Abstract:
A fan filter unit for use in cleanrooms is provided, which decreases efficiently the concentration of chemical substance existing in the atmosphere while the expansion of the air circulating space and the electric power consumption increase of the fan-driving motor are suppressed. The unit comprises an enclosure having an air inlet through which an outside air is introduced into the enclosure and an air outlet through which a cleaned air is emitted or discharged from the enclosure; a chemical filter mounted in the enclosure to remove chemical substance existing in the outside air; a dust filter mounted in the enclosure to remove dust existing in the outside air; a fan mounted in the enclosure to introduce the outside air into the enclosure through the air inlet and to emit the cleaned air to outside of the enclosure; and a bypassing path for returning part of the outside air that has penetrated the chemical filter to an upstream side of the fan without penetrating the dust filter. A damper may be additionally provided in the bypassing path for adjusting the amount of the outside air returned to the upstream side of the fan. Preferably, the damper is adjusted in such a way that the velocity of the air at the air outlet of the enclosure is set at a specific value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fan filter unit to be installed in cleanrooms of factories for manufacturing semiconductor devices, liquid-crystal panels, and films and more particularly, to a fan filter unit comprising a ventilation fan and dust and chemical filters incorporated into an enclosure, which removes efficiently dust and chemical substance existing in the atmosphere of a cleanroom by the chemical and dust filters and which enables energy saving of the fan and reduction of the air circulating space. 
     2. Description of the Related Art 
     To maintain the cleanliness of air in the cleanroom at a specific level, typically, cleanroom systems have been used. For example, whole laminar flow type cleanroom systems have been used for this purpose, which comprise fan filter units of this sort arranged on the whole ceiling surface of the cleanroom. FIG. 1 schematically shows the configuration of an example of the prior-art cleanroom systems of this sort. 
     The prior-art cleanroom system  100  shown in FIG. 1 comprises a cleanroom  140 , a ceiling chamber  147  formed over the cleanroom  140 , fan filter units  142  arranged in a matrix array on the whole ceiling surface  140   a  of the cleanroom  140 , an underfloor region  144  defined by floor panels  143  arranged on the floor of the cleanroom  140 , a cooling coil  145  for air-temperature control mounted in the region  144 , and an air circulation path  146  that connects the region  144  with the chamber  147 . Each of the fan filter units  142  includes a ventilation fan  148  and a dust filter  141 . Each of the floor panels  143  has a punched or perforated structure that allows the air to penetrate. 
     With the prior-art cleanroom system  100  shown in FIG. 1, the air existing in the ceiling chamber  147  is introduced into the inside of the fan filter units  142  by their fans  148 . The air thus introduced is passed through the filters  141  to be cleaned by the same. The air thus cleaned or filtered is emitted or blown to the inside of the cleanroom  140 . At this time, the cleaned air emitted from the units  142  form a vertical laminar flow of air that heads for the floor panel  143  from the ceiling surface  140   a  of the cleanroom  140 . The cleaned air thus emitted into the cleanroom  140  flows vertically into the underfloor region  144  through the floor panels  143  and then, returns to the ceiling chamber  147  through the cooling coil  145  and the circulation path  146 . Thereafter, the air thus returned to the chamber  147  is introduced into the cleanroom  140  again. 
     Through the above-described processes, the clean air is repeatedly circulated in the cleanroom system  100 . The cooling coil  145  serves to decrease the thermal load of the circulating air and therefore, the clean air with a fixed temperature is always supplied to the cleanroom  140 . Also, since the vertical laminar flow of the air is formed in the cleanroom  140 , the inside of the cleanroom  140  can be maintained at a specific high cleanliness level. 
     The Japanese Non-Examined Patent Publication No. 9-287791 published in November 1997 discloses a cleanroom system having approximately the same configuration as that shown in FIG.  1 . 
     Although the above-described cleanroom system  100  makes the cleanroom  140  highly clean, there is an anxiety that defects occur in the product due to contamination induced by chemical substance existing in the atmosphere in the leading-edge manufacturing processes for highly miniaturized products such as ultralarge-scale integrated circuits (ULSIs). To cope with the anxiety, fan filter units having chemical filters have been developed and used, an example of which is shown in FIG.  2 . 
