Abstract:
The invention relates to a metal sheet ( 15 ) ventilation/smoke exhaust duct section ( 10 ) for incorporation into a metal sheet ventilation/smoke exhaust duct, said metal sheets ( 15 ) being covered on the outside by a heat insulating material ( 20 ) and said duct section ( 10 ) including elongated stiffening bar members ( 30 ) located on the outside of the duct and attached to said metal sheets ( 15 ), the invention being characterized in said bar members ( 30 ) being located at a distance from said metal sheets ( 15 ), said bar members ( 30 ) being attached to said metal sheets ( 15 ) at discrete locations along the length of the bar members ( 30 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a metal sheet ventilation/smoke exhaust duct section being a part of a metal sheet ventilation/smoke exhaust duct, where the metal sheets are covered on the outside by a heat insulating material and where the duct section includes elongated stiffening bar members located on the outside of the duct, attached to the metal sheets. 
     Fire regulations require that a tight seal be established in the area where the section of the ventilation/smoke exhaust duct passes through a fire rated wall such that flames and/or toxic gases largely will not pass from one building area to the adjoining building area in case of fire in one of the areas. DIN 4102 Part 4 requires the provision of vertical internal stiffening pipes and, as the case may be, a round-going stiffening frame made up from an L-shaped profile with one leg lying flatly against the metal sheet outer surface. The mounting of the aforementioned vertical pipes inside the duct is time-consuming and also restricts the free flow of air through the ventilation/smoke exhaust duct in normal operation. Moreover, it has been found that the aforementioned stiffening L-shaped frame often applied may in fact in certain cases bring about a further loss of seal between the duct section and the wall. 
     SUMMARY OF THE INVENTION 
     By the invention it has become possible to avoid the use of any of the aforementioned vertical pipes, or alternatively to reduce the number of any pipes applied or the dimensions of any pipes applied, without compromising the fire requirements. In relation to ventilation ducts the invention may, by way of example, find particular use in connection with fire rated walls to maintain the structural stability of the duct where the duct penetrates the fire rated wall; in smoke exhaust ducts the invention may find general use for keeping the structural stability along the length of the duct. 
     According to a preferred embodiment of the invention, the bar members are connected to form a frame extending around the ventilation/smoke exhaust duct metal sheets, and the heat insulating material is arranged between the frame and the ventilation/smoke exhaust duct metal sheets, preferably to act as a spacer ensuring the proper spacing between the frame and the metal sheets such that heat transfer between the bar members and the duct is restricted and such that temperature-induced deformations of the bar members are limited, whereby deformations of the metal sheets can be restricted or limited. Preferably U-shaped or T-shaped bars are used. 
     For attaching the bars to the ventilation/smoke exhaust duct section screws are preferably used, the screws being arranged with a mutual spacing along the length of the bar members of preferably between 200 mm and 700 mm. Hence, for each meter length of the bar members as few as 2-4 screws may be used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention will now be described in details by reference to the drawings. 
         FIG. 1  shows a vertical cross-sectional view through a wall with a through-going ventilation/smoke exhaust duct section, formed in accordance with DIN 4102 Part 4, 
         FIGS. 2-4  represent partial cross-sectional views similar to that of  FIG. 1 , and showing various embodiments of the invention, and 
         FIG. 5  is a cross-sectional view, taken as shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a vertical cross-sectional view through a wall  1  separating two areas of a building and having a through-going opening  5  for a heat-insulated metal ventilation duct section  10  which is part of a building ventilation duct. By way of example only, the area to the left of the wall may contain the production facilities of a factory while the area to the right may be for the administrative facilities of the factory. 
     The metal ventilation duct section  10  has a square or rectangular cross-section and is formed from thin metal sheets  15  that delimit the duct proper, defining the vertical and horizontal sides of the duct. The two horizontal sides  16  of the duct section  10  are shown in  FIG. 1 . Heat insulating elements  20 , such as mineral fiber slabs or plates, are mounted around the ventilation duct as is conventional. The ventilation duct is typically suspended by hangers (not shown) connected to the building ceiling, and is also supported by the wall  1  at the through-going opening  5 . 
     Fire regulations require that a tight seal be established in the area  5  where the duct section  10  passes through the wall  1  such that flames and/or toxic gases largely will not pass from one area to the adjoining area in case of fire in one of the areas. Often this seal is established by arranging mineral wool heat insulating packers  25 ,  25 ′ around the duct section  10  on both sides of the wall  1  and around the duct section  10  in the opening  5  proper. 
