Although safety is the primary purpose of any laboratory fume hood, demand is also growing for hoods to have a reduced impact on the environment, as well as lower operating costs. This is understandable considering that an average 6-ft fume hood operating at a 100 fpm face velocity, with the sash fully open, will consume 70,800 cu ft of expensive tempered air every hour. A hood costs approximately $8,260 per year to operate,* and expends as much annual energy as three average North American households.
There are many advertising claims these days for “green” ventilation products, “low-flow” hoods, “high-performance” hoods, and smart mechanical systems. Understandably, many people feel lost in trying to figure out how to approach this issue for a laboratory development project. Separating the gimmicks is not all that difficult.
High-performance fume hoods
One of the most powerful tools to reduce energy consumption is the implementation of a high-performance fume hood in the laboratory. The purpose of a high-performance fume hood is to provide the highest level of containment with the lowest possible cost to operate. Synonyms for this type of hood include low-velocity or high-efficiency hood. Unfortunately, most comparisons rarely focus on the actual cost to operate, and usually focus on the face velocity. There is a relationship between face velocities and operating cost – the lower, the cheaper the hood’s cost to operate. However, to assume that two different fume hoods of the same width and with the same face velocity will have the same cost to operate is a mistake.
The cost to operate a fume hood comes from the cost to temper the laboratory room air which the fume hood simply pumps outside. So the question is, what metric should you look at in order to determine which high-performance fume hood will have the lowest operating cost and lowest energy consumption?
What we usually call “air volume” is actually a volumetric rate, usually measured in cubic feet per minute (CFM). The CFM required on a fume hood is determined by multiplying the face velocity (fpm) by the total open area on a fume hood (ft2). Unlike face velocity, the volumetric rate is directly related to the energy consumption on a fume hood.
So to achieve the lowest CFM, a hood should have a low, but safe, face velocity and/or a small opening area. The lowest acceptable face velocity currently noted in any major standard is 60 fpm, and there is currently not a written standard that would suggest it is safe to operate a fume hood below a 60 fpm face velocity in the lab. This means that the lowest acceptable face velocity is somewhat standardized, leaving opening area as the remaining variable.
Now, reducing the sash opening area that the operator has to work through is an old trick, this is usually referred to as a reduced air volume (RAV), or low-flow fume hood. This is not possible on a high-performance hood, because SEFA tells us that in order to meet the definition of high-performance, the hood must operate safely with the sash fully open (greater than 25 inches from the work surface).
Variable air volume
Variable air volume (VAV) systems are whole building ventilation automation. These systems go far beyond controlling the airflow through a fume hood. With modern VAV you can simultaneously maintain the safest minimum fume hood face velocities regardless of sash position, ensure minimum room air changes per hour are met, hold specific laboratory pressurization, as well as maintain the desired temperature and humidity.
In short, these systems maximize comfort and safety, while minimizing the energy consumption by cutting the demand for air as the fume hood sash closes, and consequently minimizing operating cost.
Unfortunately, there is no energy consumption/operating cost benefit to a VAV system, unless the operator of the fume hood closes the sash, reducing the demand for tempered air.
An intelligent sash system opens when a person is detected by an overhead sensor. When no-one is detected for the entire pre-selected delay time, the sash will close. Pairing a VAV system with an intelligent sash will ensure that the sash is closed when unoccupied, and encourage minimal sash openings when occupied, thus taking advantage of every possible opportunity to reduce the air volume demand. The result is a dramatic reduction in energy consumption and noticeable savings. In this scenario, when the sash is closed completely, a 6 foot fume hood can consume as little as 18,000 cubic feet of air per hour.†
Most Labconco fume hoods can be equipped with an intelligent sash called Intelli-Sash, but the most dramatic reduction in fume hood energy consumption can be realized when Labconco’s high performance Protector XStream fume hood is equipped with the Intelli-Sash, and installed on a VAV system. This is because the hood can safely operate at a 40% reduction in air volume over a conventional fume hood. With this scenario, the annual cost to operate is reduced by $5,810, or a 70.3% reduction in energy consumption and operating cost over a conventional by-pass fume hood. * † ‡
Life cycle cost analysis
To justify the additional up-front cost required for this reduction in energy consumption, a life cycle cost analysis is required to compare total expense for the life of each system or combinations of systems.
Though the breakeven point can vary depending on climate, energy cost, and usage, these tools provide excellent options for improving the safety and comfort in your lab while minimizing the cost to operate.
Request online at www.labcanada.com/rsc, October 2011 issue, reply card # 16.
* Based on average annual dollars per CFM of $7.00; fume hood operating 24 hours a day and 5 days per week (6,240 hours per year). Average annual dollars per CFM range from $5.00 to $12.00 depending on geographic location.
† Closed sash air volume is based on NFPA 45 recommended minimum air volume of 25 CFM per cubic feet of interior space.
‡ Based on 8 hours per day with 18″ sash opening and 60 fpm face velocity, and remaining time with sash closed.
This article appeared in the October 2011 issue of Lab Product News.