WATER COOLING FOR INDUCTION SYSTEMS: INSIDE AND OUT Water-cooling circuits are nduction water-cooling systems are as diverse as your imagina- the most neglected item in tion. Hybrid combinations con- induction maintenance tinue to be developed by compa- I nies that want to go green, save causing the most down time energy, and reduce water consump- tion and costs. Over the years, different and damage. Common OEMs and users have developed var- sense installation and ious systems depending on location, water costs, real estate, and local maintenance practices can building code restrictions. The types help reduce unexpected and combinations discussed in this ar- ticle are the most predominant. Today, down time, there by cooling systems usually consist of a increasing profits for the closed-loop recirculating system to cool the power supply, heat station, coils, user. bus, and leads. All use some sort of water-to-water heat exchanger in con- Fred Specht* junction with plant water (dirty water) Ajax TOCCO Magnethermic Corp. supplied by a cooling tower, radiator/ Warren, Ohio fan, refrigeration type chillers, city water, well water, and geothermal field. High-quality, low-conductivity water is used in the closed-loop system (clean side) to cool the power supply. Plant water (typically cooling-tower water) is considered the dirty side of the heat exchanger, but it may be ac- ceptable for cooling the power supply Fig. 1 — Water recirculating system for cooling an if the water quality is within limits and induction power supply including stainless steel all plumbing is nonferrous. pump, copper pipe, cartridge filter, nonferrous This paper is NOT intended to re- plate-type heat exchanger, and temperature place your OEM equipment manual, control to prevent condensation. which usually has a section on water cooling-system installation, mainte- tems are the life blood of an induction nance, and recirculating-water charac- system. They supply controlled tem- teristics. perature cooling for the power supply, capacitor heat station, water-cooled Recirculating Water Systems leads and bus, and the induction coil for Power Supplies (Fig. 1). The systems consist of a Closed-loop recirculating water sys- holding tank, pump, filter cartridge, Process plant water-cooling system relative costs Investment Operating Lowest achievable Type cost cost Maintenance temperature, °F Air-to-air (dry) $$ $ $$ 105 Closed-loop evaporative $$$ $$$ $$$ 85 Open-loop evaporative $$ $$ $$$$ 85 *Member of ASM International and member, ASM Heat Treating Society Refrigeration chiller $$$$ $$$$ $$ 65 HEAT TREATING PROGRESS • OCTOBER 2009 21 heat exchanger, and temperature con- sure differential switch and its func- troller to prevent condensation. Recir- tion, and always maintain a minimum culating water that is too cold (below of 30 psi differential across the system, the dew point) must be avoided to pre- which has become the industry stan- vent condensation in the power dard. To diagnose plugged paths, an supply, heat station, and coil, and to infrared (IR) survey at full power prevent damaging electrical arcs. rating can detect overly hot water It has long been understood that as paths, and it is especially useful on much as 90% of induction heating- large power supplies having dozens system problems are water related. of water paths. High-conductivity water is usually the Some older power supplies used culprit, which causes cooling-system sacrificial anodes (also know as “tar- and power-supply erosion due to elec- Fig. 2 — Calcium-clogged water paths due to high gets”) to protect the DC link. The tar- trolysis. This reaction causes the ero- conductivity water. gets were made of tungsten or stain- sion of critical copper components, and freeze damage if heat is lost in the plant less steels and over a couple of years the eroded debris collects at the loca- for an extended period of time. Ajax dissolved into the water. Their use was tion of the opposite polarity, which re- TOCCO offers MAGNE-COOL pre- found to cause clogging even with ac- duces water flow in that circuit (Fig. mixed water and glycol solution de- ceptable water quality. These have 2). Lower water flow results in higher signed specifically for use in induction fallen out of favor in the past several operating temperatures of the device, applications. decades and quality water is now the leading to premature failure due to Water that has conductivity that is norm. Another trend is toward using overheating. This is most common in too low is just as corrosive. RO and DI smaller holding tanks with fewer gal- certain water-cooling paths where high processing systems can generate water lons of water in the system, resulting electrical current potentials are present, at 0 to 10 mho/cm. This condition is in less to clean and maintain. Smaller, such as SCRs, diode heat sinks, chokes too “hungry” and causes similar cor- larger