Method and apparatus for evacuating aqueous ammonia vapor from film developing chambers
BACKGROUND OF THE INVENTION 
     For economic and other reasons, diazo film is increasingly used for making copies of microfiche masters. Generally speaking, diazo film is first exposed and thereafter it is developed in aqueous ammonia vapor. With usage, the vapor must at least be intermittently replenished with fresh vapor. This is particularly important in connection with the recently developed micro-chambers which have physical dimensions only slightly larger than those of the film because there is only a relatively small volume of vapor which, during rates of high film throughput, becomes relatively quickly spent. Thus, in connection with micro-chambers, it is necessary to supply fresh aqueous ammonia vapor on a more or less continuing basis. 
     Because of their noxious stench, ammonia vapors cannot be discharged into the atmosphere unless the volume is very small. Thus, some other means for removing spent ammonia vapors must normally be devised. In the past, several approaches were employed. In a most simple arrangement, the developing chamber is defined by a pair of opposite, spaced apart platens which are heated so as to maintain the vapor temperature in the gap between them above the dew point of the vapor. The vapors, however, were allowed to escape into a housing which encapsulates the platens. The housing walls are at a lower temperature and aqueous ammonia vapor was permitted to condense thereon. By devising properly arranged guide channels, accumulating aqueous ammonia droplets could be collected in a suitably disposed drain for discharge to a waste ammonia tank or the like. 
     A shortcoming encountered with this approach is that the overall volume of the housing into which ammonia must be introduced is relatively large so that the ammonia consumption is correspondingly high with only a small portion of the ammonia being actually used for developing the film. Further, because of the large volume, the opening of the developer for repair, maintenance and the like releases significant amounts of ammonia vapor into the surrounding atmosphere which is undesirable. Most significantly, however, the accumulation of aqueous ammonia droplets cannot be well controlled and such droplets might from time to time contact the film being developed in the chamber. Any such contact is highly detrimental to the developing process and normally renders the film unacceptable. 
     In the past, attempts have also been made to withdraw the aqueous ammonia vapor from the chamber on a more or less continuing basis and to condense the ammonia outside the chamber. Although this overcomes some of the problems mentioned in the preceding paragraph, it requires the installation of relatively complicated and, therefore, costly pumps, conduits and condensers which require constant maintenance and which, unless constantly checked, may leak and release relatively large amounts of ammonia vapors to the surrounding atmosphere. 
     A still further prior art attempt to remove spent ammonia from the developing chamber is to place one or more condenser plates into the chamber so that aqueous ammonia vapor can condense thereon. The condensate is then withdrawn by gravity through properly arranged channels, drainage holes and the like. Again, a problem encountered with such an arrangement is the fact that the removal of the vapor from the chamber requires the formation of discreet aqueous ammonia droplets. Only after the droplets have reached a sufficient size so that they can gravitationally run off the condenser plates is it possible to remove the spent ammonia from the chamber. However, the presence of such droplets in the chamber always brings with it the danger that they be contacted by the film which, as above described, damages the film. 
     From the foregoing, it is apparent that up to now difficulties have been encountered in handling the aqueous ammonia in diazo film developers. Economically feasible approaches often compromised the quality of the film and could lead to excessive rejects. On the other hand, methods for handling the ammonia which did not compromise the quality of the film were relatively expensive. Thus, there is presently a need for an efficient, low cost ammonia handling system for diazo film developers which assures high quality developed diazo film. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the shortcomings found in prior art diazo film developers and in particular the shortcomings which resulted from the manner in which the aqueous ammonia vapor in the developing chamber was replenished. Generally speaking, the present invention accomplishes this by condensing relatively minute amounts of aqueous ammonia vapor on a relatively small surface which is continuously moved into and out of the developing chamber. While the surface is disposed inside the chamber, microscopic droplets form on the surface. Before the droplets can become of such size that they may damage film being developed in the chamber the surface is moved outside the chamber and the droplets are removed therefrom. In the preferred embodiment of the invention the removal of the droplets from the surface is by evaporation, although other methods for their removal can be employed if desired. 
     Speaking in more concrete terms, the present invention contemplates the intermittent or continuous introduction of aqueous ammonia vapor into a developing chamber which is sealed from the exterior. Feed rollers for advancing film to be developed into the chamber are disposed proximate an intake opening of the chamber while exit rollers for withdrawing developed film from the chamber are disposed proximate an outlet opening of the chamber. The rollers are sealed with respect to the chamber so as to prevent the escape of vapor to the exterior and the chamber is heated to a temperature sufficiently elevated so as to prevent the formation of vapor condensate within the chamber, i.e. to above the dew point for the aqueous ammonia vapor. The feed rollers are maintained at about the same temperature as the chamber so as to prevent the formation of condensate thereon. 
