Patent Application
20120005917
This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2010 026 604.3, filed Jul. 9, 2010; the prior application is herewith incorporated by reference in its entirety.
The invention relates to a method for drying printed and/or varnished sheets in a sheet-processing machine as well as to a sheet-processing machine, in particular a sheet-fed printing press, including one or more dryers, air feed devices for the dryers, air extraction devices for the heated exhaust air and a mixing device for mixing part of the exhaust air with the dryer feed air.
Sheet-fed printing presses of that type are known per se. With rising energy prices, it has become more and more important to optimize the energy consumption of printing presses and, in particular in the area of the dryer, which has a high energy consumption in comparison with other components of the machine, to find measures for reusing the exhaust air heated up in the dryer and the heat contained therein. For instance, it is known from German Patent Application DE 10 2004 048 857 to mix warm exhaust air from part of a sheet-fed offset printing press with feed air from another part of the printing press and to feed it to a dryer in the printing press. Other proposals are aimed at heating the dryer feed air through heat exchangers, which are operated by exhaust heat from the printing press, including the warm dryer exhaust air. Such devices are described, for example, in European Patent Application EP 2 047 991 A2, German Patent Application DE 10 2005 042 956 A1 or International Publication No. WO 01/68223 A1, corresponding to U.S. Pat. No. 6,868,788.
Furthermore, it is also known from European Patent EP 1 319 506 B1 to provide movable flaps, with which the residence time of warm air in the dryer is controlled in order to bring the latter quickly to an optimal operating temperature. That control is carried out by measuring the temperature in the area of the dryer.
The known devices are, to some extent, associated with very high additional expenditure for equipment, in particular the one described in the aforementioned International Publication No. WO 01/68223 A1, corresponding to U.S. Pat. No. 6,868,788. Nevertheless, it is not ensured that the dryer exhaust air is used optimally.
It is accordingly an object of the invention to provide a sheet-processing machine with one or more dryers and a method for drying sheets, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provide equipment for sheet-processing machines with which a significant contribution can be made in a simple and cost-effective way toward increasing the energy efficiency of a dryer.
With the foregoing and other objects in view there is provided, in accordance with the invention, a sheet-processing machine, in particular a sheet-fed printing press, comprising at least one dryer, air feed devices for the at least one dryer, at least one air extraction device for heated dryer exhaust air, a mixing device for mixing the heated dryer exhaust air with the dryer feed air, and an open loop or closed loop control device controlling or regulating an extent of mixing of the dryer exhaust air with the dryer feed air by using measured variables or setting values correlated with a moisture content of the dryer exhaust air.
The invention is based on the finding that it is not merely sufficient to perform temperature measurements in order to control the air management in a dryer. Instead, the extent to which warm exhaust air that is fed back can be used once more to dry the moist sheet depends on how highly the exhaust air is already saturated with water vapor. Therefore, the invention proposes to control or to regulate the proportion of circulated air, i.e. that part of the exhaust air which is fed back to the dryer feed air or, expressed in other words, the extent to which the dryer exhaust air is mixed in, in accordance with the absolute humidity of the exhaust air. For this purpose, one or more preferably motor driven mixing valves can expediently be used, as are to some extent already present but normally without motorized activation.
The open loop or closed loop control device which controls the extent to which the dryer exhaust air is mixed in is expediently additionally connected to sensors for measuring the temperature of the dryer exhaust air, since, through the use of the simultaneous measurement of temperature and relative humidity, the quantity of water vapor contained in the exhaust air can be determined. Corresponding characteristic curves, through the use of which the mixing ratio is then controlled and regulated, can be stored in the controller.
It can also be expedient in addition to provide sensors which also measure the atmospheric humidity and/or temperature of the dryer feed air. This is because the quantity of partly moist exhaust air fed in, in particular when the feed air is taken unfiltered from the press room or the surroundings, may also depend on how high the atmospheric humidity of the surroundings already is. The control/regulation and therefore the proportion of recirculated air that is fed back from the dryer exhaust air is chosen in such a way that the energy efficiency is high and, nevertheless, the drying is not substantially impaired, due to the water absorption capacity that is still present. The efficiency of the drying operation can be determined, for example, in a Mollier diagram by using a so-called edge scale.
It is not necessary to adjust the mixing valve in the mixing device continuously. It is sufficient if the mixing valve can be adjusted in discrete stages, with it being possible for the output signal from the open loop or closed loop control device to be generated by comparison with fixed stored threshold values, for example as a signal with a staircase waveform or pulse-like waveform for driving stepping motors.
The sheet-processing machine equipped in accordance with the invention, for example a sheet-fed offset printing press, can have dryers of different types, with it being possible for the exhaust air from a dryer to be mixed with the feed air of a dryer of another type. However, it is also possible in the same way to carry out the measures described in a dryer of the same type or in the same dryer.
