FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimile machine or a multi-function machine of these machines, electrostatic recording type, or the like. Particularly, the present invention relates to a structure including a cooling device for cooling an inside of a casing in which an image forming portion is provided. 
     In the image forming apparatus, such as the copying machine or the printer, of the electrophotographic type, a toner image formed by an image forming portion is transferred onto a recording material and then is subjected to application of heat and pressure by a fixing device, thus being fixed on the recording material. 
     The heat generated by the fixing device is not only applied to the recording material but also is dissipated in the inside of the casing of the image forming apparatus, and therefore, the temperature of the image forming portion is raised by this heat dissipation. Particularly, in the case of a structure in which a two-component developer containing a toner and a carrier, it is known that frictional heat is generated by friction between the developer and a sleeve or a screw in a developing device constituting the image forming portion and thus the structure causes self temperature rise. Accordingly, the temperature in the developing device is more liable to rise in combination with the above-described heat dissipation from the fixing device. Thus, when the temperature in the developing device rises, it is known that the developer may be caked, or deterioration of the developer is considerably promoted. 
     In view of this, the image forming apparatus equipped with a means for cooling the inside of the casing has been conventionally known. As the cooling device, generally, in many cases, a constitution in which a fan is used to rake the ambient air, thus cooling the inside of the casing is employed. On the other hand, in recent years, there is also a structure in which a cooling device using vaporization as in a refrigerator or air conditioner or a cooling device using the Peltier effect is used to generate the air having a temperature lower than that of the (ambient) air outside the casing, thus reducing the temperature in the casing. For example, a cooler provided on a side surface of the main assembly of the copying machine to supply a cool air into the copying machine is devised (Japanese Laid-open Patent Application (JP-A) Sho 58-217982. 
     In the case where, as the cooling device for cooling the inside of the casing, the structure having the constitution, as described in JP-A Sho 58-217982, in which the cooling air having the temperature lower than that of the air outside the casing is generated is used, water is always generated during the generation of the cooling air and therefore drainage treatment is problematic. As a treating method of this drainage, it would be considered that the drainage is stored in a tank (container) or the like and then is periodically discarded or that pipework for the drainage is made as in the air conditioner and then the drainage is run into the outdoors or a sewer. 
     However, in the case where the drainage is stored in the tank or the like, the cooling device is provided in the neighborhood of the surface of the floor in many cases and therefore even when the tank is provided, the capacity cannot be ensured, so that there is a need to drain out the water in the tank frequently. Further, in the case where the pipework for treating the drainage is made, disposition work is on a large scale, so that a disposition cost becomes mammoth. In addition, due to convenience of routing of a drain pipe, a disposition space is also limited and therefore a slight movement of the device cannot be effected. 
     SUMMARY OF THE INVENTION 
     In view of the above circumstances, a principal object of the present invention is to provide an image forming apparatus capable of easily treating drainage generated by providing a cooling device with no need to store water in a tank and frequently drain off the water and with no need to effect pipework. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising: a main assembly for forming an image; a casing in which the main assembly is provided; a cooling device for cooling an inside of the casing by generating cooling air lower in temperature than air outside the casing; a liquid evaporator for receiving water generated during generation of the cooling air and for evaporating the water by circulation of air; and a heat exhausting device for supplying the air warmed by the main assembly to the liquid evaporator. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an image forming apparatus according to a First Embodiment of the present invention. 
         FIG. 2  is a perspective view of a disposition (installation) state of the image forming apparatus as seen from a front side. 
         FIG. 3  is a perspective view of the disposition state as seen from a rear side. 
       Part (a) of  FIG. 4  is a schematic plan view of a cooling device, and (b) of  FIG. 4  is an enlarged sectional view taken along a-a line indicated in (a) of  FIG. 4 . 
         FIG. 5  is a schematic perspective view of a drainage treatment portion in the First Embodiment. 
         FIG. 6  is a schematic sectional view of the drainage treatment portion in First Embodiment. 
         FIG. 7  is a schematic perspective view of a drainage treatment portion in a Second Embodiment of the present invention. 
