US3720002A

US3720002A - Drying sheet material

Info

Publication number: US3720002A
Application number: US00120507A
Authority: US
Inventors: P Martin
Original assignee: Wiggins Teape Research and Development Ltd
Current assignee: Arjo Wiggins Ltd
Priority date: 1970-03-19
Filing date: 1971-03-03
Publication date: 1973-03-13
Anticipated expiration: 1990-03-13
Also published as: BE764577A; DE2112706A1; ZA711340B; CA950188A; GB1337980A; AU2630571A

Abstract

Sheet material to be dried is conveyed through a drying region where a combination of radiant heat and heated gas is directed towards the material. The proportion of radiant heat relative to the amount of heated gas is adjusted through the drying region so that a higher radiant heat/heated gas ratio is provided at the end of the path than at the beginning of the path.

Description

United States Patent 1191 Martin 1March 13, 1973 DRYING SHEET MATERIAL [75] Inventor: Patrick Douglas Martin, High Wycombe, England [73] Assignee: Wiggins Teape Research 8: Development Limited, London, England [22] Filed: March 3, 1971 [21] Appl. No.: 120,507

[30] Foreign Application Priority Data March 19,1970 Great Britain ..13,317/7o [52 US. Cl. ..34/18, 34/68, 34/41 [51 [111. CI ..F26b 7/00 [58] Field of Search ..34/16, 18, 23, 34, 41, 68, 34/155 [56] References Cited UNITED STATES PATENTS 3,403,454 10/1968 Smith, Jr. ..34/68 Bakker ..34/68 Smith, Jr. ..34/68 X Primary Examiner-Charles J. Myhre 'Assistant ExaminerWilliam C. Anderson Attorney-Synder & Butrum [57] ABSTRACT Sheet material to be dried is conveyed through a drying region where a combination of radiant heat and heated gas is directed towards the material. The proportion of radiant heat relative to the amount of heated gas is adjusted through the drying region so that a higher radiant heat/heated gas ratio is provided at the end of the path than at the beginning of the path.

12 Claims, 3 Drawing Figures DRYING SHEET MATERIAL BACKGROUND OF THE INVENTION The invention relates to methods and apparatus for drying sheet material such as, for example, paper webs.

When paper has been coated with an aqueous coating consisting of solids suspended in water, a well known method of drying the paper consists of transporting the paper web through a drying zone and directing hot air of low moisture content onto the coated side of the web at high velocity. In such a method the drying rate initially increases rapidly while the web warms up. This is followed by a period of drying when the rate of evaporation is substantially constant. Finally there is a so called falling rate region of drying during which the evaporation of moisture proceeds at a continuously decreasing rate. During this latter period the remaining moisture may be below a surface which has been dried by the earlier stages of drying and consequently a barrier may be formed which reduces the effectiveness of the hot air stream in evaporating the moisture below the surface. Furthermore, during this period the moisture may migrate from the underside of the coating into the web.

It is an object of the present invention to provide improved methods and apparatus for drying and in particular which may be used to obtain improved drying results in the aforesaid falling rate region.

SUMMARY OF THE INVENTION According to the invention there is provided a method of drying sheet material comprising transporting the material to be dried along a path through a drying region, directing a combination of radiant heat and heated gas towards the material in the drying region and adjusting the proportion of radiant heat relative to the amount of heated gas along the path through the drying region so that a higher radiant heat/heated gas ratio is provided at the end of the path than at the beginning of the path.

According to the invention there is also provided drying apparatus which comprises a conveying means for transporting material to be dried along a path through a drying region, a plurality of hot gas inlets spaced along the path for directing a flow of hot gas towards the path, a plurality of radiant heating devices spaced along the path for directing radiant heat towards the path, and heat control means for controlling the proportion of radiant heat relative to the amount of heated gas directed towards the path so that a higher radiant heat/heated gas ratio is provided at the end of the path than at the beginning of the path.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic section through part of a drying apparatus according to the invention,

FIG. 2 is a schematic view of the upper part of the apparatus as seen from the line ll-II in FIG. 1, and

FIG. 3 is a schematic view of an electrical control arrangement used in the apparatus of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, the drying apparatus which may be used for drying sheet material such as a web of paper having an aqueous coating, comprises conveying means 11 for conveying the material to be dried along a path 12 through a drying region 13, a plurality of hot gas inlets 14 for directing a flow of hot gas towards the path 12, a plurality of radiant heating devices 15 spaced along the path 12 for directing radiant heat towards the path 12, and heat control means 16 for controlling the proportion of radiant heat relative to the amount of heated gas directed towards the path 12 so that a higher radiant heat/hot gas ratio is provided at the end 17 of the path 12 than at the beginning 18 of the path 12.

