8/10/2019 Water Cooling for Induction Sysytem http://slidepdf.com/reader/full/water-cooling-for-induction-sysytem 1/7 HEAT TREATING PROGRESS • OCTOBER 2009 21 Water-cooling circuits are the most neglected item in induction maintenance causing the most down time and damage. Common sense installation and maintenance practices can help reduce unexpected down time, there by increasing profits for the user. Fred Specht * Ajax TOCCO Magnethermic Corp. Warren, Ohio nduction water-cooling systems are as diverse as your imagina- tion. Hybrid combinations con- tinue to be developed by compa- nies that want to go green, save energy, and reduce water consump- tion and costs. Over the years, different OEMs and users have developed var- ious systems depending on location, water costs, real estate, and local building code restrictions. The types and combinations discussed in this ar- ticle are the most predominant. Today, cooling systems usually consist of a closed-loop recirculating system to cool the power supply, heat station, coils, bus, and leads. All use some sort of water-to-water heat exchanger in con- junction with plant water (dirty water) supplied by a cooling tower, radiator/ 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 if the water quality is within limits and all plumbing is nonferrous. This paper is NOT intended to re- place your OEM equipment manual, which usually has a section on water cooling-system installation, mainte- nance, and recirculating-water charac- teristics. Recirculating Water Systems for Power Supplies Closed-loop recirculating water sys- tems are the life blood of an induction system. They supply controlled tem- perature cooling for the power supply, capacitor heat station, water-cooled leads and bus, and the induction coil (Fig. 1). The systems consist of a holding tank, pump, filter cartridge, I WATER COOLING FOR INDUCTION SYSTEMS : INSIDE AND OUT 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 $$$ $$$ $$$ 85 Open-loop evaporative $$ $$ $$$$ 85 Refrigeration chiller $$$$ $$$$ $$ 65 Fig. 1 — Water recirculating system for cooling an induction power supply including stainless steel pump, copper pipe, cartridge filter, nonferrous plate-type heat exchanger, and temperature control to prevent condensation. *Member of ASM International and member, ASM Heat Treating Society
Transcript
8/10/2019 Water Cooling for Induction Sysytem
http://slidepdf.com/reader/full/water-cooling-for-induction-sysytem 1/7HEAT TREATING PROGRESS • OCTOBER 2009 21
Water-cooling circuits are the most neglected item ininduction maintenance causing the most down time and damage. Commonsense installation and
maintenance practices canhelp reduce unexpected down time, there byincreasing profits for the user.
Fred Specht*Ajax TOCCO Magnethermic Corp.Warren, Ohio
nduction water-cooling systemsare as diverse as your imagina-tion. Hybrid combinations con-tinue to be developed by compa-nies that want to go green, save
energy, and reduce water consump-tion and costs. Over the years, differentOEMs and users have developed var-ious systems depending on location,water costs, real estate, and local building code restrictions. The typesand combinations discussed in this ar-ticle are the most predominant. Today,cooling systems usually consist of aclosed-loop recirculating system to coolthe power supply, heat station, coils, bus, and leads. All use some sort of water-to-water heat exchanger in con- junction with plant water (dirty water)supplied by a cooling tower, radiator/fan, refrigeration type chillers, citywater, well water, and geothermalfield. High-quality, low-conductivity
water is used in the closed-loop system(clean side) to cool the power supply.Plant water (typically cooling-towerwater) is considered the dirty side of the heat exchanger, but it may be ac-ceptable for cooling the power supplyif the water quality is within limits andall plumbing is nonferrous.
This paper is NOT intended to re-place your OEM equipment manual,which usually has a section on watercooling-system installation, mainte-nance, and recirculating-water charac-teristics.
Recirculating Water Systemsfor Power Supplies
Closed-loop recirculating water sys-
tems are the life blood of an inductionsystem. They supply controlled tem-perature cooling for the power supply,
capacitor heat station, water-cooledleads and bus, and the induction coil(Fig. 1). The systems consist of aholding tank, pump, filter cartridge,
Fig. 1 — Water recirculating system for cooling aninduction power supply including stainless steelpump, copper pipe, cartridge filter, nonferrousplate-type heat exchanger, and temperaturecontrol to prevent condensation.
