fig1 a illustrates a typical arrangement of a fryer f with fryer hood fh and duct d extending through the ceiling of a commercial kitchen to the roof r of the restaurant . the gas exhaust g passes from the fryer f through the hood fh and duct d and exhaust fan ef to the atmosphere . a rectangular duct d is illustrated . in fig1 b a rectangular duct liner dl , having seal s , is shown inserted in duct d . duct d is attached to the top portion of roof curb rc . exhaust fan ef is also removably attached to the top of roof curb rc . fig1 c illustrates the duct system with exhaust fan ef removed . duct liner dl of fig1 b hangs within duct d , also being supported by the top portion of the roof curb rc . seal s is compressed between duct d and liner dl at the point of hanging . annular space a between duct liner dl and duct d , shown in fig1 b and 1c is exaggerated in the drawing for purposes of illustration . according to the preferred embodiment , duct liner dl is custom built to closely fit , but not tightly fit , duct d from the cooking hood opening 10 in the cooking hood fh to the exhaust end 20 in the roof curb rc . the process of venting is maximized when the inside diameter of liner dl is as large as possible . fig2 and 3 illustrate a round and rectangular duct liner dl , respectively , each having seal s affixed to the under side of lip 22 at the top , or exhaust end , of the liner . seal s seals between the liner and the top or exhaust end of duct d , permitting no air or gas to escape . thus , all gasses g containing oil and grease vent through fire hood fh into hood duct opening 10 and up the inside of duct liner dl . the gas is not permitted to escape through annular space a . ambient air inhibits the circulation of gasses there . to maintain duct d free of grease , the maintenance operator stands on roof r and first removes exhaust fan ef from over top exhaust opening 20 of duct d . the operator then lifts out custom built duct liner dl and , in the preferred embodiment , inserts a second clean custom built liner . in the preferred embodiment the grease laden duct liner dl is returned to the maintenance operator &# 39 ; s shop to be thoroughly cleaned and stored until the next maintenance operation . alternately , but less preferredly , the grease laden duct liner that was removed could be cleaned at the operator &# 39 ; s truck and installed again in the duct . it is believed that having two custom built duct liners for each establishment , and cleaning all grease laden duct liners at a central facility , is more efficient , cost effective and conducive to health and environment concerns . fig4 illustrates an additional detail of the present invention . liner dl , at its lower end where the duct joins the vent hood duct opening , may contain an inner lip l . inner lip l catches grease running or melting down the inside walls of liner l . fig5 a , 5b and 5c illustrate alternate details of the construction of a liner dl . in fig5 a and 5b liner dl is shown with channel 32 . within channel 32 lies a sealer - wiper 30 . channel 32 is located near the end of the liner which will , when installed , lie adjacent the vent duct opening . sealer - wiper 30 is shown in two alternate embodiments . one function of sealer - wiper 30 is to seal between liner dl and the duct d while the liner is installed . thus , sealer - wiper 30 must be of a heat resistant material sufficient to withstand duct temperatures . a second function of sealer - wiper 30 is to wipe the walls of duct d when the grease laden liner is pulled from the duct out the exhaust end . fig5 c illustrates a second channel 36 that is structured to receive temporary wiper 34 . channel 36 , designed to receive wiper 34 , is located near the end of liner dl adjacent the vent duct opening . wiper 34 may be of felt , cloth , paper or any other highly absorbent material . in operation , absorbable wiper material 34 would be attached to liner dl shortly before pulling a grease laden liner out of duct d . it is anticipated that material 34 would not need to be of high heat resistant material . the foregoing disclosure and description of the invention are illustrative and explanatory thereof . various changes in the size , shape and materials as well as the details of the illustrated construction may be made without departing from the spirit of the invention .