Step-Variable Air Volume Fume Hood Control: A Case Study

Martin J. Wendel Jr., P.E., and Sarla M. Patel, P.E., Kling Lindquist

Abstract:

We propose to present a case study of an innovative design of a fume hood control system that we developed, as a low first cost method of applying an energy savings technique to laboratory fume hood control. This design innovation combines the best features of traditional constant volume and variable volume fume hood controls, while avoiding some of their perceived disadvantages.

Step-Variable Air Volume Laboratory Fume Hood Control
Fume hoods are critical elements in the design and operation of research and development laboratories. Fume hoods are essential in many applications to protect workers from exposure to chemical fumes or other hazardous substances. At the same time, fume hoods have a significant impact on laboratory construction costs and operating costs because of the equipment and energy required to exhaust air through the fume hoods, and to provide conditioned ventilation air to the laboratory spaces.

One of the many important laboratory design issues associated with fume hoods is the selection of a fume hood control strategy, which then leads to the control strategy for overall laboratory airflow and pressurization. Constant volume fume hood control has been used in many laboratories for years, and is generally considered to be a solid, reliable approach to safeguard workers. For laboratories with large numbers of fume hoods, however, constant volume fume hood control results in enormous energy use and operating costs, since the same amount of air must be conditioned and exhausted through the fume hoods whether they are in use or not (that is, whether the fume hood sashes are open or closed). More recently, variable volume (VAV) fume hood controls have been developed and installed to reduce energy consumption and operating costs in fume hood-intensive laboratories by reducing air flow through a fume hood when the sash is closed. Many of these systems, which typically maintain a constant fume hood face velocity to capture fumes and protect laboratory workers, are also generally considered to be solid, reliable systems with good track records. VAV systems though can carry a very high first cost and, in some cases, require high degrees of maintenance and effort to set up properly due to their multiple control points and complexity.

The many issues associated with fume hood control strategy selection can be quantified and analyzed in terms of performance, installed cost, operating and maintenance costs. However, it is essential that this system selection (and most others!) include consideration of owner acceptance, including the scientific research community that will use the labs and the building engineers who will maintain the systems. The building design must be responsive to the needs of the people who will work in the building. Sometimes, as a result of the consideration of both human and technical requirements, the building design professional has to think outside the constraints of pre-packaged solutions to achieve design excellence.

This presentation discusses one example of this type of innovation, and the benefits of the resulting system design. Our intent is to present a case study profiling the implementation of a Step-VAV hood control design that provides only the required amount of air to each hood, which minimizes the capital cost (through diversity in equipment sizing) and operating costs. The control system is simple, reliable and easy to maintain. It utilizes constant volume bypass hoods. The only automatic sensing requires hood sash end switch position monitoring by the BMS, using simple proven technology. Exhaust flow for each hood is controlled in two steps: maximum flow when any sash is open; and minimum flow when all hood sashes are closed. We are, in essence, combining the best features of two of the traditional methods of fume hood control in a non-traditional manner to offer a low cost approach to energy savings.

Biographies:

Martin J. Wendel Jr., PE is an Engineering Design Principal at Kling Lindquist, a Philadelphia based Architectural-Engineering practice. He graduated from Drexel University with a Bachelor of Science in Mechanical Engineering and is a registered Professional Engineer in the state of Pennsylvania.

He has over 20 years of experience in the design of high technology facilities. He has worked for Kling Lindquist for 10 years, with a focus on the design of mechanical systems for laboratory, pharmaceutical, animal and biotech facilities. He is responsible for the conception and development of MEP systems for those projects.
Martin is a member of ASHRAE and ISPE (International Society of Pharmaceutical Engineers) and has given several presentations on facility design.

Sarla M. Patel, PE is Senior Instrumentation Engineer at Kling Lindquist, a Philadelphia based Architectural-Engineering practice. She graduated from University of Michigan with a Master of Science in Electrical Engineering and is a registered Professional Engineer in the state of Pennsylvania.

She has over 30 years of experience in the Instrumentation and control design for high technology facilities such as Pharmaceutical facilities and Computer Data centers. She is responsible for the I&C system design and its reliability evaluation.
Sarla is a member of ISA (Instrument Society of America) and has given several presentations on Instrumentation and Control design.