GB2195010A

GB2195010A - Method and apparatus for furnace atmosphere control in continuous enamel fusion furnaces

Info

Publication number: GB2195010A
Application number: GB08622061A
Authority: GB
Inventors: Geoffrey Roy Armstrong; Trevor Carr; Roger Frank Price
Original assignee: TI
Current assignee: TI
Priority date: 1986-09-12
Filing date: 1986-09-12
Publication date: 1988-03-23
Also published as: GB8622061D0

Abstract

A method and apparatus are disclosed for controlling the dew point of the furnace atmosphere in the firing chamber (3) of an enamelling, tunnel furnace (1) by feeding a stream of dry air (7c) across the mouth of the tunnel preferably by means of a vestibule (8) positioned in front of the tunnel mouth, and through which the enamelled articles enter and leave the furnace. <IMAGE>

Description

SPECIFICATION Method and apparatus for furnace atmosphere control in continuous enamel fusion furnaces This invention relates to furnaces or kilns used in the fusion or firing of vitreous enamels, and to a method of controlling the atmosphere therein.

The conventional enamelling furnace comprises an elongated, usually rectangular section, preheating chambers open at one end for the continuous feeding and removal of the articles to be enamelled and which are predipped or sprayed with the unfused enamel frit prior to entry into the furnace. At the other end the preheating chamber communicates with an enlarged firing chamber, usually a rectangular box-like chamber, through which the articles are conveyed in a generally Ushaped path emerging again through the preheating chamber. Usually the articles are conveyed through the furnace suspended from an overhead conveyor arranged in a U-shaped path extending along the preheating chamber and into the enlarged firing chamber. Alternatively, the articles are carried on a U-shaped conveyor running along the bottom of the furnace.Tunnel furnaces have also been used, in which the articles pass straight through the firing chamber, but are generally less preferred because of increased heat losses, and the need for additional heat seals, i.e. in the exit chamber as well as in the entry chamber or vestibule.

The furnaces may be heated electrically or by radiant heating tubes positioned in the firing chamber and fed with hot combustion gas.

In the preferred U-configuration, at least, the incoming articles are preheated in the preheating chamber by heat from the outgoing articles and outgoing hot air. In the tunnel furnaces the incoming articles have to be separately preheated.

Since the mouth qf the furnace is open to permit the continuous passage of articles through the furnace one or more air seals are usually provided in the preheating chamber to reduce heat losses through the open mouth of the furnace. In the case of tunnel furnaces, of course, additional air seals have to be provided at the exit from the firing chamber.

In GB-A-2,152,027 a method of enamelling is disclosed giving superior results in terms of improved adhesion and elimination of defects such as carbon boil and fishscaling, and which involves, inter alia, firing the precoated enamel ware in a furnace under controlled conditions, in particular, in a furnace atmosphere having a dew point of up to and preferably below 10"C.

In experimental work leading up to the present invention, it has been found that the injection of dry air directly into the firing chamber in order to maintain the dew point of the furnace atmosphere at up to 10 C leads to an unacceptable level of heat loss from the furnace. By contrast, and surprisingly it has been found that the dew point of the furnace atmosphere can be controlled, without unacceptable heat loss, by the simple expedient of feeding a low velocity stream of dry air across the mouth of the furnace. Surprisingly it has been found that sufficient dry air is thus drawn into the furnace to maintain the dew point of the furnace atmosphere at the desired low level.

in one aspect therefore the present invention provides a method of controlling the dew point of the furnace atmosphere in a continuous furnace, which comprises introducing into or across the furnace opening in a direction generally transverse of the direction of travel of the articles therethrough, a low velocity, high volume stream of dry air, and introducing the articles into the furnace therethrough. Preferably, the low velocity, high volume stream of dry air is fed across the mouth of the furnace opening, for example by means of a vestibule mounted in front of the furnace opening.

