fig1 shows a hoist line 1 of the type used in a wide variety of different chemical processes , including chem - milling , anodizing , metal plating , metal cleaning , and pickling operations . these types of processes typically require several separate stages to accomplish the needed chemical reactions , and a plurality of separate processing tanks 5 are normally employed . although it is possible to move the treated metal from tank to tank by hand , it is normally done using a craneway hoist 8 . one or more hoist support rails 9 ( only one shown ) are provided to create a traveling pathway , with the craneway hoist 8 mounted on a plurality of track wheels 10 to provide easy access to each of the processing tanks 5 . the craneway hoist 8 is suspended from the hoist support rail 9 on a hoist frame 12 . a hoist motor 15 is provided on the hoist frame 12 , and is used to raise and lower the materials to be processed into and out of the various processing tanks 5 utilizing a hoist line 16 attached to a hoist reel 17 ( shown in fig2 ). in the hoist line 1 according to the present invention , a workload enclosure 21 is attached to and suspended from the hoist frame 12 . the workload enclosure 21 creates a fume containment region surrounding the material that is being treated and carried from tank to tank with the craneway hoist 8 . as shown in fig1 the craneway hoist 8 is positioned over one of the processing tanks 5 , with a reciprocating cover assembly 27 shown between the processing tank 5 and the workload enclosure 21 . the cover assembly 27 , as shown more fully in the remaining processing tanks 5 shown in fig1 is a separate part of the present inventive exhaust ventilation system , providing a cover for the processing tanks 5 when the material being treated is not being placed in or withdrawn from the processing tanks 5 . each of the tanks 5 is provided with a processing solution 29 consisting of the various reagents required to obtain the chemical reactions necessary to accomplish the particular treatment . many of the processing solutions 29 are noxious , acidic or caustic materials that generate equally noxious fumes . many of the chemical reactions that occur during the treatment process require the processing solutions 29 to be heated , in turn greatly increasing the amount of fumes that would otherwise be generated . the reciprocating cover assemblies 27 dramatically reduce the amount of surface area of the process solution 29 that is exposed to the surrounding work environment . the conventional exhaust ventilation systems attempt to control the fume problem by brute force , generating an intense air flow over the processing tanks 5 in an effort to capture all of the fumes given off by the tank , entraining those fumes in the air stream for eventual treatment elsewhere . by reducing the effective amount of exposed surface area of the processing solution 29 , the reciprocating cover assembly 27 dramatically reduces the amount of air flow necessary to establish a containing air flow circulation system . whether utilizing a conventional system , or the present inventive embodiment , the entrained fumes are removed from the processing tanks 5 through one or more clateral exhaust hoods 33 located adjacent to the surface of the processing solution 29 . from the exhaust hoods 33 , the air stream passes through an exhaust pipe 35 and into an exhaust collector 39 , the collector 39 receiving the exhaust air from a number of different processing tanks 5 . although not necessary to the practice of the present invention , the embodiment shown in fig1 also provides for collecting exhaust air from within the workload enclosure 21 . an upper exhaust connecting conduit 43 is provided to form an air passageway between the interior portion of the workload 21 and the slotted exhaust duct 47 . air flows from within the workload enclosure 21 , through the connecting conduit 43 and into the slotted exhaust duct 47 . air is discharged from the slotted duct 47 into a main exhaust manifold ( not shown ). when using such an embodiment according to the present invention , an exhaust air stream for containment and control of fumes is generated by air flowing through the lateral exhaust hoods 33 and an exhaust air stream flowing from the workload enclosure 21 through the connecting conduit 43 . additional structural details of the workload enclosure 21 , and of the entire inventive exhaust ventilation system are shown in fig2 with a workload 53 shown attached to the hoist motor 15 , and suspended over the processing solution 29 . as shown by fig2 the cover assembly 27 consists of an outer cover frame 63 that surrounds and forms a central opening 67 . the workload 53 is provided access to the processing solution 29 through the central opening 67 . the outer cover frame 63 is received by and rests upon the processing tank 5 . when the cover assembly 27 does not form an integral unit with the processing tank 5 , as is the case when being retrofitted to an existing hoist line system , an exhaust spacer conduit 71 may be used to connect the lateral exhaust hoods 33 to one or more exhaust openings 73 ( see fig8 ) formed in the cover frame 63 . the materials used to fabricate both the cover assembly 27 and the workload enclosure 21 may include any of various materials able to withstand attack by the processing solutions . such materials as stainless steel , pvc , fiberglass , and the like corrosion - resistant materials are appropriate , however a preferred material is polypropylene thickness varying from 1 / 8 &# 34 ; to 3 / 4 &# 34 ;, as manufactured by dynamit nobel . fig2 illustrates a second operating position of the workload enclosure 21 . where it is