The invention relates to discharging vapor from a roller dryer.

Roller dryers comprise a rotatable roller with a heated shell to which, with the aid of applicator rollers, a thin layer of liquid product is applied. During rotation, the thin layer of product is heated on the shell surface, and moisture evaporates from the product until a dry crust is formed which is scraped from the cylinder shell with the aid of a blade. The

evaporating product moisture rises from the shell surface, and has to be exhausted.

Since a roller dryer must be properly accessible for the control of the process, the vapor is usually exhausted with the aid of an exhaust hood. To prevent the vapor from condensing, and drops of condensation from falling back onto the product, the vapor is exhausted via the exhaust hood with an excess of air. This has as a drawback that exhausting requires relatively much energy, and that large amounts of supply air have to be conditioned in order to prevent the flow of drawn-in air itself causing contamination of the product. The object of the invention is a method and an apparatus for discharging vapor from a roller dryer, wherein the drawbacks mentioned can be counteracted. To that end, the invention comprises a method for discharging vapor from a roller dryer, comprising:

collecting vapor rising from a roller dryer in a dome placed over a vapor release area of the roller dryer,

passing vapor from the dome to the condenser, and

condensing supplied vapor in the condenser. By collecting the rising vapor in a dome and then condensing it in a condenser, the vapor can be discharged without admixture of air being necessary. As a result, the energy required for discharging and conditioning large amounts of air can be saved, and via the condenser, heat can even be recovered. Collecting vapor and passing it to the condenser can take place during operation on the basis of natural draught. This natural draught occurs due to the reduced pressure resulting from the vapor condensing in the condenser. Possibly, during a start up phase the process can be primed with the aid of an auxiliary fan, and with the aid of an auxiliary fan non- condensable components of the vapor can be discharged from the condenser. The entrance velocity of air at an entry of the dome can be substantially equal to zero.

By maintaining the flow rate of condensed steam substantially equal to the flow rate of steam collecting in the dome, a continuous discharge of vapor can be realized.

The flow rate of condensed steam can be regulated on the basis of a boundary layer present in the dome between vapor having collected in the dome and air located therebeneath. This regulation can take place, for instance, on the basis of the location or movement of the stratification layer with respect to the entry of the dome, and/or on the basis of measurement of temperature and/or density of gas present in the dome. The flow rate can also be regulated on the basis of a desired zero value of the entrance velocity of air into the dome.

The flow rate of condensed steam can be regulated by controlling the condensing capacity of the condenser. This condensing capacity can be controlled by, for instance, varying the flow rate and/or the inlet temperature of the cooling water of the condenser.

The condenser can be equipped with, for instance, a conventional condensing body, such as a water-cooled plate heat exchanger. By designing the condenser with a cover, it can be utilized as a vacuum condenser. The condenser can be part of a heat recovery installation. In the condenser, cooling water can be heated, for instance, from approximately 20°C to approximately 80 - 90°C, while vapor is supplied to the condenser with a temperature of, for instance. approximately 90 - 100°C, and the non-condensable parts of the vapor are discharged with a temperature of, for example, approximately 25-30°C.

By heating the dome up to or above the dew point of the vapor, vapor can be prevented from condensing on the dome. In particular, the inner surface of the dome is heated to a temperature of 100°C or more. To prevent condensation on the inner surface of the dome, the dome can also be

manufactured from an insulating material, such as plastic. Further, the dome may be provided with discharge channels for collecting and discharging vapor having condensed on the dome.

The invention also relates to an apparatus for discharging vapor from a roller dryer, comprising a dome, to be placed over a vapor release area of a roller dryer, having a collecting space surrounded by a shell of the dome for collecting vapor rising from the roller dryer, and a condenser which is in flow connection with the dome for therein condensing vapor obtained from the collecting space.

The flow connection can be designed, for instance, as a vapor pipe. By providing the dome with a heating device, condensation of the vapor on the inner surface of the dome can be prevented. The heating device can be designed for heating the dome to at least the dew point of collected vapor, for instance 100°C or higher. The dome can be designed, for example, from metal, and can be provided with electric heating elements.

The dome can be provided with sensors for detecting the position of a boundary layer between collected vapor and air in the dome. The sensors can for instance be arranged in the shell in a height direction at different distances from an entry located at the bottom of the dome. The sensors can for instance comprise temperature meters, density meters and/or humidity meters. The dome can also be provided, near the entry, with a speedometer for measuring the supply velocity of air into the dome. The supply velocity of air via the entry of the dome is preferably virtually zero. The entry of the dome may be located above the roller dryer.

However, it is preferred that the shell of the dome overlaps the roller dryer in the height direction. For instance, the shell of the dome extends downwards to at least two-thirds, preferably at least half, of the height of the roller of the roller dryer.