       Part (a) of  FIG. 8  is a schematic sectional view of the drainage treatment portion in Second Embodiment, and (b) of  FIG. 8  is an enlarged view of an upper right portion in (a) of  FIG. 8 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     &lt;First Embodiment&gt; 
     The first embodiment of the present invention will be described with reference to  FIG. 1  to  FIG. 6 . In this embodiment, an image forming apparatus A includes an image forming apparatus main assembly B, a cooling device  100  and a drainage treatment portion  200 . First, the image forming apparatus main assembly B in this embodiment will be described with reference to  FIGS. 1 to 3 . 
     [Image Forming Apparatus] 
     The image forming apparatus A shown in  FIG. 1  is a full-color laser printer of an electrophotographic type. The image forming apparatus main assembly B constituting the image forming apparatus A forms four color toner image different in color by first, second, third and fourth image forming portions Py, Pm, Pc and Pb juxtaposed in a casing C. Then, via an intermediary transfer belt  7 , these four color toner images are transferred onto a recording material and are heat-fixed on the recording material by a fixing device  15 . That is, the image forming apparatus main assembly B forms the image on the recording material through respective processes of charging, exposure, development, transfer and fixing. A control portion  19  as a control means (controller) includes CPU and memories such as ROM and RAM. Further, when image information outputted from an external device (not shown) such as a host computer or from a scanner is inputted, the control portion  19  successively actuates the image forming portions Py, Pm, Pc and Pb in accordance with an image formation control sequence stored in the memory. 
     At each of the image forming portions Py, Pm, Pc and Pb, a photosensitive drum  1  as an image bearing member is rotated in an arrow direction at a predetermined peripheral speed (process speed). Further, an intermediary transfer belt  7  extended around a driving roller  6   a , a follower roller  6   b  and a tension roller  6   c , so as to force the photosensitive drums  1  of the image forming portions Py, Pm, Pc and Pb is rotated by the driving roller  6   a  in an arrow direction at a peripheral speed corresponding to the peripheral speed of the rotation of the photosensitive drum  1 . 
     Then, in the image forming portion Py for yellow as a first color, the outer peripheral surface (surface) of the photosensitive drum  1  is charged uniformly by a charger  2  to a predetermined polarity and potential. Next, the charged surface of the photosensitive drum  1  is exposed to and scanned with laser light generated on the basis of the image information inputted from the external device by an exposure device  3 . As a result, an electrostatic latent image depending on the image information is formed on the charged surface of the photosensitive drum  1 . The (electrostatic) latent image is developed with a yellow toner (developer) by a developing device  4 , so that a yellow toner image (developer image) is formed on the surface of the photosensitive drum  1 . 
     Similar steps of charging, exposure and development are carried out in the image forming portion Pm for magenta as a second color, the image forming portion Pc for cyan as a third color and the image forming portion Pb for black as a fourth color. 
     The toner images of respective colors formed on the surfaces of the photosensitive drums  1  in the image forming portion P are transferred superimposedly onto the outer peripheral surface of the intermediary transfer belt  7  by primary transfer rollers (transfer members)  8  disposed opposed to the photosensitive drums  1  via the intermediary transfer belt  7 . As a result, a full-color toner image is formed on the surface of the intermediary transfer belt  7 . Transfer residual toner remaining on the surface of the photosensitive drum  1  after the toner image transfer is removed by a drum cleaner  5  and the photosensitive drum  1  is then subjected to subsequent image formation. 
     On the other hand, a recording material P is fed out of a sheet feeding cassette  10  by a delivery roller  11  and is fed to registration rollers  13  along a feeding path  12   a . Then, the recording material P is fed to a secondary transfer nip Tn which is between the intermediary transfer belt  7  and a secondary transfer roller  14 , by the registration rollers  13 . Then, the recording material P is nip-conveyed in the secondary transfer nip Tn and the toner image on the surface of the intermediary transfer belt  7  is transferred onto the recording material P in this conveyance process by the secondary transfer roller  14 . The transfer residual toner remaining on the surface of the intermediary transfer belt  7  after the toner image transfer is removed and the intermediary transfer belt  7  is subjected to subsequent image formation. 