     In the case of fire, hot gases could flow inside the ventilation duct, and the duct metal sheets  15  after some time assume temperatures that give rise to metal sheet  15  deformations whereby the seal between the duct section  10  and the wall  1  becomes ineffective. The deformations typically show themselves in that the upper and lower horizontal sheets  15  flex inwardly, or sag, such that the inside vertical clearance of the duct section  10  is reduced along the centreline of the duct section  10 . Hence, the aforementioned packers  25 ,  25 ′ may be rendered ineffective along the horizontal edges of the through-going opening  5  of the wall  1 . 
     To safeguard the seal it has been proposed to arrange within the duct section  10  vertical support pipes that effectively reduce the deformations of the duct section in the area of the seal by spanning the upper and lower horizontal metal sheets  15 , the pipes being subjected to deformation generated axial loads in the event of fire. The provision of such vertical pipes and, as the case may be, a round-going stiffening frame made up from bars having an L-shaped profile  30  with one leg lying flatly against the outer surface of the duct section, is required by DIN 4102 Part 4/FIG. 84. A duct installation formed in accordance with DIN 4102 Part 4/FIG. 84 is shown in  FIG. 1 . 
     The mounting of the aforementioned vertical pipes inside the duct is time-consuming and also restricts the free flow of air through the ventilation duct in normal operation. Moreover, the applicant has found that the aforementioned stiffening L-shaped frame often applied may in fact in certain cases bring about a further loss of seal between the duct section  10  and the wall  1 . 
     By the invention, to be discussed further below, it has become possible to avoid the use of any of the aforementioned vertical pipes, or alternatively to reduce the number of any pipes applied or the dimensions of any pipes applied. 
     One embodiment of the invention is shown in  FIG. 2  which represents a partial vertical cross-sectional view similar to  FIG. 1 . In  FIG. 2 , a round-going rigid metal frame is positioned around the perimeter of a square duct section  10 , on a respective side of the wall  1 . The frame is formed from vertical and horizontal straight metal bars  30  having a U-section with the bottom of the U being located farthest from the metal sheets  15 , although a T-section or an L-section, with the larger area part, such as the top of the T being located farthest from the metal sheets  15 , could also be used. The bars  30  are connected at the ends (not shown), thus forming at the joints the corners of the round-going rigid frame. Alternatively, when the duct section  10  has a circular or similarly shaped cross-section the round-going frame may be formed from a straight band of similar cross-section and with the two ends thereof being joined. It is noted that only a single frame may be used. In smoke exhaust ducts the bars  30  may be arranged with a mutual spacing along the length of the duct of e.g. 300-600 mm. 
     The round-going frame shown in  FIG. 2  has the bars  30  positioned at a distance from the outer surface of the duct section  10  metal sheet  15  such that any direct heat transmission between the bars and the metal sheets  15  of the duct section  10  is largely prevented. Preferably, on mounting the frame the worker makes sure that after proper installation there is no contact between the bars  30  and the duct section  10  metal sheets  15  at any point around the perimeter of the duct section  10 . Normally this will follow readily from the fact that an insulating layer  20  of a given thickness covers the duct section  10  on the outside, the frame being mounted on the outer surface of the insulating layer  20 . On mounting bars  30  having a U-formed cross-section, as shown in  FIG. 2 , the worker would preferably cut two parallel round-going grooves  22  into the insulating layer  20 , the grooves  22  having a depth corresponding to ego half the thickness of the insulating layer  20  or to the height of the profile flanges, such that the profile  30  bottom bears against the outer surface of the insulating layer  20 . The worker then inserts the bars  30  into the grooves  22 , and connects the bars  30  at the ends, thus forming a rigid frame. After attaching the bars  30  to the duct section  10  as explained below he then applies any sealing packers  25 ,  25 ′ as required. 
       FIGS. 3 and 4  show partial cross-sectional views similar to  FIG. 2 , two alternative, preferred ways of attaching the frame to the duct section  10  thus being shown by way of example. The embodiments of  FIGS. 2-4  involve the use of connecting screws  40  for providing the attachment. 
     As mentioned above, any direct heat transmission between the metal sheets  15  of the duct section  10  and the frame bars  30  should be avoided, or at least be reduced to a great extend. In this manner, temperature induced deformations of the bars  30  are reduced or delayed. Hence, generally the bars  30  forming the frame will be located at a distance from the metal sheets  15  forming the duct section  10  sides  16 , the connection between the frame bars  30  and the duct section  10  preferably being provided in discrete areas or points by separate connecting means  40 , such as screws, preferably self-cutting screws that preferably may be applied by the worker after the duct section  10  has been arranged in its final position extending through the wall  1 , before or after the duct section  10  has been connected to the rest of the ventilation duct. The metal bars  30  may be provided with pre-drilled holes receiving the screws  40 , the distance between the pre-drilled holes corresponding to the required number of screws per unit length of the bars  30 , such that the worker does not apply an excessive number of screws  40 . On applying the screws  40  the worker may take advantage of pre-drilled holes in the insulating material  20 . However, in a preferred embodiment of the invention the worker would simply screw the screws  40  through the insulating material  20  and into the metal sheet  15  until the screw head engages the bar  30 , to establish a reliable connection while at the same time maintaining the required spacing between the metal bars  30  and the metal sheets  15  of the ventilation duct section  10 . 
