Clearing the Air About the Latest Fume Hoods
There are approximately 750,000 fume hoods currently in use in the United States. That translates into an annual operating cost - just for fume hoods - of approximately $3.2 billion, with a corresponding peak electrical demand of 5,000MW, according to the Lawrence Berkeley National Laboratory.
With statistics like that, it is no wonder that laboratory management, design engineers, and manufacturers are always looking for ways to reduce the amount of energy consumed by their fume hoods. They have looked at everything from reducing face velocities and exhaust rates to restricting sash openings. As expected, everyone has a different idea as to which type of fume hood works best, which method is the safest, and how much energy can be saved.
In this first article of two parts, we'll look at how today's fume hoods are defined, and how the terms used may not be the best indicator of whether or not a fume hood is safe.
Terms have changedFive years ago, the term typically used for a more energy-efficient fume hood was "low-flow" fume hood. This usually meant a fume hood with a face velocity of under 80 fpm. Use the term low-flow fume hood today, and you're likely to get some confused looks, because the terminology has changed. The term low flow is still used; however, "high performance" is also being used by many manufacturers to describe energy-efficient fume hoods.
Compounding the confusion is the fact that there are no standards or organizations that define the terminology, so manufacturers can label their fume hoods just about anything they'd like. The fume hood committee for the Scientific Equipment and Furniture Association (SEFA) is currently working on a standard for the newer fume hood designs, and supposedly the first issue SEFA is tackling is how to define the distinction between a "low-flow hood" vs. a "high-performance hood." Indeed, further discussions are taking place within the SEFA subcommittee to develop different terminology than "high performance" and a corresponding definition.
At this point in time, the majority agrees that the definition that a low-flow fume hood is one that typically operates with less exhaust flow than would be required to produce 100 fpm with a full open vertical sash. Therefore, a low-flow hood would include any hood that uses a restricted sash height to reduce the exhaust flow, even if it has 100 fpm at the lower sash height.
Dale Hitchings, president of Safelab Corp. (Indianapolis), added that a low-flow fume hood is one that "performs, or claims to perform, the same function as a 'standard' fume hood of similar size/configuration but uses less exhaust volume. They can, in some cases, conserve energy and save capital costs by reducing exhaust and supply system (equipment) capacity." Safelab is a laboratory consulting firm specializing in lab ventilation and fume hood design and performance testing.
The term "high performance" indicates a hood operating with a reduced exhaust volume, giving the hood an operating face velocity of less than 80 fpm at the full open vertical sash height. In order to be a high-performance hood, the fume hood must provide equal or better performance (usually defined by an ASHRAE test at ANSI standards) with the sash full open when compared to conventional fume hoods operating at 100 fpm face velocity.
Like many other industry experts, Tom Smith, president of Exposure Control Technologies, Inc. (ECT, in Cary, NC), doesn't care for the term "high-performance hood." He said that the word "performance" indicates the ability of a fume hood to protect the operator. ECT helps research facilities achieve safe, productive, and energy-efficient laboratories by providing testing services, training of personnel, and ventilation management programs.
"If you're saying you have a high-performance hood, then to me that would mean that the hood does the job of a 6-ft hood operating at 100 fpm, it just does it a lot better. This means you could use it for even more hazardous materials, but its containment capabilities would be dramatically greater. But that doesn't indicate anything to do with face velocity," stated Smith. In fact, Smith doesn't like labeling any fume hood based on face velocity alone, because there are so many variables involved in making sure a fume hood is safe.
Regardless of the terms used, it's clear that low-flow hoods are here to stay. Although Dr. Gerhard W. Knutson, president of Knutson Ventilation, Inc. (Edina, MN), noted that while there is a desire for low-flow hoods, "We will have to let history determine if there will be a need for low-flow hoods." Knutson Ventilation does consulting in industrial ventilation, including laboratory ventilation. Dr. Knutson codeveloped the most widely used fume hood performance test method, the ASHRAE 110 test.
