The present invention relates to a method and an apparatus for removing
solvent vapours from a vehicle body.
When painting vehicle bodies, such as car bodies, with solvent-base
paints in e.g. spray booths, solvent evaporates from the paint, both when the paint
is applied and when it is drying. The resulting solvent vapours spread inside
as well as outside the vehicle body. Usually, the solvent vapours outside the vehicle
body are removed from the spray booth by ventilation air flowing continuously
therethrough and entraining the vapours, optionally after they have been slightly
concentrated, to e.g. an incinerator.
In US-A- 4 616 594 a spray booth with two ventilation air supply
systems for removing paint mist and solvent vapours outside the vehicle body from
the spray booth is described. By using two air supply systems it is possible to
only supply air having an appropriately controlled temperature and humidity to
the zone in the spray booth through which the vehicle body to be painted is conveyed.
However, the solvent vapours inside the vehicle body are not removed
by the ventilation air, but instead accompany the vehicle body when moved into
the succeeding drying unit where they may condense on the walls. If condensate
then drops on to the vehicle body, the surface layer thereof will be ruined.
In order to check the quality of the surface layer of the vehicle
body, it is often desirable to manually inspect the vehicle body before it enters
the drying unit. Owing to the high content of solvent vapours inside the vehicle
body, such an inspection before the solvent vapours have been removed from the
vehicle body however constitutes a health hazard.
In order to remove the solvent vapours inside the vehicle body before
this enters the drying unit, robots provided with exhaust means adapted to be introduced
into the vehicle body for sucking off the solvent vapours therefrom have been
arranged between the spray booth and the drying unit. However, it has been found
impossible to keep the robots clean enough so as not to deposit any particles of
dust or dirt on the newly-painted and not yet dried surface layer when their suctions
means are introduced into the vehicle body.
When the vehicle body is being painted, particles may collect on
its bottom. These particles may come into contact with the surface layer of the
vehicle body and ruin it if they are entrained by the solvent vapours when these
are removed from the vehicle body.
Since the newly-painted surface layer is easily damaged, the removal
of solvent vapours from inside the vehicle body, without causing any damage to
the surface layer thereof, always involves problems.
One object of the present invention therefore is to provide a simple
and efficient method for removing solvent vapours from a vehicle body without damaging
the surface layer thereof.
Another object of the invention is to provide a simple apparatus
for carrying out this method.
According to the present invention, the solvent vapours are removed
from the vehicle body in that air is supplied into said body through a first means
provided at the outside thereof, at such a speed and such a temperature that it
pushes aside the solvent vapours which are caused to flow towards a second means
provided at the outside of said body for sucking off the solvent vapours from
the interior of the vehicle body.
Depending on whether the solvent vapours collect in a cavity in the
upper or lower portion of the vehicle body, the air is heated or cooled to such
a temperature that its density, respectively, becomes lower or higher than that
of the solvent vapours. Owing to this difference in density between the air and
the solvent vapours, the air is able to push aside the solvent vapours from the
cavity, whereupon the vapours are caused to flow towards the suction or exhaust
means by the kinetic energy of the air and the suction effect of said exhaust means.
Preferably, the air is heated or cooled to a temperature which, respectively,
is 2 - 20°C above or below the temperature of the solvent vapours. The range 6
- 10°C has been found particularly advantageous for obtaining a difference in
density sufficient to produce a satisfactory pushing aside of the solvent vapours,
while reducing the heating or cooling costs.
Preferably, the air is supplied into the vehicle body at a speed
of 1 - 4 m/s, especially about 2 m/s, whereas the solvent vapours and the air are
sucked off through the exhaust means at a speed of about 10 m/s. Consequently,
the air and the solvent vapours will be flowing through the vehicle body at a
speed below 1 m/s, thus ensuring that no particles present in the body are entrained
by the gases.
Preferably, the vehicle body is advanced at a speed of about 0.025
m/s along a rectilinear path transversely of the direction of flow of the air and
the solvent vapours.
The solvent vapours and the air are preferably sucked off from the
interior of the vehicle body for some time after the supply of air into said body
has ceased.
To remove the solvent vapours from the vehicle body by the above
method, there are provided adjacent to the vehicle body a first means for supplying
air into said body, and a second means for sucking off solvent vapours from the
interior of said body, whereby the air is supplied by the first means at such a
speed, i.e. 1-4 m/s, and such a temperature, i.e 2-20°C above or below the temperature
of the solvent vapours, that it pushes aside the solvent vapours towards the second
means.
Preferably, said first means consists of a funnel-shaped supply hood
and an inlet duct connected to the end of the supply hood having the smallest cross-sectional
area. The opposite end of said hood is placed adjacent to the vehicle body.
To ensure that the flow configuration of the air flowing into the
vehicle body is such that the solvent vapours are efficiently pushed aside and
removed, said opposite end of the supply hood can be covered with a plate having
suitably shaped openings.
Preferably, said second means consists of a funnel-shaped exhaust
hood and an outlet duct connected to the end of the exhaust hood having the smallest
cross-sectional area. The opposite end of said hood is placed adjacent to the
vehicle body.
To obtain a more uniform suction effect in the exhaust hood, said
opposite end may be covered with an apertured plate serving as a throttle means
for the solvent vapours and the air. The openings occupy about 10% of the surface
of the plate and may consist of elongate slots and/or round holes.
Preferably, the cross-sectional areas of the supply and exhaust hoods
are circular or rectangular.
The invention will be described in more detail below, reference being
had to the accompanying drawings, in which
- Fig. 1 is a schematic front view of an apparatus according to the invention,
which is arranged adjacent to a car body,
- Fig. 2 is a top view of the apparatus and the car body in Fig. 1,
- Fig. 3 is a front view of a component part of the apparatus in Figs 1 - 2,
and
- Fig. 4 is a front view of another component part of the apparatus in Figs 1
- 2.
