EP1167908B1 - Process and apparatus for non-thermal drying of objects freshly painted with a water-based paint, in particular car bodies - Google Patents

Description

The invention is based on a device for non-thermal drying with a water-based paint freshly painted objects, in particular motor vehicle bodies according to the preamble of claim 1 as For example, from DE 196 44 717 A1 discloses as known.

The invention also relates to a method for operating such a device.

Standard painted objects, in particular vehicle bodies are painted in several layers, being for environmental reasons at least for the primer and the filler coat mostly waterborne basecoats are used. These paints However, they can not be applied wet to wet, but instead must before each subsequent application of another lacquer layer the previously applied aqueous basecoat layer on certain maximum permissible residual moisture of about 5 to 10 wt .-% dried become. Higher residual moisture would be at the final thermal burn-in of the entire layer composite of the paints cause cracking of the paint, because the amount of off the residual moisture released water vapor not by the layer composite can diffuse through quickly enough.

So far, the intermediate drying of the individual layers takes place in particular of water-based paints thermally in one Dry tunnel in which the paint on the vehicle bodies heated by hot circulating air and / or by heat radiators and so on The contained water is carefully evaporated. The moisture-laden Exhaust air is - possibly after a heat recovery - withdrawn as a whole the drying process. This is a very energy-intensive drying process, not at times only the drying air running at high temperature, but also heated the vehicle bodies first and then must be cooled again.

Therefore, one has a less energy-intensive drying process at least for water-based paints considered, which in principle described in the cited document is. In the beginning, this proposal is correct. One The problem not solved therein is a different one Accumulation of moisture on the part of the freshly painted Items. In everyday production, the freshly painted Objects not always in constant chronological order introduced into the paint drying plant, but it is in this regard with a change, e.g. with more or less long Interruptions or with a new start of production too expected.

From DE 295 04 040 U1 is an industrial dryer with a Air drying device known, the two heat exchangers, a Air cooler and an air heater includes. Also the DE 197 81 220 C1 describes a device for drying thin Layers that have a cooling device for condensing and a Heating device for reheating the circulating air has. outgoing There is a need of these refrigerators Air drying even more effective and highly accurate adjust.

The object of the invention is based on the generic contraption and the method of operating the device improve that too at a different and above all rapidly changing supply of moisture on the part of the painted Always apply this to a specific residual moisture content can be dried and yet a constant Residence time of the lacquered objects in the paint drying plant can be complied with. A transgression of the Target residual moisture in the dried paint, so an insufficient Drying of the paint, caused during later Lackeinbrennen Lackaufbrüche with time- and cost-intensive rework and with serious logistical problems. On the other hand should The paint does not get overdried because this is for a good connection of the subsequently applied lacquer layer unfavorable is. Apart from that, overdrying would be too Energy and cost considerations uneconomical. on the other hand may be for logistical reasons a different residence time of the varnished objects in the paint drying plant not be allowed.

The task mentioned becomes through the Features of claim 1 or of claim 8 solved.

Decisive for a reliable adherence to the target residual moisture at least in a permissible tolerance field is a rapid adaptation of the drying performance of the system to the changing moisture supply. A quick adaptation will According to the invention air side by a change of a dried and an untreated partial air flow achieved. With high humidity on the part of the painted objects a high proportion of dried air is driven; of the untreated proportion of recirculated air is correspondingly lower. At low Moisture supply is the other way round; then the dried air fraction in favor of the untreated recirculation component reduced. The air drying device is with her Drying capacity designed so that even at the maximum required, dried partial air flow with safety still the circulating air target moisture can be complied with. At reduced share on air to be dried, the air is indeed due to the installed dryer capacity over the above mentioned Condition overdried, but due to mixing with the untreated portion is not harmful; In any case can quickly maintain the desired circulating air target moisture become. Overdrying of the dried air is energetic Viewpoints readily acceptable, because the Energy balance, even with overdried portion not essential different than in the design case. It only has to be done once installed a corresponding capacity of a drying capacity become.