Further advantages in relation to the energy efficiency result if the waste heat from the part of the dryer exhaust air that is not fed back is fed to other additional devices in the machine, in which the exhaust air can be used expediently. In this case, this can be one or more heat exchangers, for example, over which the parts of the dryer exhaust air that are not fed directly to the dryer feed air are led. As a rule, additional measures are needed for such further improvements in the energy efficiency and the expenditure therefor generally rises with a higher desired level or extent of the efficiency. In a further step, for example, a heat exchanger can heat the dryer feed air, preferably at a point before the dryer exhaust air is mixed with the dryer feed air. This heat exchanger can be operated with the exhaust air that is not fed back or else through the use of the waste heat which is produced, for example, by sheet-carrying elements or impression cylinders in the area of the dryer. However, by using the dryer exhaust air that is not fed back, it is also possible for other regions of the sheet-processing machine to be supplied with process heat, for example back-pressure or impression cylinders having a temperature which can be controlled, which are used for the purpose of preheating the printed sheet before entry into the respective dryer area, as described for example in German Patent Application DE 10 2007 056 899 A1, corresponding to U.S. Patent Application Publication Nos. US 2008/0134915 and US 2009/0277352, the full contents of which are incorporated herein by reference at this point.
The dryer exhaust air is used quite particularly efficiently if the part of the dryer exhaust air that is not fed back directly and the dryer feed air are in each case led over two heat exchangers, with it being possible for the first heat exchanger in the flow direction to be a simple air-air heat exchanger, while the heat exchanger following in the flow direction in each case is connected to a compressor which connects the two heat exchangers following in the flow direction to each other in the area of the dryer exhaust air and the dryer feed air. In this way, the exhaust air, for example blown into the open, can be cooled down until within the range of the surrounding air, while the energy content is simultaneously raised through the compressor in accordance with the principle of the heat pump to a temperature level from which only very little electrical energy is still needed by the heating matrices in order to bring the air to the temperature required for the drying. As a result, the compressor undertakes the task of the heating matrices, but with a very much higher efficiency. At the same time, as a result of the cooling of the moist warm dryer exhaust air, the temperature falls below the dew point, so that it is easily possible at this point to provide devices for dewatering and/or for removing solvents from the dryer exhaust air. The exhaust air dewatered and/or made free of solvents in this way can consequently be fed to the feed air through a second circuit or the result is a closed system without emissions.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a sheet-processing machine with one or more dryers and a method for drying sheets, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawings in detail and first, particularly, to
A printing press of this type is, for example, offered by Heidelberger Druckmaschinen AG under the designation Speedmaster XL105-6-LYYLX3.
In the dryer units 10a and 10b, fresh air which is blown in is subsequently heated by heating matrices. An exhaust air duct 12, which is illustrated above the dryer devices 10a and 10b, has a blower which extracts moist warm exhaust air from the dryer unit and leads it out of the press room or print shop into the open through a non-illustrated pipeline system.
The dryers 11a-11d in the delivery 5 of the printing press 1 are implemented in the form of cassette-like withdrawable units. These withdrawable dryer units can be hot air dryers, infrared dryers, UV dryers or so-called combined dryers, which act on the sheet to be dried both with hot air and with radiation energy. The exhaust air from the withdrawable dryer units 11a-11d is led into the open through an exhaust air duct 13, which is likewise illustrated in the figure.
A first exemplary embodiment of the invention will be explained in more detail by using a simplified electromechanical block diagram in
Furthermore, the controller 28 uses the signal from the humidity sensor rF and the temperature sensor T in the combined sensor 26a to determine the extent to which the exhaust air is still able to absorb moisture. In the simplest case, this is done by comparing the measured humidity values with stored values at a specific temperature value in each case. The valves 29a and 29b can then be adjusted by the controller from the result of the comparison and thus the mixing ratio of feed air and exhaust air that is fed in mixed form to the blower 24a at the point M can be changed. An improvement in the control strategy is obtained if, in addition, as illustrated in
Combined infrared/hot air dryers have four setting variables which it is necessary to optimize:
