Laboratory Design Newsletter 2012 Selected Abstract

HVAC Ring Concept for Total Flexibility

Peter Dockx, Van Looy Group

Abstract

There is one big difference between HVAC systems for laboratories and HVAC systems for offices, classrooms, etc.: HVAC for laboratories is a dynamic process.

Daily fluctuations are the first reason why HVAC is a dynamic process. Ventilation in a laboratory has to deal with laboratory exhaust, supply air to compensate all that exhaust, keep the minimum ventilation rates required for that laboratory, and often maintain the temperature in the laboratory. Depending on the time of the day, one of these criteria will determine the ventilation rate in a laboratory. Especially in laboratories with several fume hoods, equipped with VAV systems, the dynamic can be great. If all fume hoods are closed, the air exhaust will be low. If all fume hoods are open, the air exhaust demand will be high. For good air management, modern laboratories are equipped with a so-called "lab controller" to deal with the fluctuations in air demand.

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The second reason is the constantly changing setups and needs in a laboratory, such as new equipment with new needs for exhaust and ventilation. The flexibility of a laboratory is determined by not only how easy new equipment can be installed on the benches but also how easy it is to adapt the ventilation to the new needs.

Classic HVAC systems are designed in a so-called "antenna" system. This means that designers start from the air-handling units or exhaust fans with a large duct, narrowing downstream to small ducts for the last diffuser or exhaust point. They always have to design the last ductwork so that it can handle the biggest expected airflow. If ventilation demand changes due to changes in laboratory setup, they have to start dimensioning the system all over again. In practice, this is never done. Instead they just install some new ductwork and fans to deal with the new situation. After 15 to 20 years, a lot of new ductwork and extra fans will have been installed, which makes an overview and maintenance of the installation a hard job.

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Since the air demands in a laboratory are not constant and can vary from laboratory to laboratory in time, you can design your ventilation system for certain diversity. For example: If you have 10 fume hoods with a VAV system installed in a laboratory, but only seven people at most work in this laboratory, you can design the HVAC system for use of seven fume hoods at maximum flow and three fume hoods at minimum flow. This is a so-called diversity of 70 percent. The next step takes place at the floor level. If you have 50 fume hoods on one floor and at most 25 people work on the floor, you can design your system for a diversity of 50 percent. If you design your HVAC ductwork in an antenna system, you always have to design it to handle the 70 percent diversity until the last laboratory, because you never know up front where the 70 percent capacity will be needed. If you design your HVAC ductwork in a "ring" system, you design it only for that diversity of 50 percent. The "ring" system will automatically be balanced since the air will split to the two sides of the ring on the path of less resistance. Dealing with changes is much easier. A ring system is "plug and play": new exhaust and supplies are "plugged in" using CAV or VAV valves hooked up in the laboratory controller and it works.