The drying apparatus is housed between an upper casing 19 and a lower casing 20. A hot air supply shown schematically at 10 is connected to a main chamber 21 and this communicates with a number of subsidiary chambers 22 through apertures 23. The apertures 23 are adjustable by the heat control means 16, which in theexample shown in FIG. 1 consists of flap-valves l6 controlled by slidable rods 24 which project through the upper casing 19 to allow adjustment of the valves 16. A subsidiary chamber 22 is provided for each hot air inlet 14 so that the amount of hot air supplied to each inlet 14 can be individually adjusted. The inlets 14 are formed as elongated narrow slits extending across the width of the path 12 as shown in FIG. 2. The slits 14 are formed between adjacent tunnel members 25 which also extend across the entire width of the path 12. A rod-like radiant heating element 15 is mounted in each tunnel member 25 below a semi-cylindrical reflector 26 arranged to direct radiant heat evenly and directly over a relatively large area of the path 12. The reflectors 26 are fixed to the tunnel members 25 by mounting brackets 27. A similar set of rod-like radiant heating elements 15 and reflectors 26 are mounted at spaced positions below the path 12 and arranged to direct radiant heat onto the lower face of material passing along the path 12. Elongated perforated plates 28 (shown more clearly in FIG. 2) are positioned between the sides of the reflectors 26 above the path 12 and the sides of the tunnel members 25. The apertures in the plates 28 form exhaust outlets from the region 13 for the air after drying the material. The exhaust air passes through the plates 28 to a region 31 bounded by the reflectors 26 and the tunnel members 25, and is then drawn along the length of the tunnel members 25 to outlet ducts, at one or both sides of the apparatus, which are connected to suitable pumps 32 for producing sub-atmospheric pressures on the exhaust side. The exhaust air passes from the pumps 32 back to the supply 10 for recirculation, fresh air being mixed with the exhaust air as necessary at the supply 10. The conveying means 11 consist of a number of spaced rollers adapted to support the lower face of a paper web. One roller 11 is positioned between each pair of heaters 15 and immediately below each inlet 14 so as to prevent distortion of the paper web by the air jet.

It will be seen that the rollers 11 contact only the lower surface of the paper web so that the upper coated face remains untouched.

As the air inlet slits 14 are positioned between adjacent radiant heaters 15 and they project closer to the path 12 than the radiant heaters 15, substantially no incoming air passes over the radiant heaters 15 to thereby cool them. Furthermore, the exhaust apertures in the plates 28 are so arranged between the inlets l4 and radiant heaters 15 that substantially all the exhaust air is sucked up from the paper web without passing over the radiant heaters 15.

In this particular example, the radiant heaters are electrically fired units giving peak radiation in the range 3 microns to 5 microns. Radiant heater control means 29 shown in FIG. 3 is provided with control knobs 30 to adjust the temperature of the radiant heaters and thereby the wavelength of radiation emitted. Other types of radiant heater, such as gas fired units, may alternatively be used.

In use of the apparatus, material to be dried is transported along the path 12 through the drying region 13, a combination of radiant heat and heated gas is directed towards the material in the drying region 13 and the proportion of radiant heat relative to the amount of heated gas is adjusted along the path 12 so that a higher radiant heat/hot gas ratio is provided at .the end 17 of the path 12 than at the beginning 18 of the path 12. A paper web is fed into the drying apparatus from the right hand side as seen in FIG. 1 and hot air is fed into the main chamber 21 from the right hand side. The temperature of the air is at least 200 F and its velocity in the range 2,000 18,000 ft/minute on passing through the inlets 14 at the beginning of the path 12. The valves 16 are adjusted to reduce the apertures 23 progressively along the path so that the velocity and temperature of the air passing through the inlets 14 gets lower towards the end 17 of the path 12. In this way the proportion of radiant heat relative to heated air directed onto the paper web is higher towards the end 17 of the path 12. The radiant heaters 15 above the web may be adjusted by suitable setting of the control 29 so that they do not all provide the same amount of radiant heat. Similarly the radiant heaters 15 below the web can be controlled by the control device 29 so that they do not all provide the same amount of heat. The control means 29 may be arranged to allow adjustment of the, heaters 15 so that the drying apparatus may be varied to provide different drying conditions for different types of sheet material. The total amount of heat directed towards the web at the end 17 may be less than the total amount directed towards the web at the beginning 18 but in all cases the proportion of radiant heat to heated air is higher at the end 17.

