Sustainable lab planning: Changing fan speeds

Hundreds of thousands of fume hoods in North American labs consume more than $4 billion in energy each year. That’s because fume hoods and the make-up air supply they demand cause laboratories to use up energy 5 to 10 times as quickly as the average office building, and some specialty labs may even use a hundred times as much (EPA). The EPA asks lab design engineers to “Carefully consider the number, size, location, and type of fume hoods; each one uses as much energy as an entire house.” So to be sustainable, it’s vitally important for labs to optimize energy use, and the fume hood is at the front line of laboratory energy consumption.

The EPA concludes that if laboratories across the United States reduce their energy demand by 30%, they would save as much energy as 840,000 households consume. So how do we get there? To create optimized sustainable laboratories, lab planners, architects, and engineers need to leverage airflow volume and fume hood face velocity. Airflow volume is typically measured in cubic feet per minute (CFM), and the best way to optimize for sustainability is to reduce the CFM used by laboratory fume hoods. 

Sustainable Lab Exhaust Practices

The EPA directs lab planners and lab designers to “Select architectural and engineering professionals with laboratory experience and a proven record of sustainable design” with the goal of reducing U.S. lab energy consumption by 30%. Their “Laboratories for the 21st Century: Best Practice Guide,” tells us that fume hood exhaust energy-efficiency improvements “in the range of 50% reduction” can be obtained by modulating fan speed through stepped fan operation (p5). For larger fume hood arrays, this can be accomplished by implementing a Variable Air Volume (VAV) system; however, VAV systems may not be an affordable option for a smaller quantitiy of fume hoods. When VAV is not the appropriate choice, multi-speed blowers can be used in combination with sash height sensors and other features to achieve similar energy savings. 

What is a VAV system?

A variable air volume system monitors the face velocity of fume hoods and varies the lab’s air handling equipment. The two main components of the air handling equipment are the exhaust fans that remove air from the lab through fume hoods, and the HVAC units that supply make-up air. Raising or lowering a fume hood’s sash during operation will change the volume of air consumed, and the VAV monitoring system can attenuate the fume hood fans and the make-up air supply to maintain the proper airflow velocity at the sash opening. 

What is a multi-speed blower?

A multi-speed fan has various rpm set-points, so it can pull air through the fume hood at a safe face velocity across multiple sash positions. As with a VAV, a lower sash height requires less air, which provides a lower energy cost. Reducing volumetric rates allows more flexibility within the lab, compared to constant air volume systems. Since room air is not regulated by the fan, an alternative supply air configuration may be necessary to maintain proper room pressurization. 

What are high efficiency, low-flow fume hoods?

High performance fume hoods that minimize the required CFM are a key building block for lab energy efficiency. There are many names for high performance hoods, such as low flow, low velocity, low exhaust, high efficiency and energy efficient. But not all hoods are created equal. Pay particular attention to the hood manufacturer’s specified CFM and containment test data. To get the best possible energy savings, you may need to set the exhaust volume (CFM) to a specified face velocity and restrict the sash operating height. The available energy savings depend on the CFM capabilities of the specific hood you select. 

The aerodynamics of the high performance, low-flow fume hood design play a significant role in reducing CFM. The choice of lower air foil and type of rear baffle fume removal system are key. Aerodynamically designed, curved lower air foils can reduce CFM by as much as 10%, compared to typical flush foils. Furthermore, an efficient, aerodynamic rear baffle fume removal system (baffle pattern pictured below) can reduce CFM by promoting more uniform face velocity and airflow laminarity. 

ANSI Z9.5 recommends a minimum exhaust volume based on 150-375 ACH (air changes per hour) going through the fume hood. In VAV systems, minimum exhaust volume becomes critical to decreasing annual operating costs. For example, the attached chart illustrates the effect of using high performance hoods at lower face velocities, with lower sash heights (with or without sash intelligence), resulting in significant annual energy savings. These features can be combined with VAV or multi-speed blowers to use 15% to 50% as much energy as a standard constant air volume (CAV) exhaust system. In addition, sash intelligence systems that automatically close the sash save enough energy to pay for themselves many times over a hood’s expected 15-year (average) lifetime. 

Automatic sash positioning systems

Even well-intentioned fume hood users occasionally leave the sash open when they’re away from their hood. Automatic sash positioning systems help to cut down on wasted air volume by using motion detection to cause the sash to move up or down automatically. Since VAV and multi-speed blower systems require the sash to be lowered to enhance energy savings, automatically lowering the sash when the user is away is extremely beneficial. After a programmed delay, the fume hood exhaust fan can be turned down at every possible opportunity. 

To justify the additional up-front cost required for the eventual energy savings, a life cycle cost analysis can be done to compare total expense. 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: 

• XStream High Performance fume hood operated at 60 fpm: 40-60%
• VAV System on standard fume hood: 30-50%
• XStream High Performance hood with VAV System and Intelli-Sense Automatic Sash Positioning System: 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. (https://www.labconco.com/articles/high-performance-lab-ventilation)

Annual Operating Costs and Initial Capital Costs

Annual operating costs for fume hoods relate directly to the number of CFM exhausted from the lab. Operating costs range from $4 to $12 per CFM and average around $7 per CFM every year. But initial capital costs for equipment such as the cooling system, heating system, reheating, exhaust fan, supply air handler with variable frequency drive, and exhaust ductwork can total $25 to $30 per CFM to exhaust air from the lab and to supply make-up air for the amount of air exhausted. 

Initial Capital Costs per CFM

Annual Operating Cost: $4 - $12 per CFM ($7 average)

As an example, a recent laboratory upgrade at the University of Kansas included 34 high efficiency fume hoods (photo above courtesy of Randy Braley). Since the hoods demand 10% less airflow volume, they each save 60 CFM from the lab’s overall demand. Supply air and exhaust air for the laboratory design will require 2040 CFM less airflow volume at around $25 to $30 per CFM. So the high efficiency hoods save $51,000 to $61,200 in initial capital costs in addition to 10% lower operating cost. Thus, the choice of high performance hoods and their specification becomes vitally important to both initial capital costs and annual operating costs, without compromising safety. 

Installations with fewer fume hoods can realize similar saving per hood by using multi-speed blowers in spaces where a VAV system would be cost prohibitive to install. 

The initial investment in a building is only a fraction of the cost over its lifetime. Inefficient fume hoods can become an enormous recurring expense hidden in the energy bills. Always pay attention to the performance specifications of fume hoods and other equipment specified in the lab, and especially the airflow volume (CFM), to optimize annual energy usage. Also pay attention to the operating sash height and explore ways to lower the effective sash height, which affects airflow volume. Exploring ways to reduce the CFM required will help save on initial capital costs and heating and cooling costs, making high performance the right choice. 

Lastly, explore ways to use newer technology, such as enhanced high performance hood features, direct replacement LED lighting, new multi-speed fans, and new filtered hoods and ventilated enclosures to ensure lab sustainability in both new construction and existing labs. Being a good steward of energy also generates a return on investment for the building owner. 

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