A place where the word "safety" takes on significant meaning is in the world of fume hoods and laboratory ventilation systems. Here, safety can mean the difference between life and death. In this specialized sphere, researchers rely on engineers to design safe systems that will contain carcinogenic, radioactive, biological, or flammable fumes, while engineers rely on researchers to use the equipment safely and properly. A weak link on either side can mean sick researchers and/or engineers in court.
While a researcher's work practices are a bit out of an engineer's control (although education of endusers is a must), there are many points an engineer must consider to ensure a laboratory ventilation and fume hood system is doing its job and containing harmful material. It seems as if the concept of containment would be unambiguous; however, there is much debate over just what makes a system safe.
Not For Everbody
Before diving into the do's and don'ts of safe laboratory ventilation system design and proper containment, one of the most important factors to consider is experience of the design team. Designing a lab ventilation system is not for every engineer. There are many who believe that planning and designing laboratories requires special skills that may not be available in a "generalist" firm.Affiliated Engineers (Madison, WI) is one of several recognized firms that have provided significant laboratory design. The consulting engineering firm has been designing laboratories for over 25 years and has several major laboratory projects under design at any given point in time. To ensure proper lab design, this firm has come up with its own engineering standards, so all guidelines relative to fume hood systems, system design, supply air system design, diffuser location, component selection, ductwork velocities, etc., are readily available to all engineers on staff.
John Nelson, P.E., president and ceo of Affiliated, says that any new engineers coming into the firm are shown where to find those resources. The new hires must then undergo a series of in-house tutorials with senior staff, as well as attend vendor seminars to understand the range of products available. According to Nelson, engineers at Affiliated Engineers are never assigned project responsibility without a senior engineer involved who is in control of their work.
"There's a phrase in our business called 'responsible charge,' which is the professional registration term that means an engineer can make the final decisions and be in responsible charge for a design," says Nelson. "While people really vary in how quickly they assimilate this stuff, it's at least five years before an engineer is able to be in responsible charge of a laboratory design."
Barry Finkelstein, P.E., is director of engineering at CUH2A (Princeton, NJ), a consulting firm that specializes in pharmaceutical research and development. He believes that an engineer must work in the area of lab design for several years before becoming proficient.
"I have concerns with younger engineers or those transferring from another field," he says.
Finkelstein believes that attending professional seminars is one way to obtain knowledge about the field, while another is to go on factory tours to see actual mock-up laboratories.
Ken Kolkebeck, president of Tek-Air (Danbury, CT), agrees that engineers must have experience before taking on a lab project. He can describe numerous "horror show" labs in which engineers who normally design hvac systems for commercial office space or fast food restaurants have attempted to design a lab space.
"You can smell a mile away a job that's been laid out by somebody who does not do this as a routine," he says.
Meanwhile, Swiki Anderson, president of Swiki Anderson and Associates, Inc. (Bryan, TX), agrees with Nelson in that there is no replacement for experience. His firm has its own fully instrumented mock-up laboratory where concepts, hoods, hardware, and schemes can be evaluated before and during design.
"I go one step further by having all young engineers participate in full-scale laboratory air flow mock-up studies and ASHRAE 110 testing of fume hoods," says Anderson, who is known to speak his mind regarding the subject of ventilation and fume hoods. "All are taught to question which vendor represents them and that one good experiment often overcomes many future shortfalls."
Look At The Big Picture
Once engineers are experienced and ready to design a lab system, there are three "big picture" issues they need to consider, says Nelson. The first is to design an appropriate ventilation system for the function that's being served. That seems like a simple concept, but it involves understanding the usage and the application for the science or functions that will occur in the space.Because laboratory uses do change and because materials used within labs can also change (and become even more harmful), Anderson takes the view that the design should maximize containment, regardless of chemicals used, especially in university or research labs.
The second big issue is capacity.
"You need to put enough capacity in the system to handle the anticipated different operating points," says Nelson.
Adds Anderson, "We now have proven technology that can be used, based on being able to automatically close the hood sash when a user is not in front of a hood. Gaining 'minimum but adequate' capacity design can be achieved."
The third big issue is operability.
"Operability deals with simplicity, it deals with maintainability, it deals with accessibility," says Nelson. "One of the cautions to engineers is to not come up with the most perfectly engineered system, which is never able to be operated in the real world in a safe manner. It's easy for these systems to get pretty complicated quickly."
