EP0487342A1 - Hotte de laboratoire avec flux laminaire - Google Patents

Hotte de laboratoire avec flux laminaire Download PDF

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Publication number
EP0487342A1
EP0487342A1 EP91310742A EP91310742A EP0487342A1 EP 0487342 A1 EP0487342 A1 EP 0487342A1 EP 91310742 A EP91310742 A EP 91310742A EP 91310742 A EP91310742 A EP 91310742A EP 0487342 A1 EP0487342 A1 EP 0487342A1
Authority
EP
European Patent Office
Prior art keywords
jet
air
face opening
flow
working space
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP91310742A
Other languages
German (de)
English (en)
Inventor
Bernard Etkin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
AEROSPACE ENGINEERING AND RESEARCH CONSULTANTS Ltd
Original Assignee
AEROSPACE ENGINEERING AND RESEARCH CONSULTANTS Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by AEROSPACE ENGINEERING AND RESEARCH CONSULTANTS Ltd filed Critical AEROSPACE ENGINEERING AND RESEARCH CONSULTANTS Ltd
Publication of EP0487342A1 publication Critical patent/EP0487342A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
    • F24F9/00—Use of air currents for screening, e.g. air curtains
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B08—CLEANING
    • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
    • B08B15/00—Preventing escape of dirt or fumes from the area where they are produced; Collecting or removing dirt or fumes from that area
    • B08B15/02—Preventing escape of dirt or fumes from the area where they are produced; Collecting or removing dirt or fumes from that area using chambers or hoods covering the area
    • B08B15/023—Fume cabinets or cupboards, e.g. for laboratories