necessary to obtain access to the workload 53 , for example to initially load it on the hoist mechanism , or should the workload 53 shift during the treatment process , a movable canopy 74 is installed on a plurality of canopy support posts 77 . when the canopy 74 is in its fully extended position , as is shown in fig1 the workload enclosure 21 fully contains all fumes being generated by the processing tank 5 located below the craneway hoist 8 . when in its fully retracted position , as shown in fig2 access to the workload 53 is provided . a canopy motor 81 is provided to extend and retract the canopy 74 . the motor 81 may conveniently be any type of rotating motor system , including pneumatic and conventional electric motors . a particularly advantageous motor is fractional horse power electric motor of the type manufactured by w . w . granger . as mentioned previously , fumes from the workload enclosure 21 are conveyed into the slotted exhaust duct 47 through the connecting conduit 43 . as shown in fig2 the connecting conduit 43 is provided with a canoe - shaped discharge duct 87 , of a size suitable for entry of the connecting conduit 43 in the slotted duct 47 . the connecting conduit 43 may also be constructed out of polypropylene , fiberglass , or the previously listed corrosion - resistant materials . the extended position of the canopy 74 is perhaps better shown by fig3 . the canopy 74 consists of a plurality of separate window sections 93a , 93b , 93c , and in the extended position , the window sections 93a , 93b , 93c hang from one another forming sealed relationships therebetween . referring momentarily to fig8 each of the window sections 93a , 93b , 93c , consist of a support frame 95 that surrounds and receives a transparent pane 96 . the support frames may conveniently be fabricated our of polypropylene or any of the previously mentioned corrosion resistant materials , and the transparent panes 96 may suitably be acrylic . the upper and lower portions of the support frame 95 form an upper sealing strip 101 and a lower sealing strip 103 that , when the canopy 74 is in its fully extended position , interengage with one another as shown in fig8 forming a sealed interengagement between the window sections 93a , 93b , 93c . a canopy skirt is attached to the window section 93c that is adjacent the cover assembly 27 when the canopy 74 is in its fully extended position . the canopy skirt 97 is preferably constructed of a resilient material , and forms a temporary sealing interengagement between the canopy 74 and the top surface 98 of the outer cover frame 63 . neoprene is a preferred resilient material for the canopy skirt 97 . fig4 and 5 illustrate the drive mechanism used to retract and extend the canopy 74 of the workload enclosure 21 . the exposed portions of this drive mechanism are shown in fig4 wherein the canopy motor 81 causes rotation to occur in the motor shaft ( not shown ), which in turn is translated within a gear box 113 to cause the rotation of a first canopy drive shaft 117 and a second canopy drive shaft 118 . the first drive shaft 117 is supported at one end by the gear box 113 and at its other end by a first canopy journal box 123 . likewise , the second canopy drive shaft 118 is supported by the gear box 113 and at its other end by a second canopy journal box 124 . although the drive shafts may be fabricated out of any of the preceding , suitable materials , in an alternate embodiment , the drive shafts 117 , 118 consists of a c pvc piping with an optional polyurethane foam filling . the pipe shaft is of the type supplied by ryan - herco , burbank , calif . the rotational motion of the drive shafts 117 , 118 are translated into linear motion to raise and lower the canopy 75 by one or more pairs of flexible strips , with one end of the strip attached to the rotating drive shaft and the other attached to the furthest extended window section . in the embodiment shown in fig5 a first pair of flexible canopy strips 128a , 128b and a second pair of flexible canopy strips 129a , 129b are provided . these flexible canopy strips may be fabricated out of polypropylene having dimensions of 11 / 2 &# 34 ; wide by 1 / 8 &# 34 ; thick for use with a workload enclosure . from the fully extended position of the canopy 74 shown in fig5 rotation of the drive shafts 117 , 118 in the direction shown by arrow a causes the attached flexible canopy strips 128 , 129 to wind around and accumulate on the canopy drive shafts 117 , 118 . this effectively shortens the canopy strips 128 , 129 , which in turn causes the furthest extended window section 93c to begin to retract . at this point , the nested upper and lower sealing strips 101 , 103 , respectively , separate , with the upper sealing strip 101 riding along the outside of the adjacent window section 93b ( not shown in fig5 ) as the window section 93c retracts . a corner platform 133 is provided in each corner of all but the uppermost window section 93a . the corner platforms 133 ( only one is shown in fig5 ) are attached to the window section and move upwardly therewith as the canopy 74 is retracted . a bottom surface 134 of the lower sealing strip 103 ( see fig8 ) is received by the corner platform 133 as the lower , adjacent window section is retracting , thus nesting the second window section 93b within the outer , retracting window section 93c . continued rotation of the drive shafts 117 , 118 results in the two nested window sections continuing to retract as a unit , the upper sealing strip 101 of the second window unit thereafter breaking its sealing interengagement with the lower sealing strip of the upper , third window section 93a . where more than three window