Structured packing is known for low pressure drop and high efficiency, i.e., large dry area per unit volume. It is fabricated by crimping sheet metal (usually perforated to encourage cross-mixing between the vertically aligned sheets and embossed to enhance liquid spreading across the surfaces—see photograph). The crimp height to wavelength ratio is nearly the same for all sizes. Size is generally designated by dry area per cubic meter (m2/m3) so a 250 size packing would be nominally 250 m2/m3. Major suppliers of structured packing include Raschig (Super-Pac®), Sulzer ChemTech (Mellapak™ and MellapakPlus™) and Koch-Glitsch (FLEXIPAC®).