     The prior-art fan filter unit  250  shown in FIG. 2 comprises an enclosure or casing  252  having a first cylindrical part  252   a  and a second cylindrical part  252   b  that are coaxially connected together. The first part  252   a  is smaller in size than the second part  252   b . The bottom end of the first part  252   a  is connected to the top end of the second part  252   b . The inner space of the first part  252   a  communicates with the inner space of the second part  252   b.    
     An air inlet  252   c  is formed at the top end of the first part  252   a . A ventilation fan  253  is mounted in the first part  252   a . The fan  253  is driven by a motor (not shown) provided in the part  252   a . An air outlet  252   d  is formed at the bottom end of the second part  252   b . A dust filter  254  for removing dust or particles and a chemical filter  251  for removing chemical substance are mounted to be vertically apart from each other in the second part  252   b . The dust filter  254  is fixed to the bottom end of the second part  252   b  so as to close the air outlet  252   d . The chemical filter  251  is fixed to the inner wall of the second part  252   b  over the dust filter  254  at a specific distance. A partition plate  255  having holes  255   a  in its peripheral area is fixed to the inner wall of the second part  252   b  over the chemical filter  251  at a specific distance. The plate  255  divides the inner space of the enclosure  252  into upper and lower ones. The upper and lower spaces thus divided are connected to each other through the holes  255   a  of the plate  255 . 
     With the prior-art fan filter unit  250  shown in FIG. 2, the outside air  261  existing in the outside of the unit  250  is introduced into the enclosure  252  through the air inlet  252   c , forming the air  262 . The air  262  thus introduced into the enclosure  252  flows to reach the chemical filter  251  through the holes  255   a  of the partition plate  255 . The chemical filter  251  removes chemical substances contained in the air  262 , forming the chemical-removed air  263 . The air  263  thus filtered further flows to the dust filter  254  and penetrates the same. The dust filter  254  removes dust or particles contained in the air  263 . As a result, the purified air  264  is emitted from the outlet  252   d  of the enclosure  252  to the outside of the unit  250 . 
     The prior-art unit  250  shown in FIG. 2 can be used as the fan filter unit  142  of the prior-art cleanroom system  100  shown in FIG.  1 . In this case, the concentration of chemical substance existing in the atmosphere of the cleanroom  140  can be lowered, because the unit  250  includes the chemical filter  251 . To further decrease the concentration of chemical substance in the cleanroom  140 , there is the need to increase the flow rate of the purified air  264  emitted from the unit  250 , thereby raising the flow rate of the air  262  that penetrates the chemical filter  251 . 
     However, if the flow rate of the purified air  264  emitted from the unit  250  is increased, the overall amount of the circulating air within the cleanroom system  140  increases. This raises a problem that the air circulating space (i.e., the air circulation path  146  and the ceiling chamber  147 ) needs to be expanded. 
     Also, to allow the increased circulating air to penetrate the path  146  and the air-cooling coil  145 , the fan  253  needs to provide higher static pressure. Thus, there is a problem that electric power consumption of the motor for driving the fan  253  is raised. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a fan filter unit that decreases efficiently the concentration of chemical substance existing in the atmosphere while the expansion of the air circulating space and the electric power consumption increase of the fan-driving motor are suppressed. 
     The above object together with others not specifically mentioned will become clear to those skilled in the art from the following description. 
     A fan filter unit according to the present invention comprises: 
     an enclosure having an air inlet through which an outside air is introduced into the enclosure and an air outlet through which a cleaned air is emitted or discharged from the enclosure; 
     a chemical filter mounted in the enclosure to remove chemical substance existing in the outside air; 
     a dust filter mounted in the enclosure to remove dust existing in the outside air; 
     a fan mounted in the enclosure to introduce the outside air into the enclosure through the air inlet and to emit the cleaned air to outside of the enclosure; and 
     a bypassing path for returning part of the outside air that has penetrated the chemical filter to an upstream side of the fan without penetrating the dust filter. 
     With the fan filter unit according to the present invention, the bypassing path is provided for returning part of the outside air that has penetrated the chemical filter to an upstream side of the fan without penetrating the dust filter (i.e., without passing through the air circulating space and the air cooling coil). Thus, the necessary pressure loss occurring in the circulation of the outside air through the dust filter and other necessary members such as floor panels, a cooling coil, and an air circulating space can be reduced. As a result, the electric power consumption increase of the fan-driving motor can be suppressed. 