Fig. 1

ein schematisch dargestelltes Ausführungsbeispiel einer Tunneltrockenanlage zum nicht-thermischen, zweistufigen Trocknen eines auf Kraftfahrzeugkarosserien frisch applizierten Wasserbasislackes und

Fig. 2

die Lufttrocknungseinrichtung der Anlage nach Figur 1 in datailreicherer Darstellung und mit zugehörigen Peripheriegeräten.

Advantageous embodiments of the invention can be taken from the respective subclaims; Moreover, the invention with reference to an embodiment shown in the drawing is explained below; show:

Fig. 1

a schematically illustrated embodiment of a tunnel drying plant for non-thermal, two-stage drying of a freshly applied to motor vehicle bodies waterborne basecoat and

Fig. 2

the air drying device of the system of Figure 1 in a more datailreicherer representation and with associated peripherals.

The figures show a process plant for non-thermal Drying objects freshly painted with a water-based paint, e.g. of motor vehicle bodies 2, on body supports 3 are superimposed, which in turn on the air-permeable bottom 4 of a drying tunnel 1, 1 'with a constant Conveyor speed transported through this become. These are the illustrated embodiment around a two-stage drying tunnel with two together subsequent but separable tunnel sections 1 and 1 'of a first and a second drying stage. In front and behind the tunnel sections of the two drying stages is in each case a lockable lock chamber 14 and 14 'are provided. The isolation of the tunnel sections among themselves or to the lock chambers can by Hubtore or done by transversely movable sliding doors.

The paint drying plant shown contains for a non-thermal Drying an air drying device 11, the the moisture-laden air from the drying tunnel over a Exhaust air line and a suction fan 6 is supplied. In the Air drying device is a cold trap 15 containing where the moisture in the air is condensed. The dried air can by means of a Zuluftgebläses 31 via an air supply line 32 are returned to the drying tunnel.

The bodies to be dried are at one end in the Dry tunnel introduced, the surfaces of the objects in it forced convection with dry air and after drying the paint the bodies of the drying tunnel discharged. After the passage of air through the Drying tunnel, the moisture-laden air is sucked out of it, the air in the air drying device 11 which contained Moisture removed by condensate and the Air so to a certain target of absolute residual humidity - hereinafter referred to as "ambient air humidity" - dried and blown back into the drying tunnel.

Due to the drying process or in the drying plant the applied on the vehicle body paint on a dried certain, always constant target residual moisture become. However, on the part of the painted vehicle bodies due to different loading density in the drying tunnel with a different and u.U. also changing rapidly Supply of moisture count. From logistical Reasons can be a different residence time of the painted Vehicle bodies are not allowed in the drying tunnel become.

To the changing supply of moisture in the air dryer to be able to regulate the low-inertia are according to the invention a series of measures are envisaged. From the exhaust air stream branches at branch point 5, i. even before that Suction fan 6 from a by-pass line 30 from which the air drying device 11 bypasses and thus air untreated to Zuluftgebläse 31 directs. The suction fan 6 promotes accordingly only a maximum of about 60% by volume of the total circulated air amount of total circulated air by the Air drying device 11 therethrough. Due to a variable speed Drive 7 of the suction fan is this proportion rapidly variable. The air drying device is dimensioned so and designed so that at maximum moisture loading the exhaust air and at maximum loading of the air drying device the air treated in it to a much lower, preferably half the value of the circulating air target moisture is dryable. The untreated flowing through the by-pass line Air and leaving the air drying device, dried air unite in front of the supply air blower again, so that the dried and the untreated air in the Supply air fan mixed and together again the drying tunnel be forwarded.

By means of a supply-side (10) and an exhaust-side Humidity sensors 9 are moisture data circulated Air at the points concerned and these measured data processed in a controller 8. The controller changes automatically such the drive speed of the suction fan or its Flow rate that the desired circulating air target humidity supply side can be complied with.

Because of this Ausgestalltung the paint drying system is with unchanged high condensation capacity in the air drying device 11 the dried part air flow on one below dried residual moisture lying in the circulating air target moisture and the remaining, untreated portion of the circulated Air after mixing with the dried part air stream returned to the dryer housing. This is one rapid adaptation of the drying performance of the system to the changing moisture supply possible and it can the air circulation target moisture at least in a permissible tolerance field be reliably adhered to. The absolute air circulation target humidity is between 3.0 to 5.0 g water per kg wet Air, preferably about 4 ± 0.2 g / kg.