     The exit rollers, however, are positioned so that a first surface portion of each roller is disposed inside the chamber while a second surface portion of each roller is disposed exteriorly of the chamber. The temperature of the exit rollers is sufficiently low so that vapor in the chambers condenses on the first mentioned surface portions of the exit rollers. By virtue of the continuous rotation of the exit rollers, condensate formed on the roller surfaces is continuously moved outside the chamber in minute amounts, that is in the form of only microscopic droplets which are too small to either accumulate into larger droplets or to in any manner damage the film if they come in contact therewith. 
     Once the condensate on the rollers is on the exterior of the chamber, it is removed therefrom. Depending on the air temperature, humidity, etc. this can be accomplished by simply permitting the condensate on the rollers to evaporate before it can re-enter the chamber with the rotating roller surface. In accordance with one aspect of the invention, however, the condensate removal and the cooling of the rollers is performed in a single operation by passing an airstream over the exterior roller surface portions which has a temperature below the dew point for the vapor. Depending on the particular circumstances, air temperature, humidity conditions, etc. this may be an ambient airstream, a heated or a cooled airstream. To enhance the cooling and condensate evaporation efficiency, an air fan may be provided which directs the airstream against the exterior roller surface portions. 
     Additionally, the developing chamber of the present invention is constructed so as to assure that no condensate forms at any other location within the chamber along the film travel path. Thus, the chamber itself is appropriately heated. Similarly, the upstream feed rollers are heated, either by encapsulating them in a housing with conventional developing platens or by independently heating the rollers either internally or by subjecting them to a heated airstream, for example. When encapsulated with the platen a frequently sufficient transfer takes place between the platens and the rollers so as to eliminate the need for separately heating the feed rollers. 
     From the foregoing, it should be apparent that the present invention eliminates the need for complicated ammonia vapor withdrawal conduits, pumps, and the like to prevent the formation of aqueous ammonia droplets within the chamber which may contact and damage or destroy the film being developed therein. While achieving the same effect as vapor withdrawal systems, that is eliminating the formation of ammonia droplets which could damage the film within the developing chamber, the present invention also eliminates the need for ammonia condenser plates or walls, drainage passages and openings, etc. Instead, it employs the already present exit rollers which withdraw developed film from the chamber as a vehicle for withdrawing the spent ammonia from the chamber. By properly constructing the developer, the most that is needed is the provision of a cooling-evaporation fan which directs a roller cooling and condensate evaporating airstream against the exterior portions of the exit rollers. Thus, with little or no additional costs, the present invention accomplishes that which in the past required expensive equipment. Consequently, the present invention facilitates the economic large scale use of diazo film in connection with microfiche copiers and the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawing schematically illustrates an aqueous ammonia developer for diazo films constructed in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawing, a developer 2 constructed in accordance with the present invention for developing a diazo-type microfiche 4 in an aqueous ammonia atmosphere generally comprises a developing chamber 6 having an intake opening 8 and an outlet opening 10. The microfiche is advanced in a downstream direction, that is to the left as seen in the drawing, by a pair of feed rollers 12 disposed proximate the intake opening. A pair of corresponding exit rollers 14 is positioned adjacent outlet opening 10. They are driven by shafts 15 and they withdraw the microfiche after it has been developed in the chamber. The chamber itself is defined by parallel, spaced apart upper and lower platens 16, 18, respectively, which between them define a gap 20 of a height &#34;T&#34; and of a width dimensioned so as to permit the passage of microfiche 4 through the gap. A pump 22 has an intake fluidly connected to an aqueous ammonia supply 24 and feeds aqueous ammonia via supply line 26 to an ammonia discharge port 28 in the lower platen 18. The site of the lower platen facing gap 20 preferably includes a transverse groove 30 which communicates with port 28 and distributes the ammonia over the full width of the chamber. 
     Heaters 32 heat platens 16, 18 to a temperature which is sufficiently above the dew point for the aqueous ammonia discharged by port 28 so as to cause the discharge of the aqueous ammonia in its vapor form and to maintain the aqueous ammonia in the developing chamber in its vapor state. Further, seal strips 34 constructed of a suitable material such as teflon, for example, are provided to seal the ends of platens 16, 18 against the feed and exit rollers 12, 14 so as to seal the developing chamber from the exterior. In the illustrated embodiment the seal strips are secured to the platens and they extend over the full length of the rollers. Spring means (not shown) may be provided to urge the seal strips against the rollers. Finally, the platens 16, 18, the feed rollers 12 and the heaters 32 are encapsulated within a housing 36 which, adjacent its upstream end, includes an intake chute 38 through which a microfiche to be developed can be inserted into engagement with the feed rollers. 
     The operation of developer 2 is as follows. During an initial start-up period, heaters 32 are energized to bring the platens and the developing chamber to their operating temperature, that is above the dew point for the aqueous ammonia. After the temperature has been reached, pump 22 can be activated to introduce aqueous ammonia into the developing chamber 6. To minimize ammonia consumption and to maximize the developing efficiency, it is preferred that the gap width &#34;T&#34; is closely controlled. In a presently preferred embodiment the gap width is no more than about 0.02&#34; for accommodating a microfiche having a thickness of between 0.003 to about 0.007&#34;. At the indicated dimensions, microfiche is readily transported in a downstream direction without undesirable interference from the opposing platen surfaces. 
