1. تهیه و تنظیم: مهندس همت جو
تهویه در آزمایشگاهها
تهیه و تنظیم: مهندس همت جو
مرکز بهداشت استان آذربایجان شرقی
زمستان 1390

2. مقدمه
هدف از سیستم تهویه ایجاد فضایی مناسب و در عین حال ایمن برای کسانی است که در آزمایشگاه یا هر مکان دیگری با مواد شیمیایی سروکار دارند.
تمامی فضای کار باید به طور پیوسته و مکانیکی با دمیدن هوای تازه از محیط بیرون به داخل ساختمان وارد و از طریق خروجی های هودها خارج شود .دودکش این اگزوز ها باید به نحوی قرار گیرند که مانع از برگشت هوای آلوده به داخل آزمایشگاه یا ساختمان شوند.
مهندس همت جو - زمستان 1390

3. به طور عادی حدوداً مرتبه در ساعت باید فضای آزمایشگاه کاملاً تخلیه و جایگزین شود. البته لازم به ذکر است بنا به نوع کار و میزان آلودگی این بازه متغیر خواهد بود.
جریان هوای تازه باید از بخش اداری وارد محیط کار آزمایشگاه شود. بنابراین لازم است که فضای کار آزمایشگاهی را از سایر فضاها جدا در نظر گرفته و از هم جدا نماییم.توصیه می شود برای توسعه فضای آزمایشگاه در برنامه آتی ،توان سیستم بیش از 25 درصد ظرفیت مورد نیاز در نظر گرفته شود.
مهندس همت جو - زمستان 1390

4. هودهای مورد استفاده در آزمایشگاههای آلی برای جلوگیری از انتشار بخارات و دودهای سمی و خطرناک حاصل از مواد شیمیایی یا واکنشهای شیمیایی به داخل آزمایشگاه بوده و اولین وسیله حفاظتی به شمار می آیند. با وجود اینکه ممکن است در تمام آزمایشها از پخش مواد شیمیایی در محیط آزمایشگاه جلوگیری به عمل آید، ولی همیشه یک سری از حوادث ناخواسته اتفاق می افتد. بنابراین برای تمامی افراد آزمایش گر لازم است که آزمایش های خود را در زیر هود انجام دهند. نکته: آلودگی صوتی حاصل از عملکرد، نباید در فاصله ی 36 اینچی از دهانه ی هود بیش از دسی بل باشد.
مهندس همت جو - زمستان 1390

5. محل استقرار اگزوزهای تهویه، هودها و هواکش ها به نحوی تعبیه شوند تا آلودگی محیط کار را به حداقل برسانند. تمامی آزمایشگاهها باید مجهز به سیستم کنترلی دما، سرعت تهویه و فشار باشند که بتوان نحوه ی عملکرد را، از جمله مقدار حجم ورودی و خروجی هوا را مشاهده نمود. لازم است بر روی هود برچسبی نصب شود تا دودکش مختص به آن را مشخص نماید
هودهایی که به منظور کار با مواد اسیدی همچون پرکلریک به کار میروند باید ویژگی های مختص به خود همچون مقاوم بودن خرطوم ها و اگزوزها به مواد اسیدی و داشتن فیلترهای ویژه و سیستم آب فشان برای شستشوی آن را دارا باشند.
مهندس همت جو - زمستان 1390

6. مهندس همت جو - زمستان 1390

7. هودهای با حجم هوای ثابت
هودهای معمولی ،این لفظ برای توصیف هودهایی با عملکرد حجم هوای ثابت (CAV) به کار می رود.
از آنجا که مقدار هوای خروجی ثابت است ،بنابراین سرعت ورودی هود ((CAV با ارتفاع درب ورودی آن نسبت معکوس دارد.یعنی بیشترین سرعت را در هنگامی که درب آن در پایین ترین موقعیت قرار می گیرد،دارا می باشد.
مهندس همت جو - زمستان 1390