Definitions

  • This invention relates to a fume cabinet and to a method of operating a fume cabinet.
  • Fume cabinets are usually used to isolate experiments or tests from the environment and from the experimenter. In particular, they are usually used to protect the experimenter from emissions produced by the test process, to protect the experiment or test from contamination by unwanted gases, particulates or bacteria, and to protect the environment from the products of the test process.
  • the physical mechanisms available for transport of contaminant gasses outwardly through the front opening are molecular and turbulent diffusion.
  • the air flow into the front opening is strictly laminar, only molecular diffusion occurs, and calculations of molecular concentration show that it falls off rapidly with upstream distance.
  • the contaminant concentration may typically decrease as much as six orders of magnitude in an upstream distance of only one millimetre.
  • an air curtain fume cabinet comprising:
  • the invention provides a method of providing an air curtain barrier across the face opening of a fume cabinet having a working space accessed through said face opening, said method comprising directing an air curtain jet from one side of said face opening across said face opening to an opposing side thereof, and providing an exhaust flow at said opposing side to exhaust substantially (i) the entire flow of said air curtain jet, plus (ii) all of the air which said jet entrains from outside said face opening, plus (iii) a substantial quantity of additional air from outside said face opening, thus to increase the velocity of air from outside said face opening moving into said jet from outside said face opening adjacent said opposing side of said face opening beyond the entrainment velocity that would normally be produced by the action of said jet alone, thereby to reduce the likelihood of spillback of air from said jet to outside said face opening.
  • Figs.1 and 2 show a fume cabinet 8 according to the invention.
  • the fume cabinet includes a working space 10 defined by a lower surface 12, side walls 14, a top 16 and a back 18. At the front of the working space 10 there is a "face" or access opening 20.
  • the lower surface 12 is defined by the top of a base generally indicated at 22.
  • the base 22 includes an air inlet duct 24 which extends to the back of the base 22 (so that the front portion of the base 22 can be used for storage).
  • the duct 24 then bends upwardly and then extends forwardly and upwardly to an exit slot 26 which extends across substantially the entire width of the face 20 at the front of the lower surface 12.
  • a secondary and smaller duct 28 branches from the duct 24 and is directed to the rear of the cabinet where it joins a smaller slot 30 extending across the rear of the lower surface 12.
  • Air is drawn into the duct 24 through air filters 31 by several (e.g. three) conventional fans 32, passes through cleaning and flow smoothing screens 34, 36, and exits through slots 26, 30.
  • One or more plates 38 may be placed parallel to the flow in slot 26 to smooth and direct the flow.
  • a sash 40 extends downwardly from the front of the top surface 16 to control the size of the face or opening 20.
  • the sash 40 is moveable up and down in conventional fashion (by means not shown) to allow adjustment to the height of opening 20.
  • the sash 40 has an outwardly and upwardly turned lip 42 for a purpose to be described.
  • Duct 44 has an intake slot 46 which extends across substantially the entire width of the working space 10 and which has a substantial front to rear dimension.
  • the rear wall 48 of the duct 44 is formed as a double wall having sheets 48a, 48b joined by a smooth curve 48c, for a purpose to be described. Exhaust air is drawn from the exhaust duct 44 by an exhaust fan 50.
  • the rear ventilation of the cabinet can be omitted by eliminating secondary duct 28 and slot 30.
  • This arrangement is shown in Fig.3, in which primed reference numerals indicate parts corresponding to those of Figs.1 and 2.
  • slot 26 slants rearwardly. This is because the air curtain issuing from slot 46 is wider at its top than at its bottom, and the arrangement shown is convenient to have exhaust duct 44 swallow the entire curtain, including all the air which it entrains at least at its front, as will be explained. However the rearward slant is not necessary since the curtain will bend to accomodate itself to the flows Q′ and Q ⁇ (which flows will be described).
  • Fig.4 diagrammatically depicts duct 24, slot 26, and duct 44 with its intake slot 46.
  • Fig.2 the following symbols are used:
  • Q j represents the air curtain jet flow supplied through slot 26 by fan 32.
  • Q ex represents the exhaust flow drawn by exhaust fan 50.
  • Q s represents the flow from a contaminant source S.
  • Q en1 represents the air flow entrained into the jet from outside the space 10.
  • Q en2 represents the air flow entrained into the jet from inside the space 10.
  • Q′ and Q ⁇ represent air flows drawn into the exhaust at the top of the opening 20, from inside and outside the space 10 respectively, for the situation where the flow exhausted Q ex is greater than that required simply to swallow the jet Q and its entrained air.
  • the above flows may be expressed in any appropriate units, e.g. cubic feet per minute (cfm) or liters per minute (l/m) or cubic meters per hour (m3/h)