sections are provided , this entire retraction process will repeat itself , the successive window units nesting inside one another , until the uppermost window unit 93a is reached . at this point , the canopy 74 is in its fully retracted position . extension of the canopy 74 is merely the reverse of the foregoing process , with the drive shafts 117 , 118 rotating according to arrow b , and the window units successively de - nesting as the canopy 74 extends . as shown in fig5 the drive shafts 117 , 118 are substantially coplanar with a first lateral window section 135 . this planar relationship enables the direct translation from the rotational movement of the drive shafts 117 , 118 to the substantially linear , vertical motion of the flexible canopy strips 128b . although it is possible to provide a second pair of drive shafts in a vertically coplanar relationship with a second lateral window section 136 to obtain this same rotational / linear translation , the same effect can be achieved by attached both canopy strips 128a , 128b to a single drive shaft , by providing a strip guide 137 that is coplanar with the second lateral window section 136 , and thus translates the linear motion of the flexible canopy strip 128a into a substantially vertical linear motion . the flexible canopy strips 128 , 129 may be attached to the furthest extended window section by any conventional attachment means , a plurality of strip attachment bolts 141 are shown as the attachment means in both fig5 and 8 . these bolts may be fabricated out of type 304 stainless steel ; some applications require type 316 stainless steel . a pathway for the flexible canopy strips 128 , 129 ( not shown ) is provided within the canopy support post 77 and within a first and second upper lateral support frame 143 , 144 , respectively . fig1 and 12 illustrate a preferred manner of attaching the flexible canopy strip 128 to the window section 93c . the flexible strip 128 is received by slots 105 formed on the interior walls 107 of the canopy support post 77 . a strip retaining block 109 is attached to the window support frame 95 and the strip attachment bolt 141 passes through the strip 128 and is anchored in the retaining block 109 . in order to obtain air seals in the workload enclosure 21 , it is necessary that the clearances and tolerances permitting movement between the various separate canopy members must be fairly precise . the forces being applied to the canopy members during retraction and extension must be substantially equally applied to each of the areas to prevent jamming from occurring . this can be accomplished by having the individual canopy members lowered and raised in a horizontally level manner , not permitting any one corner or corners to reach its extent of travel prior to the others . when using the flexible strips 128 , these adjustments may readily be made in a conventional manner at the point where the flexible strips 128 are attached to the window section 93c . by utilizing slots formed in the flexible strips ( not shown ), in conjunction with the attachment bolts 141 , it is possible to effect a lengthening or shortening of the flexible strip 128 with respect to the window section 93c . a top canopy 149 overlies the canopy 74 and forms a sealed relationship therewith . the top canopy 149 and the canopy 74 together form the workload enclosure 21 , with only the bottom open to permit the insertion and removal of the workpiece 53 into the processing solution 29 . the hoist motor 15 lies above the top canopy 49 , and a sealed opening 151 is formed in the top canopy 149 to permit passage of the hoist line 16 . in the preferred embodiment shown in fig8 the top canopy 149 is formed as a half - dome . the air within the workload enclosure 21 frequently becomes saturated with fumes given off by the processing solution 29 . upon hitting the cooler , inner surfaces of the canopy 74 and the top canopy 149 , condensation frequently occurs , producing a plurality of fume droplets 153 . by providing the top canopy 149 with a half - dome shape , the fume droplets 153 will move outwardly , towards the canopy 74 prior to dropping back into the solution 29 . this action prevents the fume droplets 153 from dropping directly onto the workpiece 53 located in the center of the workload enclosure 21 . in the past , when the fume droplets 153 landed on the workload 53 , it would frequently generate a chemical reaction at the point of impact on the metal , requiring the workload 53 to be returned to the initial process step and repeat the entire process . such a result is substantially prevented by providing the top canopy 149 in a half - dome shape according to the present invention . returning to fig4 the reciprocating cover assembly 27 is shown suspended between the workload enclosure 21 and the processing tank 5 . when installed , the cover assembly 27 rests upon the processing tank 5 , with at least one pair of mounting flanges 162 ( only one shown ) received by a lip 166 formed on the top walls of the processing tank 5 . one or more equipment passages 169 are formed between the tank lip 166 and openings adjacent the corners in the outer cover frame 63 . two such openings are shown in the outer frame 63 depicted in fig4 . the equipment passages 169 are available to permit various types of pipe to be run into the tank for such things as heating , cooling , and supplying additional chemical reactants . the equipment passages 169 also enable outside air to flow into the contained area created by either or both the workload enclosure 21 and the reciprocating cover assembly 27 when the cover is fully extended . an access cover 171 may be provided in the outer cover frame 