8. مهندس همت جو - زمستان 1390

9. هود BYPASS
مسیر BYPASS در بالای درب ورودی هود قرار گرفته ،و به وسیله ی یک پنجره مشبک که به جریان هوا کمک می کند، پوشیده می شوند.
کانالهای فرعی برای تنظیم و تغییر سرعت گذر هوا ، از دهانه هود کار گذاشته می شوند.وقتی درب هود تقریباً در پایینترین موقعیت خود قرار می گیرد ،تا حدی باعث افزایش سرعت جریان هوا می شود. در کل حجم جریان هوا را بدون در نظر گرفتن موقعیت درب هود نسبتاً ثابت نگه می دارند. کانالها یفرعی در بالای درب هود قرار گرفته و به وسیله یک پنجره مشبک که به جریان هوا کمک می کند،پوشیده می شوند.این کانالها برای تنظیم و تغییر سرعت گذر هوا ،از دهانه هود کار گذاشته می شوند.
مهندس همت جو - زمستان 1390

10. مهندس همت جو - زمستان 1390

11. Constant Air Volume Fume Hood
Fume Hood Control
Constant Air Volume Fume Hood
A fume hood’s exhaust system can be either a constant air volume system (as seen in the slide here), or a variable air volume system.
The constant air volume fume hood maintains the volume of air being exhausted regardless of the size of the face opening or sash configuration. This volume of air is in units of CFM – cubic feet per minute.
The face opening is the open part of the fume hood – where the occupant accesses the fume hood. I will go into more detail later.
As can be seen above, in a constant air volume fume hood design, the exhaust air only varies from a maximum to approximately 95% as the sash (or fume hood window) is moved from a full open to full closed position.
Advantages to this type of control:
Simple to control – no sort of adjustment in exhaust air volume is needed as the sash is opened or closed. The hood is designed to always exhaust the same amount of airflow.
Less parts that make-up the control system – this means there are less parts that can break or need preventative maintenance. There really is no control system…it is just how the hood is built.
Disadvantages:
Very expensive due to energy being wasted – the exhaust system must continually pull the same amount of air volume regardless of sash position.
As can be seen, the exhaust system must continue to pull approximately the same amount of air from the fume hood no matter which position the sash is in. When the sash is fully closed, the air moves through a part of the fume hood called a bypass. The bypass opening is intended to provide an alternate open area for the makeup air to flow into the fume hood when the sash is not fully open. The bypass area should be the same as the fume hood open face area. This will prevent the air velocity entering the hood from changing significantly. The bypass area on a fume hood is typically hidden from view of the occupant – it might be covered by a set of louvers or a grill.
As I mentioned before, the exhaust fans, supply fans, cooling, heating, and humidification all cost money to operate, and this type of fume hood control will not provide any savings to those costs.
Some savings can be found if a two-position constant volume system is considered. In this type of system, when the sash is closed, the main building exhaust system will reduce to a lower speed or airflow setting. This is still not extremely efficient.
مهندس همت جو - زمستان 1390

12. Variable Air Volume Fume Hood
Fume Hood Control
Variable Air Volume Fume Hood
A better method of control is variable air volume. The variable volume fume hood uses a device (some sort of airflow control element) to vary the exhaust airflow from the fume hood based on sash position.
As can be seen, the exhaust airflow is reduced from maximum when the sash is open to approximately only 20% when the sash is closed. This translates to overall savings on heating, cooling, humidification, supply and exhaust airflow. The main building systems do not have to work as hard to get the air through the building when the hoods are not being used (when the sash is closed). This equals an overall savings in cost to keep the building running.
The bypass area is much smaller on this type of fume hood. The bypass here is used to keep a minimum exhaust airflow moving through the hood at all times. This way, any fumes from chemicals that are being stored in the hood will not move into the room.
Advantages:
Energy savings – building fans, cooling, and heating systems do not have to work as hard and typically do not have to be sized as large.
Disadvantages:
More moving parts that can possibly break or need service in the future.
Much more complex to control.