     The recording material P on which the unfixed toner image is carried is introduced into a nip of the fixing device  15  with an image carrying surface upward. Then, the recording material P is nipped and conveyed in the nip of the fixing device  15 , so that the toner image is heat-fixed on the recording material P. In the case where the image is formed only on one side of the recording material P, the recording material P discharged from the fixing device  15  is guided by a switching member  16  to a discharging roller  1  through which the recording material P is discharged onto a discharging tray  18  provided on a side surface of the casing C. In the case where the images are formed on both sides of the recording material P, the recording material P discharged from the fixing device  15  is guided by switching member  16  to a reverse feeding path  12   b  provided below. 
     In the reverse feeding path  12   b , when the trailing edge of the recording material P reaches a reversion point Rp, the recording material P is switch-backed, so that it is fed to a feeding path  12   c  for the both-sided print with the image carrying surface upward. The recording material P is fed out of the feeding path  12   c  to the registration rollers  13  along the feeding path  12   a . In this state, the image carrying surface is directed downward. This recording material P is fed to the secondary transfer nip Tn by the registration rollers  13 , thus being nip-conveyed in the secondary transfer nip Tn. Then, during this conveyance process, the toner image on the intermediary transfer belt  7  is transferred onto the recording material P by the secondary transfer roller  14 , and is heat-fixed on the recording material S by the fixing device  15 . The recording material P discharged from the fixing device  15  is guided to the discharging roller  17  by the switching member  16  and is discharged onto the discharging tray  18  by the discharging roller  17 . 
       FIGS. 2 and 3  show an outer appearance of the image forming apparatus. When the image forming apparatus A is viewed from a front surface side, at an upper-right rear portion, the cooling device  100  is provided. Further, behind the fixing device  15 , the drainage treatment portion  200  for treating water drained off from the cooling device  100  is provided. 
     The cooling device  100  is disposed above the respective image forming portions Py, Pm, Pc and Pb with respect to the direction of gravitation and generates cooling air (cool air) having the temperature lower than that of the ambient air outside the casing C. The generated cool air is smoothly sent from the cooling device  100  into the casing C via a connection duct  101  by an air supply fan  102  as an air supply means provided at a joint portion between the connection duct  101  and the casing C. The air supply fan  102  is disposed at the image forming portion Pb side remote from the fixing device  15 . The cool air sent into the casing C passes through gaps between the exposure devices  3   y ,  3   m ,  3   c  and  3   b  as indicated by an arrow a of a broken line in  FIG. 1  to extend to the periphery of the photosensitive drum  1  and the developing device  4  of each of the image forming portions Py, Pm, Pc and Pb, thus cooling these portions. 
     On the other hand, an exhaust fan  120  as an exhaust means is disposed on the image forming portion Py side remote from the air supply fan  102  to exhaust the air in the casing C to the outside of the casing C. Thus, in this embodiment, the air supply fan  102  is disposed at the remotest position from the fixing device  15  which is a maximum heat generating source in the image forming apparatus main assembly, and the exhaust fan  120  is disposed in the neighborhood of the fixing device  15 . Further, second exhaust fans  130   a  and  130   b  which are principally used for exhausting the air in the neighborhood of the fixing device  15  and are also used as a heat exhausting means are separately and independently provided in the neighborhood of the fixing device  15 . The air exhausted by the second exhaust fans  130   a  and  130   b  is exhausted to the outside via the drainage treatment portion  200  as described later. 
     Further, at the periphery of the fixing device  15  and the second exhaust fans  130   a  and  130   b , a shielding wall  15   a  is provided so as to cover the side of the air supply fan  120  and the exhaust fan  120 . Thus, by providing the shielding wall  15   a  while regulating the disposition positions of the respective fans, the cool air supplied from the air supply fan  102  is not readily sent to the fixing device  15  and at the same time the heat of the fixing device  15  is not readily transmitted to the image forming portions and their peripheral portions. 
     In this embodiment, the cooling device  100  is, as described above, disposed above the image forming portions (Py, Pm, Pc, Pd) of the image forming apparatus A with respect to the direction of gravitation and therefore the cool air is efficiently and smoothly sent to the image forming portions and their peripheral portions by using the gravitation. Therefore, in combination with less transmission of the heat of the fixing device  15  to the image forming portions and their peripheral portions, it is possible to efficiently prevent temperature at the image forming portions and their peripheral portions. 
