The invention relates to a method and to an arrangement for degassing treatment substance, especially for degassing a coating color or a sizing agent, of a fiber web. More especially the invention relates to a method according to the preamble of claim 1.

As known from the prior art in fiber web producing processes typically comprise an assembly formed by a number of apparatuses arranged consecutively in the process line. A typical production and treatment line comprises a head box, a wire section and a press section as well as a subsequent drying section and a reel-up. The production and treatment line can further comprise other devices and sections for finishing the fiber web, for example, a sizer, a calender, a coating section. The production and treatment line also comprises at least one winder for forming customer rolls as well as a roll packaging apparatus. In this description and the following claims by fiber webs are meant for example paper and board webs.

In production of fiber webs, for example of paper or board webs, treatment substances are added onto the fiber web. Sizing is used to alter the properties of a fiber web by adding sizing agents (sizing medium), for example glue chemicals onto the surface of the fiber web at the fiber web machine. In coating onto surface of a fiber web, is added a layer or layers of coating color (coating medium) at a coating station followed by drying.

The coating or the sizing of a fiber web typically utilizes a coating device - a coater - or a sizing device - a sizer. In connection with the coaters and sizers different kinds of application technology for application of the coating medium or the sizing medium on the fiber web are employed, for example curtain technology or blade coating technology or film transfer technology or rod coating technology or air brush coating technology or spray coating technology.

In the process industry, the mixing of gases, such as air, with the liquids and compounds used in the process typically causes many problems. Particularly in the coating or sizing of fiber web material, the gas and gas bubbles in the treatment substance cause unevenness on the surface of the web. Depending on the treatment substance the seriousness of the problems varies for example due to their different tendency to bind with gases. In addition, the significance of the problem depends also on the coating or sizing technology used.

It is known from prior art to degas treatment substance by vacuum de-aerators comprising a rotating drum arranged inside a vacuum tank, inside which the treatment substance is conducted, whereupon due to the centrifugal force, the coating material rises up the inner wall of the drum and discharges from the drum as a thin film colliding with the wall of the vacuum tank. One problem with prior art vacuum de-aerators is their insufficient de-aeration efficiency, especially with high-viscosity treatment substances. In WO publication 2006/128963 is disclosed a method and an arrangement to provide an improvement for vacuum-operated degassing of coating material. In the method the coating substance is arranged to rise stepwise up the wall of the drum, so that the coating substance will form a thin veil-like film on at least two different step levels, whereupon the gas bubbles in the coating substance will break and discharge. In the apparatus the inner surface of the drum is designed to be stepped, comprising at least two step levels so that the coating substance rising upwards due to the effect of the rotating motion will form a thin veil-like film on the said at least two step levels, respectively.

In EP publication 0517223 A1 is disclosed manufacturing of a coated paper to be used for a usual printing and using a curtain type coating method, in which is enabled by de-aerating a coating substance having a high concentration and a high viscosity is de-aerated under a degree of vacuum of a saturated vapor pressure or less and under a condition of applying shear to the coating substance, which is formed as a free-falling curtain in a vertical direction and a printing base paper is coated with the de-aerated coating liquid so that the free-falling curtain of the coating substance collides with the coating base paper running continuously in a direction crossing the free-falling curtain.

In DE 102005017952 is disclosed a method for degassing a coating substance, which substance is passed through a vacuum degassing unit at a temperature close to its boiling point and especially 40-60 °C under a pressure of 20-100 mbar for degassing the treatment substance in temperatures near boiling temperature. Heat is supplied by a heat exchanger or heated walls of the vacuum degasser. One disadvantage of this known method is that heating of the coating substance increases solubility of gases into the substance liquid and increases amount of steam that must be removed by the vacuum degassing unit from the system and therefore more powerful and bigger unit is needed and the energy consumption is increased.

In WO 2012/041662 A1 is disclosed a method for operating a device and to a device for directly or indirectly applying liquid or paste-like application media to a displaceable material web in which the application medium in the region of the degassing is set lower than the temperature in the region of the application device, wherein the temperature in the region of the degassing device is in the range from 5 °C to 35 °C.