     Moreover, since the amount of the air penetrating the chemical filter is increased without increasing the overall amount of the air that is circulated in the cleanroom, the concentration of chemical substance existing in the atmosphere of the cleanroom can be decreased while the expansion of the air circulating space can be suppressed. 
     In a preferred embodiment of the unit according to the invention, a damper is further provided in the bypassing path for adjusting the amount of the outside air returned to the upstream side of the fan. In this case, it is preferred that the dumper is adjusted in such a way that the velocity of the air at the air outlet of the enclosure is set at a specific value. 
     In another preferred embodiment of the unit according to the invention, a sensor or detector is further provided for sensing or detecting the concentration of chemical substance existing in the air, in which the damper is controlled on the basis of the result of sensing or detection. In this case, it is preferred that the sensor or detector is used to sense or detect the concentration of chemical substance existing in the air in a cleanroom itself or in an air circulating space of a cleanroom. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings. 
     FIG. 1 is a schematic cross-sectional view showing the configuration of a prior-art cleanroom system equipped with fan filter units on its ceiling. 
     FIG. 2 is a schematic cross-sectional view showing the structure of a prior-art fan filter unit into which a chemical filter and a dust filter are incorporated. 
     FIG. 3 is a schematic plan view showing the configuration of a fan filter unit according to a first embodiment of the present invention. 
     FIG. 4 is a schematic cross-sectional view along the line IV—IV in FIG.  3 . 
     FIG. 5 is a schematic cross-sectional view showing the configuration of a cleanroom system equipped with the fan filter units according to the first embodiment of the invention on its ceiling. 
     FIG. 6 is a schematic cross-sectional view showing the configuration of a fan filter unit according to a second embodiment of the invention, which is along the line IV—IV in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below while referring to the drawings attached. 
     First Embodiment 
     A fan filter unit according to a first embodiment of the invention is shown in FIGS. 3 and 4. 
     As shown in FIGS. 3 and 4, a fan filter unit  10  according to the first embodiment is comprised of an enclosure or casing  13  with an approximately rectangular plan shape. The enclosure  13  is formed by a cylindrical inner member  11  and a cylindrical outer member  12 . 
     The inner cylindrical member  11  of the enclosure  13  includes a first cylindrical part  11   a  and a second cylindrical part  11   b , both of which are extending vertically. The first and second parts  11   a  and  11   b  are fixed together to be coaxial with respect to a common vertical axis by way of plate-shaped third and fourth parts  11   c  and  11   d . The third and fourth parts  11   c  and  11   d  extend laterally. The first part  11   a  is entirely located inside the second part  11   b . The third part  11   c  is located over the fourth part  11   d  at a specific distance. The top end of the first part  11   a  is connected to the top end of the second part  11   b  by way of the third part  11   c . The bottom end of the first part  11   a  is connected to the bottom end of the second part  11   b  by way of the fourth part  11   d.    
     The outer cylindrical member  12  of the enclosure  13  includes a first cylindrical part  12   a  and a second cylindrical part  12   b , both of which are extending vertically. The first and second parts  12   a  and  12   b  are fixed together to be coaxial with respect to the common vertical axis for the inner member  11  by way of a plate-shaped third part  12   c . The third part  12   c  extends laterally. The first part  12   a  is entirely located inside the second part  12   b . The top end of the first part  12   a  is connected to the top end of the second part  12   b  by way of the third part  12   c . The bottom end of the first part  12   a  is not connected to the bottom end of the second part  12   b.    
     As clearly shown in FIG. 3, the second part  11   a  of the inner member  11  and the second part  12   b  of the outer member  12  are connected to each other with four supporting members  15  extending along the lateral, longitudinal axis of the enclosure  13 . Thus, the inner member  11  is fixed to the outer member  12  at a specific gap. 
     An air-returning path  2  is formed between the outer surfaces of the second and third parts  11   b  and  11   c  of the inner member  11  and the inner surfaces of the first, second, and third parts  12   a ,  12   b , and  12   c  of the outer member  12 . The path  2  extends along the whole outer surface of the inner member  11 . 