The condenser may be designed as a liquid-cooled plate exchanger. The liquid cooling of the condenser can be part of a heat recovery circuit in which, for instance, a heating load is included. The condensing capacity of the condenser can be regulable, for instance by rendering the flow rate of cooling water flowing through the condenser settable, by regulating the initial temperature of the cooling water and/or rendering the condensation surface of the condenser settable. The condenser may be provided with an auxiliary fan. With it, the collecting and condensing process can be primed, and any non- condensable components of the vapor, such as non-condensable gas mixed with the vapor, can be discharged from the condenser via a flue. When the vapor contains, for instance, a few percents by volume of air, this air which remains after condensation of the vapor can be exhausted with the auxiliary fan.

The invention also relates to a roller dryer, provided with an apparatus for discharging vapor. The roller dryer can comprise a rotatably arranged roller with a heated outer surface. The roller can be made of, for instance, cast iron, and can be heated by, for instance, condensation of steam on the inner surface of the roller. To that end, the steam can be supplied at a pressure of, for instance, 12 bar, so that the outer surface obtains a

temperature of approximately 180°C.

The roller dryer can further be provided with a feed device for feeding liquid product to be dried. The roller dryer can also be provided with one or more applicator rollers located at the outer surface for applying liquid product in thin layers onto the outer surface. Between the applicator rollers there may be a bath holding a supply of liquid product to be applied. The feed device can feed the liquid product to the applicator rollers and/or the bath. The roller dryer can further be provided with a blade cooperating with the outer surface for scraping a layer of product off the outer surface, and with a discharge device for discharging scraped-off product.

The invention will be elucidated in further detail on the basis of an exemplary embodiment which is represented in a drawing. In the drawing:

Fig. 1 shows a schematic representation of a roller dryer which is provided with an apparatus for discharging vapor with energy recovery.

It is noted that the Figure is only a schematic representation of a non-limiting exemplary embodiment.

Fig. 1 shows an apparatus for discharging vapor from a roller dryer 1. The apparatus comprises a dome 3 placed over a vapor release area 2 of the roller dryer 1. The vapor release area 2 is the area in which the roller dryer 1 releases vapor. This is, in particular, the outer surface 4 of the roller 5 of the roller dryer 1. In the context of this application, the wording "over a vapor release area 2" should be understood to mean that the dome 3 is located at least partly above the vapor releases area, and possibly at least partly overlapping the vapor release area 2 in a height direction h.

The dome 3 comprises a collecting space 6 surrounded by a shell 7 of the dome 3. The dome 3 is here provided at the bottom thereof with an entry opening 8, through which the top of the roller dryer 1 extends into the collecting space 6.

The apparatus further comprises a condenser 9, in flow connection with the dome 3, for therein condensing vapor obtained from the collecting space 6. In the exemplary embodiment represented here, the condenser 9 is brought in flow connection with the dome 3 via a vapor pipe 11 extending from an exit 10 of the dome 3.

In this exemplary embodiment, the condenser 9 comprises a condensing body 12 formed by a plate pack through which is run a cooling pipe 16. Cooling water can flow via an adjustable valve 13 from an entry 14 of the cooling pipe 16 via the cooling body to an exit 15 of the cooling pipe 16. Provided around the plate pack is a cover 19 to which, via vapor pipe 11, the vapor to be condensed is supplied. Non-condensable components can be discharged via an exit 20 of the condenser 9. The exit 20 can be connected to, for instance, a flue 17 in which an auxiliary fan 18 is provided. The flue 17 can further be provided with, for instance, a filter (not represented) and can open into, for instance, the outside air. The exit 15 of the cooling pipe 16 can be connected to, for instance, the entry of a further heat exchanger of a heat- consuming process. In turn, the exit of the further heat exchanger can be connected to the entry 14 of the cooling pipe 16 of the condenser 9. Thus, the condenser 9 forms part of a heat recovery circuit.

The shell 7 of the dome 3, the vapor pipe 11 and the cover 19 of the condenser 9 screen the vapor from the surroundings. In order to prevent premature condensation of the vapor, the shell 7 of the dome 3 and the vapor pipe 11 can be provided with, for instance, heating elements which heat the shell 7 of the dome 3 and the vapor pipe 11 to above the dew point of the vapor. In this exemplary embodiment, this is, for instance, 100°C or more. If any air has admixed with the vapor, the dew point may be, for example, at

approximately 9o°C. The condenser 9 is arranged above and next to the exit 10 of the dome 3. This can prevent condensate flowing back from the condenser 9 to the roller dryer 1.

