline agent until it falls within the allowable specified limits. The measure of pH of a solution is a measure of acidity of the solution. Acid solutions have a pH of less than 7.

* Alkalinity--In circulating water-cooling systems, alkalinity is one of the major factors contributing to the precipitation of calcium carbonate in the form of scale. In general, the higher the alkalinity, the higher the tendency to form scale and the higher the water's acid-neutralizing ability. If the alkalinity is determined to be in excess, treating the water with acid may be necessary.

* Slime and Algae Biological Fouling--These can occur in once through and open-circulating systems, and are formed by the excessive growth or accumulation of lower forms of plant life (algae, fungi and bacteria). Chemical treatment, such as chlorine, may be used to control these growths to avoid a reduction in heat transfer capabilities and to minimize biological fouling on metal surfaces. The growth of organisms on piping creates differential oxygen concentration cells that result in serious pitting of the metal surfaces. Closed systems may also suffer from this condition if the cooling media is not periodically changed.

Water Requirements

The quality and quantity of water required to cool a coreless induction melting system should be specified in the equipment manufacturer's literature or quotation. If a new coreless induction melting system is to be installed into an existing facility with established plumbing, several design factors relating to water flow and pressure must be considered.

Water Supply--Does additional water supply exist within the plant? If it does, is there adequate flow and pressure to satisfy the equipment manufacturer's specifications? Do the present water quality characteristics meet the specifications of the induction furnace manufacturer?

Steady Supply of Water and Pressure--Will the addition of the new system affect flow and pressure to the existing and new system in the plant? Will a booster pump elevate the problem? If existing plumbing is operating at capacity, a separate supply may be required. If a new line is required, it should be designed to eliminate friction losses.

Beyond these questions, all coreless induction furnace systems must have a source of emergency water to supply cooling water to the furnaces during power outages or pump failures. Many water systems also have a standby pump in case of a primary pump failure. In the emergency water system, enough water volume must be available to remove a significant amount of stored energy from the molten metal and the refractory system. If this energy isn't removed, the cooling water may turn to steam, damaging the system. If this situation does arise, the furnaces must be emptied to remove the largest amount of stored energy from the furnace.

There are four primary sources of emergency water:

* City Water--This is the most common and must be used as a backup to any other emergency water system. Its disadvantages are that foundries must dump treated water to drain and the furnace water system must be retreated (after using city water) before bringing the furnace back on line.

* Battery/Generator Powered Pump--This system does not require dumping water to drain, but there is a remote chance that it can fail if not properly maintained.

* Turbine Pump--Although this option doesn't require dumping water to drain, it does require a continuous supply of city water.

* Elevated Storage Tank--This system requires dumping waste water to drain and re-treatment of the water system before bringing a furnace back on line.

This article was adapted from a paper (01-143) presented at the 2001 AFS Casting Congress. Conference proceeding are available from AFS Publications at 800/537-4237.

The authors would like to thank Ajax Magnethermic Corp. for their assistance with this article.

Table 1. Water Impurities and the Water Property They Affect
                       Hardness  Alkalinity  Dissolved  Conductivity
                                              Solids
Calcium Bicarbonate      Yes        Yes         Yes         Yes
Magnesium Bicarbonate    Yes        Yes         Yes         Yes
Sodium Bicarbonate        No        Yes         Yes         Yes
Calcium Sulfate          Yes         No         Yes         Yes
Magnesium Sulfate        Yes         No         Yes         Yes
Sodium Sulfate            No         No         Yes         Yes
Calcium Chloride         Yes         No         Yes         Yes
Magnesium Chloride       Yes         No         Yes         Yes
Sodium Chloride          Yes         No         Yes         Yes

Examining Water System Terminology

Water treatment and quality is far removed from the daily routine of foundrymen. But understanding the terminology will help foundrymen in maintaining their induction furnace water system.

Corrosion is the destructive attack of a metal by electro-chemical reaction with its environment. A current (called corrosion current) flows through the electrolyte (the cooling water). Corrosion occurs where current enters the electrolyte from the metal, which is the anode. Flow of corrosion current can be caused either by an externally applied voltage, such as that generated by a power supply, or by the voltage due to galvanic action from dissimilar metals in the piping.

Erosion is deterioration caused by physical means such as high water velocity (more than 10 ft/sec).

Electrolysis is the separation of an electrolyte into its constituents by passing a current through it. Some people mistakenly use this term to mean corrosion.

Conductivity is the most important characteristic of cooling media for induction systems. It is measured in micromhos/cm. Minimum and maximum limits for conductivity vary with the specific type of equipment that it is cooling. The exact requirements can be found in the equipment manufacturer's specifications.

Total dissolved solids refers to the amount of ions in the water. This is generally measured in parts per million (milligrams per liter). Since ions are good charge carriers, higher total dissolved solids means higher conductivity water.

Relationship of total dissolved solids, hardness, alkalinity and conductivity. While there are often trace amounts of various minerals in water, three cations (calcium, magnesium and sodium) and three anions (bicarbonate HCO, sulfate SO and chloride Cl) account for nearly all impurities in water. These ions can be combined in nine possible ways.

Hard water refers to water that contains magnesium or calcium ions, which combine with fatty acids in soap to produce an insoluble precipitate. Softening the water (an ion exchange process) just replaces these ions with sodium. The sodium fatty acid compounds are soluble. This does not change the conductivity.