Laboratory Ventilation

Introduction

This unit will cover how air moves through a laboratory building. We will begin at the point where outdoor air enters the building and discuss the major components that would be encountered along the travel path through the building.

The principal components whose function will be discussed include:

Air Handling Units
Supply and Return Ducting
General and Local Exhaust Ventilation Equipment
Exhaust Ducting (for potentially hazardous exhausts)
Rooftop Exhaust Fans and Stacks

We will conclude by providing some practical knowledge that should help in understanding basic design and system performance issues along with the inherent system capabilities and limitations.

Air Handling Unit

The Air Handling Unit (AHU) should be the first component encountered as air enters the building from outdoors.

To ensure all supply air is properly conditioned, areas in buildings that directly connect to the outside must be maintained under a slightly positive relative pressure and the AHU should be the only point of intake.

AHUs first filter then heat and cool incoming air to remove particulates then adjust temperature and humidity to programmed levels.

There are four basic parts in most of our industrial AHUs:

Particulate Filters
Heating Coils
Cooling Coils
Supply Fans (or Fan Walls)

Supply Ducting

Conveys air from the AHUs to all conditioned spaces within the building.

Size and configurations are based on air velocity and static pressure requirements and are specified by the HVAC engineer during design.

Heating,
Ventilation, and
Air
Conditioning

Design and installation intentions are aimed at ensuring adequate flow while limiting turbulence. Component selection and placement will include measures to isolate/minimize noise and vibration transmission to occupied spaces.

Return Air Ducting

Carries air from non-laboratory spaces, where no hazardous contaminants are expected to be present, back to the AHU.

This previously conditioned air is mixed with outside fresh air prior to being recirculated. This ensures carbon dioxide, heat, moisture, and other contaminants are diluted sufficiently to maintain indoor air quality.

In a normal office building, average supply air contains about 80% return air and 20% fresh outside air.

Laboratories and other areas containing potentially hazardous emissions are never returned or recirculated, these areas are 100% exhausted!

Supply and Return Ducting

These ducts are usually rectangular in shape and constructed of galvanized metal.

External insulation is applied to prevent condensation and control energy loss.

Local Exhaust Ventilation Equipment
(Devices located within the laboratory rooms)

General

These are the registers/grills/vents, usually located in ceilings or walls, which exhaust the mixed air from the general space that personnel occupy.

The components chosen and their placement have major influences on function
It is essential to maintain adequate flow directions, volumes, and pressurization schemes
Automation equipment and controls require routine inspection (setbacks, occupancy sensors, air monitoring, sash position sensors, sash closers, etc.)

Direct
(Local)

The aim of this equipment is to carry any generated contaminants directly out of the area before people inhale them. Devices of this type are covered in more detail in a separate module but include:

Fume Hoods – which are most important for user exposure control
Snorkels, Canopy Hoods, Slot Hoods, etc. – are adjuncts used mainly for heat and nuisance dust or odor control
Gloveboxes – when used for employee (vs. product) protection, need to be routinely inspected and maintained

Laboratory Clearance Times via General Ventilation
(assuming ideal mixing in a single pass system)

Fume Hood Clearance Times
(assuming ideal mixing)

Exhaust Ducting

When located in areas where hazardous materials are used, conveys the air collected from general and direct ventilation devices to rooftop exhaust fans.

Must always be impervious to the contaminants being used, be fire rated, and be free from unnecessary interior obstructions.

As opposed to supply ducting, is usually round and uninsulated.

May be constructed of galvanized, spiral wound, or welded stainless steel. Welded stainless steel affords the highest degree of protection but also costs the most.

Joints must always be welded or sealed.

Duct Materials

Fume hoods require exhaust ducting that is compatible with products intended to be used.

Since future use in research laboratories is hard to predict, welded stainless steel is preferred for almost all installations.

Ducts using highly corrosive chemicals must be inspected periodically, especially in close proximity to points of use.

Some operations such as heating perchloric acid, require special decontamination and clearance before any maintenance can be performed.

Exhaust Fans and Stacks

Must be placed outside the building envelope so that contaminants are drawn out of the building and exhaust ducts are maintained at a negative pressure relative to the occupied spaces within the building they pass through.

Must be capable of producing a minimum 2,000 feet per minute (fpm) discharge velocity and a 10 foot effective stack discharge height above any roofline interference, to ensure falling rain does not enter the exhaust duct and that contaminants are discharged above the roof wake boundary zone.

Should be of strobic, tall cylindrical, or similar design. Squat style upblast fans cannot usually meet these requirements.

Good Laboratory Exhaust Stacks

Are designed to discharge contaminants above and away from buildings with as few potential obstructions as possible.