The reason for this is that when the web enters the drying apparatus with a wet coating on its upper surface, the heated air is particularly effective in the early stages of drying and the proportion of radiant energy needed is less. As the web progresses through the drying region 13, the falling rate drying region is reached where the remaining moisture may be trapped in the fibers of the web. It is in this region that the radiant energy becomes more effective and a higher proportion of radiant heat produces more effective drying in this region. Up to the falling rate region the radiant heat on the top of the web helps by augmenting the heat transfer to the water film and it also causes turbulence in the boundary layer in contact with the web. This causes better mass transfer of water vapor from the web and improves the heat transfer fromthe hot air to the water film. Furthermore, by suitable selection of the radiation wavelength, the radiant energy may penetrate the water film to cause earlier decrease in water viscosity adjacent the web and thereby provide some control over the migration of the water into the web. This effect can be tuned in or out by adjustment of the radiant heaters. The radiant heaters below the web may also affect the capillary forces and thereby the migration of water into the web. This effect may also be tuned in or out by suitable adjustment of the lower radiant heaters. During the falling rate region, most of the heat transfer to the moisture is effected by the radiant heat and as the quantity of vapor to be transported away is much reduced in these later stages of drying, the amount of hot air required is less.

By providing the integrated heating system combining hot air and radiant heaters, higher drying rates are provided at improved power consumption.

Although the example shown in FIG. 1 has only radiant heaters below the web, it is possible to provide an integrated system combining hot air inlets with radiant heaters below the web, similar to that shown above the web, when necessary or desirable for the type of material to be dried.

Although reference has been made above to use of the apparatus for drying paper webs, it will be understood that the apparatus may be used for drying other sheet material.

When drying materials which have a coating which is chemically or heat sensitive, the wavelength of the radiation used may be tuned to a value which is absorbed by water and not by the sensitive coating. In this way, the moisture may be evaporated while maintaining the coating at a lower temperature during the drying process. Conversely, the wavelength may be tuned so that the coating is heated to a higher temperature than the water.

Although reference has been made to treating coated paper, it will be appreciated that the drying process may be used on uncoated paper. The above-mentioned range of 3 microns to 5 microns for the wavelength of radiation has been found the most effective in drying aqueous based coatings. However radiation outside this range may be used for drying other material. For example, wavelengths in the range 3 microns to 8 microns may be used and microwave radiation may also be used instead of infra red radiation.

In some instances it may be desirable to increase the power of radiation at a particular part of the drying cycle without changing the wavelength. Since power and wavelength are interdependant, this effect can be achieved by having more than one rod element in a radiation source and switching on the required number of elements as desired.

The radiant heat/heated gas ratio may be adjusted at any part of the drying cycle by providing a plurality of radiant elements in each radiation source and selective switching means for controlling the number of elements which are operative. This can be done by providing a number of rod like elements 15 side by side in front of each reflector with each element extending across the path 12. Each element may be connected separately to the control 29 for selective operation.

It has already been described with reference to the above example, how the heating characteristics can be adjusted along the length of the drying apparatus to achieve desired drying rates and effects. Similarly it is possible to adjust the heating characteristics across the web being dried. That is transverse to the web 12. The

chambers 22 and heaters may be subdivided in a direction across the path 12 so that the subdivided parts may be separately controlled to achieve the required effect on the moisture across the web.

Although the reflectors 26 are shown in the drawings as being semi-cylindrical, they may alternatively be formed with a parabolic shape in cross section.

I claim:

l. A method of drying sheet material comprising transporting the material to be dried along a path through a drying region, directing a combination of radiant heat and heated gas towards the material in the drying region and adjusting the proportion of radiant heat relative to the amount of heated gas along the path through the drying region so that a higher radiant heat/heated gas ratio is provided at the end of the path than at the beginning of the path.

2. A method of drying sheet material according to claim 1 in which the amount of heated gas directed towards the material is reduced towards the end of the path.

3. A method of drying sheet material according to claim 1 in which a combination of radiant heat and heated gas is directed towards the material from only one side of the path and radiant heat is directed towards the other side of the material.

4. A method according to claim 1 in which the heated gas comprises air heated to a temperature of at least 200 F and the air is directed at high velocity towards the material through a number of inlet openings spaced apart along the path.

5. A method according to claim 1 in which the radiant heat comprises infra red radiation in the range of 3 microns to 5 microns.

