Robert K. Haugen, Ph.D.
Director of Product and Technology Development
Flow Sciences, Inc.
I. VAV (Variable Air Volume) Fume Hoods Defined:
According to Northwestern University Office for Research Safety1, variable air volume fume hoods are:
(Fume hoods that) maintain a constant face velocity regardless of sash position. To ensure accurate control of the average face velocity, VAV hoods incorporate a closed loop control system. The system continuously measures and adjusts the amount of air being exhausted to maintain the required average face velocity. The addition of the VAV fume hood control system significantly increases the hood’s ability to protect against exposure to chemical vapors or other contaminants. Many VAV hoods are also equipped with visual and audible alarms and gauges to notify the laboratory worker of hood malfunction or insufficient face velocity.
It is also true that as VAV hoods reduce exhaust volume, they can significantly increase the energy efficiency and sustainability of the lab exhaust operation. 2
We will focus on fume hood exhaust CFM in this paper. An architectural approximation $10 per CFM per year (fan use, air conditioning energy, and heating expense) will be used to estimate this exhaust expense over time.
II.VAV Savings using a face velocity of 100 FPM:
What follows is a comparison of exhaust volume and energy savings using a 6’ classical constant volume fume hood and the same hood using VAV controls:
A. Constant Volume Math:
Using a constant volume 72” wide fume hood running at 100 FPM @ 28”high sash opening for one day:
24 Hours X 60 min/hour X 1245 CFM catalog exhaust volume = 1.8 Million Cubic Feet per day exhausted
B. Variable Air Volume Math:
A VAV hood reduces volume as sash is lowered to maintain a constant face velocity above a minimum air change rate, which we will assume for this exercise is 5 air changes / minute, or 300 air changes per hour. Generally speaking, such a number is regarded as safe to prevent explosions and interior hood corrosion. 3
- At full open & 100FPM, hood will exhaust 8 million Cubic feet per day, just like the constant volume hood reviewed above.
- At 18” & 100 FPM, hood will exhaust this calculated reduced air volume:
((21.5” X 62.5”) / 144 sq.”) X 100 = 933 CFM = 933 CFM, or 1.3 Million Cubic Feet per day exhausted
Note: Very First term includes 18” sash opening plus 3.5” of airfoil and bypass opening
- At completely closed, hood will only exhaust:
((3.5 X 62.5)/144) X 100 =152 CFM= 0.22 Million Cubic Feet per day (CFD) exhausted
- Here’s where calculating VAV exhaust and energy savings becomes imprecise. Will lab personnel keep the sash down, operate at 18” sash opening, or operate at full open sash? We cannot compute effectiveness of VAV without knowing the answer to this question. For the sake of argument let’s assume the average sash position is the mathematical average of the three numbers calculated above: (1.8 + 1.3 + 0.22)/3 = 1.1 Million CFD:
1.8M -1.1M =700,000 CFD savings or 478 CFM average savings. At $10 per cfm per year, this is annually $4780/year savings on one hood.
Can VAV Save Money?
A 72” VAV fume hood can therefore generate average annual savings of ~ $4780 per year, however, such savings require not overtaxing the make-up air machinery designed to “feed” this system!
What’s The Caveat Emptor?
A reduced volume exhaust system has several “gambles” built into it based upon assumptions about human behavior! If the researchers do not close the VAV sashes or behave improperly in other ways, far less energy will be saved than the calculations predict.
If building designers downsized HVAC make-up air based on aggressive VAV assumptions, the building may not be able to heat or cool itself properly when temperature conditions are very hot or very cold and fume hoods are simultaneously wastefully run with sashes full open. If behavior does not match expectations, the building may not be able to maintain intended thermostatic conditions.
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III. VAV Savings using a face velocity of 40 FPM:
Mechanical engineers might consider additional measures to augment VAV savings when designing a fume hood exhaust system. Two likely targets in the hunt for savings are face velocity and maximum operating sash position. We have recently seen several large jobs where the VAV face velocity is specified not at 100 FPM, but at 40 FPM with a max. sash position of 18”, not 28”. Let’s see mathematically what happens at this reduced face velocity and sash opening to VAV savings:
A. Constant Volume Math:
Using a constant volume hood running at 40 FPM and18” sash opening for one day:
24H X 60 min/hour X 374 CFM = 539,000 Cubic Feet per day exhausted. Notice that the original constant volume annual cost calculated at 100 FPM and a 28” open sash was 1,800,000 Cubic Feet per day exhausted. We instantly save 1.26 million Cubic feet of exhaust per day, before we even add VAV!
B. Variable Air Volume Math:
A VAV hood reduces volume as sash is lowered to maintain a constant face velocity above a minimum hood cavity air change rate, which we will assume for this exercise is 5 air changes / minute, or 300 air changes per hour. Generally speaking, such a number is regarded as safe 3 to prevent explosions and interior hood corrosion.
- At 18” & 40FPM, hood will exhaust 374 CFM or 539,000 Cubic feet per day, just like the Constant volume hood reviewed above.
