CLEAN BENCH

Abstract: A clean bench includes a working space (1) being formed with air blowing ports (18) and air suction ports (7), an air circulation flow passage (11) provided so as to communicate the air blowing ports with the air suction ports, a first blower (3) for producing a circulating air current in an air circulation system composed of the working space and the air circula­ tion flow passage, a filter (2) mounted in the air circulation system, a second blower (15) for feeding air of the outside of the air circulation system, an air curtain blowing port (13) for supplying the air fed by the second blower to an opening (17) for working formed in the working space to provide an air curtain, and exhaust means (8, 12) located in the air circulation flow passage to release a portion of the air current to the atmosphere.

Description: CLEAN BENCH BACKGROUND OF THE INVENTION This invention relates to clean benches each equipped with an air curtain, and more particularly it is concerned with a construction of a clean bench suitable for shutting off the noxious gases generated in a working space from the outside of the working space. One constructional form of a clean bench of the prior art for handling noxious gases is disclosed in Japanese Utility Model publication No. 23514/69, and its construction is shown in Fig. 1, in which a working space 1 has a HEPA filter 2 mounted in its upper portion and air in a chamber 9 in which the clean bench is installed is directed to the HEPA filter 2 through a fan 3, so that clean air can be supplied to the working space 1. The working space 1 is enclosed by side plates 4 and a back plate 5 on three sides except for the front in which a shutter 6 with vertical sliding movement is mounted. The air supplied to the interior of the working space 1 is drawn by suction into a suction port 7' formed in the bench in a lower portion of the working space 1 together with dust produced in the working space 1 and gases, such as an organic solvent in vapor form, which should not be released to the atmosphere. The air and gas are vented by an exhaust fan 8 through an exhaust duct 12 communicating with the suction port 7'. In this case, the volume of the exhausts should be grater than the volume of air supplied to the working space 1 through the HEPA filter 2 in order to prevent the noxious gas in the working space 1 from being released to outside the working space 1 even if the shutter 6 is opened during operation. Thus, the volume of the exhausts usually set at a level 1-1.3 times as great as the volume of air supplied to the working space 1. Owing to the great volume of the exhausts, the air in the vicinity of the working space 1 has been exhausted in large volumes to the atmosphere, applying a high load to an air conditioning unit 10 in the chamber 9. Also, when the shutter 6 is opened, air containing a large volume of dust has been admitted to the working space 1, causing a reduction of the cleanliness in the working space 1. Thus, it has hitherto been usual practice to minimize the opening in the front of the working space 1 by moving the shutter 6 as little as possible. This has caused a reduction in operability. To solve this problem, a proposal has been made to mount an air curtain at an opening 17 for working as shown in Fig. 2B as disclosed in Japanese Patent Publication No. 34959/79. Figs. 2A and 2B show one embodiment of the invention disclosed therein, and Fig. 3 shows another embodiment thereof. In the embodiment shown in Fig. 3, a HEPA filter 14' for air curtain is mounted in addition to the HEPA filter 2 mounted in an upper portion of the working space, and clean air is blown through a blowing port 13' to provide an air curtain, so that introduction of dust from the outside into the working space can be avoided even if the shutter 6' is fully opened. This is conductive to improved operability. However, the problem that the volume of exhausted air is great and increases the load applied to an air conditioning unit, not shown, mounted in the chamber 9. In the embodiment shown in Figs. 2A and 2B, the air supplied to the working space and the air supplied for providing an air curtain both flow in circulation through the system. This embodiment offers the advantage that the volume of exhausted air is small and no increase is caused to the load applied to the air conditioning unit. However, gases cannot be removed by the HEPA filter and noxious gases might find their way into the air for providing the air curtain. When this is the case, if the operator stuck his arm into the working space, air containing noxious gases might flow out of the working space through the operator's arm, or when the air curtain is broken by a sudden movement of the operator1 noxious gas might flow out of the working space. SUMMARY OF THE INVENTION The object of this invention is to provide a clean bench equipped with an air curtain capable of preventing mutual contamination of the inside and the outside of a working space at a low load applied to the air conditioning unit, and particularly capable of avoiding release of noxious gases generated in the working space to the outside of the working space. According to the invention, there is provided a clean bench comprising a working space formed with air blowing ports