Heat protection hood
BACKGROUND OF THE INVENTION 
     The invention relates to a heat protection hood for heated rolls, in particular rapidly running rolls in paper machines. 
     For increasing the smoothness of paper it is often necessary to use heated rolls in the smoothing or glazing mechanism. The surface temperatures of over 200° C. which are sometimes necessary result in correspondingly high heat losses through convection and radiation. 
     It is possible to substantially reduce the radiation losses through the use of known protective hoods with thermal isolation. However, the convection which then predominates as a result of the rolls being made to rotate ever more rapidly cannot be effectively countered. Environmental air is sucked in at one side of the protective hood is drawn along by the roll and is blown out again at the other side in the heated state. 
     Allround sealing also does not lead to any significant improvement as a result of the surface wear and also the pressure conditions which thereby build up. In addition to this comes the fact that the heat loss promoted by the pressure differences over the end faces of the roll lead to a non-uniform temperature profile along the length of the roll. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the object of providing a heat protection hood for rapidly running heated rolls which substantially reduces the heat losses through convection and radiation and additionally offers the possibility of influencing the axial temperature profile of the roll. 
     In accordance with the invention this object is satisfied by a heat protection hood for heated rolls, in particular fast running rolls in paper machines which are partly surrounded by the hood, characterized in that an outer screen is arranged partially surrounding the roll and extends concentrically to it, with the outer screen forming, together with a likewise concentrically extending partition screen disposed between it and the roll, an inner gap and an outer gap. The deflection zones for the air drawn along by the roll in the inner gap adjoins the ends of the partition screen, with inbuilt deflector units attached there directing the air flow from the one gap into the respective other gap. 
     The resulting forced circulation of the air which is drawn along by the roll within the heat protection hood, which is naturally thermally insulated, substantially reduces the losses through radiation and convection. 
     Nevertheless, the pressure difference which thereby arises between the deflection regions ensures that at these regions a small amount of air is blown out or sucked in. To take account of this the outer gap should be broader than the inner gap (radially broader) and should in each case increase towards the ends of the gaps in order to reduce the speed in the deflection regions. Full pressure compensation can for example be achieved if a part flow is branched off from the outer gap, is accelerated via a fan and is supplied again to the outer gap. 
     It is furthermore of advantage to subdivide the roll into a plurality of axial zones via at least one partition web which extends in the radial direction close up to the roll surface and which also increases the static strength of the heat protection hood. The zonewise control of the temperature and thus also of the thickness profile of the roll can be made possible in this arrangement by providing a branch duct zonewise in the course of the outer gap. This branch duct supplies a part of the air flow via fans, via a controllable heating unit and also via an injector which opens into the outer gap. It is also of advantage if control flaps for cooling are present zonewise in the deflection region at the paper outlet side, with these control flaps permitting the admission of environmental air into the inner gap and simultaneously permitting the discharge of hot air to the environment out of the outer gap. 
     In addition, a holder for a scraper, which may eventually be necessary, can be integrated into the heat protection hood. It is also of benefit, above all for heat protection hoods which surround more than half the roll, to make the heat protection hood in two parts. The partition joint between the two halves should then preferably be located in the region of the opening of the injector, since the narrowing of the cross-section which is necessary for the provision of joint sealing surfaces lying transverse to the flow has little effect at this position. 
    
    
     The invention will be explained in further detail in the following with reference to an embodiment. In the accompanying drawing the figure shows a cross-section through the heat protection hood. 
    
    
     A heated roll 1 and a counter roll 3 jointly form the smoothing or glazing mechanism for the paper web 2. Approximately two thirds of the roll I are surrounded by an outer screen or shield 7 which extends concentrically to it. The outer screen or shield 7 together with a partition screen 4, which is present between it and the roll 1 and which likewise extends concentrically to the roll 1, jointly form an inner gap 5 and an outer gap 6. The outer gap 6 is thereby approximately twice as wide as the inner gap 5 with the widths of the gaps 5 and 6 respectively increasing towards their ends, i.e. their ends adjacent the circumferential ends of the partition screen lying parallel to the axis of the roll. These ends of the partition screen 4 are adjoined by respective deflection regions 9 for the air drawn along by the roll 1 in the inner gap 5. The inbuilt deflecting units 10 and 11 which are mounted there have rounded contours and direct the air flow into the respective other one of the gaps 5 and 6. That is to say the air moving out of the inner gap 5 at the left hand side of the drawing is turned so that it can move back along the outer gap 6, while air moving through the outer gap 6 at the right hand side of the drawing is turned so that it can move in the direction of the arrow shown there along the inner gap 5 under the influence of the roll 1 which rotates in the same direction as that arrow, i.e. in the opposite direction to the arrow shown on the roll 3. 
     In order to influence the temperature profile the roll i is subdivided into several axial zones by several separating webs 21 which extend in the radial direction close up to the roll 1. The webs 21 extend radially across both the outer gap 6 and the inner gap 5. In the zones formed between adjacent webs, or between the web or webs and side plates at the axial ends of the rolls, there is in each case provided a branch 12 and a branch duct 13 in the course of the outer gap 6. Each branch duct 13 supplies a part of the air stream via deflection blades 14 and a fan means 15 to a controllable heating unit 16 and also to an injector 17 which again opens into the outer gap 6 of the respective zone. Furthermore, a respective rotatable control flap 22 is provided in each zone in the deflection zone 9 at the paper outlet side. The control flaps 22 make it possible to suck in colder air and also to blow out warm air. The outer shield 7 and also the branch duct or ducts 13 are surrounded by a thermally insulating layer 8. 
     In the vicinity of the control flap 22 there is provided a scraper 23. The scraper carrier 24 has the form of a rail and forms a unit with the heat protection hood. The partition joint 18 of the thermal protection hood is located in the region of the opening of the injector 17. This permits the two parts to be moved apart in the radial directions 25 by means of non-illustrated guide rails and hydraulic cylinders for the purpose of repair and cleaning. Wedge-like built in units 19 present at the partition joint 18 form sealing surfaces for the resilient sealing elements 20 which lie transverse to the flow. 
     The reference numeral 30 schematically represents a microprocessor controlled regulating system which is connected by respective lines 32, 34 and 36 to the fan means 15, the heating means 16 and the controllable flap means 22 for each zone of the roll 1. 
     The reference numerals 38, 40 and 42 indicate lines which supply temperature signals to the microprocessor 30, e.g. for the surface temperature of the roll in each zone and/or for the air temperature in the inner and/or outer gaps 5, 6. The corresponding temperature signals can be taken into account by the microprocessor in determining the control signals for the fan means 15, the heating means 16 and the controllable flap means 32. Further inputs can also be provided, such as 44, for other relevant parameters, such as the speed of rotation of the roll, and indeed this speed of rotation can also be controlled by the microprocessor. It is particularly preferred when a microprocessor present for controlling the operation of the smoothing mechanism is also used for the control purposes discussed above. 
     The reference numerals used in the claims are purely intended by way of explanation and are not intended to restrict the scope of the claims in any way.