*Member of ASM International and member, ASM Heat Treating Society
heat exchanger, and temperature con-troller to prevent condensation. Recir-culating water that is too cold (belowthe dew point) must be avoided to pre-vent condensation in the powersupply, heat station, and coil, and toprevent damaging electrical arcs.
It has long been understood that asmuch as 90% of induction heating-system problems are water related.
High-conductivity water is usually theculprit, which causes cooling-systemand power-supply erosion due to elec-trolysis. This reaction causes the ero-sion of critical copper components, andthe eroded debris collects at the loca-tion of the opposite polarity, which re-duces water flow in that circuit (Fig.2). Lower water flow results in higheroperating temperatures of the device,leading to premature failure due tooverheating. This is most common incertain water-cooling paths where highelectrical current potentials are present,such as SCRs, diode heat sinks, chokes(reactors), and transformers referredto as the dc link.
For example, one side of an SCRheat sink is positive and the other sideis negatively charged at the exact mo-ment in time. Both sides of the SCRshare the same water, and the hoselength between these electrical poten-tials isolates the high voltage from thenext component in the water circuitand from the electrically groundedwater manifold. As the conductivity
of the water increases, electrical leakageoccurs down the water path, becauseelectrical current always takes the pathof least resistance to ground, or its op-posite polarity. Voltages are alwayslooking for their return path to ground.The conductive water makes the hoselength seem shorter, which makes theelectrical leakage current run moreeasily to ground, and electrolysis occursfaster. A hose can be thought of as awater-cooled resistor. Ideally, when thehose is new, the resistance is very high(in the multiple Mohms). As water
quality deteriorates, the effective resist-ance of the hose decreases allowingmore leakage current to flow.
Some OEMs recommend differentwater types (the most common beingreverse osmosis, or RO, deionized, orDI, and distilled), and most recom-mend an additive such as glycol. A30% percent ethylene glycol (uninhib-ited, nonconductive type) mixed withwater serves as a buffer and has afilming agent that keeps the waterfrom becoming too aggressive to pre-vent corrosion. Also, it protects from
freeze damage if heat is lost in the plantfor an extended period of time. AjaxTOCCO offers MAGNE-COOL pre-mixed water and glycol solution de-signed specifically for use in inductionapplications.
Water that has conductivity that istoo low is just as corrosive. RO and DIprocessing systems can generate waterat 0 to 10 mho/cm. This condition istoo “hungry” and causes similar cor-rosion in the DC link. Circuits that useac voltages still require the same highquality water.
Closed-loop systems that cool thepower supply should be constructedof nonferrous materials, such as PVC,copper, brass, nonmagnetic stainlesssteel, and nonconductive rubber hose.Never use black iron, galvanized,carbon steel, or garden hose in theplumbing of the power supply coolingcircuits. One single piece of any ferrous
material in the closed loop will causewater conductivity to rapidly rise andclog the small paths in the powersupply. When replacing hose, nevershorten the hose length as the lengthis pre-engineered by the OEM. Thelength is based on recommendedwater conductivity and the amount of voltage present at the device beingcooled. When replacing hose clamps,use only stainless steel with stainlessscrew posts.
The use of lake, well, river, or citytap water directly into an induction
power supply or heat station can causesevere damage. Even good qualitywater by its nature will breakdown asit reacts with copper over time. Quar-terly checks are recommended for pHand conductivity. Water in the powersupply closed loop should be changedevery year to 18 months depending onthe conductivity readings. Drain, flushwith clean water, and refill on a sched-uled maintenance program. Justadding clean make-up water may notkeep the conductivity at low enoughlevels. When refilled, check the pres-
sure differential switch and its func-tion, and always maintain a minimumof 30 psi differential across the system,which has become the industry stan-dard. To diagnose plugged paths, aninfrared (IR) survey at full powerrating can detect overly hot waterpaths, and it is especially useful onlarge power supplies having dozensof water paths.
