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The pad filter (often called a cassette) is a special case of the range of panel filters, all of which are made in standard sizes, to fit air-conditioning installations. The pad, as its name implies, is a thick flat sheet of fibrous filter medium, made either by wet-laying (as in paper) or by dry-laying (as in a felt). Felt pads are the most common, and they can be either as-laid, which would normally be the case for natural fibres such as cotton or wool, or needle-punched, for synthetic fibres.

These pads of filter medium are then placed in a containing frame to give them the necessary rigidity, and usually covered with a grille on both large faces (uncontained pads are also used, for example in aquaria). The covering grilles, made of wire mesh, expanded metal sheet or perforated sheet, give the necessary integrity to the finished element, especially in retaining the loose fibres at the pad surface, which would otherwise escape into the cleaned air flow.

The finished filter pad element is fitted singly, or in multiple arrays, into appropriately sized spaces in the dividing wall of a residential building or clean room. It is held in its space by a retaining lip around the edge of the space, on the downstream side of the element, or by clips or bolts if a more secure fixing is required. Ease of assembly, or of disassembly for cleaning or disposal, is obviously an important part of the system design.

A filter pad, which acts by depth filtration, will not easily be cleaned once it has accepted its full load of separated solid. It is possible that water washing, or the use of some other cleaning liquid, may extend its life, but it is normal to consider the
filter pad as a disposable item.

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