13. Fume Hood Face Velocity
Once the position of the sash is known, one can calculate the face velocity of the fume hood. The movement of the sash (as just seen) directly has an impact on “face velocity”.
What is face velocity? Face velocity is the speed air enters the face opening (or sash opening) of a laboratory fume hood. Face velocity is usually expressed in units of feet per minute.
Face velocity is calculated via the shown equation. The exhaust air volume is typically shown as CFM.
The unknowns in this equation make-up the basis of the project. The area of the sash is a user-defined value. The user is the lab occupant. The value is determined by the position in which they place the sash. A fume hood will always have a fixed width that does not vary. The sash movement will give the other value for the area. For this project, you will need to design a way to determine how open or closed the sash is at the present time.
Now, since the open sash area is now known, you will need to design a way to control the exhaust airflow leaving the fume hood. That is how the face velocity of the fume hood will be maintained.
This process must be highly repeatable, very accurate, and have a very quick response time. The health and safety of the lab occupant depends on it.
مهندس همت جو - زمستان 1390

14. Fume Hood Airflow Control Via Siemens Venturi Air Valve
Control Process:
1. Controller Determines Sash Position
VAV Fume
Hood
Controller
Siemens
Venturi
Air Valve
2. Controller Calculates Total Open Area
= Sash Height x Width
400 cfm
4 Ft2
+ Fixed Open Area
Airflow Sensor
3. Controller Calculates Req’d. Exhaust
= Area Ft2 x Face Velocity
(4 Ft2 x 100 Ft/Min = 400 Ft3/Min (400 CFM)
4. Measure Airflow & Compare
5. Position Cone
Sash
Sensor
Update Display Panel FPM

15. Constant Volume Bypass
Laboratory Ventilation
Air Consumption vs. Type of Hood
Constant Volume Bypass
FULL EXHAUST AT ALL TIMES
LARGE BYPASS
AREA
SASH
AIRFLOW RELATIVELY
CONSTANT AT ALL TIMES
CAV bypass hood exhaust to remain
“essentially unchanged” (less than 5% change)….Z9.5
VERY COSTLY TO OPERATE
6’ FUME 1250 CFM
ANNUAL COST*….$3750 to $6250
*Annual average cost of $3 to $5 per CFM
FULLY
CLOSED
50%
OPEN
100%
OPEN
مهندس همت جو - زمستان 1390

16. Constant Volume Bypass - 2 Position
Laboratory Ventilation
Air Consumption vs. Type of Hood
Constant Volume Bypass - 2 Position
EXHAUST REDUCTION DURING UNOCCUPANCY
UNOCCUPANCY
DAMPER W / 2 POSITION
CONTROL
LARGE BYPASS
AREA
ROOM CONTROLLER
SASH
SASH CLOSED
SAME AS REGULAR CAV
DURING OCCUPANCY.
AIRFLOW REDUCED TO 20%
DURING UNOCCUPANCY.
WORKS IN CONJUNCTION NNWITH SUPPLY AIR REDUCTION.
FULLY
CLOSED
100%
OPEN
50%
OPEN
مهندس همت جو - زمستان 1390

17. Fume Hood Shutdown Requirements
1. Maintain Required Room Ventilation Rate .
2. Maintain Room Negative Pressurization if ……..Hazardous Substances Are Present in Room.
3. Remove All Chemicals From Fume Hood.
W a r n i n g
Fume Hood is
Not Operational.
Do Not Use ! +
100.5
4. Post Sign:
W a r n i n g
Fume Hood is
Not Operational.
Do Not Use !

18. Laboratory Ventilation Control INFLOW FROM ADJACENT AREAS
CAV 2 - Position
Laboratory Ventilation Control
SUPPLY
AIRFLOW
EXHAUSTAIRFLOW
ROOM SUPPLY
AIR TERMINAL
CAV 2 - POSITION
CONTROLLER
ROOM
CONTROLLER
UNOCCUPIED
INFLOW FROM ADJACENT AREAS
Laboratory Room
CAV FUME HOOD
مهندس همت جو - زمستان 1390

19. Variable Air Volume Laboratory Ventilation
Air Consumption vs. Type of Hood
Variable Air Volume
EXHAUST MODULATION FOR CONSTANT FACE VELOCITY
VAV FUME HOOD
FACE VELOCITY
CONTROL BY
SASH SENSING
RESTRICTED BYPASS
SASH
SASH POSITION
AIRFLOW CONTROLLED TO ENSURE
CONSTANT FACE VELOCITY.
AIRFLOW REDUCED TO 20% WHEN
SASH IS FULLY CLOSED.
COST OF OPERATION TYPICALLY
40% OF CAV BYPASS FUME HOOD
FULLY
CLOSED
100%
OPEN
50%
OPEN
ALSO REQUIRES VAV ROOM
VENTILATION CONTROL.