Some older power supplies usedsacrificial anodes (also know as “tar-gets”) to protect the DC link. The tar-gets were made of tungsten or stain-less steels and over a couple of yearsdissolved into the water. Their use wasfound to cause clogging even with ac-ceptable water quality. These havefallen out of favor in the past severaldecades and quality water is now thenorm. Another trend is toward usingsmaller holding tanks with fewer gal-lons of water in the system, resultingin less to clean and maintain. Smaller,larger capacity heat exchangers havehelped this trend evolve.
During cooling-system installation,it is extremely important to place air bleeder vents at the highest point of theplumbing to remove trapped air. Air isnot a good conductor of heat and if it becomes trapped in the inductionequipment can cause component failureor reduced component life. Duringyearly maintenance, when the systemis drained and flushed, air must be bledoff after the system is refilled.
The following is an example of acooling-water specification for recircu-lating water used to cool inductionpower supplies that do not use targetsor sacrificial anodes.
Total hardness (CaCO3) 15 ppm
Total dissolved solids 25 ppm
Conductivity 20 to 50 mho/cm
Max suspended solids 10 ppm
pH 7.0 to 7.5
The following is an example of acooling water specification for recircu-lating water used in an inductionpower supply that has replaceable tar-gets or sacrificial anodes or for heat sta-tions and coil cooling.
Total hardness (CaCO3) 100 ppm
Total dissolved solids 200 ppm
Conductivity 50 to 300 mho/cm
Max suspended solids 10 ppm
pH 7.0 to 7.5
Always check your OEM manualfor their recommendations.
22 HEAT TREATING PROGRESS • OCTOBER 2009
Fig. 2 — Calcium-clogged water paths due to highconductivity water.
move heat from water and dissipatethe heat into the atmosphere. Of themany types available, three most com-monly used types are air-cooled,closed-loop, and open-loop evapora-tive towers.
Air-cooled heat exchanger-towersystems consist of a motor and pump
with an expansion tank, and tower(which consists of cooling tubes, fins,and fans). These air-cooled systems arevery low maintenance compared withevaporative systems, but they lack thecooling capabilities of evaporativetowers.
Pressurized closed-loop coolingtowers require the use of glycol to pre-vent freeze damage in the pressurized bundle, and they have a collection panheater to prevent freezing in the panarea. Heat tape is commonly used onthe tower piping to prevent pipes fromfreezing. Open towers with a gravitydrain to an inside tank do not requireglycol, nor is there a pan heater. Evap-orative towers require more mainte-nance than dry towers or geothermalsystems.
It is necessary to analyze the glycolin cooling tower systems yearly for pHlevel, inhibitor, and freeze protection,and make adjustments according tothe OEM specifications. All coolingtower types need a yearly “springcleaning” with an appropriate alu-
minum fin cleaner and low-pressurerinse to remove dirt, and calcium, andto restore 100% cooling capacity for thehot summer months.
The following is suggested waterquality requirements for both dry andevaporative cooling towers.
Total hardness (CaCO3) 100 ppm
Total dissolved solids 200 ppm
Conductivity 20 to 300 mho/cm
Max suspended solids 10 ppm
pH 7.0 to 7.5
Always check your OEM manualfor their recommendations.
Air-Cooled Heat ExchangersAir-cooled heat exchangers cool by
means of fans blowing air across acooling bundle made of stainless steelor copper tubing. The air-to-air coolersare capable of cooling the recirculatingwater to within 5°F of ambient air tem-perature; i.e., 95°F water temperatureon a 90°F day. Major advantages in-clude low maintenance and low horse-power, which equals the lowest oper-
ating cost. A disadvantage in heat-treating applications during daytimehours in hot summer months is theneed for a trim cooler with a water-to-water heat exchanger to controlquench temperatures down to an ac-ceptable 90°F or lower quench temper-ature.
