Paint drying oven

In an oven for drying paints on metal strips or bands travelling continuously therethrough, several incinerators are mounted directly on the oven, and an external incinerator is also provided for incinerating the gases released during the drying operation, each incinerator receiving the same mixture to be incinerated.

BACKGROUND OF THE INVENTION 
The present invention relates to improvements in or relating to ovens 
intented for drying paints on continuously travelling metal bands, strips 
or tapes. 
It is known that the drying of paints used in such processes is attended, 
according to the specific nature of the paints, by the release of water 
steam or organic solvent vapours. In this last instance, highly 
combustible products are involved which must be diluted sufficiently to 
prevent their ignition within the oven enclosure. This dilution is 
obtained by using air in such proportions as to set the mixture well 
beyond the inflammability limits. 
Therefore, in hitherto known paint drying ovens one portion of the gases 
contained in the oven is discharged permanently and the organic solvents 
are burned in an incineration oven meeting a twofold requirement: on the 
one hand, eliminating any possibility of pollution by these organic 
substances, and on the other hand regenerating the heat contained therein 
in order to improve the energy balance of the drying plant. 
Nowadays, ovens of relatively large size, comprising very complicated gas 
circulation systems, are used for the continuous drying of paints 
deposited on bands or strips. A typical example of an oven of this type is 
illustrated diagrammatically in FIG. 1 of the attached drawing. 
In FIG. 1 the reference numeral 10 designates the drying oven proper. This 
oven is divided into several sections A, B, C and D in order to obtain 
different heating rates for each phase of the paint drying cycle. In fact, 
at the beginning of this cycle the primary effect is to substantially 
evaporate the organic solvents contained in the paints. Then, the paints 
are polymerized, and finally hardened. As a result, very variable solvent 
concentrations are recorded along the path followed by the band travelling 
continuously through the oven. As clearly shown in the drawing, each 
section of the oven illustrated comprises a gas blowing device 12 so that 
the gas impinges against the continuously travelling band 14 and adequate 
heat exchanges take place by convection. The gas thus blown against the 
band 14 is taken from the surrounding gas in chamber 10' of the oven and 
includes fresh air mixed with hot gases (circulating along the circuit 
shown in dash lines) from the incinerator 16 and from the smokes produced 
in a make-up burner such as 18 for keeping the mixture temperature at a 
value substantially within the range of 150.degree. to 450.degree. C. 
At a predetermined point P of the evaporation area of oven 10 a certain 
output of the gaseous mixture is removed from the oven, the solvent 
content of this mixture resulting from the solvent evaporation taking 
place during and along the drying cycle. This mixture circulating along 
the path shown in dash and dot lines is directed to the incineration oven 
16 in which the solvents are burned by virtue of the additional heat 
resulting from a preferably gaseous make-up fuel G. 
In this known arrangement, the gas temperature at the outlet of the 
incineration oven is of the order of 750.degree. C. These gases are 
directed on the one hand towards the oven sections A, B, C, and D (along 
the dash-line circuit) in which they are mixed as mentioned hereinabove 
with the surrounding gas before being blown against the metal band, and on 
the other hand towards heat regenerating means 20, for example in order to 
produce steam or hot water, for heating thermal fluids, etc.., before 
being discharged to the atomsphere via a conduit 22. The fraction of the 
gases thus rejected to the atmosphere is compensated by the ingress of 
fresh air into the oven, such as shown at the right side thereof in FIG. 
1. 
In a modified construction, not shown in the drawing, of this type of known 
arrangement, the gases recycled to the various sections of the drying oven 
are derived not directly from the incinerator 16 but consist of air heated 
by means of a heater interposed between the incinerator 16 and the heat 
regenerating means 20. 
The capacities of incinerators of the type now widely used range from about 
200 to about 500 liters of solvent per hour. However, it is contemplated 
for future plants to improve these capacities up to and even beyond 1,000 
liters/hour with a corresponding air output of the order of 70,000 
Nm.sup.3 /h. Consequently, the incineration ovens incorporated in plants 
of this type will be extremely bulky. Besides, the pipes connecting the 
incineration oven to the various sections of the drying oven will convey 
gases at temperatures of the order of 750.degree. C., so that efficient 
lagging must be used, which means an excessive increase in costs and 
dimensions. 
On the other hand, due to the considerable mass of solvents to be 
evaporated, it is very important to provide means for equalizing their 
concentration along the oven in order to minimize the dilution outputs 
necessary for keeping the mixtures below their inflammability limits. 
Moreover, these known plants are also objectionable in that the drying oven 
exploitation is subordinate to the incineration requirements, on the one 
hand, and to the energy recovery or recuperation, before rejecting the 
gases to the atmosphere, on the other hand. 
SUMMARY OF THE INVENTION 
Now it is the essential object of the present invention to provide a drying 
oven capable of overcoming most, if not all, of the above-mentioned 
shortcomings by providing an arrangement such that different relationships 
are provided between the drying of paints, the dilution of solvents, the 
incineration and the heat recuperation. 
