US 3767300 A
A pollution control system for an electrostatic copying machine employing a developer made up of toner suspended in a light, hydrocarbon liquid carrier in which polluted air from the region of the photoconductive surface enclosed in a generally closed cabinet is passed through a cold trap to produce a condensate made up of the carrier liquid and water in which the condensate is separated into its component parts and the carrier liquid is returned to the supply and in which the cleared air is fed to an air knife which removes excess developer from the photoconductive surface immediately following development.
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Description (OCR text may contain errors)
United States Patent [1 1 Brown et al.
1 Oct. 23, 1973 POLLUTION CONTROL SYSTEM FOR DUPLICATOR MACHINE  inventors: Paul I. Brown, Beachwood; Arlie L.
Brown, Parma, both of Ohio  Assignee: Savin Business Machines Corporation, Valhalla, NY.
22 Filed: June 7, 1972  Appl. No.: 260,453
 11.8. C1 355/15, 34/77, 355/3, 7 355/10  Int. Cl 603g 15/00  Field of Search 355/3, 15, 10; 34/76, 77; 62/93, 173
 References Cited UNITED STATES PATENTS 3,609,029 9/1971 Egnaczak 355/3 3,232,029 2/1966 Evans..... 62/93 X 3,676,937 7/1972 .lanson 34/77 X 3,640,617 2/1972 Fredrickson et a1 355/15 X 2,910,137 10/1959 Victor 34/77 X 3,634,007 l/1972 Verderber et al. 355/3 3,677,632 7/1972 MacDonald 355/3 3,349,221 10/1967 Schulze et al 355/3 X Primary Examiner-Richard L. Moses Attorney-Henry L. Shenier et a1.
 ABSTRACT A pollution control system for an electrostatic copying machine employing a developer made up of toner suspended in a light, hydrocarbon liquid carrier in which polluted air from the region of the photoconductive surface enclosed in a generally closed cabinet is passed through a cold trap to produce a condensate made up of the carrier liquid and water in which the condensate is separated into its component parts and the carrier liquid is returned to the supply and in which the cleared air is fed to an air knife which removes excess developer from the photoconductive surface immediately following development.
20 Claims, 2 Drawing Figures PAIENTEDIINZS ms 3. 767300 SHEET 10F 2 FLEi I0 20 5O 40 5O 60 7O 8O 9O 1 POLLUTION CONTROL SYSTEM FOR DUPLICATOR MACHINE BACKGROUND OF THE INVENTION There are known in the prior art electrostatic copying machines in which a photoconductive surface first is charged by exposure to the action of a corona or the like and then is passed by an imaging station at which the charged surface is subjected to an image of the original to be copied to produce a latent electrostatic image. After production of the image, the surface passes through a developer system in which toner particles are applied to the image so as to be deposited in the regions which retain charge, thus to develop the image. In some systems, the copy material itself carries photoconductive material. In other systems, the photoconductor is on the surface of a drum or belt orthe like and is subjected to the action of a tacky toner which later is transferred to ordinary paper to produce the copy.
Many of the machines now in use employ a liquid developer in which the toner is suspended in a light hydrocarbon carrier liquid. The carrier liquid may be of any suitable type such, for example, as lsopar G, which is the registered trademark of Humble Oil and Refining Company for a narrow-cut, isoparaffinic, hydrocarbon fraction with an extremely high level of purity. This developer incorporating lsopar G as a carrier liquid may be applied to the photoconductive surface in a number of ways. conventionally, the photoconductive surface is moved past the developing system. The developing system directs developing liquid upwardly and into contact with the image as the photoconductive surface moves past the developer unit.
'Some machines of the prior art are provided with means for removing excess developer from the surface as it emerges from the developer unit. For example, the copending application of Smith et a1, Ser. No. 200,433 filed Nov. 19, 197, discloses and claims a pneumatic assembly for removing excess developer liquid from photoconductive surfaces in which an air knife running transversely of the direction of movement of the photoconductive surface, directs a narrow stream of high velocity air toward the surface to direct excess liquid back into the developer unit as the developed image emerges from the unit.
Following development of theimage it moves either to a transfer location at which the tacky image is transferred to a sheet of paper or in the case in which the paper itself has a photoconductive surface, it moves to a drying or image-setting station. In the course of this movement the hydrocarbon carrier liquid volatilizes. At any point at which the machine is open to the atmosphere the air polluted with hydrocarbon liquid liquid is circulated into the room in which the machine is installed. While this consideration is not an important one for a relatively small machine having only periodic use, it assumes significance in a large machine producing a high volume of copies. The problem of pollution from volatilized carrier liquid is aggravated by the production of aerosols by the action of the air knife or the like.