and air suction ports, an air circulation flow passage provided so as to communicate the air blowing ports with the air suction ports, a first blower means for producing a circulating air current in an air circulation system composed of the working space and the air circulation flow passage, a filter mounted in the air circulation system, a second blower means for feeding air of the outside of the air circulation system, an air curtain blowing port means for supplying the air fed by the second blower to an opening for working formed in the working space so as to provide an air curtain, and exhaust means located in the air circulation flow passage for releasing a portion of the air current to the atmosphere. The volume of the air released by the exhaust means may be set at a range between over onefold and below twofold volume of air required for providing the air curtain. The inventon provides a clean bench equipped with an air curtain capable of preventing mutual contamination of the inside and the outside of the working space at a low load applied to the air conditioning unit, and particularly capable of avoiding release of noxious gases generated in the working space to the outside of the working space. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional side view of a clean bench of the prior art, showing its construction and the direction of flow of air currents; Figs. 2A and 2B are a front view and a side view, respectively, with certain parts being shown in section, of a clean bench of the prior art equipped with an air curtain; Fig. 3 is a sectional side view of a clean bench of another constructional form of the prior art which is equipped with an air curtain; Fig. 4A is a view showing the flow of air currents in the prior art shown in Figs. 1 and 2; Fig. 4B is a view showing the flow of air currents in the first embodiment of the invention shown in Fig. 5; Fig. 5 is a sectional side view showing a construction of a clean bench according to the first embodiment of the present invention; ; Fig. 6 is a sectional side view showing a construction of a clean bench according to the second embodiment of the present invention; Fig. 7 is a sectional side view showing a construction of a clean bench according to the third embodiment of the present invention; Fig. 8 is a sectional side view showing a construction of a clean bench according to the fourth embodiment of the present invention; Fig. 9 is a sectional side view showing a construction of a clean bench according to the fifth embodiment of the present invention; Fig. 10 is a sectional side view of the sixth embodiment of the present invention; and Fig. 11 is a sectional side view of the seventh embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 5, the first embodiment of the present invention will be described. A working space 1 is defined by a HEPA filter 2 mounted on its upper portion serving as a filter and air blowing parts 18, a bench formed by a perforated board and mounted on its lower portion including suction apertures 7 which serve as air suction ports, side plates 4 located on opposite sides thereof, a back plate 5 located on the rear and an opening 17 for working located in the front thereof. The opening 17 for working is defined by an air curtain blowing port 13 located in an upper portion and an air curtain suction port 16 located in a lower portion. The air curtain blowing port 13 is communicated through a HEPA filter 14 with an outlet port of an air curtain fan 15 serving as the second blower located on the upstream side of the HEPA filter 14 which draws air by suction from a chamber 9 to form an air curtain between the air curtain blowing port 13 and air curtain suction port 16. The air supplied through the air curtain blowing port 13 is fed from the outside of the working space 1, so that the air supplied through the port 13 to form the air curtain is free from the noxious gases even if the noxious gases are generated in the working space 1, thereby the safety of operation is increased. In this embodiment, clean air is supplied through the air curtain HEPA filter 14, so that no deterioration in the cleanliness of the working space 1 occurs even if the air currents forming the air curtain are incorporated in the air in the working space 1. When a slight lowering in the cleanliness of the working space 1 is tolerated depending on the type of work done in the working space 1, the outlet port of the air curtain fan 15 may be directly connected with the air curtain blowing port 13 without using the HEPA filter 14. The air suction ports 7 and air curtain suction port 16 are communicated with each other and a return duct 11 at the back of the working space 1 and providing an air return passage. In this embodiment, the air suction ports 7 and air curtain suction port 16 are separate from and independent of each other. However, the invention is not limited to this specific form of the embodiment and the air suction ports 7 may concurrently serve as the air curtain suction port 16. The return duct 11 is communicated with an inlet port of a fan 3 serving as the first blower which is located above the HEPA filter 2 and communicated at its outlet port with the HEPA filter 2 at its upstream side. An exhaust fan 8 located at an exhaust duct 12 branching