     A rectangular air inlet  11   e  is formed at the top end of the first part  11   a  of the inner member  11 . The inlet  11   e  is defined by the top edge of the first part  11   a . The first part  12   a  of the outer member  12  is entirely overlapped with the inlet  11   e . The third part  12   c  of the outer member  12  is partially overlapped with the inlet  11   e . Thus, only inner part of the inlet  11   e  is exposed to the outside of the enclosure  13  and the remaining part of the inlet  11   e  is exposed to the air path  2 . 
     A ventilation fan  3  is mounted horizontally in the first part  11   a  of the inner member  11 , as shown in FIGS. 3 and 4. The fan  3  is driven by a motor (not shown) mounted in the first part  11   a . The inner part of the fan  3  is exposed to the outside of the unit  10  and the outer part thereof is exposed to the air path  2 . 
     A chemical filter  1  is located in the second part  11   b  of the inner member  11 . The filter  1  is fixed to the bottom end of the second part  11   b  so as to close the bottom opening  11   f  of the part  11   b.    
     The chemical filter  1  may be made of a material such as activated carbon, activated carbon mixed with a specific chemical agent, or ion-exchange fibers, which removes alkaline gases, acid gases, or organic gases. 
     A partition plate  14  is fixed to the inner wall of the second part  11   b  of the inner member  11 . The plate  14  is located just over the chemical filter  1  at a specific distance. The plate  14 , which has penetrating holes  14   a  in its peripheral area, divides the inner space of the member  11  into upper and lower ones. The upper and lower spaces thus divided are connected together by way of the holes  14   a.    
     A rectangular air outlet  12   d  is formed at the bottom end of the second part  12   b  of the outer member  12 . The outlet  11   d  is defined by the bottom edge of the second part  12   b . The outlet  11   d  is larger than the bottom opening  11   f  of the second part  11   b  of the inner member  11  and the chemical filter  1 . 
     A dust filter  4  is located in the second part  12   b  of the outer member  12 . The filter  4  is fixed to the bottom end of the second part  12   b  so as to close the outlet  12   d . The filter  4  is apart vertically from the chemical filter  3 . 
     As the dust filter  4 , for example, a HEPA (High Efficiency Particulate Air) filter or an ULPA (Ultra Low Penetration Air) filter may be used. 
     Next, the operation of the fan filter unit  10  having the above-described configuration is explained below. 
     First, the outside air  16  existing outside the filter fan unit  10  is introduced into the inner member  11  of the enclosure  13  by way of the inner, exposed part of the air inlet  11   e  due to the action of the fan  3 , forming the inside air  17 . The inside air  17  thus formed in the member  11  is sent to the underlying chemical filter  1  due to the action of the fan  3  by way of the holes  14   a  of the partition plate  14  and then, it penetrates the filter  1 . Thus, specific chemical substance is removed from the air  17  by the filter  1 , forming the chemically filtered air  18 . 
     Part of the chemically filtered air  18  from which specific chemical substance has been removed flows to the dust filter  4  to penetrate the same toward the outside of the unit  10 . Dust existing in the air  18  is removed by the dust filter  4 , forming the cleaned or purified air  19  outside the unit  10 . The air  19  is emitted from the air outlet  12   d  of the unit  10 . 
     On the other hand, the remainder of the chemically filtered air  18  is returned to the air inlet  11   e  by way of the air-returning path  2  formed between the inner and outer members  11  and  12 . In other words, the path  2  serves as a bypassing path for returning the remaining part of the air  18  to the inlet  11   e  without penetrating the dust filter  4  and emitting to the outside. The air  18  thus returned to the inlet  11   e  is introduced again into the first part  11   a  of the inner member  11 . The chemically filtered air  18  thus returned and the outside air  16  thus newly introduced are mixed together to form the inside air  17 , which passes through the chemical filter  1 . 
     Thus, the chemically filtered air  18  that has passed through the chemical filter  1  is repeatedly circulated in the fan filter unit  10 . As a result, the chemical substance contained in the outside air  17  can be removed efficiently without increasing the flow rate of the cleaned or purified air  19  emitted from the unit  10 . 
     Next, a cleanroom system equipped with the fan filter units  10  is explained below with reference to FIG.  5 . 
