Medium Frequency Smelting Furnace

What is a medium frequency smelting furnace, and how does it differ from a high frequency one?

A medium frequency smelting furnace is a coreless induction melting station whose alternating current runs at a few hundred hertz to a few kilohertz, which is low enough for the induced current to penetrate a thick shell of the charge and to couple into a bath of several hundred kilograms or several tonnes. The consequence is not a better or worse machine but a different unit of specification: a high frequency furnace is bought by the melt, one crucible and one heat at a time, while a medium frequency furnace is bought by the bath, which stays liquid in the vessel, is fed while it is hot and is tapped repeatedly. Published band definitions overlap heavily, so the label alone never settles the question; the frequency in hertz, the bore and the kilowatts have to be read together.

Why is a medium frequency furnace specified by the bath rather than by the melt?

Because what the shop buys is availability of liquid metal, not a melting rate. Once the vessel is large enough to hold a bath, the metal exists continuously through a shift, is topped up as it is drawn off and is tapped whenever the step downstream needs it, so the figures that govern the machine become the bath mass, the specific power that keeps it hot, the hours it is held and the number of taps a shift can take. Those figures have no counterpart in a single-melt specification, and a station sized on a melting rate alone will be found holding metal for hours that nobody paid for in the quotation.

What is the difference between the bath and the charge?

The charge is the metal put into the vessel; the bath is the metal standing liquid in it. A working station never empties the vessel, so the working bath is the tap mass plus the heel that is deliberately left behind, and the charge per tap is the tap mass plus whatever is needed to replace the metal that left as slag, dust or skull. A quotation that uses one word for both is hiding the heel, and the heel is the single most common reason a melt-loss figure turns out to be wrong.

How many furnace bodies should one power supply drive?

It depends on the tap schedule rather than on the tonnage. One supply to one body means melting and tapping cannot overlap, so the station’s tap frequency is one body’s cycle and the downstream step waits through every melt. One supply to two bodies leaves one body available to tap while the other is on power, which roughly doubles tap availability without adding any melting power. One supply to three bodies takes a third body off the critical path entirely, so deslagging, a crucible change or a body refit never blocks production. Each additional body adds consumables, floor space and maintenance, so the honest answer is the smallest configuration whose tap schedule keeps the next process fed.

What is the duty split, and why does it decide the running cost?

The duty split is the division of the station’s day into melt hours, hold hours, superheat hours and charge and back-charge hours, each at its own load. Melting power is the figure every catalogue quotes and the figure that is rarely the dominant cost, because the hold load is continuous: a covered bath is losing heat through its lid, its lining and the extraction for every hour it stands, and those hours are paid for whether or not metal is poured. Superheat is paid for twice, because the bath is lifted to the tapping temperature and then gives that heat back before the next tap. A station quoted only on melting power has had the largest part of its variable cost left out.

How do I work out the bath mass I need?

Start from the step downstream and work backwards. Take the mass that step consumes per hour or per cast, which gives the mass one tap has to deliver; multiply the taps a shift needs by the tap mass to get the metal the station has to move in a shift; then add the heel the shop intends to retain, because the working bath is the tap mass plus the heel and not the tap mass alone. The bath mass is that sum, and the charge rate that follows is the bath mass divided by the melt rate the specific power can sustain between taps. The chain is short, and any proposal that will not show it should be treated as an estimate.

Why is specific power per tonne of bath the first specification rather than nameplate power?

Because the nameplate answers a question nobody asked. The specific power per tonne is what determines how fast the bath can be fed and how much heat it can make up between taps, and it is derived from the charge rate the shop feeds plus the hold load of the covered bath. The same power cabinet can serve a small bath on a short cycle or a large bath on a long one, so two stations with identical nameplate power can have entirely different outputs. Ask for kilowatts per tonne of working bath and for the charge rate that follows, and the comparison becomes possible.

Why does the working frequency fall as the bath grows?

Because the induced current is confined to a thin shell at the surface of the charge, and the coil’s field has to reach metal rather than the air above it. A wide bore puts more metal in the path of that field for the same height of charge, so a larger body couples satisfactorily at a lower frequency and the working frequency falls as the bath mass rises. The pattern is visible in published equipment: one medium-frequency line runs at 500 Hz for three-tonne, eight-tonne and fifteen-tonne bodies, at 150 to 200 Hz for thirty tonnes, at 150 Hz for forty tonnes and at 135 Hz for ninety tonnes. A quotation that offers one frequency for every bath mass is offering a compromise.

What is the heel, and how much should be left in the body?

The heel is the liquid metal deliberately left in the vessel at the end of a tap so that the next charge lands on liquid instead of on a cold lining. It is what makes the second tap of a shift faster than the first: published operating figures for a small medium-frequency melting machine describe 50 to 60 minutes for a cycle started from a cold furnace against 20 to 30 minutes started from a hot one, and the heel is the physical difference between those two numbers. The size of the heel is a design decision, set as a fraction of the working bath and recorded per tap, because a growing heel silently reduces the effective bath while a shrinking one exposes the lining and hardens the next cold start.