  • equation (1) allows for more air (Q′ and Q ⁇ ) to be exhausted than is required simply to swallow the jet Q j and its entrained air.
  • Q ex must be large enough so that Q′ is greater than zero, if no curtain air is to be spilled back into the face 20.
  • Fig.5 shows a laminar jet sheet 52 of thickness t issuing from slot 26 into still air with a uniform initial velocity v j .
  • the dividing stream lines AB and A′B′ have a precise mathematical definition and can be identified experimentally.
  • the lines AC and A′C′ are the edges of the overall jet 54 and are not as well defined.
  • the entrainment process is primarily turbulent in nature. From some distance away, the jet can be perceived as a sheet sink, drawing air inwardly, the inwardly drawn air having a velocity vector approximately perpendicular to the jet axis (as shown in Fig.4).
  • the jet edge i.e. lines AC and A′C′
  • the jet edge can be defined as the location at which the x-component of velocity becomes appreciable.
  • the jet edge can be approximately located with smoke or tufts.
  • transition zone 56 typically about 3t in length, during which the uniform velocity v j is eroded from both sides, as shown at 58 in Fig.5.
  • a cosine-squared sort of profile indicated at 60, is reached in the fully developed flow.
  • the entrainment velocity is estimated as being about one thirtieth of the original jet velocity near the jet exit, and diminishing with distance from the exit.
  • the mass transfer characteristic of the described air curtain is illustrated in Fig.6.
  • the concentration profile will then be qualitatively as shown as 62 in Fig.6, falling from concentration C0 on the right to essentially zero at a line AP.
  • the concentration to the left hand side of the jet is greater than zero and is governed there by the entrainment velocity v en and by the counterflow principle.
  • the entrainment velocity v en is about 0.05 metres per second, about 1/10 of the usual face velocity.
  • the fall off of concentration in upstream diffusion is proportional to the stream velocity, so the distance for a decrease of six orders of magnitude in a 0.05 metre per second stream may typically be 2 centimeters instead of 1 millimeter. While this appears to be a deterioration in performance, it will be realized that in actual use, laminar diffusion results are not representative. In regions such as the wake of an operator, an increase in the mean flow velocity external to the wake would result in an increase in the turbulent velocities and an expected increase in forward diffusion of the contaminant.
  • the jet 52 will issue from the slot 26, travel up the face 20, and will with its entrained air enter the exhaust slot 46 from which it is removed by exhaust fan 50.
  • the air entrained into the jet 52 from inside the working space 10 is replaced by the auxiliary air flow issuing from duct 28 through slot 30.
  • this auxiliary air flow is Q a .
  • the flow of contaminant into the working space 10 from a contaminant source S is Q s .
  • Equation 13 will be valid provided that there is no recirculation of the curtain air into the cavity, i.e. provided that there is no spill back of air from the curtain into the cavity. This requires that Q ⁇ be greater than or equal to zero or that the auxiliary flow Q a ⁇ Q en2
  • equation (14) yields: Q a ⁇ .5Q j .
  • the governing parameter is the disturbance velocity v d divided by the jet velocity, i.e. v d /v j .
  • the jet velocity v j diminishes with height above the exit slot 26, and this reduction itself depends on x/t, i.e. on the jet slot width, then the critical ratio v d /v j will also depend on the jet width.
  • Fig.8 illustrates the impact on velocity distribution when an exhaust flow Q ex of the magnitude indicated by curve 72 of Fig.7 is used.
  • the velocity of the air inflow into the curtain or jet 54 from outside was measured at the centre of the face opening 20, just in front of the curtain, and at varying heights above the lower surface 12.
  • the resulting curve is shown at 80 in Fig.8 and is plotted for a three inch thick air curtain (i.e. slot 26 was 3 inches thick).
  • a jet flow of 230 cfm was used, and the average value of v j was 4.97 feet per second at the exit slot 26.
  • the extra flow Q′ which may in a sense be considered to be a "line sink" (since it is relatively small in vertical dimension) is responsible for the higher velocities there, and is highly beneficial in controlling both the concentration of contaminants at the outside of the face opening 20, and the resistance of the air curtain to cross drafts.
  • Fig.9 The beneficial effect of the extra flow Q′ on concentration distribution is illustrated in Fig.9.
  • a "contaminant" source of helium was provided with a flow of 1 cfm.
  • the jet velocity Q j was 230 cfm and the jet thickness was 2 inches.
  • the helium source was located approximately 12 inches inside the working space 10 as measured from the left side of the slot 26, and was 1/2 inch above lower surface 12.
  • horizontal distance is plotted on the horizontal axis, with the origin or zero being at the left side of slot 26. Positive distances are measured inside the work space 10, and negative distances are distances to the left of the working space (as drawn), i.e. outside the face 20.
  • the vertical axis shows the height in inches above the lower surface 12.
  • curve 90 shows the shape of a low concentration contour (14 ppm of helium) when Q ex was 440 cfm and Q e /Q j has a value of 1.9.
  • Curve 92 shows the 14 ppm helium concentration contour when Q ex was 550 cfm and Q ex /Q j has a value of 2.4.