A number of systems or devices were tried in attempts to solve the problem of pollution in a large, high volume electrostatic copying machine. Catalytic devices with and without heat exchangers were tried. These devices per se involved the defect that an excessive amount of heat was rejected into the room in which the machine was used. They did not permit recovery of the hydrocarbon carrier. Where the devices were provided with heat exchangers they required outside air or water necessitating the use of either an air duct or a water pipe through the wall of the room for effective removal of the large amount of heat generated. They were, consequently, too expensive and too inconvenient to install. As an alternative to the catalytic devices, mechanical filters were tested. First, the filters did not operate sufficiently well to reduce the pollution to an acceptable level. Such filters are extremely expensive for that they require pumps to force vapor through a high pressure drop filter.
We have invented a pollution control system for an electrostatic copying machine which reduces pollution to an acceptable level. Our system is entirely selfcontained. It permits of the recovery and reuse of the carrier liquid. It is relatively inexpensive for the result achieved thereby. It functions secondarily to cool the machine cabinet. It is quiet in operation. It does not require extensive maintenance.
SUMMARY OF THE INVENTION One object of our invention is to provide a pollution control system for an electrostatic copying machine.
Another object of our invention is to provide a pollution control system for an electrostatic copier which permits ofrecovery and reuse of the developer carrier liquid.
A further object of our invention is to provide a pollution control system for an electrostatic copier which is self-contained.
Still another object of our invention is to provide a pollution control system for an electrostatic copier which cools the machine cabinet.
Yet another object of our invention is to provide a pollution control system for an electrostatic copier which is quiet in operation.
A still further object of our invention is to provide a pollution control system for an electrostatic copier which is rugged and requires little maintenance.
Other and further objects of our invention will appear from the following description.
In general our invention contemplates the provision of a pollution control system for an electrostatic copying machine employing a liquid developer in which polluted air from adjacent to the surface of a photoconductive element in a generally closed cabinet is drawn into a cold trap to produce a condensate made up of the liquid hydrocarbon carrier and water, which condensate is carried to a separating system which separates the carrier liquid from the water and returns the carrier liquid to the supply. The cleaned air is circulated back to an air knife or the like directed against the photoconductive surface as it emerges from the de veloper system.
BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings which form part of the instant specification and which are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:
FIG. 1 is a-plot of saturation pollution in terms of parts per million of carrier in air versus temperature.
FIG. 2 is a schematic view of our pollution control system for an .electrostatic copying machine.
DESCRIPTION OF THE PREFERRED EMBODIMENT Accordingto the manufacturers data, Isopar G and similar light hydrocarbon liquids are a health hazard at approximately 500 parts per million in air. In developing our pollution control system we first calculated the relationship between saturation pollution in parts per million of carrier liquid in air versus temperature.
According to statement 1 of Avogadros law at a given pressure and temperatureall gases have the same number of molecules per cubic foot. According to statement 2 of the law, which follows directly from statement 1, at 'a given pressure and temperature the weight of one cubic foot of any gas is directly proportional to its molecular weight. For standard conditions the volume occupied by a pound molecular weight of a gas is 379 ft.
From theforegoing it is evident that the partial pressure of a gas in atmosphere maybe expressed as:
(1) pp. (lbs gaslft. air) X (1 Mol gas/M.W.gas) X Q (379 1. /1 MOI air) If V is the volume of 1 mol of air at TF, for Isopar G having a molecular weight of 146, wemaywrite the vapor pressure in atmospheres of Isopar G for saturation at TFas:
2 v.p. (lbs Isopar G/ft air) x 1 M01 Isopar G/146) (V/l Mol air) (3) (v.p. l46/V) (lbs Isopar o/re air) The relationship between pounds per cubic foot and parts per million for any gas is given by the expression:
In order to calculate saturation PPM as a function of TF, we-first calculate V as a function of T, recognizing that at standard temperature of 60 "F V is 379 ft} Thus:
I (7) V(TF) (460 TF/460 60) 379 ft Substituting (7) into (6) we obtain:
(8) PPM saturation (TF) [v.p. (532.34 1 )/(460+ TF)] The saturation vapor pressure of Isopar G at various temperatures based on data supplied by the manufactur'er is given in the following table:
T(F) v.p. (atmospheres) 0 0.00018 Using this data in equation (8) we obtain the plot of FIG. 1 illustrating the relationshipbetween saturation pollution and temperature.