Knutson added that the current driving force for low-flow hoods is energy conservation. "The energy cost of operating laboratory exhaust systems is primarily in the energy to treat the replacement air. If the exhaust air is reduced, the supply air can be reduced (assuming the laboratory is not currently supply deficient). The cost of operating a ventilation system (supply plus exhaust) is nearly directly proportional to the volumetric flow."
Dale Sartor from Lawrence Berkeley National Laboratory doesn't agree that low-flow or high-performance hoods are in existence only for energy conservation. "High performance does not mean tweaking a standard hood and running it at a lower face velocity, or restricting the sash and running it at a lower overall volume. A hood's job is to contain, not save energy. Therefore, a high-performance hood contains better than a conventional hood. Safety is, or should be, the driving force of a high-performance hood. If it is energy efficient, so much the better."
The myth of average face velocity Smith has tested thousands of fume hoods over the years. His data indicates that anywhere from 15% to 50% of fume hoods will fail even if they have a face velocity of 100 fpm. "I don't have any faith in average face velocity," noted Smith.
That's because having an "average" face velocity automatically indicates there is a variance, but no one knows what that might mean for a particular hood. "I did a table of 1,600 hoods where we plotted the performance by face velocity, and it showed that under 80 fpm we had about a 51% failure rate, and above 150 fpm we had a 3% failure rate. The 100 fpm has always been a number that has developed a myth about it that it just equates to being safe, but our findings say that it's nowhere near enough of an indicator," he said.
Smith has come up with five general categories that need to be considered in order to make sure a hood is safe:
- Nature of the hazards. If a lab technician works with a material that has a high aggressive generation rate, that's going to be different than material that is normally going to evaporate. Therefore, the velocity required to capture those processes might need to be different.
- Hood design. If there are two identical, standard 6-ft hoods sitting next to one another and the airfoil sill is removed from the second hood, the first hood might pass the safety test, while the second would fail terribly. There is also a baffle in the back of the hood, and typical designs have two or three slots in the baffle. If the top slot is wide open and the bottom slot is closed, then again it is very likely that the hood will fail.
- Laboratory design. If a hood is put into operation and set up for enough flow to reach 100 fpm, that's only 1.1 mph. If someone walks by the hood, they're walking at two to three mph, which could generate a cross draft as high as 250 fpm. In addition, most supply diffusers that are installed in labs use a four-way, high-velocity, high-aspirating diffuser. If that diffuser is near a hood and produces a cross draft velocity above 50% of the inflow face velocity, the containment capabilities of the hood will be degraded. In some cases if the velocity is blowing directly at the hood it will cause significant escape.
- Design and operation of the ventilation system. Obviously it is necessary to have enough flow for the hood to contain, which for the sake of argument, could equate to a face velocity of 100 fpm. Because this is an average, there is the ability for flow to vary within time. Right now it could be 100 fpm, five minutes from now it might be 90 fpm, five minutes after that it might be 110 fpm, but over the course of time, the average would still be 100 fpm. If flow varies more than 10% in a short period of time, say, five seconds, which might happen with a VAV system, then there may be escape.
In addition, there is the effect of discharge temperature in the lab coming from the air supply system. If the air in the room is 72 degrees F and stable, then for some reason the lab registers an increase in temperature, there might be cold air coming out of the diffuser and mixing with the warm air near the hood face. That sets up a stratification of air temperature, which creates a rolling or cycling type of flow. Even though the face velocity still might be 100 fpm, the turbulence has increased tremendously, resulting in a potential for escape from the hood.
- Work practices. The fume hood might be set up with 100 fpm face velocity, which means 1,000 cfm for a nominal 10-sq-ft opening. If a large obstruction is placed in the hood between the lab technician and the slot, the airflow will bounce off that obstruction and come right back at the lab technician. Even the presence of a lab technician in front of an opening will cause turbulence, and this turbulence is further increased due to arm movements or withdrawing equipment from the hood.
"These and other factors will cause a loss of performance that just saying, 'I have 100 fpm,' won't cover," said Smith.
Next month we'll look at the different types of fume hood technology offered by various manufacturers. ES