When freshly painted vehicle body in continuous Sequence be introduced into the drying tunnel, is the supply of moisture very much on the part of painted objects high and can be considered maximum. Then the share the air to be dried to a high, maximum fraction - See below for more details on this design criterion - The total amount of circulated air is increased; of the untreated proportion of recirculated air is correspondingly lower. At low Moisture supply is the other way round; then the dried air fraction in favor of the untreated recirculation component reduced. The air drying device is with her Drying capacity designed so that even at the maximum required, dried partial air flow with safety still the circulating air target moisture in the total circulated air in the inlet maintained in the drying tunnel (humidity sensor 10) can be. If, however, due to interruptions or Faults in the previous body painting many Places in the drying tunnel are unoccupied and have little moisture is introduced into the drying tunnel, then the to be dried Proportion of air in favor of the untreated proportion of air, e.g. reduced to about 25 to 30% by volume, the per unit time however, the total amount of air circulated is kept constant.

Also the performance of the refrigeration cycle of the condensing acting Cold trap will be regardless of the fluctuations in the supply of moisture continued at a consistently high level. at Reduced proportion of air to be dried, the air indeed due to the installed dryer capacity over one Condition with high proportion of air to be dried over-dries what however due to mixing with the untreated portion Not only is the circulating air harmless, it is also harmless can in any case at short notice the desired air circulation target humidity be respected. An overdrying of the dried Air is not only from an energetic point of view without Another justifiable, but because of mixing with the Moist, untreated air to maintain the desired Convection target humidity even required. It only has one appropriate capacity of a drying capacity installed become.

It depends on the particular design of the paint drying system from how high in a state of operation of a highest moisture supply the maximum required proportion of too drying air is made. Is in the air drying device a very high condensation power installed and Accordingly, the air treated therein very much dried out, so needs very high moisture in the paint drying system the air drying device only with one relatively low maximum proportion of, for example, 40% by volume Dry air - based on the total circulated air volume - to be driven. On the other hand, it is at tight condensation performance the treated in the air dryer Air dried less strongly, so must at a high Moisture entry the proportion of dry air on e.g. 60 %. When treated in the air dryer Air is dried so strongly that they compared to Recirculation target moisture only about half the amount of moisture per unit of quantity contains air, then at a maximum assuming moisture in the paint drying system the proportion of dried air is about 50% of the total circulated Amount of air. With reduced entry of moisture in the paint drying plant, the proportion of dried Air is reduced accordingly, due to the Consistently high condensation performance Residual moisture of the dried air is less high, the treated Air is then dried more. By mixing the dried air with the untreated air content is in the total circulated air supply side in each case the desired circulating air target humidity maintained.

Basically, it is conceivable, the proportion of drying Air to vary in various ways, e.g. by Diverters that cross the inlet section into the by-pass duct 30 in favor or to the detriment of the inlet cross-section change into the air dryer. In which In the figures dargestelten embodiment is the suction fan 6 arranged in the inlet only to the air drying device. The proportion of air to be dried can also be due to a change the delivery rate of this fan can be varied. There are also fundamentally different possibilities for e.g. by adjusting the angle of attack of the conveyor blades or by changing the drive speed of the conveyor fan 6. It is also conceivable, the circulating air to be dried to indirectly vary in that the untreated recirculating air directly by measures of the kind mentioned above being affected.

The air drying device 11 is in principle a flow cross section greatly expanded channel of preferably rectangular cross-sectional shape, in which several heat exchangers are arranged. Due to the enlarged flow cross-section the air flows through the air drying device relatively slowly through, making it a good heat exchange in it comes. In the front part of the air dryer are the luftbeaufschlagten heat exchanger surfaces 20th and 15 cooling surfaces, whereas in the rear part the air over Heat exchanger surfaces 16 is reheated.

To produce the required low temperatures at the Cooling surfaces - the dew point of the moist air must be well below are - are different refrigerant circuits provided in the paint drying plant, by the mentioned Pass heat exchanger through.

