Determining proper laboratory ventilation
Engineers must know the required air changes per hour, depending on the building type. Whether designing a laboratory in an educational facility or hospital, knowing the ventilation rates is key.
- Provide a high-level view of the various codes and standards that affect ventilation in different types of laboratories.
- Understand that keeping ventilation amounts to a minimum while still meeting safety requirements is key to energy savings.
I was at a weeklong laboratory seminar at Harvard University years ago. When we approached the portion of the seminar that involved ventilation rates, I leaned forward in my seat with great anticipation. I was finally going to get the definitive, end-all answer to the great air changes per hour (ACH) debate. I believe it was the gentleman from the Massachusetts Institute of Technology (MIT) who stood at the podium and asked the seminar participants what the correct ACH value should be when designing a laboratory. After several answers that ranged anywhere from 4 to 18 ACH, the gentleman from MIT thanked everyone for their answers and told them that they were all wrong. I was at the very edge of my seat waiting to hear the correct answer. He continued by saying that the correct answer was, and I quote, "it depends." I fell out of my chair.
I didn't realize it at the time, but that gentleman could not have been more correct. After having spent years designing laboratories that include educational laboratories, hospital laboratories, research and development (R&D) laboratories, and compounding pharmacies, the ventilation rates depend on usage, facility standards, and code enforcement for the particular project type.
Relevant codes and standards
Several codes and standards do the majority of the heavy lifting when it comes to laboratory ventilation rate designs:
- 2014 FGI Guidelines for the Design and Construction of Hospitals and Outpatient Facilities (Includes ANSI/ASHRAE/ASHE Standard 170)
- NFPA Standard 45
- ANSI Z9.5
- ASHRAE Standard 90.1
- California Mechanical Code (this is an example of a local code).
Below is a summary of the ventilation portion of each code/standard that can be used as a quick reference tool when discussing laboratory ventilation with clients.
2014 FGI Guidelines for the Design and Construction of Hospitals and Outpatient Facilities (Includes ANSI/ASHRAE/ASHE Standard 170)
ACH Range: 4 to 10 (USP 797 requires 30)
This standard is coupled with ANSI/ASHRAE/ASHE Standard 170 to develop a comprehensive ventilation table. The laboratory ACH rates can be seen in data taken from Table 7.1. Table 1 shows a summary of the ventilation portion of the table.
The FGI guidelines also touch on compounding pharmacies, which can have 30 ACH in order to maintain the appropriate level of cleanliness for the process. This standard defers compounding pharmacy design to USP 797.
NFPA 45: Standard on Fire Protection for Laboratories
ACH Range: No range given
This standard provides the general statement that "laboratory ventilation systems shall be designed to ensure that fire hazards and risks are minimized." This means that you need to consult the chemical consultant on the project for the proper storage and dilution rates.
Spaces and hoods that contain chemicals are to be continuously ventilated under normal operating conditions.
While accomplishing this, the exhaust and supply systems must also be designed to prevent a pressure differential that would prevent doors from being opened when either system fails or during a fire or emergency scenario.
Laboratory ventilation systems designs are to ensure that chemical fumes, vapors, or gases originating from the laboratory are not recirculated.
The design should ensure that air pressures in the laboratory work areas are negative with respect to corridors and nonlaboratory areas of the laboratory spaces except for positive pressure cleanrooms, when doors are opened, fume hood sash positions are changed, and other short-term temporary activities that would affect static pressures. The other exception is in a designated electrically classified hazardous area with a positive air pressure system as described in NFPA 496: Standard for Purged and Pressurized Enclosures for Electrical Equipment.
Special local exhaust ventilation systems, such as snorkels, are to have sufficient capture velocities to entrain the chemical being released. It is important to not underestimate what is required to provide proper capture velocities. These calculations are in the ACGIH Industrial Ventilation guide and can range from 75 to 2000 fpm, depending on the configuration and process.
ACH Range: "Air changes per hour is not the appropriate concept for designing contaminant control systems." However, it suggests 4 to 10 ACH for fugitive emissions and odors.
This standard is the most comprehensive and is to be followed during any laboratory design. This standard does not prescribe an ACH rate. It simply states that the specific room ventilation rate shall be established or agreed upon by the owner or his or her designee. All contaminants should be controlled at the source.
