Fume Hood Energy Savings – Low Hanging Fruit
Dr. Robert K. Haugen, Director of Product and Technology Development
2025 Mercantile Drive
Leland, North Carolina 28451
Low Hanging Fruit
In this era of energy conservation and sustainability in new lab construction, one economic question stands above all others:
“With so many fume hood energy-saving exhaust options available, which options are the most significant?”
There are many candidates. Variable Air Volume Exhaust companies claim 63% to 80% savings if new lab facilities employ VAV.1, 2, 3 Fume Hood manufacturers claim new “low volume” exhaust hoods can save over 60% of energy.4 Companies that manufacture self-closing sashes claim their devices can save 60% of energy in conjunction with VAV. 9
Other technologies exist, such as exhaust heat reclamation, nighttime setback, weekend setback, etc. Representatives of each of these technologies all have energy savings claims, but why even consider any of these, since the technologies specifically mentioned above have already saved around 200% of our current energy use!
Obviously, something is wrong here. We can never save all the HVAC and fan power costs involved in running a laboratory exhaust system since the savings for technologies mentioned above are all interrelated! In addition, each technology has associated first costs that need consideration.
Consider the following chart that lists approximate cost savings of 20, 6’ fume hoods in a facility. The chart ignores, for the time being, interrelationships between various strategies. The author uses trade knowledge and published claims for each of these costs, realizing that approximations are involved:
To make better sense of these data, we must never “double-count” savings by technologies that share similar approaches. We need to serially apply these technologies in the same order listed above (cheapest options first; here is where “low hanging fruit” comes in), and show the “chunk” of energy saved successively in each step. In this way, we will get an idea about how to proceed with the true economies of each technology. What follows is a chart that does exactly this:
Attached below are two charts describing aspects of the data:
- The ORDER in which energy saving options are applied is critical. The expenses associated with each strategy differ widely. Claims made by advocates of each strategy need to be carefully analyzed as part of the entire energy saving package. Most importantly, employment patterns and the hourly staffing needs of the research facility should be used to frame the context in which low-hanging fruit options should be evaluated. If taken in reverse order of cost, the best bang for the buck is springing for a more expensive high efficiency fume hood, which results in the ability to dramatically lower face velocities.
- The first three basic low-tech and lower cost energy conservation steps, when applied before other technologies, result in cutting energy spent exhausting fume hoods by so much (68.0%), that the remaining four higher-cost conservation methods make relatively minor contributions (11.1%). These last four technologies cost $252,000, compared to the first three costing NEGATIVE $2,600 (since we save money by buying smaller five-foot hoods).
- By no means does conclusion #2 mean VAV is a bad investment. It’s just not the best investment. Smaller hoods, or a reduced number of hoods, may not be options in all cases. Safety representatives may wish to require hoods to operate at any sash position, regardless of what effect this has on potential energy savings. It also may not be acceptable by state and/or corporate standards to run hoods at an 18” sash height and 60 FPM. These factors may require VAV be used as a principal conservation strategy, which will increase its proportional importance.
- What the above study does unquestionably show is that low-cost, acceptable energy reduction strategies (low hanging fruit) should be considered first. The above study did not even include other possible low fruit strategies; for example:
- Allow hoods not in use to be switched off entirely.
- Use air from adjacent office areas to be part of the make-up air for lab areas.
- Target research hours for “off peak load” times.
- Tolerate higher room temperatures in summer and lower temperatures in winter.
- A word about lab design: how people work and what they are doing are inescapably important in how modern buildings conserve energy.
Have lab planners asked important questions about what research behavior will be practiced in a new building?
- Is the lab designed to function 24/7? 8/5? Unpredictably?
- Is facility to be multi-shift?
- Is building located in an urban or rural setting?
- Are wind or solar options available?
- Does the facility occupant have the option to schedule work hours to reduce energy costs?
- 80% reduction predicted by Siemens Doc # 149-976, 2003. A width reduction on interior opening width for Saf T Flow hoods used in analysis is 62.5” down to 50.5, or a reduction of volume multiplier of 0.808.
- 63% reduction claimed by Lab Design News, Ronald Blanchand,10/15/2013; in this study, original 5’ 100 FPM volume is 983 CFM, which is reduced to 590 CFM at 60 FPM. Multiplier of remaining volume is 0.6.
- 75% reduction claimed by Newtech at: http://www.newtechtm.com/aspshtml/aspsenergy.html
- 60% reduction claimed by Flow Sciences Catalog, pp 98, 99, 2014
- Costs estimated for 20 fume hoods in temperate climate. This first chart ranks savings strategies from least costly to most costly.
- Installed Cost
- Cost added for low constant volume, high efficiency fume hood
- Auto close sash saves no money unless coupled with VAV. Since 60 FPM already in force, VAV savings are based on a greatly reduced volumetric base number required to achieve 69 FPM with closed sash..
Methods of Calculating Quantities in Table 2: Note percent savings is always calculated using ORIGINAL energy cost in the denominator.
- Smaller hood energy savings: In this example, going from a 6’ hood to a 5’ hood.
Energy savings is equal to volume of exhaust. At comparable sash heights, ratio of 5’ sash width to 6’ sash width times 100 will be % of volume exhaust a 5’ hood has compared to a 6’ hood. Using the Flow Sciences standard fume hood, ratio is as follows: R = 100 * 50.47”/ 62.52” = 83%; (17.0% savings)
- Reduce Face Velocity: When face velocity is reduced from 100 FPM to 60 FPM, a 40% reduction of remaining exhaust volume is saved:
R = 100 * 60 FPM/ 100 FPM = 60%; (33.2% savings)
- Sash stop at 18”: 18/28
Full open sash = 28”; ratio of volume at 18” is (17.8% savings)
- Weekend setback to 40FPM at 18”
(previous total – (2/7)*2/3 * energy cost) = (3.1% Savings)
- Weekday night time setback based on 14 ours at 40 FPM per week night = (4.4% savings)
- Exhaust heat/AC reuse extract (0.1% savings)
- VAV and auto sash
Assume average savings of 87 CFM since velocities and other factors already accounted for; (3.5% savings)
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