from the return duct 11 cooperates with the exhaust duct 12 to constitute exhaust means. It is not essential that the return duct 11 be formed integrally with a main body of the clean bench. The return duct 11 may be formed separately from the main body of the clean bench and installed at its fixing place in such a manner that the return duct is connected with the first connection lla which communicates with the inlet port of the fan 3 and with the second connection llb which communicates with the suction port 16. The fan 3 is set to supply a volume of air O1 to the HEPA filter 2, and the air curtain fan 15 is set to supply a volume of air Q2 to the HEPA filter 14 to provide an air curtain. The exhaust fan 8 is set to exhaust a volume of air Q3 to the atmosphere. The volume of air Q1 allows an air current to flow in the working space 1 at a mean flow rate of 0.2 to 0.7 m/s which is usually used for the clean bench. The volume of air Q2 allows an air current to flow through the air curtain blowing port 13 at a flow rate of 0.7 to 20 m/s which is necessary volume to achieve the air curtain effects and which is not below a minimum flow rate of air currents necessary for preventing the organic solvents poisoning between the air curtain blowing port 13 and the air curtain suction port 16. When Q3 > Q2' it is possible to prevent the pressure in the working space 1 from becoming positive. With regard to the minimum flow rate of air currents necessary for preventing the organic solvents poisoning, it is prescribed by the "Rule for Preventing the Organic Solvents Poisoning" which is a government ordinance issued by the Ministry of Labor that it be 0.5 m/s. Thus, in this embodiment, this flow rate has been set for the air currents flowing through the working space 1. However, the invention is not limited to this specific flow rate of air currents, and any flow rate may be adopted which would conform to the rules and regulations of the respective countries. The "Rule for Preventing the Organic Solvents Poisoning" referred to hereinabove prescribes that the width of an air curtain suction port be twice that of the air curtain blowing port. Thus, by setting Q2 and Q3 at the relation Q3 = 2Q2, it is possible to keep the flow rate of air currents forming the air current at a constant, and to effectively provide an air curtain even if the flow rate of the currents introduced through the air curtain blowing port 13 is low. In the embodiment shown and described hereinabove, the ratio of the width of the air curtain suction port to that of the air curtain blowing port has been set at a value described hereinabove. However, the invention is not limited to this specific ratio and any other value may be adopted that would conform to the rules and regulations of the respective countries. If Q3 increases, then the load applied to the air conditioning unit becomes higher. Because of this it is preferred that the following realtion hold: Q2 = < ' r 2Q2. The air currents flow as shown in Fig. 4B in this embodiment. With Q2 and Q3 having the relation Q2 5 Q3 5 202, the relation Q1/3 5 Q3 < = 2Q1/3 will hold if Q2 = Q1/3 In Fig. 4A showing the flow of air currents in the prior art, the following relation holds when the volume of exhausted air is denoted by Q31:Q1 5 Q3' 5 1.3Q1. Thus, 10/39 4 Q3/Q3' r 2/3. It will be seen that the volume of air exhausted from the working space in this embodiment is about 26-678 of that of the prior art. A volume of air equal to the volume of air exhausted by the exhaust fan 8 is introduced through the air conditioning unit 10 into the chamber 9 in which the working space 1 is installed and applies to the air conditioning unit 10 a load in the form of a thermal load by outdoor air. In this embodiment, the volume of the exhausted air can be reduced to about 26-67% of that of the prior art. The provision of the air curtain eliminates the need of provision of a shutter in the opening 17 for working which would interfere with the operation, thereby increasing opeation efficiency and making it possible to connect the clean bench to the production line. Even if the shut off effect of the air curtain is transiently disturbed due to a hasty action of the operator, no problem is raised. Since the exhaust fan 8 releases to the outside the volume of air Q3 which is greater than the volume Q2 of the air curtain, the air inside of the chamber 9 in which the working space 1 is installed would immediately be drawn by suction in a volume (Q3 - Q2) through the air curtain suction port 16, so that the air leaked from the working space 1 to the chamber 9 would be quickly drawn into the air curtain suction port 16. Further, since the air curtain suction port 16 is commuricated with the air suction ports 7, vapors of organic solvents generated on the bench in the working space 1 would have its concentration reduced before reaching the fan 3, thereby keeping the concentration of gases in the working space 1 at a low level. Fig. 6 shows a second embodiment of the invention in which one air curtain is formed in the front of the working space 1 and another air curtain is formed in the rear thereof. This arrangement allows work to be done from both sides of the working space 1, thereby improving operability. Although the air curtains have been described