Energy and Cost Savings for Variable Air Volume (VAV) Laboratory Fume Hoods in Two University Research Buildings
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Date
2006-04-24
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Abstract
The volume of lab exhaust for Engineering Building 1 (EB-1) and Partners III, which are research labs located on North Carolina State University's Centennial Campus, is measured and analyzed. There are three main objectives. The first is to determine how the actual system exhaust compares to the total design capacity of the lab HVAC system. Next, the potential additional savings that would result from carefully keeping fume hoods closed when no one is working in the hood. Then, estimate the actual savings that result from using variable air volume (VAV) fume hoods in place of constant volume (CV) hoods.
Lab buildings require much more energy for the HVAC system than other academic buildings because of their high minimum airflow set points, once-through air for supply, and high static pressures for the ventilation system. VAV fume hoods can reduce the HVAC energy requirements by reducing the hood exhaust flow to about one-fourth of the full flow when the fume hoods are closed. However, it is total lab exhaust that drives energy use, and the actual reduction in total exhaust cannot be assumed to be equal to a reduction in fume hood exhaust. This is true in EB-1 because minimum airflow set points are 10-12 air changes per hour (ACH), and a general exhaust box maintains this minimum.
Thus, when all 70 fume hoods in EB-1 were closed on Friday evening before the Labor Day holiday in 2005 there was a drop of 14,166 CFM in fume hood exhaust. However, total exhaust dropped by only 8,156 CFM, which is 58% as much. Therefore, energy savings made possible by closing the fume hoods would be grossly overestimated if the drop in total lab exhaust had been assumed to be equal to the drop in fume hood exhaust.
The total exhaust rate was determined to be almost constant at about 68% of the maximum design airflow for EB-1 during the trended period. The total exhaust was relatively constant over the trended period of about one month because most of the labs were controlled by the minimum airflow set point. The measured exhaust flows for the fume hoods in three Chemistry labs of Partners III indicate that on average 49% to 65% of the hoods are open over a two-week period from August 29 through September 11, 2005. These very high values were due in part to a lack of training for the users of the lab. Many students who work in the lab did not understand the importance of closing fume hoods and some in Partners III have been specifically instructed to keep the fume hood sash at 18 inches continually.
Potential savings for keeping fume hoods closed when no one is working in the hood were estimated to be $29,040 per year for EB-1 and $52,800 per year for the Chemistry labs in Partners III. Additional savings of $12,310 per year could be obtained for EB-1 if the minimum airflow set point was reduced during unoccupied periods for eight labs where this is most beneficial. Savings for using VAV fume hoods in lieu of CV hoods is estimated at $58,910 per year in EB-1 and $42,680 per year in the Partners III Chemistry labs, but this could be increased by the amounts calculated for keeping unused fume hoods closed.
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Keywords
HVAC, Exhaust, Trend, Airflow
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Degree
MS
Discipline
Mechanical Engineering