The importance of placement or po-sitioning of the air-to-air type coolingtowers is often overlooked. Ideal place-
ment is on the north side of the building in the shade of the buildingand out of direct sunlight, facing anopen field (Fig. 3). This is not alwayspossible, but placement on the southor west side in direct sunlight will re-duce the ability of the tower to dissi-pate heat. Placement on a black roof isnot recommended due to superheatedair rising off the hot roof and directsunlight, which raise the temperatureof the enclosure. Ambient air tempera-ture can increase by 5 to 10°F on a roof,which makes placement of these
towers on a roof impractical withoutheavy trim cooling or extra large size.Elevating the air cooler off the roof asfar as possible also helps. The mainconsideration is to get fresh, cool airinto the air cooler.
Air-Cooled Heat Exchangerswith Trim Cooler
These systems use a dry-type towerwith a temperature-regulated source
of additional cooling (Fig. 4). The trimis only used when the air-cooled towercannot reduce the water temperaturelow enough; usually in the summerduring the afternoon hours when theambient temperature exceeds the inputtemperature of the power supply orquench systems by 10°F. The inductionpower supply will fault on a “hot-water input” and shut off. The addi-tional trim can be a plate type heat ex-changer with city/well water coolingthe dirty side of the heat exchangerand down the drain. Another type of
HEAT TREATING PROGRESS • OCTOBER 2009 23
Fig. 3 — Air-cooled tower with top fans.
Fig. 4 — Schematic of air-cooled tower with a water-to-water trim cooler. Courtesy Dry Coolers Inc.,Oxford, Mich.
trim is a compressor/chiller with thesame type of plate or tube and shellheat exchanger. The trim atomizedmist assist shown (Fig. 5) is onlyneeded on extremely hot days inthe summer when temperatures reach90°F. The water used in the mist assistshould be good filtered city water or better. Most dry towers can operatewithout trim use in winter, spring, andfall and during summer nights whenambient temperatures are cooler.
towers are used extensively to coolplant process water using a closedpressurized recirculating system. Scaleand corrosion formation can be greatlyreduced with a closed tower comparedwith an open type tower. Eliminating
scale and sludge formation is criticalin the water-cooling bundles (tubes).Use of a closed circuit tower preventsthe water in the tubes from cominginto contact with the spray water or at-mosphere air flowing across the tubes.
Evaporative coolers can be shippedwith factory-mounted controls thatwill cycle the fan and pump On/Off as needed to maintain the desired tem-
perature set point based on the geo-graphic location where it will be used.The system may still require some fieldadjustments. The outlet plumbing fromthe cooling tower will have two tem-perature sensing bulbs installed to con-trol the outlet temperature of the tower.One bulb will be set to turn on the spraypump about 10°F below the requiredequipment inlet water temperature. Thesecond bulb is set to turn on the blowermotor about 5°F below the requiredequipment inlet temperature. The blower will then remain on until thewater drops to a temperature less than5°F below the required equipment inlettemperature. The spray pump will stayon until the water temperature dropsto 10° F below the required equipmentinlet water temperature.
In a closed-circuit pressurized tower,a thermostatically controlled electricheater in the spray water sump panturns on at a temperature of approxi-mately 40°F to prevent freeze up incold weather (Figs. 6 and 7). A float-operated valve controls the sump
water level. These systems shouldhave bleed-off controls to add enoughwater to overflow and allow scale andcalcium to escape the tank.
Open Evaporative Cooling TowersThe use of a heater or glycol is not
required with open evaporative towerhaving a gravity-drained sump tankmounted just inside the heated building. These systems have thetower mounted high on a platformoutdoors (Figs. 8 and 9). An opentower usually costs less, but it requires
extensive cleaning, good filtering forleaves, feathers, and other debris that isdrawn into the tower. There is nocooling bundle, just a cascade of waterdown a series of fins which gravitydrains inside the building.