     Alternatively, bolts pre-mounted to the duct section  10  metal sheets  15  and arranged to extend through holes in the bars  30  and secured to the bars  30  by nuts may be used. The screws or bolts  40  may be made from a material having smaller heat conductivity, compared to that of the bars  30 . 
     Tests have shown that using ordinary 4-5 mm steel screws  40  applied at a number of two equidistant screws  40  per meter length of the bars in the case of 1000 mm by 250 mm or 1000 mm by 500 mm ducts will suffice to reliably connect the bars  30  to the duct section  10  while at the same time forming a structural reinforcement or stiffening of the duct section  10 . The steel screws  40 , i.e. the connecting means, transfer forces between the metal sheets  15  and the metal bars  30 , reducing the aforementioned sagging of the metal sheets  15  in case of fire. The limiting of the heat transfer between the duct section  10  metal sheets  15  and the bars  30  of the frame and, hence, the temperature increase of the bars  30  that would otherwise result from the high temperature gases flowing within the ventilation duct reduces temperature induced deformations of the bars  30  proper such that the reinforcement or stiffening provided by the bar  30  frame remains effective for a prolonged period of time, effectively obviating any of the cumbersome pipes required by DIN 4102 Part 4. 
     To maintain the desired spacing between the frame bars  30  and the duct section  10  metal sheets  15  during use and in case of fire, double threaded screws  40  of the type shown in  FIG. 3  may be used, or tubular spacers  50  resting on the duct section  10  metal sheets  15  and being of any desired material may be located in the area of screws  40  of conventional type, as shown in  FIG. 4 . Alternatively, a mineral wool material of a certain minimum density, such as about 150 kg/m 3 , may be selected for the insulation layer  20 . By selecting the density in accordance with the torque to be applied by the worker when inserting screws  40  into the duct section  10  metal sheets  15  the spacing between the frame bars  30  and the duct section  10  metal sheets  15  is determined i.a. by the insulating material  20  since the compression of the mineral fiber insulating layer  20  beneath the bars  30  arising from the tightening of the screws  40  is generally reduced with increasing density. 
     The frame bars  30  may alternatively be connected to the duct section  10  by small size legs (not shown) that are integral with the bars  30  and that extend out from the bars, resting on the duct section  10  metal sheets  15  like the tubular spacers  50  of  FIG. 4  and being connected to the duct section  10  metal sheets  15 , such as by soldering or welding obviating the use of screws  40 , the attachment being such that axial forces, normally tensile forces in case of the attachments at the upper horizontal side  16  metal sheet  15 , can be transferred from the duct section  10  to the bars  30 . Direct heat transfer through the legs between the frame bars  30  and the duct section metal sheets  15  should be kept at a minimum. 
     In an alternative embodiment metal bars  30  as shown and explained herein may be located on the top and bottom side  16  metal sheets  15  of the ventilation duct section  10  only, i.e. dispensing from the use of a metal bar frame. In case of a square or rectangular cross-section ventilation duct section  10  the elongated metal bars  30  would then preferably extend across at least 90% of the horizontal width of the ventilation duct section  10 . 
       FIG. 5  is a view taken as shown in  FIG. 2  and illustrating the duct  10  with metal screws  40  arranged such that the metal bars  30  are attached to the metal sheets  15  at discrete locations spaced apart by distance S. 
     EXAMPLE 
     A rectangular ventilation duct section  10  as shown in  FIG. 2  was made using 0.7 mm galvanized steel sheets  15 , the duct section  10  having dimensions of 1000 mm by 250 mm and the U-shaped metal bars  30  forming the peripheral frame arranged perpendicularly to the axial extension A of the duct section  10  having a flange height of 25 mm and a moment of inertia of 1.22 cm 4 . A mineral wool fire insulation plate  20  having a thickness of 60 mm was mounted around the duct section  10  against the metal sheets  15 , and metal screws  40  arranged with a spacing of 33 cm were used to attach the metal bar  30  frame to the ventilation duct section  10 . Parallel peripheral grooves  22  of a depth of about 25 mm were cut into the insulation plate  20  such that the innermost part of the metal bars  30  were kept at a spacing of 35 mm from the outer surface of the metal sheets  15 . In fire tests the duct section  10  proved efficient in maintaining for a period of 120 minutes a seal as mentioned above in the area of the wall  1  opening  5 , by the metal bars  30  resisting sagging of the metal sheets  15  through the connecting means  40 , i.e. the screws, transferring forces between the metal bars  30  and the metal sheets  15 .