Surviving the energy crisis in your induction melt shop: Proper operating and maintenance procedures can increase furnace utilization and efficiency to reduce your energy bill and improve the bottom line.

Until recent times, the foundry industry has been able to operate in an era of inexpensive, reliable and plentiful electrical power. Two years ago, much of that changed. Foundries in California experienced the nightmare of rolling blackouts and sky-high electrical utility rates. Foundries nationwide have been feeling the pinch of rising rates and hearing warnings of power shortages.

What can be done to combat this potential energy crisis? The first response should be to minimize your foundry's exposure by reducing your electric consumption without sacrificing profitable production.

This article examines several ways a foundry can cut its power use. Some options require significant investment while others do not. Some options generate large savings, while others produce small, incremental power economies. However, all can have an impact on a foundry operation and, more importantly, on the monthly power bill.

Operating Procedures

For many foundries, the largest energy savings can be realized by replacing their current melting equipment with more productive and energy efficient units. For numerous reasons, however, this option may not be feasible. Foundries can consider operational changes that may produce significant cuts in power use at little or no cost.

Increased Equipment Utilization--The greatest operational savings in power use may be achieved by maximizing melting equipment use. Higher equipment utilization results when holding times are short relating to time spent melting at full power, reducing energy consumption.

One investment casting foundry pays its workers on an incentive basis to underline the importance of this point. Melters are paid for metal poured over the spout, whether it goes into a ladle or into a catch basin. Pourers are paid on how much metal enters the molds. If the metal leaves the furnace but goes into the catch basin, pourers aren't paid for it. Accordingly, when a melt is ready to be poured, the melter blows a horn and starts tilting the furnace. The pourer must be in a position to catch the metal.

This situation has a tremendous impact on the utilization of the melting equipment. Little, if any, time is wasted and utilization is extremely high. As a result, energy consumed per ton of metal poured is at an absolute minimum.

Speed Up Charging--Charging practices have a dramatic affect on equipment utilization. If done by hand, charging can be a tedious and dangerous process. In most cases, manual charging is unable to keep pace with full-power melting, forcing power to be throttled back. Charging by magnetic crane can result in similar pacing problems. Long charging times cause low equipment utilization.

Ideally, charging should occur rapidly and should put metal in the furnace only as fast as the furnace is able to melt it under full power. Except in small furnaces, this requires an automated charging system.

A wide variety of systems are available; many of which are adaptable to virtually any melt shop facility. Vibrating conveyors that pivot, traverse and/or index to the furnace, depending on melt deck configuration, also are available.

Charge buckets can be used in facilities with ample overhead clearance. Belt conveyors can be used to move scrap to conveyors if the charge storage area is at different elevation than the melt deck. These systems can be combined in almost any way to meet the needs of a melt shop. As a rule of thumb, the charge conveyor should hold one full furnace charge. When circumstances make this impractical, it should hold at least 50% of a full charge.

Avoid Overcharging--Overcharging of furnace (having cold charge materials laying on the top of the furnace) causes wasted energy. This leads to energy loss as the components on the top part of the furnace will over-heat. Also, an overcharged furnace prevents the lid from being closed properly. Ideally, the furnace lid will be closed at all possible times. Typically, furnace over-charging is the greatest problem in furnaces with a short freeboard area, particularly if they are being charged with buckets since the bucket capacity is discharged at one time rather than over a period of time with a vibratory conveyor.

With any type of charging, the bath must not be allowed to become molten when cold charge materials still need to be added to the furnace. A risk of eruptions from the furnace exists if wet or damp charge materials come in contact with the molten metal. When cold charge materials are added to non-molten materials in the furnace, the heat from the top of the bath will dry any residual moisture prior to the moisture coming into contact with molten metal.

Use Clean crap--Dirty charge materials waste tremendous energy and increase electrical consumption. Sand has twice the heat content of iron, thus every pound of sand in the charge equals 2 lb of iron not melted. Another way to look at this is that for every pound of sand that goes into the furnace, two times the amount of energy is required to bring that sand up to the same temperature as the metal.

Excess slag is a good indicator of dirty charge materials. Foundries should compare the cost of cleanerscrap with the excess energy consumption required to heat the dirty scrap.

Remove Slag Quicker--Reduce the time needed to remove slag during the melt cycle. For furnaces more than 5 tons in size, back-slagging mechanisms allow slag to be removed more easily, more effectively and quicker. For larger furnaces, mechanical slag grippers can be effective tools for fast slag removal.

Streamline Temperature Measurement and Sampling-Melt temperature measurement can be time consuming and costly, particularly if the operator frequently undershoots or overshoots target temperatures and must make adjustments that require additional readings. Computerized melt control systems can increase the ability to hit desired temperature levels.

