Design Issues

Diversity   

For a laboratory HVAC system to operate to its full potential, equipment must be sized properly for the expected loads and airflows. A VAV laboratory can be expected to have lower airflows than other laboratory types. Taking diversity into account means sizing building equipment for expected airflows instead of the maximum possible airflows. 

This does entail a level of risk however, for the needed airflow will not be available should all devices require it. When predicting VAV airflows in a laboratory building, diversity levels must be decided on a case-by-case basis. No two laboratories will experience exactly the same usage patterns. Professionals must be consulted to determine the level of hazard to be present in a laboratory. Only after these steps have been performed can a realistic estimate of the building airflows be determined. 

The savings from diversity result from the assumption that not all laboratories will require full airflow at the same time. These savings take the form of downsizing capital equipment. Much of the cost of a buildings HVAC system is in the chillers, fans, air handlers, and other large equipment. Sizing this equipment for expected loads instead of maximum possible loads can cover much of the cost premium of VAV controls. Operating expenses may also decrease, because HVAC equipment is most efficient at full-flow operation—equipment sized for expected airflows will run at close to peak capacity instead of part-load.

Design Issues

Face velocity of 100 fpm, which is approximately 1 mile per hour. Room air currents have a large impact on a hood’s ability to contain. Practices to eliminate the effect of room air currents, on fume hoods are:

• Choosing and placing supply diffusers to keep room air currents less than 30% of the face velocity. For a 100 fpm face velocity, room air currents at the hood must be limited to a 30 fpm maximum. This may also require ensuring that each hood has sufficient space around it.

• Placing fume hoods away from walkways within the lab. People commonly walk at a speed of 2 to 3 miles per hour, creating a “wake” that can draw air out of a hood.

• Locating fume hoods away from doors. Laboratories are normally held at a lower pressure than surrounding areas. When the door is open, air will enter the laboratory, potentially upsetting the fume hood. Additionally, it may not be safe to force people to pass by the fume hood to exit the laboratory should there be a spill or other accident in the fume hood.