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Measurement and analysis of vitiation of secondary air in air distribution systems (RP-1276).

Article Excerpt
INTRODUCTION

ANSI/ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality, (ASHRAE 2007) specifies minimum ventilation rates and other measures intended to provide indoor air quality conditions that will be acceptable to occupants of buildings and that minimize the potential for adverse health effects. Standard 62 was first published in 1973 and has under-gone many revisions, including substantial changes to the amount of outdoor air that must be provided to occupied areas, and to the procedure for calculating these amounts. One of these revisions was the introduction of the multiple spaces equation (MSE).

The MSE can be applied to buildings that use a recirculating system--one that recirculates some portion of air returned from the occupied zones in its supply air. The MSE is based on a mass balance. It accounts for the fact that only the critical zone--a zone that requires the highest fraction of ventilation air in its supply air--will produce fully vitiated air (where fully vitiated air is air from zones that receive exactly the minimum ventilation rate required by the standard, and ventilation air is a combination of outdoor air and recirculated return air that is not fully vitiated). All other zones receive more outdoor air than the minimum required, so that the return air from those zones contains some unvitiated air. This unvitiated air is recirculated, so the amount of new ventilation air that must be introduced is lower than if this effect were ignored.

Originally, the MSE accounted only for single-path air distribution systems. Warden (1995) pointed out that a secondary path--one that does not pass through the central air handler--could also be used to help ventilate the critical zone. He presented a general case of the MSE that accounted not only for the primary path, as the earlier MSE does, but also for secondary paths, such as dual-fan dual-duct systems, fan-powered terminal units, and transfer fans. Ke (1997) also developed a generalized MSE with a different form, but consistent with Warden's derivation. The general case of the MSE was included as Equation A-2 in the 2004 version of ASHRAE Standard 62.1.

To use the general case of the MSE for ventilation calculations requires knowledge of two additional variables not previously considered. In the nomenclature of the standard, these are [E.sub.p] (the primary air fraction to the zone, i.e., the ratio of primary airflow to total discharge flow) and [E.sub.r]. The first of these variables is straightforward, a fraction of two quantities typically known by a designer. The variable, [E.sub.r], however, is more complex and is the subject of this paper.

DEFINITION OF [E.sub.r]

ASHRAE Standard 62.1-2007 defines [E.sub.r] as the "fraction of secondary recirculated air to the zone that is representative of average system return air rather than air directly recirculated from the zone." No quantitative definition is provided, but guidance in selecting a value is given in a note:

For plenum return systems with local secondary recirculation (e.g., fan-powered VAV with plenum return), [E.sub.r] [less than or equal to] 1.0. For ducted return systems with local secondary recirculation (e.g., fan-powered VAV with ducted return), typically [E.sub.r] = 0.0.

The general case of the MSE (Equation A-2 in the standard) is shown in expanded form in Equation 1.

[E.sub.vz] = [[([E.sub.p] + (1 - [E.sub.p])*[E.sub.r]) + [X.sub.s]*[E.sub.p] - [Z.sub.d]*[1 - (1 - [E.sub.z])*(1 - [E.sub.r])*(1 - [E.sub.p])]]/[[E.sub.p] + (1 - [E.sub.p])*[E.sub.r]]] (1)

Where

[E.sub.p] = [V.sub.pz]/[V.sub.dz]

[E.sub.vz] = the ventilation efficiency of the zone in question

[E.sub.z] = the effectiveness of the zone air distribution system in delivering ventilation air to the breathing zone (zone air distribution effectiveness)

[V.sub.dz] = discharge flow rate to the zone

[V.sub.ou] = uncorrected outdoor...

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