Consequently, this invention is directed to provide an improved oven 
arrangement for drying paints on continuously travelling metal bands, 
strips or tapes, the oven according to the present invention comprising a 
plurality of heating areas in order to provide different heating rates 
during each phase of the heating cycle, and wherein one fraction of the 
gases filling the oven enclosure is discharged permanently and is burned, 
this oven being characterized essentially in that it comprises on the one 
hand at least one incinerator mounted directly on the oven and adapted to 
deliver the smokes produced therein directly to one or several drying 
areas, and on the other hand at least one final external incinerator 
adapted to incinerate the gases discharged from the drying oven, each 
incinerator receiving the same mixture to be incinerated. 
According to another feature characterizing this invention, each one of the 
above-mentioned oven areas may be either equipped with a separate 
incinerator, or supplied from an incinerator common to several such areas. 
According to a further feature characterizing this invention, one or 
several oven areas may be provided with a make-up burner adapted to 
regulate the blowing temperature in the area concerned.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 2, the oven according to the instant invention is designated in 
general by the reference numeral 24. In this Figure the pipings for 
circulating the various gases are designated by the same numerals or 
letters, and the same dash lines and dot-and-dash lines as in FIG. 1. The 
dash-and-dot lines show the path followed by the solvent-rich mixture 
issuing from the oven after the incineration, and the dot lines illustrate 
the gas circulation after the regeneration. 
The oven 24 is also divided into several heating areas which in this 
example are four in number and are denoted A', B', C' and D'. As in known 
oven arrangements mentioned in the foregoing with reference to FIG. 1, 
each area includes means 12 for blowing gas against the metal band or 
strips 14 travelling continuously through the oven. 
According to this invention, the oven comprises one or a plurality of 
incinerators mounted directly on the oven in the portion thereof where the 
solvents are evaporated. In the embodiment illustrated, three such 
incinerators 26, 26', 26", connected directly to the oven at the level of 
areas A', B', C', respectively, are provided, such incinerators performing 
the combustion of the solvents with the assistance of the heat resulting 
from the combustion of a gaseous fuel. These areas of the drying oven are 
supplied with hot gas from the incinerators, and in such areas the 
combustion gases are mixed immediately with the re-circulated oven gases 
before being blown against the painted metal strip. The fraction of the 
gaseous mixture within the oven which is delivered to incinerators 26, 26' 
and 26" is taken from different points or locations P.sub.1, P.sub.2, 
P.sub.3 and P.sub.4 disposed along the oven, i.e. where the solvents are 
released in different manners. In fact, the bulk of this solvent release 
takes place in the first areas of the oven where the organic solvent are 
effectively evaporated, before the paints are polymerized and hardened. 
The gases from these various gaseous release points P.sub.1, P.sub.2, 
P.sub.3 and P.sub.4 are collected at a point P' to constitute a single 
mixture the solvent content of which does not exceed the inflammability 
limit. 
With this arrangement it is possible on the one hand to reduce the amount 
of dilution air necessary for adjusting the solvent concentration to the 
value set by safety regulations, and on the other hand to deliver a 
uniform gaseous mixture to the incinerators 26, 26' and 26". 
The other or residual fraction of the gaseous mixture taken at P.sub.1, 
P.sub.2, P.sub.3 and P.sub.4, which is compensated for by the volume of 
cool or fresh air sucked into the oven, is directed towards a final 
external incinerator 28 of which the smokes, after a preliminary heat 
recovery in a recuperator 20', are discharged into the surrounding 
atmosphere via a chimney 22'. 
From the above description it appears clearly that in the paint drying oven 
according to this invention the elimination, by means of incinerator 28, 
of any atmospheric pollution by solvents, is a function separate from the 
drying of paints in the oven. 
In a plant equipped with an oven according to this invention, the 
utilization of the recovery or recuperation heat is not subordinate to the 
conditions of operation of the oven. Moreover, the oven incinerators 26, 
26' and 26", and the final incineration oven 28 may operate at different 
temperatures without producing any interaction between the production of 
the drying oven and the external pollution. Finally, with the present 
invention, the pipes are not subjected to high temperatures, since the 
only gas circulation outside the oven (in the dash-and-dot line circuit 
for the high-solvent mixture) takes place at a relatively low temperature 
(400.degree. C. in this example) and therefore under reduced volume 
conditions. 
Of course, this invention should not be construed as being strictly limited 
by the specific form of the embodiment described and illustrated herein, 
since various modifications and changes may be made thereto without 
departing from the basic principles of the invention as set forth in the 
appended claims. Thus, each oven area or section may be either equipped 
with a separate incinerator (in the case of areas A', B' and C'), or 
supplied from an incinerator common to several areas or sections. One or 
more areas of the oven may also be equipped, if desired, with only one 
make-up burner 18' (area D') for regulating the blowing temperature in 
this area. 
Finally, the oven incinerators 26, 26', 26" and the final incineration oven 
28 may operate at different output temperatures.