The shell 7 of the dome 3 is provided, near the entry 8, with two sensors 21 designed as density meters, with which the position of the boundary layer 24 between vapor 22 having collected in the dome 3 and air 23 present underneath with respect to the entry 8 can be determined. These sensors 21 are here connected to a control 25 with which the supply valve 13 of the cooling pipe 16 of the condenser 9 can be controlled. Thus, with the aid of the control 25, the condensing capacity can be set such that the boundary layer 24 between vapor and air 23 is substantially at the same height in the dome 3.

The downwardly directed parts 26 of the shell 7 of the dome 3 may link up with a conditioned space which surrounds the roller dryer 1, for instance a set of walls 27 placed around the roller dryer 1, with passages with which the air supply to the conditioned space surrounded by the walls can be controlled.

Besides a rotatably arranged roller 5 with a heated outer surface 4, the roller dryer 1 further comprises a feed device (not represented) for feeding liquid product to be dried. The roller dryer 1 is further provided with one or more applicator rollers 28 located at the outer surface 4 of the roller 5 for applying liquid product fed by the feed device in thin layers on the outer surface 4 of the roller 5. Between the applicator rollers 28 a bath may be located with a supply of liquid product to be applied. The feed device feeds the liquid product to the applicator rollers 28 and/or the bath. The roller dryer 1 is further provided with a blade, cooperating with the outer surface 4 of the roller S, for scraping a layer of dried product off the outer surface 4 of the roller 5, and with a discharge device for discharging scraped-off product.

The apparatus works as follows. To the interior of the roller of the roller dryer 1, under pressure, steam is supplied which condenses on the inner wall of the roller. The condensate is discharged from the roller as hot water. Meanwhile, the roller rotates to the right at a velocity of rotation of, for instance, 1 or 2 revs per minute. From a feed device, a liquid product to be dried is fed to the applicator rollers 28. The products to be dried on a roller dryer 1 can be edible products as well as non-edible products, and comprise, for instance, breakfast mush, starch slurries, viscous pastes, and wet salts. In this exemplary embodiment, the product to be dried can for instance be a starch slurry with a moisture content of approximately 60 percent by weight. Then, per applicator roller 28, a thin layer of product to be dried is applied to the outer surface 4 of the roller 5, for instance in four layers having a total thickness of 0.2 mm. The applied layer of product to be dried is then heated on the outer surface 4 of the roller 5, for instance to a temperature above the boiling point of the product to be dried. Through this heating, moisture exits the layer of product as vapor, whereby a layer of dried product is formed. The moisture content of this dried product can for instance be approximately 5 percent by weight. Before the product on the outer surface 4 of the roller 5 has once again reached the applicator rollers 28, the now dried product is scraped from the outer surface 4 of the roller 5 by means of a blade. The product can then come off the outer surface 4 of the roller 5, for example, as a dry sheet or as flakes, and be discharged with the aid of a discharge device (not

represented).

During operation of the roller dryer 1, vapor rises from the roller dryer 1, in particular from the outer surface 4 of the roller 5, which is released by the product. The rising vapor collects in the dome 3 placed over the vapor release area 2 of the roller dryer 1. Since the ambient air has a greater density than the vapor, the vapor collects in the top of the dome 3. Therefore, in the dome 3, a boundary layer 24 forms between vapor and ambient air located underneath. The vapor is led via the exit 10 of the dome 3 and the vapor pipe 11 to the condenser 9. In the condenser 9 the vapor condenses on the condensing body 12 of the condenser 9. As a result of condensation, the volume of the vapor strongly decreases. Thus, a natural draught of vapor from the dome 3 to the condenser 9 is formed. This natural draught is further supported in that the vapor seeks to rise owing to the lower density. Non-condensable components of the vapor, for example odors, and gases other than water vapor can be exhausted to the outside environment via a flue 17 connected to an exit 20 of the condenser 9. In this exemplary embodiment, the vapor is collected at atmospheric pressure, and the temperature of the vapor is

approximately 100°C. Upon exiting the condenser 9, the non-condensable components of the vapor have a temperature of approximately 30°C. In the condenser 9, the cooling water is heated from a temperature of

approximately 20°C to a temperature of approximately 90°C. The flow rate of condensed vapor is maintained substantially equal to the flow rate of vapor collecting in the dome 3. In this exemplary embodiment, this is done by adjusting the flow rate of cooling water of the condenser 9 with the aid of the supply valve 13, on the basis of the position of the boundary layer 24 determined with the sensors 21. During operation, the vapor is discharged from the dome 3 substantially continuously.

It is noted that the invention is not limited to the exemplary embodiment represented here. For instance, the roller dryer may be provided with a larger and smaller number of applicator rollers, and, for instance, also a double roller dryer can be used. Further, several domes can be placed over a vapor release area of one roller dryer or, conversely, several roller dryers under one dome. Also, the condenser can be operated without heat recovery.

Such variants will be clear to the skilled person and are understood to be within the scope of the invention as set forth in the following claims.