Reducing the risk of Legionella bac-teria can be accomplished by thoroughcleaning on a regular basis. If a tower isto be idle for an extended time, itshould be completely drained. If draining is not practical, a chemicalshock with a biocide may be requiredto kill the bacteria.
24 HEAT TREATING PROGRESS • OCTOBER 2009
Fig. 5 — Air-cooled tower that uses an atomized mist assist option. Courtesy Dry Coolers Inc., Oxford, Mich.
Inlet Outlet
Outdoors
Tees and nozzles
Slope
header
f or
gravity
drain
½ in. hose
½
in.
ball
valve
¾
in.
city
water supply,
80
psi
¾
in.
plug
to
drain
misting
system
in winter
Normally
closed
solenoid valve
¾
in.
× 10
micron
water
f ilter
Fig. 6 — Counter-flow closed-loop pressurizedevaporative cooling tower; requires a pan heater.
Fig. 7 — Schematic of a closed-loop pressurized evaporative cooling tower. Courtesy Dry Coolers Inc.,Oxford, Mich.
for multiple induction systems (over500 kW), which eliminates the possi- bility of a breakdown of more thanone induction system if a tower orpump has failed (Fig. 10). Smallertowers are easier to clean than a singlelarge tower. This is solely an economicdecision, because multiple smalltowers cost more to install and takeup more real estate, but potentialdowntime concerns and cleaning costneed to impact that decision.
Placement of Cooling Towers
Avoid placing any outdoor towernear gravel or dirt parking lots andsand blast or shot-reclamation systems,which could produce dust that issucked into the tower, requiring addi-tional maintenance and reducing itsefficiency (Fig. 11). Positioning of thecooling tower is of the utmost impor-tance to reduce the chances of air re-circulation, which occurs when someof the moist, hot exhaust air leavingthe tower is drawn back in the freshintake vent. Placement and positionrelated to the prevailing wind in the
summer for the particular geographic
area and distance and height relatedto any buildings can all cause recircula-tion (Figs. 12 to 14).
Avoid placing hurricane fencing orchain link fence with aesthetic inserts,as they will restrict fresh air intake andcause recirculation of moist hot air (Fig.15). The heat-laden exhaust air is sat-urated and can be at a 10 to 15°Fhigher wet-bulb temperature than theambient wet bulb. The cooling capacityof the unit is drastically reduced whenrecirculation occurs.
Indoor installations of evaporative
HEAT TREATING PROGRESS • OCTOBER 2009 25
Fig. 8 — Open evaporative cooling tower withgravity drain to holding tank indoors. Fig. 9 — Schematic of open evaporative tower with emergency city water option. Courtesy Dry Coolers
Inc., Oxford, Mich.
Inside
Outside
Warm
moist
air out
Propeller f an
½ in. bleed Evaporative to
drain
cooling
tower
Thermostatic
control
valve
Gravity
return
Duplex process
Stainless pumps
steel tank
20
-mesh
strainer
Induction f urnace pumping station City water
Tower pumping station
Drain
Induction furnace
Fig. 10 — Multiple cooling tower installation; one tower for each in-duction system.
Fig. 11 — Avoid chain link fences with aesthetic inserts (reduces fresh airflow) and loca-tions next to gravel parking lots.
Fig. 12 — Incorrect installation position due toprevailing wind causes recirculation of moist air.Courtesy EVAPCO Inc., Taneytown, Md.
Fig. 13 — Incorrect installation position related toheight of the building and prevailing wind
direction. Courtesy EVAPCO Inc., Taneytown, Md.
Fig. 14 — Correct installation position related toheight of the building. Courtesy EVAPCO Inc.,Taneytown, Md.
cooling towers are commonly used inextreme northern locations where tem-peratures can reach -40 to -50°F (Fig.16). Canvas drapes (not shown) arehung on the exterior wall in winter tothe exhaust side to prevent cold-air back drafts into the plant. The biggestadvantages are to prevent freezing andfrost build up of the tower, and sup-plemental heat that can be used to heatthe plant. Inside installations of coolingtowers require larger motors withmore horsepower due to the increased backpressure of the venting ductwork.Placement of the fresh air intake andexhaust are critical to prevent recircu-lation of moist laden air.
