Reducing fume hood energy consumption
Though safety is the primary purpose of any laboratory fume hood, there is an ever increasing demand for hoods to also have a reduced impact on the environment, as well as a lower cost to operate.
This is understandable considering an average 6 foot fume hood operating at a 100 fpm face velocity, with the sash fully open, will consume 70,800 cubic feet of expensive tempered air every hour. The hood costs is approximately $8,260 per year to operate,* and expends as much annual energy as three average American households.
There are a lot of advertising claims these days for “green” ventilation products, “low flow” hoods, “high performance” hoods, and smart mechanical systems. Understandably, many people are lost trying to figure out how to approach this issue for a laboratory development project. Separating the gimmicks is not as difficult as you may think.
High Performance Fume Hoods
One of the most powerful tools to reduce energy consumption is the implementation of high performance fume hoods in the laboratory. The purpose of a high performance fume hood is to provide the highest level of containment, at 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 that the fume hood simply pumps outside.
So the question is, what metric should you look at to determine which high performance fume hood has 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 Volumetric Rate (CFM) required on a fume hood is determined by multiplying the face velocity (fpm) by the total open area on a fume hood (feet squared).
Unlike face velocity, the Volumetric Rate is directly related to the energy consumption on a fume hood. Volumetric Rate (CFM) = Face Velocity (fpm) x Area Opening (feet squared) So to achieve the lowest Volumetric Rate (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 to meet the definition of High Performance, the hood must operate safely with the sash fully open (greater than 25" from the work surface).
While exceeding the ASHRAE performance standards outlined by SEFA for High Performance fume hoods, Labconco’s high performance fume hood, the Protector XStream, offers the lowest operating cost in the industry, without sacrificing precious sash area opening.
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.
Labconco fume hoods can be prepped to accept the required equipment from virtually any VAV manufacturer. Because the Protector XStream requires comparatively little air volume, the payback of your VAV system is faster with the Protector XStream, regardless of your desired face velocity.
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. The Labconco Intelli-Sash™ opens when a person is detected by the overhead Passive Infra-Red Occupancy Sensor. When no one is detected for the entire pre-selected delay time, the sash will close.
Pairing a VAV system with the Intelli-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.
Though most Labconco fume hoods can be equipped with the Intelli-Sash, the most dramatic reduction in fume hood energy consumption is realized when the Labconco Protector XStream fume hood is equipped with the Intelli-Sash, and installed on a VAV system.
This is because the Labconco Protector XStream can safely operate at a 40% reduction in air volume over a conventional fume hood. With this scenario, 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 much 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.
However, when compared to a standard fume hood operating with a face velocity of 100 fpm with the sash fully open, here are typical energy use reductions:
- Protector XStream High Performance Fume hood: 40-60%
- VAV System on standard fume hood: 30-50%
- Protector XStream with VAV System and Intelli-Sash: 70-75%
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.
* Based on average annual dollars per CFM of $7.00; fume hood operating 24 hours a day and 5 days per week (6240 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.