as being provided in the front and rear of the working space 1, the invention is not limited to this specific form of the embodiment, and air curtains may be provided to the front and one side or to opposite sides. The number of the air curtains may be 3 or more. Fig. 7 shows a third embodiment of the invention in which the exhaust fan 8 is mounted in the same case as the fan 3, and the exhaust duct 12 is connected to the outside of the working space 1. This arrangement eliminates the need to perform an operation for providing wiring to the exhaust fan 8. Fig. 8 shows a fourth embodiment of the invention in which the fan 3 is mounted in a case including a portion projecting through the back of the chamber 9 to the outside for supporting the exhaust fan 8. This arrangement eliminates the need to provide an exhaust duct and does away with an operation for installation, thereby facilitating the operation of installing the clean bench. Fig. 9 shows a fifth embodiment of the invention in which the air currents are made to flow horizontally and a return duct is installed on a bottom surface of the working space 1. This arrangement allows the operator to do work through a top surface of the working space 1. Fig. 10 shows a sixth embodiment of the invention which suits a working space in which it is not essential to maintain the cleanliness at a high level. In this embodiment, the filter 2 is mounted on the upstream side of the fan 3. By providing the air blowing ports 18 with a perforated board or other member capable of regulating the flow of air, it is possible to obtain a uniform rate of flow of air currents at the blowing ports 18. Fig. 11 shows a seventh embodiment of the invention in which the fan 3 has a high capacity. In this embodiment, the exhaust fan 8 need not be mounted in the exhaust duct 12. By providing the exhaust duct 12 with a damper 19 for regulating the volume of air, it is possible to readily effect adjustments of the volume of exhausted air and the volume of air flowing in circulation through the working space 1. Additionally, a check valve, not shown, may be mounted in the exhaust duct 12. In all the embodiments shown and described hereinabove, the first blower, the second blower, the air curtain blowing port, the air curtain suction port and the air suction ports are contained in the same case as a unit. However, the invention is not limited to this specific form and the system may be split in which case the first blower, the second blower and the air curtain blowing port may be in one package and the air curtain suction port and the suction ports may be in another, so that the two packages can be installed in one building operation.

Claims: WHAT IS CLAIMED IS: 1. A clean bench comprising: a working space (1) being formed with air blowing ports (18) and air suction ports (7); an air circulation flow passage (11) provided so as to communicate the air blowing ports with the air suction ports; a first blower means (3) for producing a circulating air current in an air circulation system composed of the working space and the air circulation flow passage; a filter (2) mounted in the air circulation system; a second blower means (15) for feeding air of the outside of the air circulation system; an air curtain blowing port means (13) for supplying the air fed by the second blower to an opening (17) for working formed in the working space to provide an air curtain; and an exhaust means (8, 12) located in the air circulation flow passage to release a portion of the air current to the atmosphere. 2. fi A clean bench as claimed in claim 1, wherein the volume of air exhausted by said exhaust means is set at a value above onefold volume of air providing the air curtain and below twofold volume of air providing the air curtain. 3. A clean bench as claimed in claim 1 or 2, further comprising a filter (14) interposed between said second blower and said air curtain blowing port to supply clean air to provide the air curtain. 4. A clean bench as claimed in claim 1 or 2, wherein said second blower and the air curtain blowing port are provided to at least two planes of the working space. 5. A clean bench comprising: a working space (1) being formed with air blowing ports (18) and air suction ports (7); an air circulation flow passage (11) including an exhaust means (8, 12) for releasing a portion of an air current to the outside; a first connection (lla) communicating with said air blowing ports and a second connection (llb) communicating with said air suction ports which are capable of being connected with an air circulation flow passage (11) for communicating said air blowing ports with said air suction ports; a first blower means (3) for producing said air current flowing through an air circulation system composed of said working space and air circulation flow passage; a filter (2) mounted in said air circulation system; a second blower means (15) for feeding air of the outside of the air circulation system; and an air curtain blowing port means (13) for supplying the air fed by the second blower to an opening (17) for working formed in the working space to provide an air curtain.

Inventors: YOSHITOMI, TATSUO
Applicants: HITACHI, LTD.
Application year: 1984
Main classification: F24F9
IPC codes: B08B15, F24F3, F24F9, B25H1