Richards of Hull Ventilating a Small Darkroom

In an old or architecturally sensitive building where it would not be desirable to have a cowl on view, it might be possible to employ a decorative iron or teracotta grille and a 4/5" hole in the wall. Internally, a couple of bends should be enough to block light transmission, with an inline centrifugal fan - a bathroom fan would not be adequate. Equally important is the point of extract; rather than having an elephant's trunk duct dangling over the dishes a box canopy could be constructed in contiplas or mdf.

A canopy such as this not only ensures that air is drawn to the back, away from your face, but provides a fixing point for the duct, which may be convoluted flexible or (better) smooth bore 4" rigid PVC tube. ( be aware that some grades of flexible duct are translucent and allow light transmission ) Painted black inside it forms a final light trap.

Dimensions The canopy box could anything from 12" to 3ft long and the slot at the back between 1/2" and 1" wide (not less than 12 sq ins) - back-to-front anything up from 9" minimum.
The void above the baffle forms an essential plenum chamber to even out the flow along the slot. It should be a minimum of 4" but the deeper the better the longer the slot. Similarly, the skirt surround could be anything from 3" to 6" and forms a fume collection zone.

Components required: Square to Round Connector - ref 56 08 56 Powerflow Inline Centrifugal Fan - ref ACP100-12B The moulded flanged square-to-round transformer is only available for 4" (100mm) duct - to use 6" duct other components need to be specified

Midrange of its performance curve the Powerflow 100 extracts about 140 cubic metres per hour; this is sufficient to provide just more than 10 air changes per hour in a room 8ft x 6ft x 8ft, and which meets the current "best practice" guidelines. This performance will be less if subject to a convoluted run of duct but, if the extraction is localised to where it's at, will probably still be more than adequate.