     [Cooling Device] 
     Next, the cooling device  100  will be described with reference to  FIG. 4 . In this embodiment, the cooling device  100  uses a constitution of heat of evaporation. The cooling device  100  comprises mainly, as shown in (a) of  FIG. 4 , a compressor  105 , a condenser  106 , an evaporator  108  and a drier  107 . 
     As a mechanism of the cooling device  100 , first, the coolant is compressed by the compressor  105  into a high temperature and high pressure state. The high temperature and high pressure coolant dissipates heat in the condenser  106 , so that the temperature of the coolant is lowered and the coolant is liquefied. At a portion of the condenser  106 , heat is generated. Then, from the liquefied coolant, impurities such as water (moisture) content and contaminant are removed by the drier  107 , so that the coolant is sent to the evaporator  108 . In this evaporator  108 , the coolant is reduced in pressure and expands. That is, in a low pressure and low temperature state, the coolant evaporates, and at this time it deprives the heat (of evaporation) of the neighborhood thereof. Thus, the cool air is just generated. 
     The air (ambient air) outside the casing C is, as indicated by an arrow b, drawn in by the fan  112   a  and passes through the evaporator  108  to be cooled to low temperature. Then, the cooled air (cool air) passes through the connection duct  101  and is supplied into the casing C by the fan  102  described above. 
     On the other hand, when the ambient air passes through the evaporator  108  in the cooling device  100 , the ambient air is cooled, so that condensation occurs at the surface of the evaporator  108 . This condensed water content is dropped onto a drain pan (tray)  109  disposed at a lower portion of the evaporator  108  and is sent to an evaporation pan  110  through a pipe  109   a . On the evaporation pan  110 , as shown in (b) of  FIG. 4 , on evaporation sheet  111  is provided. The evaporation sheet  111  is constituted by a nonwoven fabric or a foam member and sucks up and holds the water (content). To this evaporation sheet  111 , by the fan  112   b , as indicated by an arrow c, the air (warm air) which passes through the condenser  106  and then passes through the neighborhood of the compressor  105  is sent. As a result, by using the heat generated from the condenser  106  and the compressor  105 , a part of the water held by the evaporation sheet  111  is evaporated. Here, the water which cannot be evaporated passes through a drainage pipe  150  to be sent to the drainage treatment portion  200 . 
     [Drainage Treatment Portion] 
     Next, the drainage treatment portion  200  will be described with reference to  FIGS. 5 and 6 . The drainage treatment portion  200  includes sheet members  201   a  and  201   b  formed with the nonwoven fabric as a liquid evaporation means, second exhaust fans  130   a  and  130   b  as a heat exhaust means, and a third exhaust fan  203  as an exhaust means. 
     The sheet members  201   a  and  201   b  retain the water generated during the generation of the cooling (cool) air exhausted from the cooling device  100  and evaporates the retained water by circulation of the air. In this embodiment, the sheet members  201   a  and  201   b  are prepared by forming a water (moisture) absorbing nonwoven fabric into sheet shape and then by bending the sheet in an L-like shape. Then, a short portion  201 S having a short length of each of the sheet members  201   a  and  201   b  is disposed in the substantially horizontal direction and a long portion  201 T having a long length hangs down from an end of the short portion  201 S. 
     The sheet members  201   a  and  201   b  are disposed in plurality in the direction of gravitation. In this embodiment, two sheet members are vertically arranged so that a lower end portion of the upper sheet member  201   a  overlaps with an upper end portion of the lower sheet member  201   b . That is, the upper and lower sheet members  201   a  and  201   b  overlap with each other at a part thereof. Incidentally, in an example illustrated in  FIGS. 5 and 6 , the upper sheet member  201   a  and the lower sheet member  201   b  have different shapes but may also have the same shape. Further, the number of the sheet members to be disposed may also be three or more or one. Further, below the lower sheet member  201   b  with respect to the direction of gravitation, a drainage receiving tray  204  as a receiving member for starting the water which is not evaporated by and is dropped from the sheet members  201   a  and  201   b  is disposed. 