Under that umbrella of issues comes a whole host of important details. These include furniture placement, fume hood location, cross drafts, room pressures, work practices, diffuser location, face velocity, hood selection, etc. Basically, it's impossible to look at just fume hoods in a lab.
"Fume hoods should be selected like any engineer-based airside product," says Anderson, "and not on an aesthetic basis that often does not recognize this need."
As Nelson notes, "Laboratory planning requires a highly integrated teamwork approach, where the engineer and the architect and the owner's safety people and the owner's users are all together developing this jointly."
To do otherwise can compromise safety.
"I will go one step further," says Anderson, not one to shy away from controversy. "Most users of labs really do not understand the containment factor and the owner's engineering rep most often also fails to consider this. As a result, they tend to want to influence design to replicate what they had before simply because they are familiar with it. As an example, if they had a paper towel in the hood that wiggled when the hood operated, they assumed this was a good indication that the hood was doing a good job, even if the face velocity was 200 fpm-plus."
Nelson does say that fume hoods are the place to start when designing a lab.
"You start by placing the fume hoods in a safe but functional location, then you work back and lay everything else out around that and all the systems around that."
Adds Anderson, "You start by selecting the hoods on maximum-containment performance basis. Containment-performance basis is most important."
Everyone agrees that fume hood location is also extremely important, because if it is located in a high-traffic area, those cross drafts can pull the fumes from the hood. For this reason, fume hoods should be located away from doors and heavily traveled areas.
"The other problem is the turbulence created by exterior conditions, such as supply air movements, auxiliary air hood conditions, etc.," says Steve Tassini, director of marketing, Phoenix Controls (Newton, MA).
Diffusers are just one of those exterior conditions that can cause serious cross drafts and compromise safety. According to Kolkebeck, the accepted rule of thumb is that the diffuser terminal discharge velocity at the hood face should be no more than one-third to one-half of the design face velocity. Meanwhile, others, like Anderson, believe there should be no terminal velocity imparted by the diffuser at the hood face.
Kolkebech notes that people routinely specify diffusers with a throw, as opposed to the more preferable perforated-type diffusers.
"A normal diffuser is designed to shoot the air along the ceiling line in order to induce airflow," says Kolkebech. "The problem is that the air goes along the ceiling, down the soffit on the face of the hood, and across the face, which draws the fumes out of the fume hood."
Open vs. Closed Loop
One of the bigger debates over what makes a fume hood safe is whether the control system operates on an open loop or closed loop principle. (And, believe it or not, Anderson has problems with the terms "open loop" and "closed loop," too - but that is an entirely different story.)Typically, an open loop control system is defined as one in which the control system does not directly measure the face velocity of the fume hood. Instead, it uses sash measurement, valve position, and other factors to determine that a safe face velocity is being maintained.
A closed loop provides for the direct measurement of face velocity by feeding the air quantity, flow, or face velocity information into a digital controller. That measurement value is read and compared against the setpoint, and flow is adjusted accordingly.
The reason measuring face velocity in the hood sash is such an issue is because it has been and continues to be the standard used in the U.S. to determine whether or not a system is containing fumes. Many believe that 100 fpm is a "safe" face velocity and will pull all fumes into the hood. Others believe that there is no one "right" answer, and the correct face velocity will depend on many factors mentioned in this article, such as cross drafts, work practices, etc. People create the bulk of problems with cross drafts just by working in front of the fume hood, says Tassini.
Ironically, this same issue was discussed and tossed around at a forum at ASHRAE's recent Summer Meeting in Seattle. If there was a consensus from that forum, titled "Is Fume Hood Face Velocity the Right Measurement?", it was this: It's not the only parameter.
Whether an open or a closed loop system, all agree that if the face velocity drops below a predetermined level, indicators and alarms should notify the user of an unsafe condition. Those who argue for closed loop systems say that it is impossible to know whether or not the measured (actual) face velocity is at a safe level in an open loop system, because the control system doesn't measure and control the hood exhaust flow in response to a calculated (or indirect) measured face velocity.
Joe Lisowski, head of Controls and Automation at CUH2A, states that there is no direct positive feedback of face velocity in an open loop system. Therefore, if an air flow problem exists in the central system, there is no way to know that face velocity is decreased until the hood gets to its alarm setting.