In terms of sampling, distance and time are key. Button molds for sampling should be near every furnace to be sampled so operators can make samples quickly and send them to the metallurgical lab. In the lab, analysis should be performed efficiently to allow the melt deck to proceed with distribution of the melt to pouring.

Optimize Pouring-While a furnace is pouring, it is not being used for melting. In fact, if tapping takes too long, the metal may require reheating, which is another waste of electrical power. The tapping practice should be evaluated to determine if it is being completed as quickly as possible. Could larger ladles be used to reduce the number of taps?

A large Midwestern jobbing foundry increased its melt furnace utilization tremendously by increasing the width of the trough in the transfer launders from the melters to holders. This allowed it to increase the pouring rate, therefore holding time and pouring temperature were reduced. This cut energy consumption and increased refractory life.

Another way to reduce energy consumption is to pour at the coolest possible temperature and avoid temperature overshooting. If metal that could have been poured at 2750F (1510C) is allowed to rise to 3000F (1650C) (less than a 10% increase), heat losses are boosted by 33%. Pouring at the lowest temperature also reduces refractory wear and alloying element losses.

Schedule/Control Power Use--Beyond poor melt deck practices, many melt systems are under-utilized due to a lack of metal demand from the mold line(s). Electrical costs can be cut considerably by reducing the peak output power of the melt power supply, reducing demand charges. However, it is critical to verify with the melt equipment supplier that the melt equipment will not be adversely affected.

Another option is to melt fewer days per week and more hours per day. This alternative will save energy because the foundry reduces the time holding molten metal in the furnace at temperature. In addition, refractory life will increase. Refractory life is affected by metal temperature and throughput, as well as the number of hot/cold cycles it encounters.

Many foundries also realize significant savings in both their energy and demand charges by melting during off-peak hours. Most of these operations will melt and pour simultaneously during the off-peak hours, but others have changed their equipment to allow for melting off-peak and pouring during the regular day shift. For example, a medium size malleable iron foundry in New England added a fourth 8000-lb furnace to its melt deck to provide a total of 32,000 lb of metal ready to pour when the day shift started (on peak). As each furnace is emptied during on-peak, its holding power is added to the fourth furnace. This allows the foundry to perform additional melting in that furnace. By the time the first three furnaces are emptied, a significant amount of melt power is available for the fourth furnace while maintaining an even, low demand charge.

Maintenance Saves Power

Close attention to normal maintenance procedures can produce surprising energy savings at minimal additional cost.

Furnace Lids That Fit-Seventy-five percent of the heat loss in an induction furnace with an open lid is radiated into the air from the surface of the molten bath. The other 25% is lost via conduction through the refractory walls and floor.

The best way to reduce heat lost through radiation is by keeping a lid on the furnace. This means closing the furnace lid as quickly as possible after the addition of charge materials and after taking temperatures or adding alloying materials. In addition, if the lid doesn't fit properly due to warping, improper installation or a worn top cap and/or lid refractory, it will not be as effective. Regular maintenance to ensure correct lid fit will save the energy that would otherwise be needed to replace radiant heat losses.

Radiated heat losses rise exponentially with metal temperature (Fig. 1). As previously stated, a 10% increase in molten metal temperature results in a 33% increase in radiated heat losses. Inexpensive, manually operated, precast lids are readily available for retrofit.

Maintaining Specified Refractory Thickness--A common misconception among metalcasters is that thicker refractory linings in induction furnaces will extend lining life and save energy due to the higher insulating value of the increased refractory. In reality, this increases melt cost. Thicker refractory means that the metal will be further away from the coil, which causes a lower coil power factor in the furnace coil, leading to greater electrical losses.

With more electrical losses in the coil, less energy is available to melt metal. As a result, every melt will take longer than it would with a standard refractory thickness. This creates more conducted and radiated heat losses, further increasing the amount of energy consumed. The best refractory design is the one the manufacturer provides on the furnace cross-section drawing that comes with the equipment.

As refractory heats and cools, it expands and contracts. Over time, the diameter of the furnace coil will increase due to the pressure being exerted on it by the refractory. This results in a thicker lining and a lower efficiency as outlined above. It also means that more refractory will be required with every reline.

The solution is to periodically have the coils rebuilt and resized to ensure the diameter remains where the manufacturer intended it. The coil growth process can be slowed by regularly tightening the shunts in the furnace. This will help prevent the pressure of the expanding refractory from distorting the furnace coil. However, this maintenance cannot be performed on every furnace; check with the manufacturer.

Configuring Leads--A furnace's water-cooled leads can produce unnecessary electrical losses if not properly maintained or configured. If the leads are old and have gone through many cycles of furnace tilting, broken cables could be unseen inside the leads. This causes higher resistance in the lead and higher electrical losses.

Lead length also is important. Each foot of flexible water-cooled leads adds another increment of electrical losses. Make sure that furnace leads are no longer than they absolutely must be.