In this context, a main cooling circuit is initially closed mention that from a refrigerant compressor 17, a plurality of parallel to each other arranged by the refrigerant flowed through Refrigerant condensers 16, from a throttle 18 and out another group of heat exchangers, namely the refrigerant evaporators 15, which constitute a cold trap exists. Around a sufficiently low temperature in the air and at the To reach heat exchanger surfaces is in the air flow direction before the refrigerant evaporators 15 a number of Pre-cooler 20 arranged in the air drying device, the also act as a refrigerant evaporator, so part of a another refrigerant circuit, namely a secondary cooling circuit are. The main components of this sibling refrigerant circuit are also a refrigerant compressor 22, a plurality of each other in a waste heat air duct 25 arranged Refrigerant condensers 21, a throttle 23 and the already mentioned, located in the front of the air dryer Refrigerant Evaporators 20. Fluidic for the refrigerant in parallel there is another refrigerant evaporator 20 ', in an even further to be treated excess air line 35 is arranged and promoted therein Cools excess air.

Through the refrigerant evaporator 20 of the secondary cooling circuit becomes the air flowing into the air drying device pre-cooled. The extracted heat is in the waste heat air line 25 arranged refrigerant condensers 21 to uninvolved Indoor air discharged by means of the cooling fan 24 through the refrigerant condensers and through the waste heat air line finally through the hall roof 26 ins Free is encouraged.

When the air enters the rows of refrigerant evaporators 15 of the main cooling circuit is this thanks to the effect of the secondary cooling circuit already at temperatures near freezing pre-cooled. The refrigerant of the main cooling circuit has in the refrigerant evaporators - depending on the strength of the air the air drying device - a temperature from -10 to -20 ° C. At high air flow turns one higher temperature than at lower air flow. In each Case, the cooling circuits regardless of the air flow through the air dryer at about the same Power operated. Due to the low temperature the evaporator surface separates there in the Air moisture contained by condensation from and stores attached to the surfaces of the heat exchanger fins. The on the surface collecting and accumulating condensate liquid agglomerates into larger drops due to gravity along the smooth-surfaced heat exchanger fins run down into a collecting trough and out of this can be derived via the condensate line 27. The absolute Residual moisture of the dried air is at maximum Throughput of air to be dried according to the design of the Air drying device and the paint drying system in total at a value in the range between 1.5 to 2.5 g / kg, preferably at about 2 ± 0.1 g / kg.

The dehumidified air is initially quite cold. she is in the air dryer through the heat exchanger rows the refrigerant condensers 16 of the main cooling circuit passed through, in the interior of which passed condensed but still condensed vapor refrigerant is and on the outside of the air flowing around about Room temperature is heated. This will increase the relative humidity lowered and their ability to absorb moisture, increased.

After the freshly applied water-based paint its moisture initially easier in the drying tunnel to the flowing around Air gives off, as if the paint already dried a bit is seen in Figure 1 continuously operated paint drying plant with a drying tunnel two successive drying stages with air-technically separated Air drying devices 11 before. When presented Embodiment are the one inside the drying tunnel following dry steps are approximately equal to each other long formed, which approximates in two stages equal length of stay for the bodies to be dried result. For both stages, an approximately equal Recirculating air target moisture, e.g. the already mentioned value of 4 ± 0.2 g / kg complied with. However, the two differ Drying stages by the intensity of ventilation. In the first drying step, the painted objects are gentle, i.e. at a lower speed than in the second drying stage, for example at about 2 to 5 m / s, preferably about 3 m / s forcibly convective with dried air applied. In the second drying stage, the air hits at about three times the exit velocity, preferably about 10 to 12 m / s on the already dried and solidified Paint on. Due to the greater flow intensity of the dry air in the second opposite the first stage improved conditions for a moisture transfer from the paint to the air.

In leading to the drying tunnel supply air line 32 opens a Excess air line 35 a. This is with a surplus blower 36 connected, which via an intake 37 air sucked from the environment of the air drying device. Thereby is operated at the continuously and with a Dry tunnel provided paint drying plant in the drying tunnel continuously a small overpressure compared to the ambient air pressure generated and so an uncontrolled entry of air from adjacent Treatment zones in the drying zone of the drying tunnel prevented. Relative to the total circulated Amount of air is constantly a proportion of about 1 to 8 vol .-%, preferably about 3% by volume of ambient air to that in the drying tunnel total recirculated air added. The excess air can pass into the lock chambers 14, 14 'and exit from there into the hall environment. The indoor air is subject due to the size of the hall and due to the circumstance that it is heated or controlled ventilated, only very slight fluctuations in temperature and humidity. Through the continuous addition of excess air is at the same time the in the paint drying plant circulating air running though slowly renewed. This does not lead to accumulations of Solvent vapors or the like.