"We don't feel comfortable with the concept," says Lisowski.
Those for open loop systems say that they allow for faster reaction time than closed loop systems.
"A sash sensing system gets its command immediately," says Tassini. "It knows where to go and some systems can get to the proper flow very quickly. It may be unconventional to some, but with proper engineering, this approach is very safe."
However, others disagree and claim that rapid repositioning of a valve does not ensure proper flow and thus proper face velocity. And then there are those who believe that both principles can work, if they are applied correctly.
The issue shouldn't be closed loop or open loop, insists Scott Moll, P.E., a principal with Affiliated Engineers. In his eyes, it should be containment, because ultimately, that's what's important. He says that vendors promote different features in order to give themselves a perceived market advantage.
"We don't think there's a universal answer," says Moll. "The major laboratory controls players today are mature in the industry. They've done laboratory controls now for 10 to 15 years, so they've worked out the bugs to make systems safe."
Nelson adds that an open loop system is a little bit of a departure from classic control theory.
"In an open loop system you never really measure and control the critical variable," he says. "You rely on an indirect set of actions to provide you with the result you want. In closed loop systems you measure the variables, but then you put measurement devices in a contaminated air stream, so there isn't one system that's necessarily superior to the other."
Low Flow The Way To Go?
This brings us to the next controversy, which is the low flow strategy.As noted earlier, fume hood safety in the U.S. revolves around face velocity. Nelson notes that countries in Europe and around the world are moving away from measured hood face velocity as the indexing means of judging the containment performance. Instead, a containment challenge test is becoming a required standard. It should not be a surprise, of course, that there are some that believe neither reflects on airflow patterns in the hood, how air enters the hood, how air is exhausted from the hood, and what the actual functions are inside the hood.
"The discussion needs to be containment, not face velocity," says Nelson, "because there is no right answer there. There is a wrong answer: Too high a face velocity can break down containment, and obviously too low a face velocity can prevent anything from being contained. But there's a wide range of right answers between those two extremes."
Labs are terrific users of energy, and the low-flow strategy is touted as a way to save money. In a low-flow setting, one control strategy allows for the face velocity to drop down to 60 fpm when a researcher walks away from the fume hood (and doesn't close the sash). When the researcher returns, the fume hood automatically jumps back up to 100 fpm, or whatever the predetermined setpoint is.
Seems like a good plan. However, according to Kolkebeck, all the conditions have to be perfect in order for the 60 fpm to work.
"The ventilation system has to be perfect, the diffusers have to be perfect, the location of the fume hood has to be perfect, the work practices have to be right. It's easier just to train the researchers to close the sash," he says.
Kolkebeck adds that people walking by fume hoods cause a cross draft, and even 100-fpm face velocity will not contain a cross draft.
"That is a simple fact. A lot of people will have you believe that it will - if you run the fume hood at a lower face velocity and as long as you bump it up to 100 when somebody walks by, it'll be safe. That is absolutely not true, and it's very easily demonstrated with smoke. Somebody walking by an open hood at a normal pace will suck the smoke right out of the hood," he says.
Meanwhile, Anderson believes the only strategy that can be touted as the way to provide maximum containment and, at the same time, save money are by having constant face velocity and variable exhaust flow with the sash guaranteed at its minimum "occupied" position, and closed when "unoccupied."
Alan Jones, sales engineer and product manager of Triatek (Norcross, GA), notes that the low flow strategy is controversial, because there are many labs that have a large number of hoods in them. If one researcher leaves his hood and the face velocity subsequently drops to 60 fpm, there may still be other researchers working in the area.
"There are certain kinds of cross drafts that occur from the dynamics of the room that could possibly pull hazardous or noxious fumes out of the fume hood and into the area that is occupied," he says.
He does note that the face velocity reset strategy can be successful when the room is totally unoccupied, when no one is present that can be subject to poor ventilation system containment.
"In that case, as long as they're controlled to 60 fpm and if someone enters the room all the hoods are brought up to normal safe levels, typically 100 fpm, then it can work," believes Jones.