In EP 0517223 A1 is disclosed method of manufacturing a coated paper to be used for a usual printing and a printing base paper is coated with the deaerated coating liquid.

In EP 2474369 is disclosed a curtain coating apparatus and a curtain coating method for supplying a curtain film of a coating material onto a surface of a web being conveyed to coat the surface of the web with the coating material, in which the coating material cooled to around 20°C before it is supplied into a container.

In JP 2001087698 A is disclosed a color deforming apparatus in which coating color is sent under pressure from a return tank to be introduced into a vacuum defoaming machine and subjected to defoaming treatment, in which a water cooling jacket is provided to the vacuum defoaming machine.

In EP 1498544 A1 is disclosed an arrangement in cellulosic products having a barrier coating applied thereto.

In some cases heating of the treatment substance might, for example due to chemical reactions initiated by the heat or due to non-tolerance of heat of some components of the treatment substance, deteriorate properties required from treatment substance. For example thermal coatings will lose brightness already at temperatures 50 °C and some latex coatings will suffer instability in temperatures over 45°C in longer run.

The heating of the treatment substance, which typically is already in rather, even too high temperatures, for example 40 - 50 °C, when passed to the degassing, creates more steam into the substance. In degassing the gases and the steam is typically removed by a suction pump of the degassing unit from the treatment substance, which might in some cases be problematic since some suction pump types, for example dry screw vacuum pumps, which are operable in vacuums over 22 mbar, and liquid ring vacuum pumps, which are operable in vacuums over 35 mbar, do not function well when steam is present.

An object of the invention is to create a method and an arrangement for degassing treatment substance of a fiber web in which the above problems and disadvantages are eliminated or at least minimized.

Another object of the invention is to provide a method and an arrangement for degassing treatment substance of a fiber web in which high temperatures of the treatment substance are avoided.

A particular object is to prevent harmful chemical reactions in treatment substance during degassing.

A further particular object is to provide a method and an arrangement for degassing treatment substance of a fiber web in which the temperature of the treatment substance is controlled during degassing.

A further particular object is to provide a method and an arrangement for degassing treatment substance of a fiber web in which the suction pump type of the degassing unit choice is not limited by steam tolerance.

In order to achieve the above objects the method for degassing treatment substance of a fiber web according to the invention is mainly characterized by the features of claim 1.

According to the invention in the method for degassing a treatment substance, in particular a coating color or a sizing agent, of a fiber web the treatment substance is degassed in a degassing unit and temperature of the treatment substance is adjusted to temperature below 35 °C by cooling the treatment substance in a temperature control unit before degassing of the treatment substance in the degassing unit. The arrangement for degassing treatment substance of a fiber web configured to carry out the method comprises a degassing unit and a temperature control unit, advantageously a cooler or a heat exchanger, before the degassing unit for adjusting temperature of the treatment substance to temperature below 35 °C before degassing, a feed tank and a feed channel for passing the treatment sub-stance to the temperature control unit and another feed channel for passing the treatment substance from the degassing unit to a working tank from which the treatment substance is passed to a treatment device, in particular to a coating device or to a sizing device, and a control unit receiving measurement information from a temperature sensor, from a pressure sensor in the feed channel and from a pressure sensor in the degassing unit for controlling the temperature of treatment substance in the temperature control unit and the pressure of the degassing unit.

According to the invention temperature changes of the treatment substance are evened out in the temperature control unit before degassing of the treatment substance in the degassing unit.

According to the invention in the method the temperature control unit maintains the temperature changes of the treatments substance flow, for example about 1°C/15 min.

According to an advantageous feature in the method when the treatment substance is cooled in the temperature control unit, steam in the gas of the treatment substance is condensed and vacuum is created, since the steam volume decreases due to cooling and condensing of the steam.

According to an advantageous feature the treatment substance is cooled to temperatures 20-30 °C before degassing.