     Such sheet members  201   a  and  201   b  are provided outside (behind) the second exhaust fans  130   a  and  130   b  as the heat exhaust means of the fixing device  15 . These second exhaust fans  130   a  and  130   b  are also arranged and provided in the direction of gravitation. Further, the upper second exhaust fan  130   a  is provided in a plurality of exhaust fan portions (four in the example illustrated in  FIGS. 5 and 6  as shown in  FIG. 1 ) and these exhaust fan portions are arranged and provided in the horizontal direction. On the other hand, the lower second exhaust fan  130   b  is a single exhaust fan having a large size than that of each of the fan portions of the upper second exhaust fan  130   a.    
     The thus disposed second exhaust fans  130   a  and  130   b  airy the air warmed by the image forming apparatus main assembly B to the sheet members  201   a  and  201   b . Particularly, in this embodiment, the second exhaust fans  130   a  and  130   b  are, as described above, disposed in the neighborhood of the fixing device  15 . Therefore, the second exhaust fans  130   a  and  130   b  airy the air principally warmed by the fixing device  15  to the sheet members  201  and  201   b.    
     Further, with respect to the sheet members  201   a  and  201   b , at an opposite side of the second exhaust fans  130   a  and  130   b , a plurality of the third fans  203  are disposed. That is, the drainage treatment portion  200  is provided with the sheet members  201   a  and  201   b  disposed so as to cover the second exhaust fans  130   a  and  130   b  provided at the rear portions of the casing C and is provided with a second casing  200   a  so as to cover the sheet members  201   a  and  201   b . Further, at a part of the second casing  200   a , the third exhaust fans  203  are disposed downstream of the sheet members  201   a  and  201   b  with respect to an air flow direction. 
     In this embodiment, the third exhaust fans  203  are provided in six in total by being arranged in two parallel rows each including three fans arranged in the direction of gravitation. Further, the disposition position of these third exhaust fans  203  is a side wall of the second casing  200   a  perpendicular to an air feeding direction of the second exhaust fans  130   a  and  130   b . Such third exhaust fans  203  exhaust the air passing through the sheet members  201   a  and  201   b  to the outside of the casing C. 
     In the case of the thus constituted embodiment, as described above, the drainage sent from the drainage pipe  150  drops on the short portion  201 S of the upper sheet member  201   a . The dropped water moves in the sheet member  201   a  by the capillary action and then slowly moves in the long portion  201 T vertically extending in the direction of gravitation along the drop direction. Further, the water which is not evaporated in the sheet member  201   a  is sent to the lower sheet member  201   b  via the above-described casing (overlapping) portion. 
     In this case, the air exhausted from the second exhaust fans  130   a  and  130   b  is, as indicated by arrows d in  FIG. 5 , blown onto the sheet members  201   a  and  201   b . Thereafter, by the third exhaust fans  203 , as indicated by arrows e in  FIG. 5 , the air passing through the sheet members  201   a  and  201   b  flows so as to turn to the right. As described above, the air exhausted from the second exhaust fans  130   a  and  130   b  is dried by the high temperature heat of the fixing device  15 . Therefore, the high temperature air is supplied to the sheet members  201   a  and  201   b  by the second exhaust fans  130   a  and  130   b  and the third exhaust fans  203  and flows in the neighborhood of the sheet members  201   a  and  201   b . As a result, the water in the sheet members  201   a  and  201   b  is efficiently evaporated. The air containing the water evaporated from the sheet members  201   a  and  201   b  is exhausted to the outside of the image forming apparatus by the third exhaust fans  203 . 
     Further, in this embodiment, a water amount detecting sensor  205  as a water amount detecting means for detecting an amount of the water stored in the drainage receiving tray  204  disposed below the lower sheet member  201   b  with respect to the direction of gravitation is provided. This water amount detecting sensor  205  detects the amount of the water stored in the drainage receiving tray  204 , e.g., by detecting the weight of the drainage receiving tray  204 . As such a water amount detecting sensor, in addition to the weighing structure, e.g., a structure in which the drainage receiving tray  204  is provided with a float. In this structure, the float comes up by an increase of the amount of the stored water and when the float comes up in a predetermined amount, the sensor detects that the water in the predetermined amount is stored. As the water amount detecting sensor  205 , that having another structure may also be used. 