Referring now to FIG. 2, one type of electrostatic copying machine, indicated generally by the reference character 10, with which our pollution control system can be used, includes a cabinet indicated generally by the reference character 12, having a base 14, sides 16, a front 18, a back 20, and a top 22. We so construct our cabinet 12 that the interior space 24 is substantially closed to the surrounding space. That is to say, the cabinet is not open to the atmosphere save for the opening through which the completed copy is discharged in a manner to be described hereinbelow.
The machine 10 includes a drum 26 supported for rotarymovement on a shaft 28. The drum 26 has a surface 30 which is photoconductive owing to the presence of a thin film of a suitable photoconductive material on the drum. In operation of the machine 10, the surface 30 of the drum 26 first moves past a charging station at which we locate a'corona 32 which applies a uniformelectrostatic charge over the surface moving thereby.
After leaving the corona 32 the surfacecarrying the charge passes by an exposure station at which a suitable projection system 34 focuses an image of the original to be copied onto the surface. Since the details of the lens system and the original transport system do not, per se, form any part of ourinvention, they will not be described in detail.
' After leaving the exposure station at which a latent electrostatic image of the original is formed on the surface, the surface moves through developer apparatus indicated generally by the reference character 36. As is pointed out hereinabove, our system employs a liquid developer comprising a toner of any suitable type suspended in a carrier liquid such, for example, as Isopar G. The developer liquid is fed from a supply tank 38 by means of a pump 40 to an inlet line 42 leading into the apparatus 36. Developer from the apparatus 36 travels through a return line 44 back to the supply tank 38.
In the machine illustrated in FIG. 2, we provide an air knife 46 of the type described in the copending Smith et-al. application referred to 'hereinabove for directing a thin curtain of air toward the photoconductive surface so as to cause return of excess developer to the apparatus 36. In the course of our study we have discovered that the air knife 46 may produceaerosols which increase pollution within the cabinet 12 over that which is the result of evaporation of carrier liquid from the photoconductive surface 30.
Where the machine is of the contact transfer type we provide a roll 50 of paper 48 supported on a shaft 52. Such a machine is described more fully in the copending application of Smith et al. Ser. No. l55,l08, filed June 21, 1971. Paper 48 from the roll 50 is directed by a guide roller 54 into contact with the surface of the drum 26 carrying the developed electrostatic image. Owing to the greater affinity of the toner for the paper 48 than for surface 30, the image is transferred from the surface 30 of the drum 26 onto the surface of paper 48. After image transfer takes place, take off rollers 56 and 58 direct the paper along a path between cutter blades 60 and 62 and through an outlet 64. The cutter blades 60 and 62 are operated in any manner known to the art to cut the copy to length. As has been mentioned hereinabove, the outlet 64 is substantially the onlyv communication between the space 24 formed by cabinet 12 and the outside atmosphere.
Our control system, indicated generally by the reference character 66, includes a cooling unit having a cabinet 68 mountedin the wall 20 of the machine cabinet 12. We provide cabinet 68 with a partition forming a vaporbarrier which divides the cabinet into an evaporator section, indicated generally by the reference character 72, and a compressor section, indicated generally by the reference character 74. A compressor piston 76 mounted for reciprocating movement in a cylinder 78 is adapted to compress a refrigerant to a liquid state in a manner to be described. Cylinder 78 has an inlet 80 to a subchamber having a valve 82 which permits the expanded refrigerant to flow into cylinder 78 above the piston 76.
In operation, as piston 76 is reciprocated by any suitable means known to the art, gas entering the cylinder 78 through the valve 62 is compressed to the liquid state and is fed through a check valve to coils 84, one of which is illustrated in the drawings, provided with fins 86 for cooling the condensed refrigerant A fan 88 within the section 74 draws air from the room inwardly through an opening 90 provided with louvres 92. This incoming air passes over the fins 86 and outwardly through an opening 94 provided with louvres 96. In this way the'heat generated in the course of the compressing operation is dissipated'into the room in which the machine is installed. As will be apparent from the description given hereinbelow, the amount of heat dissipated into the room is not excessive.