- The minimum 300 air changes per hour is 300 X (62.5 X 24 X 48)/1728 = 12,500 cubic feet per hour = 300,000 Cubic feet per day
- Average low and high cubic feet per day to obtain average total daily cubic feet.(539,000+300,000)/2 = Average volume exhausted using VAV= 419,500 CFD
- Savings is difference between line 1 and line 3 = 539,000 – 419,500 = 119,500 CFD
- This 82.9 CFM average reduction is about $829/year in energy savings.
These parallel calculations of energy savings make a serious point: The lower the baseline acceptable face velocity and maximum sash position are, the greater the energy savings is BEFORE we consider VAV’s contribution. As the third technology (after face velocity and sash position), VAV still saves energy, but dramatically less than when it is considered the first technology.
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IV. Preventing Dangerously Low Air Changes within the Fume Hood Cavity
Let me cite an experience from a very early VAV system I checked out in 1984. This customer was dissolving small samples of limestone in dilute hydrochloric acid (HCl) on hotplates inside a VAV fume hood with the sash closed.
The hood interior environment became hostile and corrosive. The hotplates corroded. The stainless steel sash frames corroded on the interior-facing side. The customer called us in to “fix” the hood.
The first-gen VAV installed in this lab hood ramped exhaust down so face velocity was always 100 FPM, right down to full sash closure. Velocity checked out at 100 FPM all the way down. The problem was, at full closure and no bypass, the only air route into the hood was the 1” slot under the airfoil.
A. On this 6’ hood, this meant exhaust volume was
CFM = (1” * 62”/144 sq. inches per sq. foot )*100 FPM = 43 CFM.
B. Hood internal volume was Vol = 48” * 62” * 22” / 1728 cu inches per cu foot = 37.9 Cu Ft
C. Internal Air changes were therefore ACM = 43/37.9 = 1.13 ACM, or 1.13 * 60 = 68 ACH
All the corrosive issues appeared to be caused by LOW AIR CHANGES at full sash closure. We experimentally proved this on site. At the time I did this research, I discovered corrosion in this application stopped happening if minimum airflow was increased to about 5 ACM (300 ACH). Other engineers were noticing similar issues. Over time, air change rates themselves became controversial to the extent that now a “suggested range” of 150 to 375 ACH is cited in ANSI/AIHA Z9.5 – 2012 3. Other researchers also note theoretically a danger of explosive vapors building up at air change rates lower than the range set forth in Z9.5.
Most VAV manufacturers now allow the inclusion of a minimum air change rate into the VAV algorithm defining exhaust demand at all sash settings. For a representation of this new controller function, note the contrast between the two lines on the chart below where blue line represents a VAV unit where face velocity is constant down to sash closure and orange line represents exhaust reduction only down to minimum air change rate:
Notice how much one must limit the air savings VAV achieves to get our 300 ACH minimum. The last nine inches of sash travel earns not one extra CFM of energy savings on the orange line.
Check out the next graph! Many designers now wish to lower the sash upper limit to 18” rather than 28”. This limits maximum CFM as one raises the sash and also allows most hoods to pass ASHRAE 110 containment since the sash rail at 18” remains below the average operator’s breathing zone during all procedures. In this VAV approach, it is not recommended to use the sash during an active experiment above 18”.
Same shape, right? What’s the big deal?
Notice that the VAV system now only modulates airflow (orange line) over 8” of sash travel from 18” down to 10”! Again, as we reduce maximum sash opening and increase minimum air changes, the operational influence of VAV on fume hood exhaust becomes less and less.
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- A variety of different methods for reducing fume hood exhaust volume exist. VAV fume hoods are a prominently mentioned method of saving exhaust CFM and gaining significant financial and environmental benefits. An example used in this paper demonstrates large savings using a VAV hood operating at 100 FPM and 28” sash opening.
- As other, simpler, modifications are made to fume hood applications, it becomes apparent much of the savings these methods achieve are the same dollars saved by VAV that were discussed in conclusion one.
- VAV technology requires careful consideration of what happens if fume hood interior air changes drop too low. There is no agreement in the literature about where this air change magic number exactly lies.3 It very well may be inside the range AIHA Z9.5 cites between 150 and 375 ACH, but exactly where? The “right” minimum air change rate also depends on the challenge rate of fumes introduced into the hood, which obviously depends on the process/application being undertaken. We shouldn’t guess at this number! The most current ACH reference of 150 to 375 has a range of 250%! In my opinion this is like posting a speed limit sign of 50 to 125 MPH! Safety assessment of the minimum air change rate should be application-specific and empirically tested with fume hoods that are on site.
- In this limited study, it appears much of the calculated energy savings attributed to VAV may be achieved by alternate means. If our architectural design objective is to run an energy-efficient lab that is also safe, we should focus on the best mix of many available technologies. Another paper in this series, Low Hanging Fruit 5, focuses on seven widely used strategies, picking the least expensive alternates first.
Finally, all hoods are not created equal.4 Some high efficiency hoods may require a lower minimum air change rate than others! Do we decide to pick the “safest” hood regardless of price, or the least expensive system with the highest ROI? The facilities planner and scientist have already lost if they believe such a choice is valid or necessary. Picking a fume hood that contains well at lower velocities and selecting energy savings objectives that match the applications being used in the lab in question are both possible in the same assessment and should be unwaveringly advocated.