     In either structure, in the case where the water in the predetermined amount is detected by the water amount detecting sensor  205 , the control portion  19  lowers the cooling performance of the cooling device  100 . For example, the control portion  19  increases a set temperature of the cooling device  100  and widens a driving interval of the cooling device  100 . As a result, an air of the drainage from the cooling device  100  is decreased, so that overflow of the water from the drainage receiving tray  204  is prevented. 
     That is, in this embodiment, as a countermeasure to leakage of the drainage in the event of accident, the drainage receiving tray  204  is provided below the sheet member  201   b  disposed at the lowest portion. Further, the drainage receiving tray  204  is provided with the water amount detecting sensor  205  for preventing the overflow, thus preventing the overflow of the drainage from the drainage receiving tray  204  in the case where the drainage is abruptly increased in amount due to some factor. Specifically, in the case where the water amount detecting sensor  205  detects a full amount (predetermined amount) of the drainage, an instruction to drain off the drainage is provided to an operating portion and at the same time the operation of the cooling device  100  is limited. Incidentally, in the case where the drainage cannot be drained off and the water amount is increased even when the operation of the cooling device  100  is limited, e.g., in the case where the water amount detecting sensor  205  detects a second predetermined amount which is larger than the above-described predetermined amount, the cooling device  100  may also be stopped. 
     Further, in this embodiment, as shown in  FIG. 1 , a temperature sensor  20  as a temperature detecting means for detecting the temperature in the casing C is provided. In the case of the example illustrated in  FIG. 1 , the temperature sensor  20  is disposed in the neighborhood of the image forming portion Py remotest from the air supply fan  102 . Further, the control portion  19  lowers productivity of the image forming apparatus main assembly B in the case where a predetermined temperature is detected by the temperature sensor  20 . For example, an image forming speed is lowered by widening an interval of image formation. As a result, an amount of heat in the image forming apparatus main assembly B is reduced, so that temperature rise in the casing C can be suppressed. 
     Particularly, as described above, in the case where the operation of the cooling device  100  is limited, the temperature in the casing C can be increased and therefore in such a case, the temperature sensor  20  detects a predetermined temperature and then the operation of the image forming apparatus main assembly B is limited. In the case where the temperature rises even when the operation of the image forming apparatus main assembly B is limited, a user may also be notified of a massage of that effect by giving a warning or the like. 
     Incidentally, in this embodiment, the lower sheet member  201   b  extends to the neighborhood of the bottom of the drainage receiving tray  204  and therefore, even in the case where the drainage on the drainage receiving tray  204  is not drained off, the sheet member  201   b  sucks up the water stored in the drainage receiving tray  204 . Then, as described above, the air is supplied to the sheet member  201   b  by the second exhaust fans  130   a  and  130   b  to dry the sheet member  201   b . For this reason, the amount of the water on the drainage receiving tray  204  can be reduced with time. 
     SPECIFIC EXAMPLE 
     A specific example in this embodiment will be described. First, as the air supply fan  102 , two axial-flow fans each having substantially  120  mm-square shape as seen from the front side are used, and an amount of supply air is about 1-3 m 3 /min. Further, as the exhaust fan  120 , a single axial-flow fan having substantially 120 mm-square shape as seen from the front side is used, and the amount of supply air is about 0.5-1.5 m 3 /min. Further, as the second exhaust fans  130   a  and  130   b  for exhausting principally the heat of the fixing device  15 , two axial-flow fans each having substantially 120 mm-square shape as seen from the front side are used, and the amount of the supply air is about 1-3 m 3 /min. Further, as the third exhaust fans  203  for exhausting the air in the drainage treatment portion  200 , six axial-flow fans each having substantially 60 mm-square shape as seen from the front side are used, and the amount of supply air is about 1.2-3.3 m 3 /min. 
     Further, the performance of the cooling device  100  is such that the air outside the casing C is lowered in temperature by about 5-10° C. and then is supplied. For this purpose, the power of the cooling device  100  is about 500 W and the rated power of the compressor is about 150 W, and as the coolant, R134a, improved in operation efficiency, of HFC as an alternative to CFCs (chlorofluorocarbons) is used. Such a cooling device  100  drains off the water in an amount of about 0.3 litter per hour (about 0.3 L/h) in an environment of 27° C. and 70% RH. 
