Enameling Furnaces

Fusing of the porcelain enamel coating to its metal substrate or to a ground coat can be done in a continuous, intermittent, or batch furnace. The furnace may be heated by oil, natural gas, propane gas, or electricity. With oil heating, a muffle furnace is required to prevent the products of combustion from contaminating the enamel coating. Gas-fired furnaces are either muffle or radiant-tube, with a limited number of luminous-wall, direct-fired furnaces being used.

Continuous furnaces are of either straight-through or U-type (Fig. 12) design; furnaces of both designs use air curtains to prevent heat losses through the end openings. A laydown wire-mesh-belt conveyor is used for small signs, dials, microcircuitry, and other small flat pieces. Most continuous furnaces, however, are equipped with overhead monorail conveyors located above the furnace roof. Alloy hook or drop rods extend down from the conveyor trolleys through a narrow slot in the furnace roof to transport the parts. Sealing of this slot is accomplished by the use of articulated steel or alloy seal plates carried by the conveyor. When over the furnace, these plates ride on a cast or fabricated alloy seal track that is incorporated into the furnace roof. Cycles for continuous-furnace firing of ground coats and cover coats on a number of different types of steel parts are given in Table 11.

Table 11 Cycles for firing ground-coated and cover-coated sheet steel parts in a continuous furnace

Type of part

Gage of steel

Operating temperature(a)

Firing time, min(b)

°C

°F

Architectural panels

16-22

805

1480

2-4

Home laundry equipment

18-22

805

1480

4-5

Water heater tanks

7-16

870

1600

8-12

Range equipment

18-24

805

1480

3-5

Sanitary ware

14-18

815

1500

4-6

Temperature varies with composition of frit. Time in hot zone of furnace

Fig. 12 Coating and firing installation using a U-type continuous furnace. Personnel stations: A, loader and stoner; B, spray operator; C, brusher; D, loader; E, unloader and inspector

Intermittent furnaces are equipped at both ends with split side-opening or articulating doors, air-cylinder operated, and have an overhead monorail conveyor similar to that of the continuous furnaces. With this type of furnace, however, the conveyor moves in increments, so that when one load is discharged, a new load enters the furnace where it remains until the firing cycle is completed. Doors and conveyor are electronically interlocked so that the firing cycle can be timer controlled. Figure 13 illustrates the small difference in temperature between the center and bottom of an intermittent 5 m (16 ft) radiant tube furnace set at 825 °C (1520 °F) for firing a load of sheet steel parts.

Fig. 13 Temperature and time data for an intermittent furnace. Difference in temperature between two sheet steel parts during a 10 min firing cycle at 825 °C (1520 °F) in a 4.9 m (16 ft) intermittent furnace with radianttube heating. Indicating thermocouple was 710 mm (28 in.) below the top of the conveyor and 580 mm (22.75 in.) above the differential thermocouple. Both thermocouples were attached to the parts.

Batch furnaces used for porcelain enameling are of two types. One type has a vertical-lift single door through which the load is charged and withdrawn by means of a charger fork. The second type has a slot in the roof and a manually operated overhead conveyor. The load for this furnace design is supported by alloy rods from overhead trolleys and is manually pushed into the furnace for firing.

Furnace construction for aluminum enameling generally requires the use of stainless steel inner liner sheets, low-density wall insulation, and plain carbon steel exterior shell. Current technology for either retrofitting existing furnaces or erecting new porcelain enameling furnaces, both continuous and batch units, uses a thin-wall lining; it is typically 150 to 205 mm (6 to 8 in.) of ceramic fiber in conjunction with radiant tubes in both the preheat and hot zones. This permits very short heat-up and cool-down periods with no resulting refractory damage. Consequently, these furnaces can be turned off during nonproduction hours, which achieves dramatic fuel savings.

Forced convection is the preferred method of heating furnaces for firing porcelain enamel on aluminum. The heat is provided by quartz-tube electric heaters, metal-sheath heaters, or electric package heaters; all are designed for operation at high ambient air temperature. Quartz-tube and metal-sheath heaters are adapted to the furnace so that radiant heat is available in the firing zone along with forced circulation. Package heaters are placed far from the firing zone to eliminate direct radiation and hot spots. Heat from the package heater requires adequate air circulation to maintain a temperature uniformity of ±1% of the normal operating temperature throughout the furnace.

Forced convection heating is also accomplished with gas-fired radiant tubes as the heat source. The tubes are baffled from the work or firing zone so that air circulation provides the same advantages as in electric-package forced convection heating.

Firing temperatures for enamel on aluminum range between 525 and 550 °C (980 and 1020 °F); cycles are shown in Table 12. To control the color and gloss of the enamel within acceptable limits, the temperature throughout the work must be held to ±1- °C (± 2- °F). 2 2

Table 12 Cycles for firing porcelain enamel on aluminum

Type of part

Section thickness, 0.025 mm (001 in.)

Firing time, min

Firing temperature

°C

°F

Any configuration

26-40

5-61 2

540

1000

Any configuration

51-64

7-8

540

1000

Extrusions

125

10

550

1020

0 0

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