6. A method of drying a web of sheet material having a coating on one side which comprises the steps of:

a. passing the web through an elongate drying zone at a speed such that a predetermined time is consumed by the passage of any prescribed point on the web from the entrance to the exit of said zone and adjusting the prescribed time such that as the web passes through the drying zone the drying rate initially increases rapidly followed by a period of drying when the rate of evaporation is substantially constant and finally reaches a falling rate region of drying during which the evaporation rate decreases continuously before reaching the exit of the drying zone;

. directing heated gas against the coated side of said web at a plurality of transverse regions spaced between the entrance and exit of said drying zone while simultaneously withdrawing gas from the drying zone at either side of each such region;

c. directing radiant heat energy against said coated side of the web at a plurality of second transverse regions spaced between the first transverse regions whereby the coated side of the web is subjected throughout said drying zone to a combination of radiant heat drying and heated gas drying;

. adjusting the heated gas drying and radiant heat drying adjacent said entrance of the drying zone to provide a selected ratio of radiant heat drying to heated gas dryin ;and l progressively refiucmg the quantity of heated gas directed against the coated side of the web in the first transverse regions of step (b) from the entrance to the exit end of said drying zone while maintaining the radiant heat energy directed against the coated side of the web in the second transverse regions of step (c) such that the ratio of radiant heat drying to heated gas drying is greater within said falling rate region adjacent said exit of the drying zone than said selected ratio.

7. A method as defined in claim 6 including the step of directing radiant heat energy against the uncoated side of said web at least within said falling rate region of drying to attenuate migration of moisture from the underside of the coating into said web.

8. Drying apparatus comprising conveying means for transporting material to be dried along a path through a drying region, a plurality of hot gas inlets spaced along the path for directing a flow of hot gas towards the path, a plurality of radiant heating devices spaced along the path for directing radiant heat towards the path, and heat control means for controlling the proportion of radiant heat relative to the amount of heated gas directed towards the path so that a higher radiant heat/heated gas ratio is provided at the end of the path than at the beginning of the path.

9. Drying apparatus according to claim 8 in which a combination of radiant heating devices and hot gas inlets is provided along one side of the path and a plurality of radiant heating devices alone are provided along the other side of the path.

10. Drying apparatus according to claim 8 in which some of the sources of radiant heat comprise a plurality of separately operable elements which may be selectively operated to vary the amount of radiant heat.

11. Drying apparatus according to claim 8 in which the hot gas inlets are spaced from the radiant heating devices so that substantially all the incoming hot gas may reach the material to be dried without flowing over the radiant heating devices.

12. Drying apparatus according to claim 11 in which a plurality of gas outlets are provided between the gas inlets and the radiant heating devices so that substantially all the gas, after flowing over the material to be dried, may pass to an exhaust outlet without flowing over the radiant heating devices.

Claims

1. A method of drying sheet material comprising transporting the material to be dried along a path through a drying region, directing a combination of radiant heat and heated gas towards the material in the drying region and adjusting the proportion of radiant heat relative to the amount of heated gas along the path through the drying region so that a higher radiant heat/heated gas ratio is provided at the end of the path than at the beginning of the path.

4. A method according to claim 1 in which the heated gas comprises air heated to a temperature of at least 200* F and the air is directed at high velocity towards the material through a number of inlet openings spaced apart along the path.

6. A method of drying a web of sheet material having a coating on one side whicH comprises the steps of: a. passing the web through an elongate drying zone at a speed such that a predetermined time is consumed by the passage of any prescribed point on the web from the entrance to the exit of said zone and adjusting the prescribed time such that as the web passes through the drying zone the drying rate initially increases rapidly followed by a period of drying when the rate of evaporation is substantially constant and finally reaches a falling rate region of drying during which the evaporation rate decreases continuously before reaching the exit of the drying zone; b. directing heated gas against the coated side of said web at a plurality of transverse regions spaced between the entrance and exit of said drying zone while simultaneously withdrawing gas from the drying zone at either side of each such region; c. directing radiant heat energy against said coated side of the web at a plurality of second transverse regions spaced between the first transverse regions whereby the coated side of the web is subjected throughout said drying zone to a combination of radiant heat drying and heated gas drying; d. adjusting the heated gas drying and radiant heat drying adjacent said entrance of the drying zone to provide a selected ratio of radiant heat drying to heated gas drying; and e. progressively reducing the quantity of heated gas directed against the coated side of the web in the first transverse regions of step (b) from the entrance to the exit end of said drying zone while maintaining the radiant heat energy directed against the coated side of the web in the second transverse regions of step (c) such that the ratio of radiant heat drying to heated gas drying is greater within said falling rate region adjacent said exit of the drying zone than said selected ratio.