     Therefore, by the exhaust of the air from the second exhaust fans  130   a  and  130   b , the drainage may only be required to be evaporated in the amount of 0.3 liter per hour. Here, an exhaust temperature of the air from the second exhaust fans  130   a  and  130   b  is about 60° C., and a flow rate of the air exhausted from the second exhaust fans  130   a  and  130   b  is about 150 m 3 /h. Further, in the case where the specific gravity of the water is 1.0 g/cm 3 , the water of 0.3 L (300 cm 3 ) is 300 g. For this reason, the water of 2 g per 1 m 3  (300 g/150 m 3 ) of the amount of the air of the second exhaust fans  130   a  and  130   b  is evaporated. The saturated vapor volume at 60° C. is 129.86 g/m 3  and therefore 2/129.86=0.015 . . . (about 2%). 
     From the above, the air exhausted from the second exhaust fans  130   a  and  130   b  passes through the sheet members  201   a  and  201   b  and when the humidity of about 2% is raised, the whole drainage from the cooling device  100  is evaporated in calculation. In other words, when the material and area of the sheet members  201   a  and  201   b  are determined so as to satisfy such a condition, the whole drainage from the cooling device  100  can be evaporated. For this purpose, as the sheet members  201   a  and  201   b , a 5 mm-thick non-woven fabric which uses, as a starting material, aramid fiber having a standard (official) moisture regain of 5.5% and which is 0.25 g/cm 3  in standard density is used. However, the material for the sheet members  201   a  and  201   b  is not limited to such a material but may also be a material formed of, e.g., a foam member so long as the material has properties of high water absorption, high water retention and high humidity retention. 
     According to this embodiment, the water drained off from the cooling device  100  is retained in the sheet members  201   a  and  201   b  and the air warmed in the image forming apparatus main assembly B is supplied to the sheet members  201   a  and  201   b , so that the water retained in the sheet members  201   a  and  201   b  can be efficiently evaporated. For this reason, there is no need to frequently store the water in the tank and then drain of the water and is no need to effect pipework, so that the treatment of the drainage generated by providing the cooling device  100  can be effected easily. Particularly, in this embodiment, as the air (the heat to be exhausted) warmed in the image forming apparatus main assembly B, the exhaust heat of the fixing device  15  is principally used and therefore is helpful in recycling of heat. 
     &lt;Second Embodiment&gt; 
     Second Embodiment of the present invention will be described with reference to  FIGS. 7 and 8 . This embodiment is the same as that in First Embodiment except that a structure of a liquid evaporation portion  210   a  as a liquid evaporation means of a drainage treatment portion  200 A is different from a corresponding structure in First Embodiment. For this reason, portions similar to those in First Embodiment will be omitted or simplified from description and illustration. In the following, the portion different from that in First Embodiment will be principally described. 
     In this embodiment, a plurality of liquid evaporation portions  210  each constituted by a tray  210 H and a sheet member  210 V are disposed along the direction of gravitation. The plurality of trays  210 H and the plurality of sheet member  210 V are alternately disposed along the direction of gravitation. 
     Of these members, each of the plurality of trays  210 H is disposed slightly inclined from the horizontal direction so as to be directed downward with a distance close to an end of a bottom plate  210 Ha. Further, the directions of the inclination of the bottom plates  210 Ha are alternately opposite from each other. Further, the respective trays  210 H are arranged in the direction of gravitation with a predetermined interval. Further, each bottom plate  210 Ha is provided with a cut  210 Hb at a lower end portion with respect to the inclination direction, so that the water which reaches the surface of the bottom plate  210 Ha flows along the inclination direction of the bottom plate  210 Ha and then is dropped from the cut  210 Hb. 
     Further, the sheet member  210 V is formed in a sheet shape and is disposed so as to hang from the cut  210 Hb of each of the above-described plurality of the trays  210 H, and a lower end of the sheet member  210 V is close or contacted to an upper end portion of the lower tray  210 H with respect to the inclination direction. Further, the sheet member  210 V is formed with a mesh-like member so that the drainage is easily evaporated by decreasing a drop speed of the drainage in the sheet member  210 V by the capillary action and by increasing the surface area of the sheet member  210 V. The mesh used in this case may desirably have the number of meshes of 50-100 per inch and an aperture of about 30-40%. Incidentally, the sheet member  210 V may also be the nonwoven fabric or the foam member similarly as in First Embodiment described above. 
