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  • Alex Mathers

Fume Hood Airflow Design Considerations

Theory is when everything is simple and assumed ideal. Practice is when complex imperfection ruins your day. In theory, a fume hood face velocity requirement of 100 FPM seems simple enough – but in practice, 100 FPM is rarely simple.

The primary function of a fume hood is to safely capture, contain and remove airborne contaminants. A fume hood is a ventilated enclosure where harmful or toxic fumes or vapors can be safely handled while protecting the laboratory technician. The laminar flow, negatively pressurized environment of a hood is achieved by an exhaust system which “pulls” air from the laboratory room into and through the hood and exhaust system.

Air travels at a certain speed or velocity. The more air that is pulled through a fume hood’s openings, the faster the air will travel. The airflow of a fume hood is measured in the plane of the sash, and referred to as the face velocity, measured in feet per minute (fpm). Standards and Codes reference face velocities at which fume hoods may be operated.

It has been proven that faster air is not necessarily safer. The airflow across the face must be laminar. Many laboratory standards specifically state that operating at high velocities (above 150 fpm) can create a safety risk and health hazard due to turbulent air. A fume hood face velocity of 100 fpm is considered acceptable in standard practice by OSHA, ANSI and SEFA. This baseline is used in the California Code of Regulations:

CCR Title 8 (CalOSHA) §5154.1. Ventilation Requirements for Laboratory-Type Hood Operations.

(1) Laboratory-type hood face velocities shall be sufficient to maintain an inward flow of air at all openings into the hood under operating conditions. The hood shall provide confinement of the possible hazards and protection of the employees for the work that is performed. The exhaust system shall provide an average face velocity of at least 100 feet per minute with a minimum of 70 fpm at any point…

A mechanical engineer of record (EOR) designing a lab exhaust system connected to a fume hood must calculate the amount of air volume (in CFM) to correspond to a code required face velocity of 100 FPM at the operating sash opening. Exhaust Air valve selection and sizing is critical for the correct operation of fume hoods and the safety of the personnel working in them.

The amount of air volume required for the hood (CFM) is equal to the size of the sash opening (or openings). Hoods are generally sized by the width in feet (4-foot, 6-foot, 8-foot). The engineer must also know the operating height of the sash. This is sometimes 12”, 18” or 24”. For example, if the hood is a 6-foot hood with an 18” operating sash height, then the hood’s required air volume is (9 sq.ft. x 100 FPM) = 900 cubic feet per minute (CFM).

Easy, simple, done? Not so fast. The above example is for a constant volume, vertical sash only fume hood and assumes no leakage, no other openings and a perfect balance.

Complications of even simple systems

Duct leakage – Exhaust ducts connecting a fume hood to an exhaust air valve is typically stainless-steel round duct. It is low pressure ductwork and therefore can be SMACNA Seal Class C or Leakage Class 8. Even with a short distance between the valve and the top of the hood, the allowable leakage can be ~15 CFM. If an Exhaust Air valve is sized for 900 CFM per the example above and leaks at the allowable rate, the fume hood average face velocity would be 98 FPM, or less than allowed by code.

Other openings – There is a common assumption that the fume hood is airtight except for the sash opening. This is not true. Shipping, moving and installing fume hoods creates small opening around the casing which can let air in. Penetrations for lab gasses and process air also adds risk of bypass air. This is usually minor but should be checked prior to testing.

A much bigger issue is the electrical pass-through. These are sometimes multiple 2”x3” openings for power for equipment in the hood to be plugged into receptacles outside the hood. The open area is not accounted for in the CFM sizing of the system and reduces the resulting face velocity below the acceptable amount.

Perfect Balance – Balancers typically use a VelGrid for use in the measurement of general face velocity conditions which provides multiple reading points. However, ASHRAE and NEEB do not allow the use of this device to verify the airflow face velocity because the mass of this device partially blocks the fume hood opening and results in a higher reading than using the allowed hot wire anemometer.

I will regularly see 5% - 10% lower FPM readings using the hot wire anemometer when a system was balanced using a VelGrid.

Summary of “simple system” issues – Between leaks, holes, and unknown balancing methods, even a simple CAV fume hood valve should be sized and have a higher upper limit of airflow in the event real world conditions require more airflow than assumed ideal conditions.