How long does one tap take on a medium frequency furnace?

There is no single figure, because a tap time is the sum of four things: the time to melt down the replacement charge, the time to lift the bath to the tapping temperature, the tapping time itself, and the time taken to recharge the body afterwards. Only the first is the melting rate, and it depends on the specific power, the alloy and the coupling of the charge. Ask instead for the cold-start and hot-start cycle figures, the assumed charge mass and alloy, and whether the figure includes charging, deslagging, composition adjustment and temperature measurement, because a published figure usually assumes a hot lining, a closed cover and no residual melt.

Can a medium frequency station hold a bath overnight or over a weekend?

It can hold a bath for as long as the shop is prepared to pay for the hold load, and the constraint is not the power supply but the vessel and the metal. A covered bath held at a temperature window loses heat continuously and that loss is purchased hour by hour, so a long hold is an operating decision with a cost attached rather than a technical limit. The practical constraints are the crucible and the lining, which accumulate thermal exposure, the alloy, which may take up gas or lose a volatile element while hot, and the tap schedule, which decides whether holding actually buys anything. Where a long hold is unavoidable the machine is specified with the hold regime written into it rather than discovered afterwards.

Can one station run several alloys?

Yes, and the changeover is a procedure rather than an obstacle. Because the vessel is never fully emptied, a change to a chemically different alloy is normally run by drawing the bath down to the heel, charging the new alloy against the retained metal, verifying the composition before the tap, and recording the changeover against the tap record. Where the two alloys are incompatible, the heel is drained and the body and crucible are cleaned or changed. The equipment side of the question is the crucible grade and the tapping temperature window, because a grade that suits one alloy’s liquidus and reactivity does not automatically suit the next one.

Can a medium frequency induction furnace do reduction smelting with a heavy slag?

Not as its primary duty. The coil couples to the conducting charge, so it heats metal and not oxide: a slag layer above the bath is warmed only by contact with the metal underneath it and acts as an insulating blanket over the surface it covers. The flux practice that follows is a thin reactive skin that gathers oxide and is removed at a slag door, which is a refining aid rather than a slag process. A duty whose purpose is a heavy-volume reduction slag, or which needs a separate metallic phase to gather the values, belongs to the furnace family built to hold one, and a station is routed there rather than oversold.

How much metal is lost in a melt, and how is that figure produced?

The only honest melt-loss figure a station has is the unexplained term in its own metal balance: charge plus back-charge, less the tap, the change in the heel, the slag and dross, the captured dust, the crucible and lining skull and the retained sample. Every one of those terms has to be weighed and assayed, and the heel has to be measured rather than assumed, which is why the figure can only be produced per tap. An annual estimate cannot separate a real loss from a weighing error, an unrecorded heel or a sample that was never entered; in a recovery shop the melt loss is the largest controllable operating cost and the one that stays invisible longest.

How is the bath verified before the tap?

By stirring for the mixing time written into the recipe and then taking the sample at the position the recipe specifies, because a sample drawn from a quiet corner, from the surface or from the first metal out of the tap belongs to a different composition from the bath. The stirring is what makes the verification meaningful: at medium frequency the induced current penetrates a thick shell of the bath, so the force acts on a large volume of liquid and a tonne-scale bath reaches compositional uniformity in minutes rather than hours. What stirring cannot do is remove an element that is in solution, so a bath whose charge was wrong stays wrong, and the verification is a check on uniformity rather than a repair.

What site services does a medium frequency melting station need?

A station is a set of units rather than one box, and each has its own requirement. The power source needs the connected electrical load that the duty split and the specific power produce, together with the rectifier, the harmonic treatment and the power-factor correction the local supply authority requires. The cooling circuit needs treated water to the published chemistry limits, with the flow, pressure and return temperature the station draws and a route for its heat rejection. The bodies need a floor that will carry them standing full of liquid metal, drainage and containment beneath them, and a lifting arrangement for a loaded crucible. Extraction is needed over the bodies, the tap and the refill point, and the lay-out has to keep liquid metal, cooling water and electrical equipment physically separated, with the pour aimed away from the coolant circuit.

What charge will you refuse, and why?

Wet or damp charge, because moisture flashes to steam against a bath and throws metal; sealed or hollow containers, for the same reason; mercury-bearing material, because the vapour is a health and environmental hazard that a melting station cannot capture adequately; nitrate-, chlorate- or peroxide-bearing material, because it can react violently with liquid metal; moist halide salt, which is a similar hazard; oversized pieces that will not enter the body or will bridge above the bath; and material that cannot be identified or assayed, because a bath charged with an unknown composition is a bath whose product cannot be certified. The refusal is issued with the physical mechanism attached, so that the customer can correct the material rather than argue with the decision.