To many people, the subject of heat in the iron and steel industry conjures the thought of vast furnaces smelting the raw ingredients to produce the iron and steel. Thudding electrodes, snaking cables carrying thousands of amps and showers of sparks complete the scene.
There are other parts of the process, which require more modest temperatures to prepare the resultant metal for the consumer. Gas fired re-heat furnaces prepare the steel for the rolling mills to produce sections and plate required by industry. Once the fabricators have turned the metal into the items ordered by their customers, heat treatment may be necessary to change the hardness to the required specification or relieve stresses caused by welding. This will require a furnace capable of raising the temperature of the article at a well-controlled rate, holding at a specific level for a period of time and then cooling, again at a predetermined profile.
For these furnaces, gas is the most favoured fuel, allowing the temperature control to be as precise as the process requires. There are many excellent controllers available on the market, which can be programmed to control the burners to achieve the required temperature profile. However, these cannot do their function accurately without the burner gas and air pipe-trains having suitable valves, which can respond appropriately to the signals generated by the controllers.
Even in these days of Profibus and other communication protocols, the 'good old' 4 to 20 mA control loop is the most likely to be found in use for burner control. In some cases, slidewire feedback is still considered to be the most reliable method. Whatever system is employed, the purpose is to control the gas and airflows, thus adjusting the heat release of the burner throughout its firing range. This 'turndown' is usually quoted by the manufacturer and is the difference between the maximum and minimum firing rates (usually expressed as a ratio). Above and below these positions, the flame becomes unstable and, at the lower end, is likely to extinguish itself.
The control of these valves can be either by a mechanical linkage, coupling the gas and air valves together, with a common actuating motor; or a 'zero governor' may be employed. This system uses an actuated butterfly valve in the combustion air supply and an impulse line taken from the controlled side and connected above the diaphragm of the zero governor (aka ratio regulator). Changes in air pressure in the impulse line open or close the gas valve accordingly.
In certain applications, required by the iron and steel industry, the atmosphere in the furnace has to be completely free of oxygen. In this instance, the burners are set to fire rich. That is, there is no free oxygen in the exhaust gases once combustion has taken place. This 'off ratio' firing needs careful control and is best achieved by using a specialised controller, such as the Fireye Nexus, which has the facility of controlling the combustion air and gas valves independently. Once configured by the commissioning engineer, the air/gas ratio is controlled to a very fine degree throughout the firing range of the burner. It is even possible to incorporate an O2 trim, which will take into account the varying oxygen levels in the atmosphere, due to prevailing weather conditions.
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