Preferably the dry air stream is a low velocity, high volume air stream, e.g. having a velocity of from 5 to 100 m per minute, preferably from 15 to 60 m per minute. The dry air stream may be at ambient temperature, or, if desired, it can be preheated. Before feeding across the mouth of the furnace, the air will be predried, for example, to a dew point of less than 10"C, preferably less than 0 C, in any suitable manner, for example, by using desicant drying agents. Volume flow rates will generally be in the range 200 m3/hr to 12,000 m3/hr, preferably 400 m3/hr to 6,000 m3/hr.

In a second aspect, the present invention provides a continuous enamelling furnace comprising a firing chamber, a preheating chamber in open communication with the firing chamber and the furnace opening, and means for continuously conveying articles to be enamelled from the furnace opening through the preheating chamber to the firing chamber, through the firing chamber and back out through the same or different opening, wherein means are provided adjacent the furnace opening(s) for feeding into or across the opening in a direction generally transverse to the direction of travel of the articles therethrough a low velocity, high volume stream of dry air.

Preferably the enamelling furnace is of the type comprising an elongated preheating chamber open at one end and communicating at the other with an enlarged and otherwise enclosed firing chamber, and comprising a conveyor extending into said furnace around a U-shaped path for the continuous feeding of enamel ware through the preheating chamber to the firing chamber and back through the preheating chamber to said opening. Prefera bly, the means for feeding said stream of dry air into or across the furnace opening(s) com prise a vestibule (or post-chamber) mounted in front of, or behind, said opening, as the case may be, and having a through opening for the passage of enamel ware therethrough into and from said opening, and one or more air out lets in said vestibule for feeding a stream of dry air into said vestibule transversely of the through opening therein.

Whilst the invention is particularly designed for the control of furnace atmosphere in the conventional U-configuration furnace, the prin cipal thereof may also be adapted for the con trol of the furnace atmosphere in a tunnel fur nace, in which case a similar low velocity, high volume dry air stream will be fed into or across the furnace exit e.g. by means of a similar vestibule or post-chamber adjacent the furnace exit.

The method and apparatus of this invention is further described with reference to the ac companying drawings, in which: Figure 1 is a diagrammatic view of a con ventional continuous enamelling furnace; Figure 2 shows the furnace as modified in accordance with this invention; and Figures 3a, 3b and 3c are side, front and plan views of the vestibule which is mounted in front of the furnace opening in accordance with one aspect of the present invention.

Referring to Fig. 1, the conventional continu ous enamelling furnace comprises an elon gated, rectangular preheating chamber (1), in this embodiment having a stepped configura tion, and which is open at one end (2) and at the other communicates with an enclosed rec tangular box-like firing chamber (3). A U shaped conveyor (4) extends along the roof of the furnace to convey enamel ware continu ously into the furnace, through the preheating zone (1), through the firing chamber (3), and back through the preheating zone (1) and back out through the furnace opening (2).

To reduce heat losses from the furnace, conventional air seals or air curtains are pro vided at the opposite ends of the preheating chamber. These comprise pairs of air outlets (5, 6) located on opposite sides of the pre heating chamber and through which the hot furnace atmosphere is recirculated to form a moving curtain of air extending across the in terior of the preheating chamber and through which the articles pass on their way to and from the firing chamber. Nevertheless, as the arrowed air flow pattern (7) shows, there is still a considerable loss of hot furnace atmos phere by hot gases flowing along the roof of the preheating chamber from the firing cham ber and exiting (7a) from the open furnace mouth.Similarly, there is a considerable in gress of moist ambient air (7b) into the fur nace mouth and which travels along the bot tom of the furnace through the preheating chamber into the firing chamber.

In accordance with this invention this moist ambient air flow is reduced or eliminated by placing a rectangular vestibule (8) in front of the furnace opening and having a through opening axially aligned with, and of comparable cross-sectional area with, the furnace opening (2). As shown more clearly in Figs.