     A cleanroom system  29  shown in FIG. 5 comprises a cleanroom  20 , a ceiling chamber  25  formed over the cleanroom  20 , the fan filter units  10  arranged on the whole ceiling surface  20   a  of the cleanroom  20  in a matrix array, an underfloor region  22  defined by floor panels  21  arranged on the floor of the cleanroom  20 , a cooling coil  23  for air-temperature control mounted in the region  22 , and an air-circulation path  24  that connects the underfloor region  22  with the ceiling chamber  25 . Each of the fan filter units  10  has the configuration according to the first embodiment as described above. The floor panel  21  has a punched or perforated structure that allows the air to penetrate. 
     With the cleanroom system  29  shown in FIG. 5, the air existing in the ceiling chamber  25  is introduced into the inside of the fan filter units  10  due to the sucking action of the fans  3 . The air thus introduced is cleaned or purified by penetrating the chemical filters  1  and the dust filters  4  of the units  10 . The air thus cleaned or purified is then emitted to the inside of the cleanroom  20 . At this time, the cleaned air emitted from the units  10  form a vertical laminar flow of air that heads for the floor panel  21  from the ceiling surface  20   a  of the cleanroom  20 . The cleaned air in the cleanroom  20  flows into the underfloor region  22  through the floor panels  21  and then, returns to the ceiling chamber  25  by way of the cooling coil  23  and the circulation path  24 . Thereafter, the air existing in the chamber  25  is introduced into the cleanroom  20  again. 
     Through the above-described processes, the clean air is circulated in the cleanroom system  29  as shown by arrows in FIG.  5 . The cooling coil  23  serves to decrease the thermal load of the circulating air and therefore, the clean air with a fixed temperature is supplied to the cleanroom  20 . 
     The fan filter units  10  remove efficiently desired chemical substance from the air and thus, the removing or filtering efficiency of desired chemical substance is raised. This means that the flow rate increase of the air emitted from the units  10  is not necessary. Accordingly, the air-circulation path  24  and the ceiling chamber  25  (i.e., the air circulating space) need not to be expanded for raising the efficiency of removing the chemical substance. 
     Subsequently, to compare the cleanroom system  29  including the fan filter units  10  according to the first embodiment of the invention with the prior-art cleanroom system  100  using the prior-art fan filter units  250 , the inventor calculated the pressure drop of air. 
     In this calculation, 50% of the chemically filtered air  18  was set to be returned to the inlet side of the unit  10  without passing through the dust filter  4 . The pressure losses L 1 , L 2 , and L 3  of the chemical filter  1 , the path  2 , and the dust filter  4  for the air were set at 3 mmAq, 1 mmAq, and 10 mmAq, respectively. The pressure losses L 4 , L 5 , and L 6  of the floor panels  21 , the cooling coil  23 , and the returning path  24  for the air were set at 2 mmAq, 3 mmAq, and 2 mmAq, respectively. 
     When the prior-art fan filter units  50  were used, the total pressure loss L of the air of the cleanroom system  100  is given as the sum of the pressure losses of the chemical filter  251 , the dust filter  254 , the floor panels  143 , the cooling coil  145 , and the path  146 . Thus, the following equation (1) is established. 
     
       
           L=L   1 + L   3 + L   4 + L   5 + L   6   (1) 
       
     
     As a result, total pressure loss L of the cleanroom system  100  with the units  50  is equal to 20 mmAq (i.e., L=20 mmAq). 
     On the other hand, when the fan filter units  10  according to the first embodiment of the invention were used, 50% of the air  18  is returned to the inlet side of the unit  10  without passing through the dust filter  4 . Accordingly, the total pressure loss L of the air of the cleanroom system  100  is given as the average of the sum of the pressure losses of the chemical filter  1  and the path  2  and the sum of the pressure losses of the chemical filter  1 , the dust filter  4 , the floor panels  21 , the cooling coil  23 , and the path  24 . Thus, the following equation (2) is established.              L   =         (     L1   +   L2     )     +     (     L1   +   L3   +   L4   +   L5   +   L6     )       2             (   2   )                                
     As a result, total pressure loss L of the cleanroom system  100  with the units  10  is equal to 12 mmAq (i.e., L=12 mmAq), which is (⅗) times the value with the prior-art units  50 . 
     As explained above, with the cleanroom system  29  using the fan filter units  10  according to the first embodiment, the total pressure loss L is lowered and as a result, the electric power consumption of the motors driving the fans  3  can be decreased. 