20. VAV Laboratory Ventilation Control INFLOW FROM ADJACENT AREAS
SUPPLY AIRFLOW
EXHAUST AIRFLOW
ROOM SUPPLY
AIR TERMINAL
VAV FUME HOOD
CONTROLLERS
EXHAUST
Laboratory
Room
ROOM
CONTROLLER
INFLOW FROM ADJACENT AREAS
VAV FUME HOODS
مهندس همت جو - زمستان 1390

21. Laboratory Room Ventilation
Negative Room Pressurization
Inward
Airflow =
 Airflow Tracking
Airflow Tracking
Diff. Pressure Sensing
Airflow Tracking
EXHAUST
SUPPLY

22. Dynamic Graphics FUME HOOD 046 مهندس همت جو - زمستان 1390 SASH HEIGHT
28 Inches
SASH HEIGHT
18 Inches
SASH HEIGHT
0 Inches
EXHAUST
250 CFM
EXHAUST
525 CFM
EXHAUST
800 CFM
FACE VELOCITY
64 Ft./Min.
FACE VELOCITY
103 Ft./Min.
FACE VELOCITY
NORMAL
مهندس همت جو - زمستان 1390

23. Average Face Velocity Recommendations
ACGIH
NIOSH
ANSI (Standard Z )
NFPA
OSHA
NIH
SEFA
80 to 100 fpm
100 to 150 fpm
80 to 120 fpm
60 to 100 fpm
100 fpm
ACGIH – American Conference of Governmental Industrial Hygienists
NIOSH – National Institute for Occupational Safety and Health
ANSI – American National Standards Institute
NFPA – National Fire and Protection Agency
OSHA – Occupational Safety and Health Administration
NIH – National Institutes of Health
SEFA – Scientific Equipment & Furniture Association
Governmental and industry organizations have adopted fume hood standards.
Standards are intended to designate face velocities that are high enough to contain fumes but not so high as to cause air turbulence between a hood’s face and a worker standing at the hood’s face.
As can be seen, the 100 feet per minute is a very good estimate for average face velocity.
مهندس همت جو - زمستان 1390

24. طبقه بندی هود براساس میزان خطرات مواد شیمیایی:
Face velocity
Hazard rate
طبقه بندی هود
125 fpm
High A
100 fpm
Moderate B
75 fpm
low c
مهندس همت جو - زمستان 1390

25. Fume Hood Testing – ASHRAE* 110 Standard
Measurements of Face Velocity
Air Flow Visualization Test (Smoke Test)
Tracer Gas Containment Test
There are many recommendations for average face velocity from industry and government groups. There are also many different fume hood manufacturers and equally as many types of laboratory layouts (as determined by the consulting engineer for a new building).
How can a lab occupant be certain that their fume hood is operating correctly and maintaining an average 100 fpm face velocity?
ASHRAE has come up with a “standard”.
ASHRAE - American Society of Heating, Refrigerating and Air-Conditioning Engineers
ASHRAE 110 Standard – Method of Testing Performance of Laboratory Fume Hoods (a protocol for fume hood testing)
A reproducible method for testing laboratory fume hoods
This protocol does not specify a performance level fume hoods should meet, it simply provides a complete protocol for fume hood performance testing.
Three part test:
Face velocity measurements and testing
Air flow visualization test (commonly known as a smoke test)
Tracer Gas Containment Test
* American Society of Heating, Refrigerating and Air-Conditioning Engineers
مهندس همت جو - زمستان 1390

26. Measurements of Face Velocity
Even though face velocity testing is just one of three tests called for in the ASHRAE 110 test, it is often the only test performed once a fume hood is installed in a laboratory.
Some reasons why: the full ASHRAE 110 test is very expensive. The cost of the recommended tracer gas and the equipment that tests for this gas can approach $20,000. The full test is also quite time consuming to complete (approximately 2 hours per hood). And face velocity is also the only item required to be tested by most regulatory organizations.
How face velocity is tested:
The fume hood face is divided into a 1 foot square grid pattern
Face velocity readings are taken at the center of each grid space with a velocity meter (e.g. anemometer)
The tester records average, minimum, and maximum face velocity readings
مهندس همت جو - زمستان 1390

