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A blast wave damper comprises a section of duct, with a multiplicity of rigid elements each extending across the duct. All the elements are parallel to each other, and they are arranged in an array consisting of a multiplicity of lines, each such line extending across the duct. The elements in one l

A blast wave damper comprises a section of duct, with a multiplicity of rigid elements each extending across the duct. All the elements are parallel to each other, and they are arranged in an array consisting of a multiplicity of lines, each such line extending across the duct. The elements in one line are staggered relative to the elements in an adjacent line, and the gaps between successive elements within a line are no wider than the widths of the elements. There may be ten columns of tubular elements in a regular array. This may be combined with a louver mechanism arranged to shut if the pressure drop exceeds a threshold.

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1. A blast protection damper for suppressing a blast wave in an air flow duct carrying a flow of air for cooling or ventilation, where the blast wave is a pressure wave which is such as to cause injury to people, the damper consisting only of a section of duct adapted to form part of the air flow du

1. A blast protection damper for suppressing a blast wave in an air flow duct carrying a flow of air for cooling or ventilation, where the blast wave is a pressure wave which is such as to cause injury to people, the damper consisting only of a section of duct adapted to form part of the air flow duct, the section of duct having opposed walls, and a multiplicity of substantially rigid elements of steel, titanium, or titanium alloy, each extending across the section of duct and being fixed to the opposed walls at each end, the elements being of cylindrical shape and so providing only cylindrical surfaces to scatter blast-waves, the elements not comprising sound absorbing or attenuating material, and not being arranged for through-flow of any fluid, the elements all extending parallel to each other and being arranged in an array consisting of a multiplicity of lines of said elements, each such line extending across the section of duct, the elements in each such line being staggered relative to the elements in each adjacent line, and each gap between successive elements within each such line being no wider than the width of each element, such that, in the event of a blast wave in the section of duct, the damper restricts the overpressure downstream of the damper. 2. A damper as claimed in claim 1, wherein the lines are straight lines, and within each line the elements are equally spaced. 3. A damper as claimed in claim 1, wherein there are between eight and fifteen lines of elements in the array. 4. A damper as claimed in claim 3, wherein there are ten lines of elements in the array. 5. A damper as claimed in claim 1, wherein the elements within each such line occupy at least 50% of the projected area as viewed in a direction parallel to the flow direction along the duct. 6. A damper as claimed in claim 1, wherein the elements are of diameter about 60 mm, and in each line are arranged at center-to-center spacing of no more than 120 mm. 7. A blast protection damper comprising a damper as claimed in claim 1, wherein the section of duct also comprises a support plate at an intermediate position across the section of duct, so the elements extend through the support plate, the support plate supporting the rigid elements at an intermediate position along their length. 8. A blast protection structure for suppressing a blast wave in an air flow duct to carry an air flow for cooling or ventilation, where the blast wave is a pressure wave which is such as to cause injury to people, the blast protection structure comprising a damper consisting only of a section of duct adapted to form part of the air flow duct, the section of duct having opposed walls, with a multiplicity of substantially rigid elements of steel, titanium or titanium alloy, each having two ends, and extending across the duct, the ends being fixed to the opposed walls at each end, the elements being of cylindrical shape and so providing only cylindrical surfaces to scatter blast-waves, the elements not comprising sound absorbing or attenuating material, and not being arranged for through-flow of any fluid, the elements all extending parallel to each other and being arranged in an array consisting of a multiplicity of lines of said elements, each such line extending across the section of duct, the elements in each such line being staggered relative to the elements in each adjacent line, and each gap between successive elements within each such line being no wider than the width of each element, such that, in the event of a blast wave in the section of duct, the damper restricts the overpressure downstream of the damper, and the structure also comprising a louver which is arranged to shut if the pressure in the duct exceeds a threshold. 9. A blast protection structure as claimed in claim 8, wherein the louver is downstream of the damper, and comprises an array of louver blades which in normal operation are oriented at an acute angle to the gas flow direction, and are pivotally supported to be able to rotate into a closed position in which the louver blades are oriented across the duct, each louver blade resting against the adjacent louver blade, and the louver blades being connected to a latch, such that if the pressure forces acting on the louver blades exceeds a threshold value, the latch is released so that the blades rotate into the closed position under the effect of the gas pressure. 10. A blast protection damper as claimed in claim 8, wherein the air flow duct also comprises a plenum of a larger cross-sectional area than the section of duct downstream of the louver. 11. A method for providing a flow of air for cooling or ventilation along an air flow duct, where there is a risk that ignition of a flammable mixture may cause propagation of a blast wave along the air flow duct, where the blast wave is a pressure wave which is such as to cause injury to people, which method comprises: (A) installing in the air flow duct a blast protection damper consisting only of a section of the air flow duct, and a multiplicity of substantially rigid elements of steel, titanium or titanium alloy, each extending across the duct section and being fixed at each end, the elements being of cylindrical shape and so providing only cylindrical surfaces to scatter blast-waves, the elements not comprising sound absorbing or attenuating material, and not being arranged for through-flow of any fluid, the elements all extending parallel to each other and being arranged in an array consisting of a multiplicity of lines of said elements, each such line extending across the duct section, the elements in each such line being staggered relative to the elements in each adjacent line, and each gap between successive elements within each such line being no wider than the width of each element; and(B) causing air for cooling or for ventilation to flow along the air flow duct;whereby, in the event of a blast wave in the air flow duct, the damper restricts the overpressure downstream of the damper. 12. A method as claimed in claim 11, wherein the method also comprises: (A) installing a louver in the air flow duct adjacent to the duct section; and(B) arranging the louver to shut if the pressure in the duct exceeds a threshold. 13. A method as claimed in claim 12, wherein the louver is installed downstream of the damper, and comprises an array of louver blades which in normal operation are oriented at an acute angle to the gas flow direction, and are pivotally supported to be able to rotate into a closed position in which the louver blades are oriented across the duct, each louver blade resting against the adjacent louver blade, and the louver blades being connected to a latch, such that if the pressure forces acting on the louver blades exceeds a threshold value, the latch is released so that the blades rotate into the closed position under the effect of the gas pressure. 14. A method as claimed in claim 12, wherein the method also comprises installing in the air flow duct downstream of the louver, a plenum of a larger cross-sectional area than the section of duct. 15. A method as claimed in claim 11, wherein the elements are tubes of diameter about 60 mm, and in each line are arranged at center-to-center spacing of no more than 120 mm. 16. A method as claimed in claim 13, wherein there are ten lines of elements in the array. 17. A method as claimed in claim 13, wherein the elements within each such line occupy at least 50% of the projected area as viewed in a direction parallel to the flow direction along the air flow duct. 18. A method as claimed in claim 11, wherein the elements are tubular.