The primary function of a fume hood(s) is to gather contaminated air and thereafter contain, dilute, and discharge chemical contaminants safely. A blower is used to pull the contaminated air out through the fume hood and the ducting system. Two important parameters are the face velocity at the fume hood opening and the rate at which the air volume is being pumped out.
Starting with the basics, if the face velocity at the fume hood opening is 100 fpm (feet per minute-optimum for safe working) and the hood opening is 7.5 square feet, the air volume pump-out rate would be (Area X Velocity)100X7.5 =750 cubic feet per minute. This is usually true for a typical 5 feet fume hood. Now let us understand two terms CAV (constant air volume) and VAV (variable air volume). These terms relate to the ways we handle the air through the fume hood(s). We must bear in mind that these terms are for the exhaust system as a whole and are not limited to a fume hood description.
What is a Constant Air Volume System (CAV)?
In our example, a constant volume (rate in cfm) of air is pumped out; 750 cfm in a CAV system. This is regardless of whether the fume hood is in use and its sash is open or closed. This means that the air velocity through the hood would menacingly increase and might spoil the equipment set up if the sash is fully closed. It might even force back the contaminated air into the room. It would also burden the HVAC system as an unnecessary volume of good-conditioned air would be pulled out of the fume hood(s). That means higher loads on the HVAC system and costs.
What is a Variable Air Volume System (VAV)?
In contrast, a VAV system reduces the volume (rate in cfm) of air being pumped out when the fume hood is not being used and the sash is closed (about 250 cfm). Thus, the total airflow would be greatly reduced overcoming the limitations of a CAV system, while keeping the face velocity constant. VAV systems are driven by a combination of electro-mechanical valves and electronic feedback systems. These are more complicated than CAV but offer a lot of cost savings and other advantages. These days, when most modern laboratories have many hoods and the atmosphere is controlled by HVAC systems, the use of VAV systems becomes unavoidable.
What is an intelligent VAV system?
As our laboratories get modern in line with the ever-changing nature of research and development, the complexities are bound to increase. Today, we have Air-conditioned (HVAC) labs with several fume hoods and other exhaust equipment (Spot extractors, Biosafety Cabinets, Bespoke ventilation devices etc.). This means a lot of electronic controls are involved and these controls keep building data and communicating with the BMS. BMS is a building management system, where HVAC, Fire Alarms, Gas Alarms and other controls merge. A lot of sophistication is built into modern HVAC controls turning them intelligent and responsive in a fraction of a second. Consequently, smarter and fast responding VAV systems and controls for hoods are available. Thus, the term intelligent VAV system.
What are the advantages of VAV over CAV?
1. Energy Efficiency
This is the foremost advantage that is directly reflected in saved costs. A CAV system would keep on exhausting 750 cfm, as in our example. The VAV system on the other hand would exhaust 750 cfm only when the sash is fully opened but reduce it to about 250 cfm when the sash is closed. It is assumed that on a typical 8 hours day, the sash is fully open for about 2 hours and closed for 6 hours.
We can compare the 8 hours of duty cycles as follows:
Volume Exhausted = 8 X60 X750 = 3,60,000 cubic feet, for CAV.
Volume Exhausted = 2X60X750+6X60X250=1,80,000 cubic feet, for VAV.
Thus, the Volume reduction (and proportionate energy savings) over a day = 1,80,000 cubic feet. Which translates to about 50% energy savings. The cost-benefit is obvious in the case of VAV.
2. Reduced HVAC Capex costs:
Revisiting our example, the HVAC system design would entail sizing the system to 3,60,000 cfm for CAV. Whereas, adopting a VAV system would require the HVAC package to be sized for only 1,80,000 cfm. Thus, significantly reducing the Capex. A possible argument could be about the additional costs of VAV components. That is true, however, the additional investment does get paid off over time in terms of energy savings, More the number of hoods, the higher the savings.
ROI on VAV systems easily offsets the system components’ costs.
3. Reduced Carbon Emission:
As VAV systems use much less carbon energy (electrical power) thus reducing emissions, a distinct feature in times of global warming.
4. Improved Safety:
VAV systems are fast responding and nowadays are aided with automatic sash controllers. They offer higher safety by offering better containment than CAV systems.
5. Comfort:
CAV systems tend to drastically fluctuate the lab temperature in open and closed positions. In the case of VAV, these fluctuations are smoothly evened out. VAV systems with RH control are also available. Therefore, there is increased comfort for the occupants inside the lab.
6. Reliability:
Modern VAV systems are very reliable, serviceable and long-lasting Their sensors and feedback system is extremely reliable.
What are the limitations of VAV systems?
There are very few limitations. One is of course the higher initial costs. However, higher cost (investment) is quickly paid back owing to higher energy savings. The higher the number of fume hoods in the lab, the higher the saving. The other limitation could be system complexity and maintenance needs. This is easily overcome with efficient system design and proactive service management.
What are the recent advances in VAV systems?
The modern VAV fume hood controllers are much smarter in responding and processing data. This means there is better communication with the BMS and better comfort with increased energy savings. These controllers log data and help build up a diagnostic history too. Those are digital and not analogues anymore. A sash control system works in tandem with modern VAV controls, offering higher energy efficiency and increased safety. The user interface is ergonomic and simpler.
Thus, VAV system fume hoods are a necessity in a modern research laboratory. Initial costs may be higher but ROI is better with enhanced safety. A comfortable and ergonomically designed lab certainly improves the research output and keeps the scientific community stay focused on research. The advantages far outweigh the limitations.
We at LabGuard are willing to collaborate on your next project right from inception. Remember, a good system design is a foundation on which VAV systems are built. We have the necessary expertise and experience to design and deliver a perfect VAV system in the shortest possible time. Do get in touch with LabGuard for your VAV fume hoods requirements.