The best designs have redundancy, are not overtaxed, use unconditioned air to augment demand supplied air to maintain >3,000 fpm discharge velocity, and are located to prevent contaminant reentry into the served or neighboring building air intakes.

Poor Laboratory Exhaust Stacks

Are capped or otherwise direct contaminants back down towards the roof or other nearby buildings.

Do not minimize rain entry into the stack and/or do not drain rain that has entered properly.

Do not have high enough effective stack heights to overcome roof wake boundary zones.

Non-critical applications with lower required flow rates, such as bathroom vents, will generally use these types of fans.

Fume Hood Density

Fume hoods are critical engineering controls and the primary means utilized to protect personnel from exposure to airborne contaminants. But...

How many should a typical laboratory have?

Can there be too many?

How should they be integrated into the laboratory design for the most benefit without wasting resources?

The following scenarios should help you understand some of the factors that need to be considered when a laboratory is designed and programmed. Fume hoods are critical and valuable laboratory tools but the operating costs are significant. Designs have to be informed, balanced, and realistic.

Historic Design Philosophies & Criteria

A fume hood should be available for every 2 lab personnel (NFPA 45)
25 ft³/ft² of horizontal fume hood work surface, minimum flow volume is required (NFPA 45)
4 ACH is the minimum in any laboratory with chemicals present (NFPA 45 guidance)
Lab ventilation rate range is 4-12 ACH, 8-12 is typical for occupied settings (OSHA)
Potentially hazardous exhaust cannot be recirculated (NFPA 45)

Note: Keep in mind that studies consistently show fume hoods are actually only "in use" <5% of the time and ventilation rates for unoccupied demands should be much less than during use. These criteria are changing due to increased awareness and sustainability efforts.

Laboratory Ventilation
Fume hood density in an untaxed lab environ (with VAV or CV2 controls)

Click to enlarge

Laboratory Ventilation
Approaching maximum practical fume hood density

Click to enlarge

Laboratory Ventilation
Excessive fume hood density

Click to enlarge

Recognizing Problems

Trust your instincts, always report conditions that do not seem right to you!

Sometimes it doesn’t require any special knowledge to notice abnormalities but usually the signs/symptoms are subtle.

Those of you that routinely spend time in these spaces and actually use this equipment should be best able to notice slight changes and problems.

Ventilation Component Maintenance

Corrosion anywhere on laboratory ventilation systems should be cause for concern and must be promptly reported to facility managers or the Facilities Department.

FSU Facilities Customer Service Center:
Phone: 644-2424
On-line: https://www.facilities.fsu.edu/requests/serviceCenter/

Ventilation Component Maintenance

This is a close-up of one the exhaust fans seen in the previous slide. The corrosion was so bad that you could see the fan rotor through the casing.

The Architect specified “weather resistant construction” in the original plans but this was obviously not installed, nor corrected during decades of subsequent maintenance.

Proper and routine maintenance should identify this type of deficiency, but don’t rule out the potential.

Ventilation Component Maintenance

Correct sound/vibration insulating materials must be used and maintained, especially when exposed to harsh weather conditions.

The flexible pieces connecting fans to ducts and rubber drive belts are often the most vulnerable component in exhaust systems.

Recognizing Failures Requires Knowledge of the Overall System Operation

External signs may not always be evident, particularly when internal components are less robust than exterior materials.

This occurred in a cleanroom where concentrated acids were frequently used in a fume hood. The hood served by this exhaust fan exhibited proper face velocity, normally indicative of proper operation. In this case though, it was due to positive room pressurization (excess supply ventilation) pushing air through the hood. This was a difficult problem to diagnose until a thorough system inspection was performed!

Recognizing Failures Requires Knowledge of the Overall System Operation

Pay attention and report any unusual condition such as rust rings on roofs by exhaust stacks, these must be investigated further.

User Fixes and Modifications

Will probably be functional but may be highly inappropriate...always contact Facilities for repairs.

Pull strings are not a sufficient solution for a bad windshield wiper motor.

Formal System Installation Verification and Performance Testing

Commissioning

A comprehensive and independent verification that building systems or structures were installed and are performing as specified by the designers. This commonly includes the HVAC mechanical, electrical, control, and plumbing systems

Test & Balance

Consists of third-party verification that the ventilation system airflow and components were installed as specified and are working within prescribed bands
Must be performed before other validations such as commissioning or fume hood ASHRAE-110 testing is performed
Is performed after all work that may affect system performance has been completed

ASHRAE-110 (1995)

Quantitative and Qualitative Testing of fume hood containment performance under several static and dynamic configurations using a neutrally buoyant tracer

Practical Application - Note the Following Conditions

Assessing equipment and system performance by noting obvious indicators.