1. The output from the infrared radiator
2. The temperature of the hot air
3. The quantity of air (blown air) which is blown onto the sheet
4. The proportion of exhaust air which is fed back into the dryer again (recirculated air).
Normally, the output of the infrared radiator and/or the hot air temperature are controlled in such a way that a preset sheet or stack temperature is reached. Furthermore, the quantity of blown air is set to be as high as possible or at least just sufficiently high that there is no detrimental influence of the air stream on the sheet run. According to the invention, the proportion of the recirculated air is now changed, starting from a preset value or standard value, in such a way that the specific energy used per kg of evaporated solvent (water) does not exceed a predefined value. This value describes the efficiency of the dryer process. The value can be determined, on the basis of job data, by computation by making practical assumptions about physical variables entering into the calculation, as explained below. About 2500 kJ are needed in order to evaporate one kg of water, in the ideal case. In the real industrial printing process of a sheet-fed offset printing press, these values frequently lie around 5-10 times higher. This is associated with energy losses but primarily also with the fact that the air masses used for the drying are able to absorb little water vapor, since the warm exhaust air is removed from the process after impinging on the printed sheet once and is replaced by newly heated air. The high quantities of blown air which result from the blown air velocity and the area of the blown air field are, however, absolutely necessary for good drying of emulsified varnish during the relatively short time during which the sheet is in the dryer. However, only a small part of the air blown in reaches the surface of the sheet and only this part is also saturated with solvent (water vapor). This is because the part of the air stream which impinges on a sheet guide plate or cylinder or further machine parts does not pick up any moisture at all. In many cases, this is half or more of the quantity of hot air being used, depending on the sheet format of the sheet running through.
The efficiency of the energy use can now be calculated through the psychometric state variables of the feed and exhaust air. This can be completed geometrically or graphically, for example in a Mollier diagram, which is illustrated in
In order to carry out the determination of the evaporation enthalpy it is, of course, possible, instead of the graphical procedure outlined in the Mollier diagram, to determine the evaporation enthalpy by computation through the respective formula and to program this in the controller 28.
However, it is then not possible to achieve the theoretical value of 2500 kJ/kg water through the higher and higher admixture of exhaust air with feed air, since the more moist exhaust air is mixed with the feed air, the lower the level of drying of the sheet to be dried also becomes. This relationship is illustrated in
The course of the two curves depends on very many parameters, with it being possible for the reference points illustrated therein to be determined firstly from humidity and temperature measurements in the exhaust air and feed air of the dryer and on the dried sheet but, secondly, also by using the parameters from a print job. These characteristic curves depend, amongst other things, on the sheet format, the application of varnish to the sheet, the infrared output from the radiator in the combined IR/hot air dryer, the machine speed and the ink coverage of the printed sheet, to name only a few of the most important parameters. In principle, it is therefore also possible, instead of the humidity measurements described previously, to determine the characteristic curves for efficiency and level of drying from these parameters. The operator would then have it in his or her power, as indicated by an arrow at 60% recirculated air, to accept a slight penalty in terms of the level of drying but, in return, to increase the efficiency of the dryer by about 50% and therefore to reduce the required electrical power accordingly.
The controller 28 can, however, also be equipped for various operating modes. For non-critical print jobs, in which for example little varnish or rapidly cross-linking varnish is applied, the proportion of recirculated air can be very high (energy saving mode). On the other hand, in the case of critical print jobs, the operator sets the proportion of recirculated air to be lower, i.e. he or she gives up the energy saving mode or sets it back by one or more steps. Furthermore, it is possible to store in the controller the fact that with blown air temperatures set higher, it is also possible to run with higher proportional quantities of air in the recirculated air (for example at temperatures T>70° C.) than in the case of very moderate air temperatures of, for example, T<60° C. These relationships can also be derived from the Mollier diagram illustrated in
In general, the controller 28 therefore ensures that the energy efficiency of the dryer is as high as possible but the drying is not substantially impaired, in that the mixture of fresh air and exhaust air formed through the valves 29a and 29b still has a sufficiently high absorption capacity for water.
The exemplary embodiment according to
Nothing changes in the absolute air humidity of the feed air after passing through the heat exchanger 40, so that the humidity values from the combined sensor 36b for the feed air also continue to apply to the feed air at this point downstream of the heat exchanger. On the other hand, the temperature changes after the passage through the heat exchanger 40, which raises the feed air from the throttle valve 39a to a higher level. Accordingly, the electrical energy for the heating matrix 33 can be reduced, since the feed air already enters the heating matrix 33 at a considerably higher temperature than in the case of the exemplary embodiment according to
By using the measures described herein, the energy requirement of a dryer can be reduced from typically 100 kW in full operation, i.e. at a machine speed of 16,000 sheets per hour in the 75×105 format, by about 20-30 kW, with the controlled feedback of the exhaust air and the use of the heat exchanger 40 accounting for approximately 10-15 kW in each case. The sheet guide plate 46 or the sheet guide plates in the region of the dryer 31 are blackened, in order to improve the absorption characteristics of the guide plate surface and to increase the effectiveness of the additional feed air heating in this way.
A further reduction in the dryer power by about 10 kW is possible by preheating the sheets to be dried with process heat from exhaust air being transferred to impression or back-pressure cylinders having a temperature which can be controlled, or by the exhaust air that is not fed back transferring part of its heat content to the fresh air through an air/air heat exchanger. This case is illustrated in the embodiment according to
In contrast to
The fresh air fed to the throttle valve 79a is likewise led over the heat exchanger 80 after passing the measuring point 76b and picks up a large part of the heat contained in the exhaust air thereby. The heating power which the heating matrix 73 has to provide is correspondingly lower, as explained above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 026 604.3 | Jul 2010 | DE | national |
