Preheaters save energy.

Foundry scrap is discarded metal, much of it garden variety junk. In its as-delivered condition, or because of its storage conditions, scrap can contain water, rust, oil and grease, slag, wood and other materials.

These contaminants are a potential hazard to the crews responsible for charging induction melting furnaces. Adding wet, rusty and oily charge materials to a molten bath of iron in an electric furnace is risky. The presence of water is critical. The result can be an explosive reaction sending molten metal hurtling out of the furnace, seriously damaging furnace linings and obviously endangering lives.

Why Preheat?

The melter's best precaution against a potentially dangerous charging misadventure is to preheat each electric furnace charge. Sufficiently high temperatures will eliminate any free water and other volatile contaminants in the scrap, lessen the chance of accidental damage to melting equipment and add to melt shop safety margins.

Preheating also conserves electric energy required for melting as less costly oil or gas can be used for preheating.

* Other advantages include: increased production from tap-to-tap time;

* the use of lower cost scrap available for melting;

* decreased electrode consumption (in electric arc furnaces);

* reduced furnace smoke emissions by vaporizing or burning nonmetallic inclusions, paint, oils and lubricants found in scrap;

* lower slag production and prolonged furnace lining life.

Types of Preheaters

The design of a preheating system is dependent upon the type of scrap used, the foundry's charge handling equipment and the intended effect of preheating.

Preheating systems are usually referred to as being either driers or true preheaters. The temperatures required for a drier are usually in the 210-57OF (22-67C) range and primarily intended to eliminate moisture since temperature (in that range) has little effect on the melt rate or the economics of electric furnace energy consumption. Today's true preheating practice uses temperatures well above this, generally between 570-1380F(67-168C).

There are four main types of preheaters, each offering advantages with different types of scrap. These include bucket, vibrating conveyor, continuous and revolving drum preheaters.

Bucket preheaters-The preheater consists of a unitized bucket/heating element that can be part of a conveyorized charging system that fills the bucket, heats the charge and empties it into the furnace in a continuous cycle lasting 15-30 minutes. It works best with uniformly large scrap pieces because the blast of heat can permeate the entire scrap load. Small, dense scrap loads will require greater soaking time.

Vibrating conveyor preheater-This system uses a feed conveyor to load a vibrating conveyor that mo low bed of charge material through a furnace containing roof-mounted high velocity burners. By adjusting the vibrations of the loading and furnace conveyors, the operator can vary the scrap bed height to accommodate the type of scrap being charged. Typical heating times are 4-6 minutes with a total cycle time of 8-10 minutes, raising the scrap temperature to about 1022F(124C).

Continuous preheater-When scrap is in bales, it is important to heat the core of each bale high enough to evaporate water and incinerate organics. The continuous preheater is similar to a continuous heat treating furnace in that the burners usually are not in the furnace chamber in order to provide a low (930-935F/112-113C), constant temperature over a longer period to minimize charge oxidation. Long dwell times (up to an hour) assure a scrap bale core temperature between 300-400F(3346C), sufficient to evaporate any moisture and most residual oil and grease.

Revolving drum preheater-Cast iron borings, chips and turnings are low-cost sources of scrap charging materials and removal of moisture and cutting fluid residues before use is required. Because of their tendency to gather into high density layers or packs, they need to be kept as shallow and loose as possible to be preheated as quickly as possible in the range of 300500F(33-58C) for about 3-5 minutes. This system uses a heated revolving drum to accommodate a single charge load. The heat source can be idled during loading and put on 'high fire' after the charge is loaded.

All of these preheating systems are effective only if adequate methods of quickly transferring the scrap from the preheater to the electric furnace are taken.