Liquid refrigerant from the coil 84 passes through the partition 70 and through an expansion valve 98 in the connecting tube so as to expand into the evaporator coils 100 of the unit. We provide the coils 100 with fins 102. The gas expanding into the coils 100 forms a cold trap which is adapted to condense both lsopar G and the water from air entering the section 72.
We form the section 72 with an inlet opening 104 through which polluted air is adapted to enter the section 72. It will, of course, be appreciated that the level of pollution in the air entering the cold trap 72 must be sufficiently high to allow dew point of the lsopar to be above the freezing point of water vapor to prevent the cold trap from freezing up. In order to ensure that the air entering the trap 72 has a relatively high level of pollution, we provide a duct 1106 leading from adjacent to the surface of drum 34) into the inlet opening 104 of the cold trap 72.
We provide a pipe or line 110 leading from an outlet opening 108 of the cold trap chamber 72 to carry away air which has been cleared of vaporized lsopar and water. It will readily be appreciated that the action of the cold trap 72 in condensing the pollutants cools the air passing through the chamber 72. We find it preferable to pass this clean air through a heater M2. The heated air is directed bya blower M4 to the manifold or plenum 116 associated with the air knife 46.
Condensate from the coils 100 of the cold trap flows to the bottom of the chamber 72 and thence outwardly through a drain pipe 11% extending to a point adjacent to the bottom of a tank 120. Owing to the fact that the lsopar G has a specific gravity of about 0.75, it separates from the water in the tank 120 :so that the tank 120 has an under layer of water 124 and an upper layer of lsopar G 122. When the level of liquid in the tank 120 reaches a point adjacent to the top of the tank the lsopar G from the upper layer 122 flows through a pipe 126 back to the developer supply tank 38.
We provide our separation system with means for eliminating the water 124. A line 128 adjacent to the bottom of the tank has a valve 130 therein adapted to be opened upon energization of a solenoid (not shown) to cause water in the bottom of the tank to flow outwardly of the cabinet B2.
A float 132 and its associated structure to be described, has such a weight that the float floats at the parting line between the layers of liquid 122 and 124. A rod 134 on the float extends upwardly out of the tank 120. A switch 136 is provided with a toggle arm 138 adapted to be actuated in one direction to close the switch to energize the solenoid of valve 130 and adapted to be actuated in the other direction to open the switch to de-energize the solenoid associated with valve 130. Arm 138 carries a collar 140 surrounding the rod 134. We secure an upper actuator 142 to the end of the rod 134 above the collar 140. A second actuator 144 is secured to the rod at a location between the collar 140 and the float 132. When the float 132 reaches a level such, for example, as that indicated by the arrow a adjacent to the tank 120, actuator 144 engages collar 140 to close switch 136 to open valve 130 to permit water to flow outwardly from the tank 120. When the water reaches a low level such as that indicated by the arrow b adjacent the tank, actuator 142 engages collar 140 to open switch 136 to permit valve 130 to close.
The general operation of our pollution control system 66 will be apparent from the description hereinabove. As the machine 10 operates, cylinder 26 rotates in a clockwise direction so that the surface 30 thereof first moves past the corona 32 at which it receives a uniform electrostatic charge. Next, it is exposed to an image of the original through a lens system 34 to cause the charge to leak off in areas which are exposed to light to result in a latent electrostatic image. As the drum 26 continues to rotate the latent electrostatic image moves through the developer applicator 36 and liquid developer made up of tacky toner in a light hydrocarbon carrier liquid is applied to the surface of the image. Thus the image is developed. As the surface of the developed image emerges from the applicator 36, it is subjected to the action of the air knife 46 to cause excess developer to be driven back toward the applicator 36. Next, the image is brought into intimate contact with the paper 48 by means of the roller 52 and transfer of the image from the drum to the paper takes place over the arc between roller 54 and roller 56. Rollers 56 and 58 carry the length of paper having the developed image thereon outwardly of the machine through aperture 64 and past knives 6t) and 62 which are operated to out the copy to length.