     In this embodiment, as indicated by an arrow f in (b) of  FIG. 8 , the water dropped from the drainage pipe  150  onto the uppermost tray  210 H flows along the inclination direction toward the lower side of the bottom plate  210 Ha to reach the cut  210 Ha. Then, by the capillary action, the water gradually moves in the sheet member  210 V. Further, the water which is no evaporated by the sheet member  210 V is sent to the tray  210 H present below the sheet member  210 V. 
     In this embodiment, these operations are repeated plural times. Then, to the thus constituted plurality of liquid evaporation portions  210 , the high temperature air (the exhaust heat) of the fixing device  15  is blown through the second exhaust fans  130   a  and  130   b . Thus, the high temperature air passes through the gaps between the trays  210 H provided in plurality along the direction of gravitation and then passes through the sheet members  210 V as indicated by arrows d in  FIG. 7 . In this way, by passing the high temperature fixing exhaust heat through the neighborhood of the liquid evaporation portion  210 , the water on the liquid evaporation portion  210  is evaporated. The exhaust air which absorbs the water of the liquid evaporation portion  210  is exhausted to the outside of the image forming apparatus by the third exhaust fans  203  as indicated by arrows e. 
     Incidentally, also in this embodiment, similarly as in First Embodiment, the amount of the water stored in the drainage receiving tray  204  disposed below the lowermost sheet member  210 V with respect to the direction of gravitation is detected by the water amount detecting sensor  205 . Other structures and actions are the same as those in First Embodiment. 
     &lt;Other Embodiments&gt; 
     In the above-described embodiments, the exhaust heat of the fixing device  15  is used but irrespective of this, the water retained by the liquid evaporation means may also be evaporated by using the air warmed in the image forming apparatus main assembly (e.g., the air warmed by the developing device or the various driving motors). For example, the air exhausted by the exhaust fan  120  may also be sent to the drainage treatment portion. 
     Further, the sheet members  210   a ,  210   b  and  210 V is not limited to the nonwoven fabric (including the mesh-like member) and the foam member but may also be those by which the liquid can be evaporated by retaining another liquid and by circulating the air. Further, the sheet members may also be formed by appropriately combining the nonwoven fabric, the foam member or another member. Further, the mesh-like member in Second Embodiment may also be applied to the sheet member in First Embodiment. 
     Further, in the embodiments described above, the plurality of the second exhaust fans for feeding the air to the liquid evaporation means are disposed along the direction of gravitation but the constitution of these (plural) fans may preferably be such that the amount of the air supplied to the liquid evaporation means present at an upper side with respect to the direction of gravitation becomes smaller. By employing such a constitution, the amount of the air supplied to the upper-side liquid evaporation means with the large water content becomes small and the amount of the air supplied to the lower-side liquid evaporation means with the small water content becomes large. As a result, the water of the upper-side liquid evaporation means with the large water content is blown off without being evaporated, so that it is possible to prevent, e.g., wetting of the outside of the image forming apparatus. On the other hand, even when the air in the large amount is supplied to the lower-side liquid evaporation means which is decreased in water content, the water is evaporated sufficiently. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 004916/2011 filed Jan. 13, 2011, which is hereby incorporated by reference.
An image forming apparatus includes an image forming unit configured to form a toner image on a recording material, a fixing unit configured to fix the toner image on the recording material by heat, a cooling air generating unit configured to generate a cooling air, and an air supplying unit configured to supply the cooling air toward the image forming unit. In addition, a primary water absorbing portion is configured to absorb water generated by the cooling air generating unit, and a secondary water absorbing portion is disposed at a position lower than the primary water absorbing portion in a gravitational direction and configured to absorb the water supplied through the primary water absorbing portion. An air blowing unit blows air warmed by the fixing unit toward the primary and secondary water absorbing portions to evaporate the water absorbed in the first and secondary water absorbing portions.