Due to the constant and intense swirling of the circulated Air in powerful blowers with a moderate Efficiency is constantly a not inconsiderable amount of benefits into the air, which leads to a warming of the Air leads. The air would tend to become impractical soon heat high temperatures. To the circulated air permanently at a lower temperature level in the area stabilize from about 25 to 35 ° C, this is the added Excess air also exploited by these before their Add to the circulating air to about 5 to 15 ° C, preferably on about 10 ° C is cooled. To cool the injected excess air is in the excess air line 35 already mentioned Heat exchanger, namely another refrigerant evaporator 20 'of the secondary cooling circuit arranged.

Because constantly from the hall air, which also contain dust particles can, new air is entered into the air cycle, is to protect the paint against dust accumulation the entire recirculated air before entering the drying tunnel filtered. For this purpose is in the to the drying tunnel leading supply air line 32 an air filter 34 is arranged.

Also from the drying tunnel dust particles in the drying process and be introduced into the circulating air, the at the partially wet heat exchange surfaces of the Air drying device 11 attach and the heat exchange can affect. To prevent this, is also the to be dried portion of the circulated air before its entry filtered into the air drying device. Accordingly, in the exhaust air line leading to the air drying device also an air filter 33 is arranged.

Claims (19)

1. Device for non-thermally drying a water-based paint freshly applied to objects, in particular vehicle bodies (2),

   with a dryer housing (1, 1') temporarily accommodating the freshly painted objects, in particular a drying tunnel,

   with a discharge air system containing a suction blower (6) discharging moisture-laden air from the dryer housing (1, 1') and delivering it to an air-drying unit (11),

   also having said air-drying unit (11), which contains at least one coolant evaporator (15) constituting a cold trap and at least one coolant condenser (16) which heats the air, which air-cooling unit (11) extracts the moisture from the circulating air by condensation and dries the air to a specific target value of absolute residual moisture - hereafter referred to as "circulating air-target moisture level",

   with an inlet air line (32) containing an inlet blower (31) which returns the dried air to the dryer housing (1, 1'),

   with a bypass line (30) branching off from the discharge air flow at a point before the suction blower (6), bypassing the air-drying unit (11) and delivering air untreated to the inlet air blower (31), which bypass line (30) merges with the line carrying the dried air as it leaves the air-drying unit (11) before the inlet air blower (31),

   which suction blower (6), provided with a speed-variable drive (7), conveys only a variable, maximum proportion of the circulated air amounting to approximately 60 % by volume of the total circulated air through the air-drying unit (11),

   and which air-drying unit (11) is so dimensioned and designed that when the discharge air is charged with a maximum moisture level and the air-drying unit (11) is being operated at maximum rate, the air treated in it can be dried to a significantly lower value, preferably half the value, of the circulating air-target moisture level,

   characterised by the combination of the following features:

   a controller (8) is provided for processing the measurement data from a moisture sensor (9) at the inlet air end (10) and/or at the discharge air end, by means of which the conveying power of the suction blower (6) can be automatically varied by means of the drive speed so that the desired circulating air-target moisture level can be maintained at the inlet air end,

   the at least one coolant evaporator (15) and the at least one coolant condenser (16) heating the circulating air are component parts of a common main cooling circuit (17, 16, 15, 18) and