Refrigerant ChillersRefrigerant chillers cool by mechan-
ical-refrigeration principals. Air-or water-cooled condensers areavailable with non ozone-depletingrefrigerant (Fig. 17). These systemsare predominantly used on smallinduction systems less than 200 kW.They can be open or closed-loopedwith an integral evaporator or
secondary heat exchanger. Thesesystems are the most expensive,and have high operating costs, but very low maintenance. Theyusually are indoors and dischargehot air into the plant, which cansupplement plant heating.
The advantage of a chiller is they arecompact and can take the place of thewater-to-water heat exchanger, pump,and controls shown in Fig. 1. A closed-loop quench system with its own heatexchanger is still needed to control thequench system temperature. Chillerrecirculating temperature can be set at85°F, which eliminates condensationin the power supply.
The following is an example of thecooling water specification for a re-frigeration chiller with nonferrouscooling bundle and holding tankused to directly cool induction powersupplies.
Total hardness (CaCO3) 15 ppm
Total dissolved solids 25 ppm
Conductivity 20 to 70 mho/cm
Max suspended solids 10 ppm
pH 7.0 to 7.5
Fig. 15 — Gang of cooling towers placed too close together with no regard to fresh air intake.
26 HEAT TREATING PROGRESS • OCTOBER 2009
Fig. 16 — Out-door view of indoor closed-loopevaporative towers with fresh air inlet openings
at bottom and exhaust out just below the roofline.Roof mounted exhausts fans at this Minnesota
foundry supply extra plant ventilation to reducethe humidity inside the plant.
Fig. 17 — Refrigeration chiller with filter panelsremoved used to direct cool an induction power
supply. Fig. 18 — Geothermal cooling field, using plastic 2.5-in pipe shown in foreground.
work very well when less than 300kW needs to be dissipated. Over thiskW rating, the size of the field is con-sidered too large. The ground is at aconstant 55° F ambient temperature below the freeze line. A geothermalsystem consists of a pump, heat ex-changer, expansion tank, inlet valve,
controls, and air bleed vents at thehighest points of the plumbing. Thewater temperature sensor that wasused to open the solenoid to the heatexchangers now is used to turn on thepump, and the solenoid is eliminatedto allow a free flow of water. A mix-ture of 30% glycol is still recom-mended to buffer the water.
The underground plumbing usu-ally contains dual parallel loops thatconsist of special geothermal poly/plastic materials and glued fittings.One pipe run is buried at about 5 ftand the other at 7 ft, about two linearfeet apart. These depths depend onthe freeze line at the particulargeographic location. In Florida, a 2to 4 ft depth is adequate, while a 6 to8 ft depth is required in Canada.The geothermal installation shown in
Fig. 18 is located in central Michiganand has been in service for over tenyears.
SummaryMany factors can affect performance
of water-cooling systems used for in-duction heating systems. Poor main-tenance can cause the long-term relia- bility of the induction system to be
reduced. The cooling tower placementrelated to prevailing winds and sur-rounding buildings can affect its per-formance, and cost of operation.
Water-cooling circuits are the mostneglected item in induction mainte-nance, causing the most down timeand damage. The damage causedis insidious as the damage beingdone cannot be seen. The systemoperates until partial plugging of awater path occurs, and expensive de-vise failure follows. These SCR, diode,and transformer failures can costthousands of dollars, but to the un-trained eye cannot be directly linked back to the high conductivity of thewater.
Common-sense installation andmaintenance practices can help re-duce unexpected down time, thereby
increasing profits for the user. Newand improved hybrid systems are being used in various combinationsto reduce costs. Great success incooling the induction heating systemat a low cost over decades of serviceeven on the hottest days can beachieved if these simple rules areimplemented. HTP
