A radial U-flow adsorption unit for purifying air by removal of at least one contaminant selected from the group consisting of water, carbon dioxide, hydrocarbons and NO x, said unit comprising: an outer tubular side wall having a longitudinal axis and comprising a top end and a bottom end opposite said top end, said top and bottom ends being closed by top and bottom end walls (14, 16) respectively, said outer tubular side wall having an inner diameter, A; an elongated annular bed comprising at least one selectively adsorbent material (30, 32), said annular bed being located co-axially within said outer tubular side wall thereby defining a first annular space between said side wall and said annular bed, said annular bed defining a central channel in fluid communication with said first annular space through said annular bed; at least one baffle (50, 58, 60, 64, 66, 68, 76) for reducing gas flow maldistribution in said first annular space; an impermeable annular skirt supporting said annular bed on said bottom end wall, said annular skirt being located co-axially within said outer tubular side wall thereby defining a second annular space between said side wall and said annular skirt, said second annular space being in fluid communication with said first annular space, said annular skirt having an inner diameter, C; at least one gas inlet located in said outer tubular side wall at said bottom end and in fluid communication with said first annular space through said second annular space; and a gas outlet located in said bottom wall of said unit and in fluid communication with said central channel, wherein the ratio of the inner diameter of the outer tubular side wall, A, to the inner diameter of the annular skirt, C, is from 3:1 to 11:10. an outer tubular side wall having a longitudinal axis and comprising a top end and a bottom end opposite said top end, said top and bottom ends being closed by top and bottom end walls (14, 16) respectively, said outer tubular side wall having an inner diameter, A; an elongated annular bed comprising at least one selectively adsorbent material (30, 32), said annular bed being located co-axially within said outer tubular side wall thereby defining a first annular space between said side wall and said annular bed, said annular bed defining a central channel in fluid communication with said first annular space through said annular bed; at least one baffle (50, 58, 60, 64, 66, 68, 76) for reducing gas flow maldistribution in said first annular space; an impermeable annular skirt supporting said annular bed on said bottom end wall, said annular skirt being located co-axially within said outer tubular side wall thereby defining a second annular space between said side wall and said annular skirt, said second annular space being in fluid communication with said first annular space, said annular skirt having an inner diameter, C; at least one gas inlet located in said outer tubular side wall at said bottom end and in fluid communication with said first annular space through said second annular space; and a gas outlet located in said bottom wall of said unit and in fluid communication with said central channel, wherein the ratio of the inner diameter of the outer tubular side wall, A, to the inner diameter of the annular skirt, C, is from 3:1 to 11:10.