It will be appreciated that there is evaporation of liquid-carrier from the surface of drum 26 from the surface of the paper carrying the transferred image and from the applicator 36. In addition, we believe that aerosols are generated by the action of the air knife 46. The overall result is that the space 24 normally would become full of highly polluted air. From FIG. I it can be seen that the saturation pollution level at temperatures which can be anticipated in the machine cabinet and in the surrounding space far exceeds the dangerous level of pollution. Our machine not only prevents the escape of any large amount of highly polluted air to the 7 surrounding space but also reduces pollution within the cabinet 12'from the extremely high level which otherwise would exist to a level which ensures that pollution of the space in which the machine'is used will be negligible even after a long period of continuous use of the machine. Since only a minor portion of air can escape from the outlet 64, our pollution control system 66 does not permit any large amount of polluted air to escape to the atmosphere. Moreover, it closely controls pollution in the space 24. Blower 14 draws air through the duct 106 from adjacent to the surface 30 of drum 26 to the cold trap space 72. This highly polluted air is brought into contact with fins 102 on the evaporator coils 100 to produce a condensate containing both lsopar G and water which flows downwardly in the chamber 72 and outwardly through pipe 118 to tank 120.
Owing to thefact that the ls'oparG is lighter than is the water, the condensate separates intank 120 into an upper layer 122 of separated lsopar G and a lower layer 124 of water. When the level of liquid in the tank 120 is above pipe 126, lsopar G flows outwardly through the pipe 126 and back to the developer supply tank 38. When the level of water 124 in tank 120 is sufficiently high actuator 144 strikes collar 140 to close switch 136 to cause valve 130 to open to permit water to flow outwardly from the bottom of the tank 120. This outflow of water continues until the water level drops to a point at which actuator 142 strikes collar 140 to open the switch to close valve 130 to shutoff the outflow of water.
i From the description hereinabove, it will be remembered that a dangerous level of pollution of lsopar G in air is about five hundred parts per million. A particular example of my system which held pollution in the room in which the machine was operated towell below a dangerous level employed a modified 5,000 BTU per hour air condition unit. First, the thermostatic control of the unit was bypassed so that the compressor ran continously. Secondly, the partition 70 was installed to form a vapor barrier between the evaporator and condenser sections to prevent leakage of pollution from the cabinet 12 to the room. Thirdly, the condensate drain was relocated to the point shown under the evaporator coil which will permit condensed carrier liquid and water to flow into the tank 120. In that arrangement, air was circulated through the section 72 at about 200 cubic feet per minute. The unit maintained the inside of the cabinet 12 at slightly below room temperature, well below the danger level.
In a typical test of the particular unit described above, the flow from the air knife 46 was regulated to about cubic feet per minute at 150 miles per hour. Drum 26 was rotated at a surface speed of about 60 feet per minute. The gap between the knife 46 and the surface 30 of the drum was arranged to be about 0.063 inches. The air heater 112 was operated at 600 watts. Under the conditions just described, pollution in the test room leveled out at an average of about 12 parts per million per 960 cubic feet or 23 parts per million for the specified spaced of 500 cubic feet. Thus, pollution of the space in which the machine was used was negligible. At the same time pollution within the cabinet was about 700 parts per million. From FIG. 1 it can be seen that this is far below the saturation pollution which would be expected at the normal interior temperature of cabinet l2.
In analyzing the results of the test, it was discovered that somewhat more lsopar G was collected than could be accounted for by a theoretical analysis. It is believed that this result was owing to the fact that air knife 46 produced some aerosol not accounted for in the theoretical analysis. a
--'lt will be seen that we have accomplished the objects of our invention. We have provided a pollution control system for an electrostatic copying machine. Our system confines appreciable pollution to the machine cabinet. It controls the level of pollution within the cabinet. Our system permits of the recovery and reuse of developer carrier liquid. It is once more effective and lessexpensive than are systems of the prior art. It is entirely self-contained.
It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of our claims. It is further obvious that various changes may be made in details withinthe scope of our claims without departing from the spirit of our invention. It is, therefore, to be understood that our invention is not to be'limited to the specific details shown and described.
Having thus described our invention, what we claim 1. In a pollution control system for an electrostatic copying machine utilizing a liquid developer made up of toner in a liquid carrier, apparatus including a cabinet, a reservoir of said liquid developer, means in said cabinet providing a photoconductive surface adapted to receive a latent electrostatic'image, means in said cabinet for applying developer from said reservoir to a latent image on said surface, a cooling unit having a housing, an evaporator section forming a cold trap in said housing, a compressor section in said housing,
means for circulating a compressible refrigerant through said evaporator and compressor sections, a vapor barrier in said housing between said evaporator sections, and means for flowing air in said cabinet through said cold trap of said evaporator section.
2. Apparatus as in claim '1 including means for re turning condensed carrier liquid from said cold trap-to said reservoir.