   another auxiliary cooling circuit (20, 22, 21, 23) is provided, incorporating a coolant evaporator (20) for pre-cooling the air flowing into the air-drying unit (11) and a coolant condenser (21) for discharging the extracted heat to uninvolved workshop air.
2. Device as claimed in claim 1,
   characterised in that
   an air filter (33) is disposed in the discharge air line leading to the air-drying unit (11).
3. Device as claimed in claim 1 ,
   characterised in that
   an air filter (34) is disposed in the inlet air line (32) leading to the dryer housing (1, 1').
4. Device as claimed in claim 2,
   characterised in that
   a surplus air line (35) opens into the inlet air line (32) leading to the drying tunnel (1, 1'), which is connected to a surplus blower (36) sucking in ambient air from the area around the air-drying unit (11).
5. Device as claimed in claim 1,
   characterised in that
   a heat exchanger (20') is disposed in the surplus air line (35) for cooling the surplus air circulating in it.
6. Device as claimed in claim 1,
   characterised in that
   when the dryer housing in the form of a drying tunnel (1,1') is continuously operated, at least two successive drying stages having air-drying units (11) with separate air systems are provided.
7. Device as claimed in claim 6,
   characterised in that
   the successive drying stages inside the drying tunnel (1,1') are more or less of the same length as each other.
8. Method of operating a device as claimed in claims 1 to 7,
   characterised in that
   the device is operated in such a way that when the incidence of moisture is at a maximum and accordingly a maximum proportion of air has to be dried, but still keeping the condensation power of the air-drying unit (11) constant, i.e. not increasing it, the residual moisture of the dried air is approximately 40 to 60 %, preferably approximately 50 %, of the circulating air-target moisture level and the residual moisture of the dried air lies below this if the proportion of air to be dried is less high, so that the treated air is then dried to a more pronounced degree.
9. Method as claimed in claim 8,
   characterised in that
   the variable proportion of the air to be dried is adjusted by means of a speed-dependent change in the conveying power of an appropriate conveying blower (6).
10. Method as claimed in claim 8,
    characterised in that
    the surfaces of the cold trap (15) to which air is applied are maintained at a constant temperature in the range of from -10 to -20°C, in the stationary state, in the state when it is necessary to dry a maximum proportion of air and below this when a lesser proportion of air has to be dried.
11. Method as claimed in claim 8,
    characterised in that
    the circulating air-target moisture level is in the range of between 3.0 and 5.0 g of water per kg of moist air, preferably approximately 4 ± 0.2 g/kg, and the residual moisture of the dried air of the treated proportional flow when a maximum proportion of air has to be dried is in the range of between 1.5 and 2.5 g/kg, preferably approximately 2 ± 0.1 g/kg.
12. Method as claimed in claim 8,
    characterised in that
    the dried air of the treated proportional flow is mixed with the air of the untreated proportional flow by means of the conveyor blower (31) commonly conveying the two proportional flows before introducing the inlet air into the dryer housing (1, 1').
13. Method as claimed in claim 8,
    characterised in that
    the circulating air-target moisture level is maintained at approximately the same level in both stages.
14. Method as claimed in claim 8,
    characterised in that
    the dwell time of the objects (2) to be dried is maintained at approximately the same length in both stages.
15. Method as claimed in claim 8,
    characterised in that
    the dried air is applied to the painted objects (2) in the first drying stage by forced convection at a lower rate than in the second drying stage.
16. Method as claimed in claim 8,
    characterised in that
    the outlet speed of the dried air applied to the painted objects (2) in the first drying stage is approximately 2 to 5 m/s, preferably approximately 3 m/s and in the second drying stage approximately three times this value, preferably approximately 10 to 12 m/s.
17. Method as claimed in claim 8,
    characterised in that
    as the dryer housing designed as a drying tunnel (1, 1') is operated continuously, i.e. the objects are fed in from one end and fed through and out from the drying plants at the other end, there is a surplus of air at the inlet air end over and above the air extracted at the discharge air end due to the continuous addition of ambient air introduced into the air re-circulated in the drying tunnel (1, 1'), thereby preventing any uncontrolled ingress of air in the drying zone of the drying tunnel (1,1') from adjacent processing zones and at the same time renewing the circulated air continuously, albeit slowly.
18. Method as claimed in claim 17,
    characterised in that
    a proportion of approximately 1 to 8 % by volume, preferably approximately 3 % by volume, of ambient air - relative to the total quantity of circulated air - is added to the air circulated in the drying tunnel (1, 1').
19. Method as claimed in claim 17,
    characterised in that
    the ambient air is cooled (evaporator 20') to approximately 5 to 15°C, preferably to approximately 10°C, before being added to the circulating air.

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