3a, 3b, 3c, to one side of the vestibule (8) there is a conical air inlet duct (9) through which a stream of dry air is fed into the vestibule across the mouth of the furnace opening, by a conventional blower or fan (not shown).

Also not shown, is a drier, either upstream or downstream of the blower, for drying the air stream which is fed into the vestibule (8) via the inlet duct (9).

As will be seen in Fig. 2, the conveyor (4) is extended through the vestibule for the continuous feeding and withdrawal of the enamel ware to the furnace through the transverse stream of dry air introduced via the inlet (9).

As indicated by the pattern of air flow arrows (7c) a portion of the transverse dry air flow is diverted outwardly of the furnace to reduce or prevent the ingress of moist ambient air along the bottom of the furnace, whilst a remaining portion is drawn into the furnace, thereby to provide the firing chamber with the required dry furnace atmosphere.

Instead of mounting a separate vestibule in front of the furnace opening, it is also envisaged that the control of the furnace atmosphere within the firing chamber, and particularly its dew point, may be effected by using a dry air stream to provide the air curtain (6) at least at the upstream end of the preheating chamber, if not a both ends.

Claims

1. A method of controlling the dew point of the furnace atmosphere in the firing chamber of an enamelling furnace into which the articles to be enamelled are continuously fed through a furnace opening into a preheating chamber and from thence into a firing chamber and from which the enamelled articles are removed through the same or different opening, which comprises introducing into or across the furnace opening in a direction generally transverse of the direction of travel of the articles therethrough a low velocity, high volume stream of dry air, and introducing the articles into the furnace therethrough.

2. A method according to claim 1, wherein the low velocity, high volume stream of dry air is fed across the mouth of the furnace opening.

3. A method according to claim 2, wherein the low velocity, high volume dry air stream is fed across the mouth of the furnace opening by means of a vestibule mounted in front of the furnace opening.

4. A method according to claim 1, 2 or 3, wherein the low velocity, high volume air stream has a dew point of less than 10"C.

5. A method according to claim 4, wherein said air stream has a dew point of less than O"C.

6. A method according to any one of claims 1-5, wherein the low velocity dry air stream has a velocity in the range 5 to 100 m/min.

7. A method according to claim 6, wherein said velocity is from 15 to 60 m/min.

8. A method according to any one of claims 1-7, wherein the low velocity, high volume air stream is fed into or across the furnace opening at a volume flow rate of from 200 m3/hr to 12,000 m3/hr.

9. A method according to any one of claims 1-8, wherein the articles to be enamelled follow a U-shaped path through the furnace and enter and exit through the furnace opening and through said low velocity, high volume dry air stream.

10. A continuous enamelling furnace comprising a firing chamber, a preheating chamber in open communication with the firing chamber and the furnace opening, and means for continuously conveying articles to be enamelled from the furnace opening through the preheating chamber to the firing chamber, through the firing chamber and back out through the same or different opening, wherein means are provided adjacent the furnace opening(s) for feeding into or across the opening in a direction generally transverse to the direction of travel of the articles therethrough a low velocity, high volume stream of dry air.

11. A furnace according to claim 10, wherein the means adjacent the furnace opening(s) for feeding said stream of dry air comprise a vestibule (or post-chamber) mounted in front of-(or behind, as the case may be) said opening(s) and having a through opening axially aligned with the furnace opening for the passage of the articles to be enamelled therethrough on said conveyor, said vestibule (or post-chamber) having one or more air inlet openings therein for feeding thereinto a dry stream of air transversely of the through opening therein.

12. A furnace according to claim 10 or 11, wherein the furnace is of the type comprising an elongated preheating chamber open at one end and communicating at the other with an enlarged and otherwise enclosed firing chamber, and comprising a conveyor extending into the furnace around a U-shaped path for the continuous feeding of enamel ware through the preheating chamber to the firing chamber and back through the preheating chamber to the furnace opening.