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Constant Flow, Variable Flow, and All the Space Between

Jim Coogan, P.E., Siemens Building Technologies

Truly constant volume laboratory ventilation systems are almost obsolete, but that doesn't mean everything is fully VAV. There is a wide range of 'nearly constant' systems that respond in steps to user's demands. Sometimes called 'two-state' systems or 'two-position constant volume' the concept offers a lot of options to ventilation designers.
The first step is to identify the conditions in the room that call for differing flow rates. Often it's the presence or absence of people in the room, but it can be also be the need to use a particular exhaust device or some other aspect of the work the users do. Second, the flow rates are determined, for the particular device related to the user's task, and for the other flow devices that coordinate with it. The third step is to decide what inputs should trigger the change in flow rates. Manual switches, time schedules, occupancy sensors, and many other means have been applied. The final engineering consideration is an indicator to tell the users the state of the ventilation system. A variety of examples illustrate the thought process connecting users' needs, engineered systems, and operating procedures.


Jim Coogan, P.E., is a principal in product development and applications for Siemens Building Technologies. He has over 25 years experience designing microprocessor-based controls for mechanical systems, with 19 of those spent in the HVAC industry. Jim has served chairman of ASHRAE Technical Committee 1.4, Controls and is an active member of TC 9.8 Laboratory Systems. Publications include technical papers on room pressurization and laboratory commissioning.

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