3. Apparatus as in claim 1 in which said cabinet is substantially closed to the outside space and in which said cold trap communicates with said cabinet.
4. Apparatus as in claim 1 in which said cold trap condenses moisture together with vaporized carrier liquid from the air fed thereto to produce a condensate made up of carrier liquid and water, said apparatus comprising means for separating said condensate into water and carrier liquid, and means for returning separated carrier liquid to said supply.
5. Apparatus as in claim 4 in which said separator comprises a tank, means for conducting condensate from said cold trap to said tank, means for removing separated carrier liquid from adjacent to the top of said tank and means adapted to be actuated to remove separated water from adjacent to the bottom of said tank.
6. Apparatus as in calim 5 in which said water removal means comprises a normally closed valve, means responsive to a predetermined high level of water in said tank for opening said valve and means responsive to a predetermined low level of water in said tank for closing said valve.
7. Apparatus as in claim 1 including means for removing excess developer liquid from said photoconductive surface following application of said developer thereto.
8. Apparatus as in claim 7 in which said excess liquid removing means is an air knife and including means for recycling air from said cold trap to said air knife.
9. In a pollution control system for an electrostatic copying machine utilizing a liquid developer-made up of toner in a liquid carrier, a cabinet, means in said cabinet providing a photoconductive surface adapted to receive a latent electrostatic image, a developer reservoir, means in said cabinet for applying developer from said reservoir to said image on said surface, a cooling unit comprising an evaporator section and a condenser section and a vapor barrier between said sections, means for passing air from said cabinet through said evaporator section to produce a condensate made up of water and said developer carrier liquid, means for separating said condensate into carrier liquid and water and means for returning separated carrier liquid to said reservoir.
10. Apparatus as in claim 9 including means for directing a stream of high velocity air adjacent to said surface following the application of developer thereto to remove excess liquid from said surface.
1 1. Apparatus as in claim 10 in which said means for passing air through said evaporator section includes means for recycling air from said evaporator section to said air stream directing means.
12. Apparatus as in claim 11 including means for heating air being recycled.
13. Apparatus as in claim 12 in which said means for passing air from said cabinet through said evaporator section comprises a duct leading from adjacent to said photoconductive surface to within said evaporator section.
14. Apparatus as in claim 13 in which said means for passing air through said evaporator section comprises a blower for drawing polluted air from said cabinet adjacent to said photoconductive surface into said evaporator section and for recycling air from said evaporator section to said air stream directing means.
15. Apparatus as in claim 9 in which said cooling unit has a housing formed with respective openings leading into said evaporator section and said condenser section and means mounting said housing in a wall of said cabinet with said evaporator section opening communicating with the interior of said cabinet and with the condenser section opening communicating with the surrounding space.
16. Apparatus as in claim 15 including means for circulating air from said surrounding space through said condenser section.
17. Apparatus as in claim 9 in which said means for separating said condensate into its constituents comprises a tank, means for conducting condensate from said evaporator section to said tank, said carrier liquid having a different specific gravity than water whereby said carrier liquid and water settle into levels in said tank.
18. Apparatus as in claim 17 in which said carrier liquid is lighter than water, and in which said means for returning separated carrier liquid comprises means adjacent to the top of said tank for conducting carrier liquid to said supply.
19. Apparatus as in claim 18 including a normally closed valve adjacent to the bottom of said tank for draining water from said tank and means responsive to the level of water in said tank for operating said valve.
20. In a pollution control system for an electrostatic copying machine utilizing a liquid developer made up of toner particles in a light hydrocarbon carrier liquid, apparatus including a substantially closed cabinet, a member having a photoconductive surface adapted to receive a latent electrostatic image, a supplyof said liquid developer, means for applying liquid from said supply to a surface moving thereby, an air knife adjacent to said developer applying means for directing excess liquid on a surface moving thereby back to said developer applying means, means mounting said member for movement of said surface sequentially past said applying means and said air knife, a cooling unit having a housing containing an evaporator section and a condenser section and a vapor barrier between said sections, said housing having respective openings leading to said sections, means mounting said unit in a wall of said cabinet with the evaporator section opening communicating with the interior of said cabinet and with the condenser section opening communicating with the outside space, means for directing polluted air from within said cabinet into said evaporator section to produce a condensate of said carrier liquid and water, means for recycling air from said evaporator section to said air knife, means for separating said condensate into its carrier liquid constituent and its water constituent and means for feeding said carrier liquid constituent to said developer supply.