The following section of the standard is in complete agreement with the gentleman from MIT I referenced earlier. It states that an air exchange rate cannot be specified that will meet all conditions: "Air changes per hour is not the appropriate concept for designing contaminant control systems."
In fact it goes further to accurately state that "excessive airflows can lead to designs that are very energy inefficient."
The standard does, however, require dilution ventilation to control the buildup of fugitive emissions and odors in the laboratory. However, source exhaust ventilation should be the primary method of capturing fumes in a fume hood. Typical dilution ventilation rates can range from 4 to 10 ACH depending on heating, cooling, and comfort needs and the number and size of exposure control devices.
The standard recommends that variable air volume (VAV) fume hoods be considered to save energy in ventilation. This is a good place to save energy as air exhausted by fume hoods needs to be replaced with conditioned air. The conditioned air will require a larger air handling unit, chiller, and boiler. The Dept. of Energy (DOE) has published data that indicates that a single fume hood uses as much energy as 3.5 homes, which really promotes reducing any ventilation associated with fume hoods.
However, if VAV hoods are used, it is recommended that they be equipped with an emergency override that would permit full design flow in the event there is a spill, even if the sash is closed.
Safety and energy usage must be considered when designing the ventilation system to account for fume hood face velocities. The fume hood manufacturer must show that the hood has passed tests using ANSI/ASHRAE Standard 110: Method of Testing the Performance of Laboratory Fume Hoods at the selected face velocity.
The typical range for face velocity is 80 to 120 fpm. It is never recommended to operate below 60 fpm
Low-flow "energy-efficient hoods" have face velocities of 60 to 80 fpm, but these types of hoods must have excellent containment characteristics and operate in ideal conditions with respect to open windows, doorways, and personnel traffic flow, as these will adversely affect the containment of chemicals within a fume hood.
Airflows to achieve the face velocities must be capable of keeping vapor concentrations between 10% and 25% of the lower flammability limit (LFL) or lower explosion limit (LEL). This would typically be calculated by the chemical consultant.
Air pressure in labs is typically negative unless trying to protect a space, such as a cleanroom, in which case the space will be positive. It is very important to design the building envelope to reduce the amount of transfer air required to maintain the correct pressurization. The standard also recognizes that it is impractical to maintain a differential pressure across an open door due to the effective large leakage area. Often a door switch with an alarm delay is used to avoid nuisance alarms associated with door openings.
A method to save energy and capital costs is to not size the ventilation system to accommodate the full flow of the fume hoods. A diversity factor may be used to anticipate the actual number of hoods in use at one time. When applying a diversity factor, a designer must consider a wide range of topics, which include but are not limited to: expansion considerations, minimum and maximum ventilation rates for each laboratory, quantity of hoods and researchers, sash management (sash habits of users), and use patterns of variable volume hoods.
It is crucial that the users agree to the diversity factor and understand the limitations to the laboratory. Not all fume hoods can be used at one time if a diversity factor is used. An airflow alarm system must be installed to warn users when the system is operating beyond capabilities allowed by diversity.
The standard also requires that an emergency ventilation system be designed when a certain type and quantity of chemicals or compressed gases are present in a laboratory that would warrant emergency ventilation. This would be determined by the chemical consultant on the team and would allow for the ACH rate in the space to be raised during a spill.
In the event of an emergency mode operation or a fire mode, typical schemes that simply turn off supply to a space and maintain exhaust flows cannot be done without taking room pressurization into consideration. The ventilation system must not create a large pressure differential that would prevent a user from opening a door to leave the area.
In general, the standard states that return air is not to be used in laboratories with hazardous chemicals or biological hazards.
As an energy-saving strategy, energy recovery systems should be evaluated to reduce the energy needed to condition a large outside air intake.
Ultimately, the ventilation rate selected for a laboratory depends on the largest airflow of thermal comfort, dilution and displacement of contaminants not captured by exposure control devices, and makeup air to account for the operation of exposure control devices such as laboratory hoods, space pressurization, or minimum code occupancy ventilation. These must all be considered before settling on the ventilation rate.