     Also, as already explained above, the unit  10  has the bypass path  12  between the inner and outer members  11  and  12  of the enclosure  13  while the dust filter  4  is located in the outer member  12 . Part of the air  18  that has passed through the chemical filter  1  is returned to the inside of the inner member  11  by way of the path  12 . Accordingly, the function of the unit  10  to remove the specific chemical substance without increasing the flow rate of the air to be emitted from the unit  10 . 
     Moreover, with the cleanroom  29  using the units  10  according to the first embodiment, the function of the cleanroom  29  to remove the specific chemical substance can be enhanced without expanding the circulating space of the air. Since the pressure loss of the air in the system  29  is decreased, the electric power consumption of the motors driving the fans  3  is lowered. 
     Second Embodiment 
     FIG. 6 shows a fan filter unit  10 A according to a second embodiment of the invention, which has the same configuration as that of the fan filter unit  10  according to the first embodiment, except that dampers  36  are provided in the returning or bypassing path  2 . Thus, the explanation about the same configuration as the unit  10  is omitted here for the sake of simplification by attaching the same reference symbols as used the first embodiment to the same or corresponding elements in FIG.  6 . 
     The unit  10 A comprises the dampers  36  located in the path  2  near the air inlet  11   e , and damper controllers  37  located onto the outer member  12  of the enclosure  13  for controlling the respective dampers  36 . 
     Each of the dampers  36  has two fins. The flow rate of the air  18  passing through the path  2  is controlled or adjusted by changing the angel of the fins with respect to the flowing direction of the air  18 . 
     Each of the damper controller  37  is electrically connected to a sensor or detector  30  for sensing or detecting the desired chemical substance. The sensor  30  is located in the cleanroom  20  or the air path  24 . The sensor  30  senses the specific chemical substance existing in the air flowing in the cleanroom  20  or path  24  and then, sends a signal S to the damper controllers  37  according to the sensing result (i.e., the concentration of the chemical substance). In response to the signal S, the controllers  37  adjusted the angle of the fins of the dampers  36 . 
     For example, when the concentration of the specific chemical substance is higher than a predetermined reference value, the controllers  37  controls the corresponding dampers  36  to widen their paths for the air  18 . Thus, the flow rate of the air  18  to be returned to the upstream side (i.e., the air inlet  11   e ) is increased and thus, the removing or filtering efficiency of the chemical substance is improved. As a result, the concentration of the chemical substance existing in the cleanroom  20  or the circulating path  24  is kept at the specific level or lower. 
     As the sensor  30 , any ion chromatograph may be used for sensing the ammonia concentration and any gas chromatograph mass spectrometer may be used for sensing the organic substance concentration. 
     As explained above, with the fan filter unit  10 A according to the second embodiment of FIG. 6, the dampers  36  are additionally provided in the bypassing path  2  to control the flow rate of the air  18  on the basis of the concentration of the chemical substance sensed by the sensor  30 , thereby controlling the flow rate of the air  18  in the path  2 . Thus, the removing or eliminating efficiency of the chemical substance or substances can be further raised compared with the unit  10  according to the first embodiment. This facilitates keeping the chemical substance or substances at or lower than the specific value. 
     VARIATIONS 
     In the above-described fan filter units  10  and  10 A according to the first and second embodiments, the bypassing path  2  is formed to extend over the whole outer surface of the second part  11   b  of the inner member  11 . However, the path  2  may be located only on the opposing outer surfaces of the second part  11   b  of the inner member  11 , in other words, the cross-section of the path  2  may be determined so that the air flows through the path  2  at a desired flow rate. 
     In the unit  10 A according to the second embodiment, the dumpers  36  are controlled on the basis of the concentration of the chemical substance detected by the sensor  30 . However, any means for measuring the air flow velocity may be provided in the vicinity of the outlet  12   d  in order to measure the flow velocity of the air  19  emitted from the outlet  12   d . In this case, the dampers  36  are controlled in such a way that the flow velocity of the air  19  emitted from the outlet  12   d  is kept at a desired value or values. 
     Needless to say, the invention is not limited to the first and second embodiments. For example, the invention is applicable to a fan module unit comprising a plurality of fan filter units and a common ventilating fan, which are incorporated into an enclosure. In this case, the same advantage as those in the first or second embodiment are given. 
     While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the present invention, therefore, is to be determined solely by the following claims.