WHEN…	EXPECT …	BECAUSE…
Entering the building	To feel air moving slightly towards the outdoors as the door is opened	The interior building and non-laboratory areas should be at a slightly positive pressure. Air should enter through the ventilation supply system (AHU). This filters and conditions the air to keep pollutants, dust, humidity, radon, etc. from entering and collecting inside the building
In hallways, offices, or other non-laboratory areas	To see all laboratory doors are shut	Propped doors waste energy with no benefit to safety and negatively impact system performance and safety (fire, smoke, fugitive emissions)
	To smell no offensive odors	These are generally indicative of problems and should be reported unless a benign cause is obvious
	To feel temperature and humidity at comfortable levels	Temperature and relative humidity is normally 70°F +/- 5°F and 55% +/- 5%, respectively
	To see no condensation forming on indoor surfaces (tiles on drop ceilings are not stained, water is not pooling on sills, etc.)	This should not occur and may cause mold growth, damage to building materials, and other hazards

What you should expect

WHEN…	EXPECT …	BECAUSE…
Entering laboratories	To sense that air is drawn into the lab area as you open the door	The pressure differential between the hallway and laboratory should make this slightly apparent as the door is cracked open. Flow in the wrong direction (away from laboratory) or too much flow is indicative of HVAC system problems
	That ventilation rates will increase, if you are the first to enter and system setbacks are used and have activated	Occupancy sensors are installed in some newer labs to allow energy conservation. Sensor indications or failures must cause the system to return to a higher rate
	To smell no significant or noticeably increased odors	Odors are usually most evident when first encountered, do not tolerate them – report them.
	To see all fume hood sashes, cabinets, and chemical containers are closed if not actively being used	This ensures maximum containment and energy conservation

What you should expect

WHEN…	EXPECT …	BECAUSE…
Exiting laboratories	All hood sashes are closed	This provides maximum containment, maintains better lab air quality, and minimizes energy waste
	All unused laboratory equipment that can be turned off without ill effects is secured (ovens, hot plates, stirrers, etc.)	This will minimizes heat loads and energy use as well as reduce the potential for fires
	All storage cabinets and chemical containers are tightly closed and labeled	This will help minimize fugitive emissions, protect the laboratory air quality, and ensure product integrity
	The laboratory lights are turned off and it is locked when unoccupied	This will help protect the contents, save energy, and allow energy saving measures such as system setbacks to be effectively utilized

What you should expect

WHEN…	EXPECT …	BECAUSE…
Using fume hoods	That flow indicators show normal operation, without alarms or instabilities	Flow alarms/indicators are required on all newer fume hood installations
	The hood is in good working order with sashes clear, clean, and easy to operate	This is critical to safety and repairs must be done before subsequent use. A four or five foot hood sash should require no more than 5 pounds force to move, 6 pounds for 6 foot hoods, and 8 pounds for 8 foot hoods
	That the work area is unobstructed and clean	This will minimize the potential for turbulence, mishaps, or cross contamination
	To see a label showing the maximum working height and testing is current	This label states the flow rate at that maximum height and indicates that overall hood performance was adequate when inspected. Fume hoods are routinely tested on an annual schedule
	To find that hood flow is proper during use	Transient smells and other faulty conditions can be indicative of problems. Simple indicators such as a light tissue held or taped to the sash bottom (should be pulled inward slightly and steadily) can be used as visual aids
	That sashes are adjusted properly during use	Horizontal sliding sashes and vertical sliding sashes should be positioned to create a barrier in front of the user's face and be closed to the maximum degree possible for protection. Combination sashes require the vertical sliders to be shut before being lifted horizontally
	That chemical operations are not left unattended or have appropriate pre-approved controls	Unattended operations should be avoided, but if required they must be engineered to provide automatic override and shutdown controls to prevent fires or explosions and periodic surveillance must also be scheduled
	Potential turbulence causing events are minimized	The fume hood air flow which offers protections is fragile and susceptible to disturbances. Personnel walking by, opening of nearby doors, or similar situations can create turbulence and vortices
	To see that testing is current	ASHRAE -110 testing is performed during installation as verification and is used as a baseline for annual face velocity testing. Each hood should have at least one sticker noting the maximum sash working height and face velocity that has been initialed and dated within one year

Complete
Training

Complete the following form to receive credit for training.

Marked fields are required.

Your Name:
Your Phone:
Your Email Address:
Department:
Supervisor:
What is your FSU affiliation? Was this training informative? Did you experience any technical difficulties during this training?

Technical Difficulties Info:

Comments/Concern/Question: