With a majority of labs still focused on “wet” research, fume hoods are an important safety equipment staple. By definition, fume hoods are local ventilation devices designed to limit exposure to hazardous or toxic fumes in lab settings. And, for years, vendors have advanced the technology further and advertised these standard safety devices as energy-efficient devices.
Yet, with the turn from safety to energy savings, has chemical exposure become an issue?
Issues with fume hoods
“What I am seeing right now is a lot of confusion around what fume hood to specify in a given situation,” says Greg Muth, Senior Project Manager for Science and Technology at Tsoi/Kobus & Associates.
In today’s lab settings, a one-size-fits-all model of fume hoods is typically implemented. “So you are specifying hoods for a synthetic chemistry lab the same way you are specifying hoods for a biology lab, or just a general lab,” says Muth.
Looking at this issue from a sustainability standpoint, in certain areas, lab designers/planners have to provide more containment. For example, when planning and designing a synthetic chemistry lab where users are working with very high hazards and dangerous chemicals, fume hoods that can provide more containment are advisable. “But, in most fume hood applications, lab designers/planners can probably look at reducing the containment levels we are trying to achieve,” says Muth. “So, in those spaces, we really need to look at a different model of fume hoods.”
The issue is that all architects have one containment specification for fume hoods, and they don’t adapt the specifications to the science conducted in the lab. “A lot of people focus on the wrong thing when implementing fume hoods into their labs,” says Muth. “They are looking at the ASHRAE 110 test, which doesn’t set a passing standard for containment. So, people are all over the map with whether they want .05 or .01 ppm of containment or something else. Focusing on containment rather than specifying a face velocity is much more important for energy savings.”
To experience sustainability from fume hoods, this issue must be addressed in the programming stage. Here architects and lab designers/planners must understand what the need is and adapt the fume hood to meet that need, rather than trying to come back after the fact and reduce face velocity or find other strategies for reducing energy. It all starts with containment, and really looking at a programmatic approach to reduce the energy cost for the fume hood equipment.
CAV vs. VAV
With a variable air volume (VAV) fume hood there is an opportunity for greater savings but whether a lab relies on a constant air volume (CAV) or variable air volume (VAV) fume hood depends on the lab setting. A lab with a single fume hood might be best served by a CAV hood running at a low face velocity. “But as we are seeing labs reduce their air change rates, the need for VAV becomes more important,” says Muth. “With this trend, there are less opportunities to use a CAV hood in labs.”
Most fume hoods found in industrial lab settings are ducted, and CAV hoods mainly come in ducted forms. There are three main types of CAV hoods—non-bypass CAV ducted hoods, bypass CAV ducted hoods and low-flow/high-performance bypass CAV ducted hoods. Closing the sash on a non-bypass CAV hood will increase face velocity, thus the hood’s safety is largely dependent on the sash position. Bypass CAV hoods (conventional hoods) were developed to overcome the high velocity issues faced by conventional hoods, and allow air to be pulled through a bypass opening from above as the sash closes. The face velocity stays more or less constant but the exhaust rate and thus the energy usage stays constant. Low-flow or high-performance bypass CAV hoods display improved containment, by having refined aerodynamic designs to reduce turbulence and sometimes small supplemental fans to create an air-curtain barrier. They achieve their energy savings by reducing face velocity necessary to provide the targeted level of containment, but there is no energy savings from closing the sash.
VAV hoods on the other hood can vary the volume of room air exhausted while maintaining the face velocity at a set level. This means that by closing the sash the exhaust rate and the energy usage can be reduced down to the minimum flow necessary to prevent a buildup of fumes inside the hood and prevent corrosion of the liner and ductwork.
Ducted vs. ductless
To make this decision, lab designers/planners must look at the overall lab picture. Yet, the obvious benefit of ductless over ducted is the reduction of ducts in lab setting, therefore, a reduction on lab ventilation costs.
“However, if you have a ductless fume hood and aren’t filtering out more air than what is required to meet your minimum air change rates, then you aren’t saving anything,” says Muth. In fact, lab owners are paying more for a ductless hood because they will need to replace the filters every two and half years.
“I would like to see lab design moving towards the two extremes—higher levels of containment using less cfm and labs moving away from fume hoods and moving towards other ventilation control devices,” says Muth. “Not every lab needs to be fume hood-intensive. And not every ventilation device needs to be a fume hood.” This means lab designers/planners should look for alternate containment devices tailored to the use. This might mean devices with smaller openings, or modified gloveboxes for high-containment operations and more flexible devices that might have a lower face velocity for hazards with a higher allowable exposure level .
This trend is starting to take wings in lab settings as some labs are choosing equipment enclosures rather than putting equipment in fume hoods. Some lab owners are spending on the construction of custom enclosures around their equipment to minimize the airflow, rather than relying on a single fume hood model that isn’t specified to meet their needs.
To gain sustainable benefits in a lab setting, sustainability must be approached in the programming stage. Lab designers/planners must really understand the science behind the ventilation and the hazards in a lab.
“Where is the energy going?” asks Muth. “Any solution has to start in programming.”
Besides fume hoods, there is another trend on-going energy usage driven by refrigeration and heat/temperature control—incubators, refrigerators and freezers. There is a tremendous energy load coming from the plug load as well as heat rejection. “I think there is probably some sort of solution where the heat recovery from these types of equipment can balance each other out for sustainable benefits. The move to water cooled freezers is the first step in this direction,” says Muth.
And this is what lab designers/planners are looking to explore in the future.