furnace-dried

The Necessity of Furnace-Drying

When the electric furnace is newly constructed, it must be baked/dried for the first time using or after long-term deactivation. It should be carried out according to the predetermined furnace-drying curve. The purpose is to remove the moisture inside the chamber material, namely from the bricks, and make the bricks transform completely, so as to avoid the cracking and peeling of the masonry, which will affect the service life of the furnace lining.

On the other hand, by performing furnace-drying, it will test the performance of furnace heating, air circulation, water cooling system, transmission device and working temperature thus to meet the production requirements. At the same time, by which can also carry out the most realistic inspection of the manufacture or repair of the furnace.

1  Furnace-drying curve

The heating rate, holding time and the time-temperature chart that must be followed in the oven are called the oven curve.

1.1  Three stages for furnace-drying

Stage 1 Moisture excretion period: From normal temperature to 200°C is the excretion period of the moisture in the cement and part of the moisture in the masonry. At this time, keep the furnace door opened, and at 200 ° C, it must be kept for a long time to ensure the sufficient discharge of moisture.

Stage 2 Masonry expansion period: The expansion period of the masonry is from 200 to 600°C, which is the period when the masonry begins to expand and expand to deform. At this stage, the temperature should be raised slowly. If the temperature rises quickly, cracks and damages are often caused by the rapid expansion of the masonry. It is generally recommended that the heating rate should not be too fast, and the recommended heating rate should not exceed 1°C/min.

Stage 3 When the holding period: when the furnace temperature is above 600°C, the heating rate and holding time can be determined according to the different working temperature. For every 100~200°C increase, it should also be kept for a proper period of time, and it will cause the damage to the masonry if it is too fast.

1.2  Factors on the furnace-drying Curve

The drying time is related to the size of the furnace, the performance of refractory materials, the season of repairing and the construction method, for example, the time required for masonry with larger thickness is longer than that of masonry with small thickness. In short, the larger the furnace, the longer the drying time, but even the smallest oven, the drying time should not be less than 20 hours.

The heating rate of the furnace mainly depends on the thermal expansion of the masonry:

The holding temperature and holding time of the furnace/oven mainly depend on the moisture content (free water and crystal water) in masonry and the expansion and contraction of the crystal transformation of the brick.

1.3  Furnace-drying Curve

Drying curve with temperature and time
StepsTemperatureTimeRemarks
10-200°C100 min 
2200°C – 200°C50 minHolding temperatureopen the door in a small angle, around 15-30°
3200°C – 400°C100 minClose the door
4400°C – 400°C150 minHolding temperature
5400°C – 600°C150 min 
6600°C – 600°C100 minHolding temperature
7600°C – 800°C200 min 
8800°C – 800°C150 minHolding temperature
9800°C – 100°C500 minNatural cooling
Oven dried programs

2  Preparation before oven / furnace-drying

following preparations should be done carefully before the drying:

  • The furnace inner chamber and exhaust pipe should be cleaned thoroughly so that there are no broken bricks, dust or other debris left behind
  • All parts of the furnace must be thoroughly inspected, especially the cooling system and the furnace chamber, and the transmission mechanism must be tested and adjusted well. The oven can be allowed drying only after the inspection meets the technical requirements
  • Check the tightness of the furnace door, valve, air system etc. and the flexibility of opening and closing. Test the components of the cooling system to check whether the supply of circulating water
  • Check the furnace masonry, especially the roof of the furnace, whether there is any abnormal phenomenon
  • Check the control instruments, make sure it eliminated if there is any defect
  • Before the drying, check whether all the vertical and horizontal tie rods on the furnace skeleton are welded firmly, and whether the nuts are tightened
  • According to the specific conditions of the furnace, a professional person should formulate the heating curve of the furnace/oven, the operation rules and the precautions

3  Operating precautions for oven / furnace-drying

The furnace/oven must be carried out strictly according to the furnace-drying curve to ensure the temperature in the furnace chamber rises and distributes evenly. During the drying process, the actual temperature of each part should be measured with thermal instrument timely, and the actual furnace/oven curve should be recorded and drawn in detail.

Following items must be paid attention to:

  • The resistance to ground of the furnace wall should be checked before power transmission. When the voltage is 500V, the resistance to ground should be greater than 0.492
  • Adjust the controller to the set temperature, then turn on the power
  • For furnaces with quartz or alumina crucibles, the crucibles must be stacked up to 50~80mm on its bottom
  • For the oven of the mechanized furnace, when the furnace temperature reaches above 400 °C, the machinery (such as conveyor belts, rollers, etc.) must be operated to avoid parts overheating
  • Furnaces with fans. When the furnace temperature rises to 400°C, the fan must be turned on
  • After the furnace is powered on, the working condition of the resistance wire or the resistance band should be observed frequently to check whether there is parts overheating

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