Complications of variable volume systems

Variable Air Volume Fume Hood systems are designed with sash position monitors which provide a signal to a variable air volume lab air valve to control the amount of air being exhausted from the hood based on the sash position. This ensures a constant face velocity is maintained while working and contributes to superior hood performance and reduces the operating cost of the system.

The same issues noted above for simple systems apply here – with the added complexity of airflow variation. Some fume hood systems come with a Purge Mode which will increase the amount of air required. This feature exists in variable-air-volume hoods because when you close the hood sash all the way, the volume of air exhausted per unit time is reduced for energy efficiency. In the case of a spill, you'd want to override that efficiency feature to provide maximum ventilation.

The amount of CFM required for purging is not dictated by code, so it is up to the EOR to size and selecting the valve based on the use case. The air valve’s upper limit of airflow should allow for this additional flow if required.

Also, the air valve’s lower limit should be reviewed. VAV Fume Hoods with occupancy sensors or Zone Presence Sensor (ZPS) are allowed in some cases to reduce the face airflow velocity to 60 FPM.

CCR Title 8 (CalOSHA) §5154.1. Ventilation Requirements for Laboratory-Type Hood Operations.

“When a laboratory-type hood is in use to contain airborne hazardous substances and no employee is in the immediate area of the hood opening, the ventilation rate may be reduced from the minimum average face velocity of at least 100 feet per minute to a minimum average face velocity of 60 feet per minute...”

An 8-foot fume hood with a 24” vertical sash opening requires 1600 CFM of air for 100 FPM of face velocity but only 500 CFM for 60 FPM at half operating open position. Can the air valves selected turndown 30%? Some can, but not all.

Complications of different hood types

Not all fume hoods are benchtop - vertical sash only. Some fume hoods have combo sashes (vertical and horizontal openings). Some fume hoods have dual side-by-side vertical sashes. Some fume hoods are floor mounted vertical sash. Some are floor mounted horizontal sash. All present unique challenges.

The EOR needs to know if combo sash fume hoods are planned because the face opening size for combo sashes differs between the horizontal and the vertical. The EOR needs to know not only the operating height of the vertical use case (eg. 12”, 18”), but the height and the operating width in the horizontal use case. This depends on the number of sash panels and the width of the panels.

Dual sash or double sash units again present the EOR with the issue of not knowing what the operating condition will be. Will both be used at the same height, or will one sash be 25% open and the other 50% open? This creates a challenge when calculating the open area for face velocity.

Floor mounted or “walk in” hoods can require a substantial amount of air due to the size of the opening. But herein lies the rub. What is the “operating condition” of a floor mounted fume hood? The floor mounted fume hood is used so an operator can roll in large equipment or portable work stations and close the door while a process is taking place. They are not designed to be occupied – or to have someone stand in front of them to perform a task like a bench top fume is.

So what is the operating sash open area of a closed sash? It is 0 inches squared. What is the airflow volume (CFM) required? Per code, the average face velocity must be at least 100 feet per minute. The code does not distinguish between a bench top and a floor mounted fume hood. What is the CFM required for 100 FPM across an area of 0? Excel says it is #DIV/0!. I hope the EOR can find an air valve that does #DIV/0!.

The EOR must size an air valve based on an assumed opening which will not be the actual operating condition. Also, if this assumption is simplified to one panel fully open, then the surface area, and the air volume required can easily reach in the thousands.

Fume containment is critical to the safety of laboratory workers. Several factors are involved with the proper containment of fumes, including face velocity, cross-drafts, and work practices. Common industry guidelines in many modern facilities 100 FPM face velocity as the accepted standard for safe operation. The selection and accuracy of the airflow control device plays a critical role in maintaining the proper face velocity on fume hoods for operator safety, as well as space pressurization for containment and ventilation as well as comfort control.

Air supplied to labs is single pass, or once-through. It is 100% outdoor air, conditioned, filtered, delivered used once, and then exhausted. This air is not cheap and can cost an owner $7-$10 for every CFM. But above just the cost of the air, exhaust air valve selection and sizing is critical for the correct operation of fume hoods and the safety of the personnel working in them.

Several considerations of fume hood face opening, actual conditions, imperfections, equipment restrictions and operating conditions must be made as the basis for sizing the exhaust airflow equipment for fume hoods.

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