A tunnel washer, also called a continuous batch washer, is an industrial laundry

Laundry

Laundry is a noun that refers to the act of washing clothing and linens, the place where that washing is done, and/or that which needs to be, is being, or has been laundered...

 machine designed specifically to handle heavy loads.

As the name implies, the device consists of a long metal cylinder called a tunnel. A huge rotating metal Archimedes screw runs down the center of the tunnel, separating it into compartments called "pockets" and moving linen from one end of the tunnel to the other. The screw is made of perforated metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

, so laundry can progress through the washer in one direction, while water and washing chemicals move through in the opposite direction. Thus, the linen moves through pockets of progressively cleaner water and fresher chemicals. Soiled linen can be continuously fed into one end of the tunnel while clean linen emerges from the other.

Since their introduction in the late 1960s, tunnel washers have become progressively more reliable. Though extremely expensive, they are now common in major hotels, hospitals and other high-volume laundry departments.

At the start, one of the machine's major drawbacks was the necessity of using one wash formula for all items.

Modern computerized tunnel washers can monitor and adjust the chemical levels in individual pockets, effectively overcoming this problem.