Leads should be tied together. Leads that splay apart from one another will have higher electrical losses than those that are bundled together properly. Furnaces with four lead systems should be bundled with lead polarities opposite each other, creating a "diamond" type configuration on (Fig. 2) to minimize losses.

Eliminate Hot Spots--Many foundries will periodically perform an infrared scan of all of their equipment. Any hot spots on the power supply, interconnecting bus bar, water-cooled leads or furnace assembly are created by energy that is not reaching the metal. A periodic infrared scan of the equipment also reduces downtime by identifying components likely to fail due to heat build up.

Control Fume Collection Flow--Another area of energy savings is the fume collection system. The amount of flow through the fume hood should be no more than what is necessary to effectively remove the fumes. Excessive air flow simply will draw more heat off the top of the furnace, increasing energy losses.

New Equipment--Having maximized the power savings achievable by optimizing operations and maintenance, the next step should be to find energy saving opportunities related to your melt shop's capital equipment. Begin by reviewing the assumptions made when the existing melt equipment was first acquired. Pay particular attention to changes that have taken place since the equipment was installed.

For example, have the number of shifts changed? Are the same alloys and metals being melted? Has the production level changed? Are the casting sizes as anticipated? Are the same size ladies being used? Are the charge materials the same? Has the electric rate structure changed? Using this information, the current melt operation can be examined.

First, evaluate the power supplies. Many induction units built before today's solid state systems were 80% efficient, compared to electrical efficiencies of up to 97% today. Foundries must examine how all elements in their melting systems work together.

Today's induction melting systems operating at higher frequencies (resulting in smaller furnaces and lower heat losses) require 25% less connected kilowatts than systems built in the 1950s and 1960s, saving demand charges while using 38% less power to produce the same amount of metal. In addition, and more significantly, they require almost 15% less connected kilowatts and 10% less power than systems built just more than a decade] ago.

Looking at a typical two shift, 5-day/wk operation producing 6-7 tons/hr, today's power-efficient systems might save about $80,000/yr in electric costs compared to induction systems a decade ago. This is based on a current mid-range demand charge of $5/kilowatt and power use rate of $0.04/kilowatt hr.

Generators--Foundries should evaluate the purchase of generators as part of any overall energy optimization effort. However, issues involving permitting, space and noise must be addressed, and not all melt equipment can be powered by generators.

Maximizing Efficiencies

The keys to minimizing electrical costs and maximizing efficiency in the melt operation are:

* understanding the complete electrical rate structure and ho it affects the foundry operation;

* understanding how the melt equipment operates within that electric rate structure;

* optimizing the operation of existing melt equipment given that rate structure;

* maintaining melt equipment to maximize its efficiency;

* considering an upgrade to new, more energy efficient melt shop systems.

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RELATED ARTICLE: Understanding Your Electric Bill

Commercial electric power bills contain seven cost elements.

Base Charge--This is the fixed monthly charge the electric company imposes for maintaining an account.

Demand Charge--This charge is based on the highest level of power use during the month. It may be based on peak KVA use or peak kilowatt (KW) use. If the demand charge is based on KVA, check the power factor (P.F.) and see if power factor correction would help. Power companies explain that demand charges are used to pay for the capital equipment, such as generating plants and transmission lines, required to meet the power demands of their customers.

Energy Charge--This is the most straightforward item on a bill. It is the amount paid for the power used. It is based on kilowatt hours (KWH) consumed. This charge is to pay for the cost of generating the power.

SpecialAdjustments--This category includes fuel cost adjustments, tax assessments and other extra charges. Fuel adjustment is the most common charge and can be quite substantial. These charges are assessed based on power (KWH) consumption. In California, one adjustment is based on the average daily temperature during the month.

Power Factor Penalties--In some cases, foundries are charged a penalty if the line power factor drops below a specified level. Typically, this level is 80%, but may be as high as 90% in some areas.

Ratchet--This allows the power company to assess charges even if a foundry shuts down for the month. Most power companies will charge 80% of the highest monthly demand charge during the preceding 11 months as a minimum billing.

Time of Day Rate Differentials--Power companies offer lower energy rates for power consumed during "off peak" periods. These are the times when overall power demands are lower. In addition, there sometimes are intermediate "shoulder" periods where the KWH are charged somewhere in between the high "on peak" rate and the lower "off peak" rate. Power companies offer these rate discounts to help even out their loads throughout the day. Sometimes, these discounts are seasonally based as well.

Interruptible Rates--In many cases, rate discounts are available if a foundry becomes an interruptible power user as the plant will allow the power company to cut or reduce power service for a period when the power company needs to reduce its overall load.

Another type of interruptible rate sometimes offered will provide kilowatt hours at a discount during regular operations. However, during periods that the power company is short of energy, the foundry will be offered the option of shutting down or purchasing the electricity at a substantial premium.