capacity heat exchangers have (reactors), and transformers referred rosion in the DC link. Circuits that use helped this trend evolve. to as the dc link. ac voltages still require the same high During cooling-system installation, For example, one side of an SCR quality water. it is extremely important to place air heat sink is positive and the other side Closed-loop systems that cool the bleeder vents at the highest point of the is negatively charged at the exact mo- power supply should be constructed plumbing to remove trapped air. Air is ment in time. Both sides of the SCR of nonferrous materials, such as PVC, not a good conductor of heat and if it share the same water, and the hose copper, brass, nonmagnetic stainless becomes trapped in the induction length between these electrical poten- steel, and nonconductive rubber hose. equipment can cause component failure tials isolates the high voltage from the Never use black iron, galvanized, or reduced component life. During next component in the water circuit carbon steel, or garden hose in the yearly maintenance, when the system and from the electrically grounded plumbing of the power supply cooling is drained and flushed, air must be bled water manifold. As the conductivity circuits. One single piece of any ferrous off after the system is refilled. of the water increases, electrical leakage material in the closed loop will cause The following is an example of a occurs down the water path, because water conductivity to rapidly rise and cooling-water specification for recircu- electrical current always takes the path clog the small paths in the power lating water used to cool induction of least resistance to ground, or its op- supply. When replacing hose, never power supplies that do not use targets posite polarity. Voltages are always shorten the hose length as the length or sacrificial anodes. looking for their return path to ground. is pre-engineered by the OEM. The The conductive water makes the hose length is based on recommended Total hardness (CaCO3) 15 ppm length seem shorter, which makes the water conductivity and the amount of Total dissolved solids 25 ppm electrical leakage current run more voltage present at the device being Conductivity 20 to 50 mho/cm easily to ground, and electrolysis occurs cooled. When replacing hose clamps, Max suspended solids 10 ppm faster. A hose can be thought of as a use only stainless steel with stainless water-cooled resistor. Ideally, when the screw posts. pH 7.0 to 7.5 hose is new, the resistance is very high The use of lake, well, river, or city The following is an example of a (in the multiple Mohms). As water tap water directly into an induction cooling water specification for recircu- quality deteriorates, the effective resist- power supply or heat station can cause lating water used in an induction ance of the hose decreases allowing severe damage. Even good quality power supply that has replaceable tar- more leakage current to flow. water by its nature will breakdown as gets or sacrificial anodes or for heat sta- Some OEMs recommend different it reacts with copper over time. Quar- tions and coil cooling. water types (the most common being terly checks are recommended for pH reverse osmosis, or RO, deionized, or and conductivity. Water in the power Total hardness (CaCO3) 100 ppm DI, and distilled), and most recom- supply closed loop should be changed Total dissolved solids 200 ppm mend an additive such as glycol. A every year to 18 months depending on 30% percent ethylene glycol (uninhib- the conductivity readings. Drain, flush Conductivity 50 to 300 mho/cm ited, nonconductive type) mixed with with clean water, and refill on a sched- Max suspended solids 10 ppm water serves as a buffer and has a uled maintenance program. Just pH 7.0 to 7.5 filming agent that keeps the water adding clean make-up water may not from becoming too aggressive to pre- keep the conductivity at low enough Always check your OEM manual vent corrosion. Also, it protects from levels. When refilled, check the pres- for their recommendations. 22 HEAT TREATING PROGRESS • OCTOBER 2009 Cooling Towers Cooling towers are designed to re- move heat from water and dissipate the heat into the atmosphere. Of the many types available, three most com- monly used types are air-cooled, closed-loop, and open-loop evapora- tive towers. Air-cooled heat exchanger-tower systems consist of a motor and pump with an expansion tank, and tower (which consists of cooling tubes, fins, and fans). These air-cooled systems are very low maintenance compared with evaporative systems, but they lack the cooling capabilities of evaporative towers. Fig. 3 — Air-cooled tower with top fans. Pressurized closed-loop cooling towers require the use of glycol to pre- Air-ccooled heat exchanger vent freeze damage in the pressurized Air Air bundle, and they have a collection pan heater to prevent freezing in the pan area.