Tassini says that controls manufacturers are responding to customer requests by offering the low-flow option. When a system does not have enough capacity, yet a customer wants to add more hoods to a lab, they're basically stuck. That's when they ask manufacturers (and engineers) what their options are. Lowering the face velocity of fume hoods that are not being used frees up capacity for other fume hoods that need to be used. He says the low-flow option is a strategy that can ultimately help customers, if it's used properly.
Anderson disagrees. He claims that if a lab is supply-system deficient, the only way to "catch up" with the supply, save energy, and ensure containment is to maintain hood face velocity constant with variable exhaust flow as a function of the hood sash opening. He also maintains that the hood should be closed as much as possible. Since users do not necessarily do the latter, he believes this is why the automatic sash positioning system is catching on and being installed with lab vav systems.
And that's really a key issue: Using the system properly. The best system in the world won't work well if it's not used as it's intended. In addition to designing a safe system, engineers must also take the time to educate the users - or at least make sure that there is a safety coordinator at the facility who understands how the system should work.
In the end, it's careful planning and working hand-in-hand with endusers which will help engineers ensure their systems are safe. ES
'Experts' Offer Their Respective 'Pearls Of Wisdom'
ON CONSTANT VS. VARIABLE VOLUME:Is constant volume safer than variable volume? In the eyes of John Nelson, P.E., Affiliated Engineers, the answer is: Not necessarily."In fact, you can make an argument that variable volume is safer than constant volume," he says. "The problem with constant volume is that when you close the sash in a constant-volume fume hood, the face velocity tends to accelerate. There's no way you can prevent it. There's a bypass arrangement but it isn't perfect. In a system-to-system comparison, the variable-volume system can manage that issue a little better than a constant-volume system can.
"Also with a constant-volume system, the only capacity that's available is the capacity you install in the initial set up. In a variable-volume system, you have the ability to use excess capacity in a variety of different locations in the building. If a laboratory building has a variable-volume system that operates at 60% or 70% of its installed capacity simply because the users are very efficient in managing how they operate their hoods, that capacity is now available for additional hoods. If that were a constant-volume system, it wouldn't be available."
ON PRESSURIZATION: "We are advocates of tracking systems, which is where the supply and the exhaust are synchronized so that they track with a fixed offset," says Ken Kolkebeck, Tek-Air. "We are not advocates of pressure control systems, where you measure the differential pressure between the room and the corridor. We don't recommend it for laboratories, because most laboratories are just office spaces with research going on in them, so there's usually a tremendous amount of leakage.
"Basically, the spaces are not made to be pressurized. What happens is that it's difficult to control how porous a room is, so you end up adjusting the cfm differential to maintain the porosity and even in similar rooms it can be all over the map. For an industrial designer who's trying to find out where that air comes from, the air comes from the make-up systems. He wants to design a nice constant-volume make-up system that feeds the corridors. He's trying to control the pressurization of the building as a whole, as it relates to outside and you just can't do it in a pressure-controlled situation where you use room pressure.
"In the end, all you're trying to do is develop an inward flow anyway, usually for a chemical lab."
ON WORK PRACTICES: "It's really the proper use of the hood that counts," says Steve Tassini, Phoenix Controls. "That means basically keep that sash closed as much as possible. It means educating people on the proper use of hoods. Most of us, whether it be fume hoods or almost anything else in our daily lives, do what's convenient. Fume hoods are kind of like seatbelts: Your chances are better in a car if you're wearing a seatbelt. And in a lab, the chances are better if you have the sash closed. It's pretty tough not to contain if I've got the sash closed."
ON CAPACITY: "Make sure there's enough fan capacity for the exhaust and also make sure there's enough supply air for the hoods," says Alan Jones, Triatek. "One of the labs I worked on has 24 huge fume hoods in one room. It takes an enormous amount of air being driven into the room to supply the hood exhaust. In those cases you have to make sure there's an adequate supply of air and plenty of exhaust capacity. In addition, the components used to actually monitor and control the fume hood need to be proven components that can maintain face velocities that are safe."
ON WRITING A SPECIFICATION: "Understand the entire operating system as well as all the options available," says Barry Finkelstein, P.E., CUH2A. "Write a specification and clearly describe how it is to operate. Understand what procedures are taking place and why a hood is being used. The location of the hood and location and type of air diffuser have a large impact on how the hood will function. Work as a team with all parties. In hood-intensive labs, the design needs to be a collaborative effort in order for the design to be successful."