As shown in Fig. 1, the car body 1 rests on a conveyor 2 which travels
through a spray booth 3 just above the floor 4 of the booth. In the spray booth,
the car body is sprayed with solvent-base paint from which solvent evaporates,
both during spraying and during drying of the paint on the car body. The resulting
solvent vapours spread inside as well as outside the car body. The solvent vapours
outside the car body are removed from the spray booth by ventilation air flowing
therethrough. The ventilation air is supplied to the spray booth through the perforated
ceiling 5 and escapes from the booth through the floor grating 4. The polluted
ventilation air, which also entrains paint particles from the spray booth, is first
conducted to a venturi-type separator (not shown) for separating the paint particles,
then to an incinerator for combustion of the solvent vapours, optionally after
these have been slightly concentrated. However, the car body prevents the ventilation
air from removing the solvent vapours inside the car body. Instead, these vapours
will accompany the car body until it reaches the end of the spray booth, where
they are removed by means of a supply hood 6 and a suction or exhaust hood 7. These
hoods are fixedly mounted in the spray booth on a level with the side panel window
openings of the car bodies passing by in the direction of the arrow F on their
way to a succeeding drying unit (not shown).
As shown in Fig. 2, the hoods have the form of truncated pyramids,
the base of each hood being intended to cooperate with the car body. The truncated
tops of the supply and exhaust hoods are connected with an inlet duct 8 and an
outlet duct 9, respectively. The base of the exhaust hood is as wide as that of
the supply hood, but, having a greater length, it will cooperate with the car
body for a longer period of time than does the base of the supply hood. Furthermore,
since the hoods are so positioned in the spray booth 3 that their upstream ends
are located opposite one another, the car body will continue to cooperate with
the base of the exhaust hood for some time after it has ceased cooperating with
the base of the supply hood.
As shown in Fig. 3, the base of the supply hood is covered with a
metal sheet 10 having a number of circular openings 11 and arcuate openings 12.
As shown in Fig 4, the base of the exhaust hood is covered with a
metal sheet 13 having three elongate slots 14.
The function of the apparatus will be described in more detail below
with reference to the accompanying drawings. The car body is advanced through the
spray booth at a constant speed of about 0.025 m/s and, when reaching the end
of the spray booth, is caused to cooperate with the bases of the supply and exhaust
hoods, simultaneously. When the bases of the hoods are caused to cooperate with
the car body, cleaned indoor air will automatically be supplied at one side of
the car body 1 through the supply hood 6 while solvent vapours will automatically
be sucked off at the opposite side of the car body through the exhaust hood 7.
The indoor air is sucked in through a particle-separating filter
(not shown) from the premises surrounding the spray booth, e.g. a car assembly
hall (not shown), whereupon it passes through a refrigerator unit (not shown)
before being supplied to the supply hood through the inlet duct 8. In the refrigerator
unit, the indoor air is cooled to such an extent that when it is injected into
the car body, it will have a temperature which is 6 - 10°C below the temperature
of the solvent vapours. Hence, the indoor air becomes so heavy in relation to the
solvent vapours that it is capable of also pushing aside the solvent vapours which
have collected on the bottom of the car body. The indoor air is supplied into the
car body at a speed of about 2 m/s, which, in combination with the flow configuration
of the air after passing through the openings 11, 12 of the metal sheet 10, enables
it to efficiently force the solvent vapours to flow towards the exhaust hood 7
through which the vapours are then sucked off by means of a fan (not shown).
The provision on the suction hood 7 of the metal sheet 13, which
serves as a throttle means for the solvent vapours, results in a more uniform suction
effect and thus a more efficient removal of vapours from the car body. The suction
effect of the exhaust hood is adjusted in such a manner that the solvent vapours
are sucked off through the slots 14 of the metal sheet 13 at a speed of about 10
m/s. Since the slots occupy only about 10% of the surface of the metal sheet 13,
the speed of the solvent vapours, before the suction means, is not quite 1 m/s,
thus ensuring that the vapours will flow so slowly through the car body that they
do not entrain any particles that may have deposited on the bottom thereof. Then,
the solvent vapours are conducted, through the outlet duct 9 and without being
concentrated, to the above-mentioned incinerator for combustion together with the
solvent vapours removed from the spray booth by the ventilation air.
Since the base of the exhaust hood continues to cooperate with the
car body for some time after the car body has ceased cooperating with the base
of the supply hood, solvent vapours will be sucked off from the interior of the
car body for some time after the supply of indoor air has ceased. In this manner,
the last-supplied indoor air is efficiently used and the solvent vapours are almost
completely removed. Some time after the supply of indoor air into the car body
has started, indoor air is of course sucked off as well through the exhaust hood
7 together with the solvent vapours.
It goes without saying that the invention is not restricted to the
embodiment described above but may be modified in various ways within the scope
of the appended claims.
For instance, the supply and exhaust hoods can be arranged outside
the spray booth adjacent to the inlet of the drying unit, or inside the drying
unit instead of the spray booth.
Also, instead of being formed as truncated pyramids the supply and
exhaust hoods may have the form of truncated cones, or any other suitable funnel
shape.
Furthermore, the supply hood 6 may be provided with guide vanes and
a filter to replace the metal sheet 10.
Moreover, the metal sheet 13 of the exhaust hood may have round holes
instead of the slots 14.
If the solvent vapours are, for example, heated by waste heat from
the drying unit to a temperature which is 2 - 20°C above the temperature of the
indoor air, before being contacted with said air, the indoor air need of course
not be cooled before being fed to the supply hood.