According to an advantageous aspect of the invention the temperature of the treatment substance, in particular the temperature of a coating or a sizing substance, is controlled before degassing the substance in the degassing unit by cooling the substance by a cooler, for example by a heat exchanger. The cooler advantageously maintains the temperature changes of the treatments substance as slow as possible by which best environment and best conditions for degassing and for functions of the degassing unit and the vacuum system are provided. Thus improved degassing effect is achieved.

According to an advantageous aspect of the arrangement the temperature of the treatment substance is controlled by a heat exchanger, advantageously by a cooler.

According to an advantageous feature of the invention the arrangement comprises a droplet separator between the degassing unit and the vacuum system. According to an advantageous feature of the invention the arrangement comprises a dry screw vacuum pump for providing vacuum 20 - 40 mbar in the vacuum system. Advantageously there is a droplet separator before the dry screw vacuum pump. In the following the invention is described in more detail with reference to the accompanying drawing, in which

• figure 1 is a schematic process chart of treatment substance flow,
• figure 2 is a schematic example of one advantageous aspect of the invention and
• figure 3 is an air-saturation vapor pressure diagram.

According to figure 1 the process of treatment substance flow the treatment substance is delivered to a feed tank 11. From the feed tank 11 the treatment substance is passed to temperature control unit 12, which is for example a heat exchanger or advantageously a cooler. The temperature controlled, advantageously cooled treatment substance is then passed to degassing unit 13, which comprises a vacuum system 14 with a suction pump (not shown) for removing gas from the treatment substance. The degassed treatment substance is the passed to a working tank 15, from which the treatment substance is passed to a coating device or to a sizing device (not shown). The degassed treatment substance may also be passed directly to a coating head / an applicator of a coating device or to a sizing head / an applicator of a sizing device (not shown).

In the example of figure 2 treatment substance is delivered via feed channel 20 to the temperature control unit 12 from the feed tank 11 (fig. 1). Temperature of the treatment substance is measured by a temperature sensor 12A and pressure of the treatment substance in the feed channel 20 is measured by a pressure sensor 20A. Measurement information from the temperature sensor 12A and the pressure sensor 20A is transmitted to a control unit 25. From the temperature control unit 12 the treatment substance is passed to degassing unit 13, in which the gas is separated from the treatment substance and removed by the vacuum system 14. The gas flow is passed to the vacuum system via a droplet separator 16, in which the steam in the substance is cooled to condensate and moisture is separated from the gas flow. Pressure in the degassing unit 13 is measured by a pressure sensor 13A and its measurement information is transmitted to the control unit 25. The control unit 25 controls operation of the system and especially controls the pressure created by the vacuum system and temperature conditions of the treatment system by adjusting the operation of the temperature control unit 12. From the degassing unit the degassed treatment substance is fed via a feed channel 22 to the working tank 15 (fig. 1). The degassed treatment substance may also be passed directly to a coating head / an applicator of a coating device or to a sizing head / an applicator of a sizing device (not shown).

As shown in figures 1 and 2 temperature of the treatment substance is adjusted by the temperature control unit 12 and temperature changes of the treatment substance are evened out before degassing of the treatment substance in the degassing unit 13. Temperature control unit 12 is advantageously a cooler or a heat exchanger, which maintains the temperature changes of the treatments substance as slow as possible and thus best environment and best conditions for degassing and for functions of the degassing unit 13 and the vacuum system 14 are provided.

In figure 3 is an air-saturation vapor pressure diagram, which provides basic information for the control unit 25 (fig. 2) for creating required pressure level in view of the temperature of the treatment substance for desired degassing conditions. When the temperature of the treatment substance is controlled in the temperature control unit 12, a vacuum is created as the steam volume in the gas flow decreases due to cooling and condensing of the steam. The steam condensate is removed in the droplet separator 16.

11

feed tank

12

temperature control unit

12A

temperature sensor

13

degassing unit

13A

pressure sensor

14

vacuum system

15

working tank

16

droplet separator

20

feed channel from feed tank

20A

pressure sensor

22

feed channel to working tank

25

control unit