     Pump 22 may be selected so that it pumps a very small volume of aqueous ammonia which is selected to provide just enough ammonia to develop the fiches at whatever rate they pass through the chamber. Alternatively, the metering pump may be an intermittently operating pump which is selectively activated in response to an approaching microfiche to be developed. In an alternative operational mode, a valve (not shown) may be interposed in supply line 26 and be coupled with suitable sensors (not shown) to temporarily open the valve to flow the desired amount of ammonia to discharge port 28. As the ammonia approaches the discharge port, it is heated by lower platen 18 and evaporates so that it is discharged into the developing chamber in its vapor state. 
     The developer is now ready for use. After a microfiche 4 has been exposed, it is entered through chute 38 until feed rollers 12 grasp it. The rollers advance the microfiche into developing chamber 6 where the aqueous ammonia vapor develops the fiche. The leading edge of the fiche is then grasped by exit rollers 14 which withdraw the fiche from the chamber for discharge into a suitable receptacle (not shown). 
     The developing of the fiche consumes ammonia. Thus, it is necessary to remove from the chamber spent aqueous ammonia and replenish it with fresh aqueous ammonia in the above outlined manner. The present invention accomplishes the removal of spent ammonia by establishing in effect an equilibrium flow, that is by removing aqueous ammonia at the same rate at which it is introduced into the chamber. 
     Exit roller 14 accomplish the actual removal of the spent aqueous ammonia from the chamber as follows. The temperature of the exit rollers is maintained relatively low, that is below the dew point for the ammonia. This may be accomplished by placing the rollers in a relatively cool atmosphere. The cooling of the exit rollers is enhanced by providing a fan 40 which has a discharge nozzle 42 that directs an ambient airstream 44 towards exterior surface portions 46 of the cylindrical outer surface 48 of the rollers. A cooler (or heater) 47 may be provided to regulate the temperature of the airstream 44. 
     It will be observed that at all times there is an interior surface portion 50 of the exit rollers which is disposed within, i.e. which forms the downstream boundary for the developing chamber 6. The temperature of the exit rollers is below the dew point for the aqueous ammonia in the developing chamber. Accordingly, aqueous ammonia in general and spent aqueous ammonia in particular, condenses on the interior surface portions 50 of the exit rollers. Since the rollers rotate continuously and the interior surface portion 50 is relatively small, i.e. it is less than 50% of the entire cylindrical surface of the rollers, only minute amounts of aqueous ammonia condensate can form on the interior surface portion until the surface portion is rotated to the exterior of the developing chamber. The minute amounts of condensate are present on the interior surface portion in the form of microscopic droplets that are far too small to coalesce into larger droplets and which, if they contacted a fiche disposed between the exit rollers will not damage the fiche. Furthermore, the small droplet size prevents the droplets from being squeeged off the rollers along their common contact line. Their accumulation within the chamber is thus prevented. 
     In fact, the small droplets on the interior roller surface portions 50 pass with the rotating surface past the contact line between the rollers to the exterior of the developing chamber. Once the droplets are on the outside of the chamber they are readily removed, in accordance with the present invention preferably with the same airstream 44 which is used to cool the exit rollers by causing the evaporation of the minute droplets before they can re-enter the developing chamber with the rotating rollers. 
     Although the above-described removal of aqueous ammonia vapors from the developing chamber does not as such discriminate between spent ammonia and fresh ammonia, the relative remote location of the exit rollers from the aqueous ammonia discharge port 28, which is proximate the intake opening 8, has a tendency to preferentially remove spent ammonia because there is a slow circulation of ammonia vapors from the discharge port towards the exit rollers and the progressive development of a fiche passing through the chamber uses up increasing amounts of ammonia. This assures a highly efficient use of the ammonia and further minimizes the amounts of ammonia discharged to the atmosphere. 
     Although the aqueous ammonia removal from the developing chamber in accordance with the present invention is generally applicable to any developer, it is particularly well suited for the earlier discussed micro-developing chambers which have minimal chamber volumes and, therefore, minimal ammonia requirement. This translates into small volumes of ammonia condensate which must be removed from the chamber and for which the removal in accordance with the present invention is particularly well-suited. 
     To prevent the formation of aqueous ammonia condensate on the feed rollers 12, the temperature of the latter is maintained above the aqueous ammonia dew point, e.g. at about the same temperature as that of the developing chamber. For this purpose, the feed rollers are disposed within housing 36. For most cases, their placement within the housing is sufficient to maintain the necessary temperature through heat transfer from the heated platens 16, 18. However, additional heaters 52 may be provided for independently heating the feed rollers to the desired temperature.