27. Airflow Visualization Test (Smoke Test)
A smoke stream is generated at designated points within a fume hood.
This test provides a visual understanding of air flow currents that exist within a fume hood.
Smoke is discharged around the hood (front, sides, top, etc.). The subsequent smoke pattern is then observed.
A good smoke visualization would be where the smoke is completely “caught” in the fume hood and pulled away from the occupant and out of the lab. It is required that no smoke reversal happens. Reversal is when the smoke comes back out of the sash instead of being pulled out by the exhaust system.
Results are usually reported as a qualitative judgment of air flow distribution according to ratings of:
Fail – smoke is visually observed escaping from the hood
Poor – slow capture, reversal of flow evident near openings
Fair – no visible escape, limited turbulence of flow inside hood
Good – no visible escape, no turbulence in hood, good capture and clearance of smoke from hood
As an example of a “good” result of a smoke test – see video
مهندس همت جو - زمستان 1390

28. Tracer Gas Containment Test
The third part of the ASHRAE 110 test is the Tracer Gas Containment Test
This test uses a tracer gas (typically sulfur-hexafluoride (SF-6)) – It is an inert gas that does not occur naturally, with no commercial use. This means that background levels in a typical lab will be very low and easily be able to be detected.
A mannequin is used to simulate the aerodynamics of an actual fume hood user. The mannequin has a tracer gas monitoring device affixed in its breathing zone.
The test is looking for the fume hood leakage rate that a lab user would be exposed to.
Components of the test:
Tracer gas regulator
Tracer gas cylinder
Tracer gas nebulizer/ejector
The actual tracer gas cloud
Monitoring device attached to the mannequin’s breathing zone
Tracer gas analyzer
Strip chart recorder
This test typically done at the manufacturer’s company on new fume hoods.
Benefits of the tracer gas test
reduces risk of over-exposure to hazardous chemical vapors in a work process
verifies fume hood containment level in breathing zone of occupant
detects problems more thoroughly than fume hood face velocity testing and smoke testing alone
مهندس همت جو - زمستان 1390

29. Tracer Gas Test - Video Simulation
مهندس همت جو - زمستان 1390

30. توصیه های ایمنی در هنگام کار با هود ها
تمامی افراد باید به نحوه کار صحیح با هودها آشنایی داشته باشند.
همیشه در هنگام کار در زیر هودها ،حداقل به اندازه ی 6 اینچ محل استقرار لوازم آزمایشگاهی و مواد مورد نیاز در داخل هود از درب آن دورتر قرار دهید.
نباید هودها را همچون کابینت های مواد شیمیایی استفاده نمود و از قرار دادن وسایلی که مانع بستن درب ورودی آن می شود خودداری نمود.
از قرار دادن بطری ها یا ظروف بدون درپوش در داخل هودها خودداری کنید.
مهندس همت جو - زمستان 1390

31. نکات مهم در نگهداری و کار با هودها
بازرسی و تنظیم عملکرد آن طبق دستورالعمل و توسط افراد متخصص صورت می گیرد.
هودها باید همیشه در بهترین وضع عملکرد آنها همانطور که اشاره شد باقی بمانند.
از بردن سر به داخل هود جهت بازرسی و سرکشی به لوازم آزمایشی جداًخودداری کنید.
بعد از هر بار استفاده سطح داخل هود را تمیز کنید.
تمام وسایلی که ایجاد جرقه می کنند از داخل هود یا نزدیک آن دور سازید.
مهندس همت جو - زمستان 1390

32. قانون كلي صحیح (مقاومت کمتر) ناصحیح (مقاومت بیشتر)
Streamlineبودن سيستم=
كاهش توربولانس ومقاومت
صاف وسخت بودن كانال> ناهمواربودن کانال
کوتاه بودن< طولاني بودن
مستقیم بودن < زانویی داشتن
شاخه ورودىبا زاویه زیاد>شاخه ورودی با زاویه كم
شعاع انحنا زیاد>شعاع انحنا کم
قطرزياد < قطركم
کانال گرد < کانال مربعی