  • the ratio Q ex /Q j is 2.4, the 14 ppm helium concentration profile 92 stays well inside the face or opening 20.
  • the effect of increaeing the exhaust flow Q ex in reducing concentration at the face is seen from Fig. 9 to be quite dramatic.
  • Fig.10 again horizontal distance from the left side of slot 26 is shown on the horizontal axis, as in Fig.9.
  • Figs. 11 and 12 illustrate the benefits on resistance to cross flows of having the ratio Q ex /Q j substantially greater than the theoretically calculated ratio (based on average flows needed to ensure no spillback to the outside of the curtain).
  • SF6 was used as a test or contaminant source gas.
  • the cross wind speed is shown in feet per minute on the horizontal axis, and the contaminant concentration in ppm on the y axis.
  • Curve 100 shows the result with a face opening of height 27 inches
  • curve 102 shows the result when the face opening was 21 inches.
  • the concentration was measured where the face of a person would be, using the ASHRAE standard for reporting.
  • Fig. 11 the measurements were taken without a manikin, but where the manikin's face would be located, i.e. about 2 inches outside the curtain and at the height of the manikin's face.
  • the contaminant concentration at the manikin's face was measured as being .018 ppm. This level can be achieved by a conventional fume cabinet under ideal conditions. As the velocity of the cross wind increased, the contaminant level increased only very slightly, until the cross wind velocity reached 110 fpm. Then, at a face opening height of 27 inches, a very large increase in contaminant concentration at the manikin's face occurred, as indicated by curve 100. However, when the face height was reduced to 21 inches (curve 102), a cross wind of 120 fpm (the limit of the test equipment used) was unable to produce any breakdown in the curtain. The contaminant concentration at the manikin's face remained very low.
  • Fig. 12 An even better result appears from Fig. 12.
  • the Fig. 12 measurements were taken using a manikin, and using the Figs. 1 and 2 arrangement, i.e. the working space was ventilated with auxiliary air from duct 28.
  • two curves 110, 112 were plotted, both for a face opening height of 27 inches.
  • the exhaust flow Q ex was 500 cfm
  • Q ex was 700 cfm.
  • the auxiliary flow Q a was sufficient to replace air entrained into the jet from inside space 10 and was approximately 110 cfm.
  • the ratio Q ex /Q j for the ratio curtain height to jet thickness x/t up to approximately 30, is preferably between 2 and 3, and preferably between 2.4 and 3. Where the curtain is higher (x/t > 30) or where cross winds may be particularly severe, the ratio Q ex /Q j can be greater than 3, but if it is too high, more air will be exhausted (which must be cleared and which carries room heat) than is needed.
  • an exhaust flow of 700 cfm is relatively low as compared with that used in a conventional counterflow fume cabinet, where the exhaust flows are typically in the region 1000 to 1200 cfm.
  • the invention will particularly be appreciated by comparison with that shown in German Offenlegungschrift 29 17 853 (supra), and particularly Fig. 6 thereof.
  • the air curtain is shown as entraining 100 m3/h from outside the working space and 50 m3/h from inside the working space.
  • An additional boosting flow of 80 m3/h is added at the top of the air curtain and total exhaust flow from the top of the air curtain is shown as 330 m3/h. From the rear of the working space, 50 m3/h is separately exhausted.
  • the applicant's ratio x/t is typically about 15.
  • an air curtain of the height shown would try to ingest or entrain 348 m3h of air from each side.
  • the air (100 m3/h) shown as being entrained in the jet from outside is far less than that needed to provide the air curtain with the air it needs, and the exhaust flow is also far less than that required to exhaust this volume of air. The consequence is a spillback of contaminated curtain air into the room at the top of the opening.
  • the applicant's arrangement ingests significantly more air through the face than the above theoretically calculated entrainment, in order to help ensure smooth continuous inflow at the lip 42 despite momentary localized flow reversals caused by occasional intermittent bursts of turbulence.
  • both fans can be on a single shaft operated by a single motor, as shown in the German document, although additional duct work would be required in such an arrangement. In addition, such an arrangement would not deal with the possibility that the exhaust duct may become partly obstructed.
  • a sensor attached to the moveable sash which can be used to control either or both of the exhaust and curtain flows, in order to maintain them at the magnitudes and in the ratio appropriate to the sash opening.
  • the fume cabinet of the invention may be supplied without its own exhaust fan and may instead be connected to the building or laboratory exhaust fan. In that case, the air flow required for the fume cabinet exhaust will of course be specified so that the necessary exhaust flow is achieved.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Ventilation (AREA)
EP91310742A 1990-11-22 1991-11-21 Hotte de laboratoire avec flux laminaire Withdrawn EP0487342A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
CA2030579 1990-11-22
CA 2030579 CA2030579C (fr) 1990-11-22 1990-11-22 Hotte a rideau d'air; methode de realisation