63 , to permit access to the cover and cover drive mechanisms . similarly , a motor access panel 176 is provided in the outer cover frame 63 , permitting rapid access to the cover drive motor ( not shown in fig4 ). a receiving slot 174 is formed in the interior walls of the outer cover frame 63 , the receiving slot 174 guides the cover panels during their extension across the central opening of the cover assembly 27 . a cover 181 , suitable for use with the cover assembly 27 , is shown in fig6 . the cover 181 may be constructed out of many different types of materials , the key qualities for their present expected use include flexibility and the ease with which it can be extended or retracted during operation of the reciprocating cover assembly 27 . for example , a number of telescoping sections consisting of fiberglass reinforced , plastic - covered polyurethane foam panels might be used as the cover 181 . a suitable drive mechanism for such telescoping sections ( not shown ) are flexible strips of the type used with the canopy 74 , attached to the leading section with the remaining sections linked together by flanges or the like . as shown in fig6 a preferred construction for the cover 181 consists of a single sheet of polypropylene with a linear scoring pattern providing the appearance of a plurality of separate slats . however , in the preferred embodiment , the scoring 199 is only to a sufficient depth to create a living or flexible hinge 99 ( fig7 ). the scoring 199 permits the polypropylene sheet to easily bend along the scoring line , greatly amplifying the flexibility of the polypropylene sheet . flexibility of the cover 181 is desirable in order to permit the compact storage of the cover when in its retracted position , and to enable the extension thereof when required . the cover 181 is taken up by and extended from a main cover shaft 184 . the main shaft 184 is turned by a cover operating motor 187 that is located behind the motor access panel 176 ( shown in fig4 ). as shown in fig6 the operating motor 187 , through a system of gearing 188 , rotates the main shaft 184 through a shaft linkage 189 . in cases of motor or power failure , a hand crank 191 may be attached to a fail - safe fitting 195 formed on the main shaft 184 opposite to where the cover motor 187 is attached . in order to hand operate the system , it is necessary to disconnect the cover motor 187 from the main shaft 184 , as shown in phantom in fig6 . thereafter , the cover 181 can readily be manually operated -- whether extended or retracted . returning to fig8 the cover 181 is shown in its fully retracted position , lying within a cover storage chamber 204 . in addition to providing room for the storage of the cover 181 , the storage chamber 204 also provides a pathway for the exhaust air flow . air leaves the processing tank 5 through a first exhaust opening 207 formed in an inner wall of the outer cover frame 63 . after passing through the storage chamber 204 , the air is exhausted through a second exhaust opening 211 formed in the outer cover frame 63 and in the spacer conduit 71 . the conventional lateral exhaust hoods 33 are thereafter used to remove the exhaust gases . replacement air is provided the system through the equipment passage 169 . although not necessary to practice the present invention , the workload enclosure 21 may be provided with a separate ventilation system besides that provided through the reciprocating cover assembly 27 . air and entrained fumes may flow through the upper exhaust connecting conduit 43 and into the slotted exhaust duct 47 . as shown in fig9 the slotted duct 47 receives the canoe - shaped discharge duct 87 through a slot - shaped opening 237 formed in the duct 47 . an air seal is maintained about the discharge duct 87 through a sealing system consisting of an inner flexible boot 241 , backed up by a plurality of curve - molded fingers 242 . the flexible boot 241 may conveniently be formed of neoprene , and is forced together and / or against the sides of the canoe - shaped discharge duct 87 by the molded fingers 242 , which can be conveniently formed from fiberglass reinforced plastic . when received by the slotted duct 47 , the connecting conduit 43 permits passage of gases and entrained fumes from within the workload enclosure 21 , through an opening 232 formed in the connecting conduit 43 ( see fig1 ), and into the slotted duct 47 for collection by a central exhaust manifold ( not shown ). fig1 and 14 illustrate an alternate embodiment for the reciprocating cover assembly 27 , where the processing tank 5 ( not shown ) is particularly large . for these larger tank openings , the cover receiving slot 174 no longer provides sufficient support to the cover 181 to prevent substantial sagging thereof , particularly towards the middle of the open tank area . this sagging risks not only damaging the cover 181 , but also makes the extension and retraction thereof subject to hang - ups due to the binding of the cover 181 during its extension and retraction operations . for these larger processing tanks , a plurality of cover support rams 247 are provided , and simultaneously extend and retract in conjunction with the extending or retracting cover . in addition to the support rams 247 , a pair of half covers 181a , 181b can be provided with the covers 181a , 181b meeting substantially in the middle of the process opening . while i have disclosed an exemplary structure to illustrate the principles of the present invention , it should be understood that i wish to embody within the scope of the patent warranted hereon , all such modifications as reasonably and properly come within the scope of my contribution to the art .