Publications (1)

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EP0487342A1 true EP0487342A1 (fr) 1992-05-27

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EP91310742A Withdrawn EP0487342A1 (fr) 1990-11-22 1991-11-21 Hotte de laboratoire avec flux laminaire

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EP (1) EP0487342A1 (fr)
CA (1) CA2030579C (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001044724A1 (fr) 1999-12-14 2001-06-21 Georg Emanuel Koppenwallner Procede et dispositif pour separer et aspirer des fluides au moyen de generateurs de turbulences frontales
DE10015666A1 (de) * 1999-12-14 2001-06-28 Georg Emanuel Koppenwallner Verfahren und Einrichtung zum Erfassen, Trennen und Absaugen von fluiden Medien unter Verwendung von Frontalwirbelgeneratoren
CN113751452A (zh) * 2021-08-20 2021-12-07 哈工大泰州创新科技研究院有限公司 一种实验室通风柜
CN113899050A (zh) * 2021-09-22 2022-01-07 首都医科大学附属北京世纪坛医院 一种基于气流动力学模型的气幕空间屏障装置及系统

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3292525A (en) * 1965-09-13 1966-12-20 Donald D Jensen Protective air curtain for cooking area
DE1960485A1 (de) * 1969-01-13 1970-09-17 Nihon Kuki Sochi K K Vorrichtung zur Schattung eines Luftvorhanges unter Bildung einer pneumatisch nach aussen isolierten Innenzone
GB1237694A (en) * 1968-08-19 1971-06-30 Progress Jets Ltd Improvements relating to air curtains
DE2917853A1 (de) * 1979-05-03 1980-11-06 Heinrich Hilbers Gasabzugschrank mit belueftungsschleier und ablufteinrichtung
GB1582438A (en) * 1976-10-12 1981-01-07 Ici Ltd Heat treatment

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3292525A (en) * 1965-09-13 1966-12-20 Donald D Jensen Protective air curtain for cooking area
GB1237694A (en) * 1968-08-19 1971-06-30 Progress Jets Ltd Improvements relating to air curtains
DE1960485A1 (de) * 1969-01-13 1970-09-17 Nihon Kuki Sochi K K Vorrichtung zur Schattung eines Luftvorhanges unter Bildung einer pneumatisch nach aussen isolierten Innenzone
GB1582438A (en) * 1976-10-12 1981-01-07 Ici Ltd Heat treatment
DE2917853A1 (de) * 1979-05-03 1980-11-06 Heinrich Hilbers Gasabzugschrank mit belueftungsschleier und ablufteinrichtung

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001044724A1 (fr) 1999-12-14 2001-06-21 Georg Emanuel Koppenwallner Procede et dispositif pour separer et aspirer des fluides au moyen de generateurs de turbulences frontales
DE10015666A1 (de) * 1999-12-14 2001-06-28 Georg Emanuel Koppenwallner Verfahren und Einrichtung zum Erfassen, Trennen und Absaugen von fluiden Medien unter Verwendung von Frontalwirbelgeneratoren
CN113751452A (zh) * 2021-08-20 2021-12-07 哈工大泰州创新科技研究院有限公司 一种实验室通风柜
CN113751452B (zh) * 2021-08-20 2023-04-07 哈工大泰州创新科技研究院有限公司 一种实验室通风柜
CN113899050A (zh) * 2021-09-22 2022-01-07 首都医科大学附属北京世纪坛医院 一种基于气流动力学模型的气幕空间屏障装置及系统
CN113899050B (zh) * 2021-09-22 2023-07-07 首都医科大学附属北京世纪坛医院 一种基于气流动力学模型的气幕空间屏障装置及系统

Also Published As

Publication number Publication date
CA2030579A1 (fr) 1992-05-23
CA2030579C (fr) 1994-08-02

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